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X-RAY SENSITMTY AND X-RAY INDUCED

CHARGE TRANSPORT CHANGES IN STABILIZED

a-Se FILMS

A Thesis

Submitted to the College of Graduate Studies and Research

in Partial Fulfillment of the Requirements

for the Degree of

Doctor of Philosophy

in the Department of Electrical Engineering

University of Saskatchewan

Saskatoon

MARK TIMOTHY ALEXANDER NESDOLY

Saskatoon, Saskatchewan

Spring 2000

Copyright O 1999: Mark T. A. Nesdoly

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ABSTRACT

Stabilized amorphous selenium (a-Se:0.2-0.5%As7 I0 - 20 ppm C1) is currently an x-ray photoconductor that is in use in medical diagnostic radiography. Amorphous selenium is attractive because it may be deposited in large areas with relative ease. It also hoIds promise for decreasing x-ray exposure for the patient and increasing the resolution of the radiographic image when compared with conventional filmbased radiography. The x-ray sensitivity and charge transport properties of a-Se are far from ideal, however it still performs acceptably as an x-ray detector. Even though existing commercial digital radiographic units employ a-Se as the x-ray detector, there are still several aspects regarding how x-rays affect the charge transport of a-Se that had not been investigated prior to this work. For example, the behaviour of electron transport following exposure to x-rays was unknown prior to this investigation. Knowledge of how exposure to x-rays adversely affects the charge transport properties of a-Se may ultimately lead to some means of minimizing or counteracting these same changes. This one aspect-minimization of x-ray induced charge loss-is a very important factor in decreasing patient dose.

The samples used throughout this work were studied via the timesf-flight (TOF) and interrupted field time-of-flight (IFTOF) transient photoconductivity techniques. The samples were of commercial device quality as evidenced by their low charge loss via deep trapping. This is quantified by a quantity known as the Schubweg, which is a measure of the average distance a charge carrier will travel before becoming trapped and, therefore, lost to conduction. In general, the Schubweg of a charge carrier must be longer than the thickness of the a-Se film in order to ensure that little charge will be lost due to deep trapping-

The detection of x-rays depends on the generation of fiee etectrons and holes within an a-Se film, and the subsequent collection of those charges once they reach the electrodes. This study investigates the mobility and trapping of charges and the recombination of x-ray induced charges in a-Se. Changes in these parameters, caused by exposure to x-rays, were also investigated. Prior to exposure to x-rays, the mobilities and deep trapping lifetimes of both holes and electrons were found to be constant and would not vary over time. No change in the mobility of holes or electrons was found upon exposure to x-rays. However, immediately following exposure to x-rays, the hoIe deep trapping lifetime would fall - 30% while the electron deep trapping lifetime would change oniy - 10%. Mer that initial exposure, both the hole and electron deep trapping lifetimes continued to change ovet time-in some cases rising, while in others falling. These changes were tracked ovet as much as a I2 hour period. Following a 24 to 48 hour rest period after that initial x-ray expomre, the hole and electron lifetimes within the a-Se

film wouId return to a stable, unchanging state. However, these rested lifetimes would not necessarily be equal to the initial lifetime prior to the x-ray exposure one or two days earlier. It is proposed that these changes occur because of a relaxation or reordering of the atoms in a-Se, similar to changes thought to occur as a result of exposure to visible light.

Analysis of the experimental evidence suggests that intimate valence alternation pair (IVAP) charged defects are created by x-ray irradiation; the concentration of these defects was found to be proportional to the absorbed x-ray dose with higher doses creating a higher concentration of these defects, These same defects were observed to be relatively unstable, disappearing within two hours after irradiation. Since these IVAP defects were observed to disappear within two hours after irradiation while the hole and electron lifetimes continued to change for at Ieast 12 hours, it was concluded that the traditiond view of deep charge trapping into these charged IVAP defects cannot be the cause. Indeed, some researchers have long suspected that these charged defects were not the cause of the deep traps in a-Se. A new charge trapping theory, consistent with published optically induced effects, is proposed in this work to explain these observations.

Annealing, i&-red soaking and ultrasonic treatment were investigated as possible methods to restore the original charge transport properties of a-Se films that were exposed to x-rays. However, none of these methods were found to aid this recovery.

The energy required to create a ike electron-hole pair in a-Se by exposure to x- rays was calculated by observing the total charge liberated by an x-ray beam when it impinges a biased a-Se film, and dividing that charge by the energy that the x-ray beam deposits in the h. This energy, Wmp, was found to exhibit a very pronounced field and temperature dependence, while almost no dependence on the mean energy of the x-ray beam was observed. These findings are consistent with the geminate recombination theory? generally agreed to be the dominant charge loss mechanism with optical photons in a-Se.

The persistent x-ray photocmt was investigated and was found to be thermally activated below - -20 "C with an activation energy of 0.16 eV. This energy does not co~espond to the level of any known traps in the bandgap of a-Se, and this finding lends further support to the charge trapping theory developed in this work to explain the charge trapping lifetime behaviour upon exposure to x-rays.

ACKNOWLEDGEMENTS

I would first like to thank my wife Nicole whom I met at the very start of this

endeavour three years ago. Even though I was a poor student at the time, she still found

it in her heart to do me the honour of marrying me. Her support (and patience while I

monopoIized the computer) was very much appreciated. My mother was also there when

I needed it, and for that I will always be gratefid. Chris Haugen also deserves a big

''thank you" for the support and stimulating (not to mention lively) conversations

regarding the measurements. I would like to thank Dr. Hugh Wood for giving me the

opportunity to earn a little extra cash during this study by allowing me to teach at the

undergraduate level. I greatly value the experience. Dr. Narinder Sidhu deserves

mention for the many occasions that he graciously offered his time and assistance, and

for that I am grateful. I would also like to express my appreciation to NSERC and the

University of Saskatchewan for the 6nancial support during the past three years. Finally,

and certainly not least, I would like to thank Dr. Safa Kasap for his support during this

work. His insistence that I continually delve deeper for the underlying truth is

appreciated and is a lesson well leaned.

"In the field of observation, chance favors only the prepared mind" L. Pasteur

TABLE OF CONTENTS

COPYRIGHT ....................................................................................................................... i . . ABSTRACT ........................................................................................................................ n

............................................................................................... ACKNOWLEDGEMENTS iv .................................................................................................... TABLE OF CONTENTS v ...

LIST OF FIGURES ........................................................................................................ vul LIST OF TABLES ......................................................................................................... xvi

......................................................................................... LIST OF ABBREVIATIONS xvii

1 . Diagnostic Radiography and Amorphous Semiconductors ........................................... I ............................................................................................................... 1.1 Introduction 1

1 2 Introduction to Radiographic Imaging ...................................................................... 2 1.3 Advantages of Solid State Detection .................................................................... 6 1.4 Solid State Imaging Systems .................................................................................... 7

1.4. i Xerographic Mode Detection ............................................................................. 7 1.4.2 Active Matrix Array Imaging System ....................................................... 9

1.5 Research Objectives ................................................................................................ 12 ................................................... 1.5.1 Carrier Drift Mobilities and Trapping Times 12

1.5.2 Changes in Charge Transport with X-ray Exposure ........................................ 13 1.5.3 Electron Hole Pair (EHP) Creation Energy ..................................................... 14

.................................................... 1 5 4 Persistent X-ray Photocurrent Investigation 14 ......................................................................................................... 1.6 Thesis Outline 15

2 . Electronic Band Structure and Electrical Behaviour of Amorphous Selenium ............ 16 ............................................................................................................. 2.1 Introduction 16

2.2 Atomic Structure of Amorphous Solids .................................... 16 ......................................................... 2 3 Band Theory of Amorphous Semiconductors 17

2.4 Bonding Structure of Amorphous Selenium ........................................................... 22 2.5 Band Model of Amorphous Selenium .................................................................... 27 2.6 Charge Transport in a-Se ........................................................................................ 28 2.7 OpticaI Properties of a-Se ....................................................................................... 30 2.8 Summary ................................................................................................................ 32

3 . The Timed-Flight Transient Photoconductivity Technique ....................................... 34 ............................................................................................................. 3.1 Introduction 34

............................................................. 3.2 Principle of the T i f - F l i g h t Technique 34 ................................. 3.3 Principle of the Interrupted Field Time-of-Flight Technique 42

. . 3.4 Transient Trap Lmted Theory .................................................................. 43

.................................................................... 3.4.1 Monoenergetic Trap Distn'bution 13 ............................................................................................. 3.4.1.1 Low Field Case 47 ................................. 3.4.1.2 High Field Case ... 4 8

.............................................................................. 3.4.2 Extended Trap Distriiution 48 ............................... 3.5 Summary .. .......................... 52

4 . X-ray Photoconductors ................................................................................................ 53 4.1 Introduction ........................................................................................................ 53 4.2 Ideal X-ray Photoconductive Material ............ .., .................................................. 53 4.3 Practical X-ray Photoconductors ............................................................................ 55

.............................. 4.3.1 Amorphous SeIenium (a-Se) ... .......................... 56 4.3.2 Hydrogenated Amorphous Silicon (a-SiW .................................................... 57 4.3.3 Cadmium Telluride (CdTe) .............................................................................. 57 4.3 -4 Lead Oxide (PbO) ............................................................................................ 58

........................................................................................ 4.3.5 Crystalline Materials 59 4.4 X-ray Sensitivity ..................................................................................................... 60

............................................................... 4.4.1 The Energy Absorption Coeficient 6 0 .................................................... 4.4.2 Energy Absorption and Detector Thickness 63

...................................................... 4.4.3 Electron-Hole Pair Creation Energy: Wm 65 4.5 Summary ................................................................................................................. 70

. ..........................................................*............. 5 Experimental Apparatus and Procedure 71 5.1 Introduction ......................................................................................................... 71 5.2 a-Se Thin Film Preparation ..................................................................................... 71 5.2.1 Substrate Reparation ....................................................................................... 72 . . 52.2 Vacuum Deposlnon System ......................................................................... 73

.................................................................... 5.2.3 Transparent Electrode Deposition 74 5.3 TOF/IFTOF Apparatus ........................................................................................... 76

................................................................................................. 5.3.1 Nitrogen Laser 79 .................................................................... 5.3.2 MOSFET High Voltage Switches 81

5.3.3 Voltage Follower ............................................................................................. 87 . .................................................................................. 5.3.4 FPGA Tlrmng Generator 89 . . .............................................................................................. 5.3.5 Data Aquisltlon 92 5.4 X-ray Photocurrent (Wm) Measuremmts ............................................................. 93 5.4.1 X-ray Exposure System .......................... .. ............................................... 94 5.4.2 Pulse Height Spectroscopy ........................................................................... 97 5.4.3 Spectral MtswumnmbProcedure ...................................................... 100

................... ....................................................... 5.4.4 Measured X-ray Spectra .. 102 .................................................... 5.4.5 Energy Absorbed in a-Se Layer and W m 106

................................................................................................. 5.4.6 I-V Converter 110 .......................................................... 5.4.7 Low Temperature Protective Chamber 112

5.4.8 X-ray ColIimatodFilta Housing .................................................................... 113 ....................................................................... 5.5 Miscellaneous Experimental Tools 115

.................................................................................................. 5.5.1 Sample Oven 115 5.5.2 IR Light Source .......................................................................................... 116

5.5.3 Ultrasonic Waves ........................................................................................... 1 16 5.6 Summary ............................................................................................................... 117

............................................................................................... 6 . Results and Discussion 118 6.1 Introduction ................................................................................ ........................ 118

......................................................................... 6 2 Charge Transport Measurements 118 ...................................................................................... 62. L TOF Measurements 118

6.2.2 lFTOF Measurements .................................................................................... 123 6.2.3 a-Se Film Quality ........................................................................................... 134

....................................................... 6.3 X-ray Induced Changes in Charge Transport 137 ...................................................................... 6.3.1 Charge Carrier Transit Time t~ 138

6.3.2 Charge Carrier Lifetimes .......................................................................... 139 6.3.3 Light Induced Structlrral Changes ia a-Se ..................................................... 154

....................................... 6.3.3.1 Trapping Mechanisms and the Observed Results 161 ............................................................ 6.3.3 2 Irreversiile X-ray Induced Damage 171

........................................................................ 6.3.4 Electron L i m e Recovery 171 6.3.4.1 Anneding .................................................................................................... 173

.................................................................................................. 6.3.4.2 IR Soaking 175 .................................................................................. 6.3.4.3 Ultrasonic Treatment 176

6.4 Electron Hole Pair (EHP) Creation Energy - WEHP ............................................. 179 ................................................................. 6.5 Persistent X-ray Induced Photocurrent 187

6.6 Summary ............................................................................................................... 190

........................................................................... 7 . Conclusions and Recommendations 192 ........................................................................................................... 7.1 Intraduction 192

7 2 Charge Transport Study ........................................................................................ 192 ...................................... 7.3 Changes in Charge Transport Upon Exposure to X-rays 193

............................................................................................ 7.4 EHP Creation Energy 194 ................................................................. 7.5 Persistent X-ray Induced Photocurrent 194

................................................................................ 7.6 Suggestions for Future Work 194

8 . References ................................................................................................................... 197

LIST OF FIGURES

F i e 1.1. An x-ray image of the hand of the wife of Wielm Roentgen. The image was obtained with an exposure of more than 30 minutes; an eternity by modem standards. [Image obtained fiom httDJ/~~~.accessexceilence.ordAElAEClCC/hist~rical back~round.html] ........... 3

Figure 1.2, TypicaI projection radiographic medical imaging system .............................. 4 Figure 13. (a) Schematic of a-Se plate with active matrix readout and (b) a

cross-section through one pixel (not to scale). [After Rowlands J., and Kasap S.O., "Amorphous semiconductors usher in digital x-ray imaging", Physics Today, 50, 1997, pp. 24-30.] ..................................................................................... 10

Figure 1.4. An x-ray image of a wrist phantom obtained by an experimental active matrix array imaging system (reduced h m original; courtesy of Dr. J. Rowlands, Sunnybrook Health Science Centre). ................................................. 1 1

F i i 2.1. Two dimensional representatioa of the structure of (a) a crystalline solid; (b) an amorphous solid. Spheres marked " 0 and 'W' represent over-coordinated and under-coordinated atoms, respectively. ................................. 1 7

F i i e 22. Density of states for a crystatline semiconductor. ........................................ 18 F i e 23, DOS models as proposed by (a) Mott; (b) Cohen, Fritzsche and

Ovshinski (CFO); (c) Marshall and Owen. The hatched regions denote localized energy states. ............................................................................................. 20

Figure 2.4. Selenium chain and the definition of the dihedral angle 9. The dihedral angle is defined as the angle separating the two planes defined by atoms 123 and 234. It may be observed by looking down the bond connecting atoms 2 and 3 [26]. ................................................................................ 23

Figure 25. The random chain model of the structure of a-Se showing localized regions that are ring-like and chain-like [26] ............................................................ 24

F i e 2.6. Structure and energy of simple bonding configurations for seienium atoms. Straight lines represent bonding orbitals, lobes represent lone-pair (nonbonding) orbitals, and circles represent antiinding orbitals. The energy

................................. of a lone-pair is taken as the zero energy. [Adapted h m 381 25 ......................... Figure 2.7. Illustration of the a-Se structure detailing an IVAP defect. 26

F i e 2.8. Experimentally determined density of states function for amorphous selenium [39]. .......................................................................................................... 28

F i i e 2.9. Absorption coefficient a (solid line) and quantum efficiency q (broken lines) as a function of the incident photon energy hv for various applied electric fields [I 7,461. ................................................................................. 3 1

F i e 3.1. (a) Simplified schematic and (b) small signal ac equivalent for the TOF transient photoconductivity technique. C, is the sample capacitance. ............ 35

F i e 3.2. The motion of a charge q through a distance dx in the sample induces a charge dQ to flow in the external circuit ................................................................ 38

Figure 33. Signals from the TOF experiment where (a) is the I-mode signal and (b) is the V-mode signal. Solid lines represent expected signals in a trap-free solid while dashed lines represent signals in a solid with deep traps only. .............. 41

F i e 3.4. (a) A typical TOF waveform and (b) a typical ETOF waveform. The interruption time is ti; the magnitude of the photocurrent immediately before and after interruption is denoted il and iz .............................................................. 42

Figure 3.5. Current flow with trapping and reIease processes in a thin slice of semiconductive material. ....................................................................................... 44

Figure 3.6. Sketch of a Gaussian distribution of shallow energy traps that lay immediately below the conduction band edge. ........................................................ 5 1

Figure 4.1 An attenuator is placed in an x-ray beam. ..................................................... 6 I Figure 4 3 A number of different interactions are possible when an x-ray photon

enters a material. ................................................................................................ 6 1 Figure 4 3 Energy absorption coefficient vs. photon energy for various

................................................................................. photoconductive materials [8 11 64 F i e 4.4. EHP creation energy vs. bandgap for a selection of photoconductive

materials [8 11. ........................................................................................................... 67 F i e 45. Schematic representation of the different types of recombination that

are posslible in an a-Se photoconductor. The cylinders represent the tracks of primary electrons. Bulk recombination occurs outside the tracks between charges that originated in different tracks. Geminate recombination occurs between the original hole and electron. Columnar recombination takes place between electrons and holes h m different pairs, but within the same trac k........... 69

Figure 5.1. Schematic diagram of vacuum deposition system ....................................... 74 F i r e 52. Schematic diagram of the electrode sputtering system. ................................. 75 F i e 53. SimpIified view of the TOFAFTOF apparatus. Two PCs were used;

the 8088 PC connected to the CCD m e r a (anaiog oscilloscope) was necessary because the antiquated video h e storage card could not be used in the modem pentium PC. ......................... ,., .......................................................... 78

Figure 5.4. Potential energy mes for the lowest triplet states in the N2

molecule [98]. .................................................................................................... 80 F i r e 5.5. Blumlein circuit used for rapid excitation of Nz laser. ................................. 8 1

....................... F i r e 5.6. Trigger timing requirements of the LN 103 C nitrogen laser 82 F i e 5.7. Application and removal of the high voltage bias during the lFTOF

........... experiment induces large switching transients across the sampling resistor. 82 Figure 5.8. The MOSFETloptoisolator gate driver pair that forms the basic

building block of the switches built for this work. MOSFET: Motorola MTP3NIOOE n-channel enhancement-mode rated at 1000 V, 3 A, R,,, = 4 R, Q,, = 32.5 nC. Optoisolator Hewiett Packard HCPL-3101 Power MOSFETnGBT Gate Drive Optocoupler rated at 5000 Vac isolation and 0.4

........... A peak output current. A calculated switching time of 8 1 ns is achievable. 84 ............................................................... Figare 5.9. Schematic of the high side switch, 85

Figure 5.10. Schematic of the low side switch ..................... .. ............................. 86 F i i 5.11. Unloaded switching transients of the high side switch. ............................ .. 88

Figare 5.12. Photograph of the low side switch ............... ,. .................................... 89 Figure 5.13. Circuit schematic of the voltage follower ................................................ 89 Figure 5.14. Gain vs. hquency response of the voltage follower. ................. -....... ....... 90 Figure 5.15. A simplified diagram of the connections to the Ahera UP1

Education Board showing the fIow of data into and out of the board. .........-......... .. 90 Figure 5.16. A block diagram of the chained counters responsible for generating

the timing waveforms necessary to control the TONIFTOF apparatus, and the timing waveforms thus created (not to scale). ................................................... 9 1

Figure 5.17. Sample connections for measurement of (a) hole transport and (b) ..................................................................................................... electroa transport. 93

Flgure 5.18. Schematic diagram iilustrating the major components of a rotating anode x-ray tube ............................ .. ........,,..... 94

Figure 5.19. Typical electron interactions with a target. (a) Electron suffers ionizational Iosses, giving rise to delta rays and heat. (b) Electron ejects K shell electron leading to characteristic radiation. (c) Collision between nucleus and electron of energy E leads to bmnstmhlung radiation of energy hv. The electron recedes h m the collision with energy E - hv. (d) Collision where electron is completely stopped by a collision with the nucleus. The full energy of the electron is released as bremsstrahlung radiation. ......................... 96

F i i e 5.20. The principle of pulse height spectroscopy. The photon with the highest energy is emitted fiom the radiation source first and strikes the detector first, generating the highest photocurrent pulse. The process is repeated, in turn, for the lowest and intermediate energy photons, which create the lowest and intermediate photocurrent pulses, respectively. ..................... 98

F i e 5.21. A photograph of the pulse height spectroscopy measurement unit used in the course of this work. The photograph on the left shows the DART A/D unit and the Pb shielded amplifier and detector. The photograph on the right is a top view of the amplifier and detector alone, with the top portion of the Pb shielding removed to reveal the cylindrical detector atop the amplifier. ....... 99

Figure 5.2 2. Measured spectrum of the x-ray tube at 50 kVp and 15 mA. The e6ect of PPU completely obscures the real output spectnrm of the x-ray unit . - ......................*.. ..............*...*.*..........................**..*. when no filtettng rs present. ,., 10 1

F i e 5.23. Raw spectral data obtained fiom the pulse height spectroscopy unit for the 39.2 keV beam. PPU is present, but it may be used as a check on the

......................................................................................................................... data 104 ..................... Figure 5.24. (a) Raw data and (b) filtered spectra of the 39.2 keV beam 105

Figure 5.25. The manually filtered spectra of the four beams used throughout the ............ course of this work with mean energies of 32.8,39.2,47.1 and 58.2 keV. 106

.................. Figare 5.26. Diagram of the apparatus used to measure the WUrp of a-Se. 1 10 F i e 5.27. Circuit schematic of the I-V converter built for this work with a

.................... variable conversion gain selectable in decades from 16 to 10' V/A. I 1 1 Figare 5.2%. Diagram of the low temperature protective chamber flooded with

Ar ............................................................................................................................ 113 Fie 5.29. A cutaway view of the modified collimator attached to the x-ray

unit to restrict scattered radiation from reaching the samp1es. ............................... 1 14

Figure 530. Simple oven for sample annealing using a temperature controller and a heating element. A thermocouple connected to tbe temperature controller provides a means to measure the temperature of the aluminum block .................................................................................................................. 1 15

Figure 531. Equipment employed to create ultrasonic waves in the a-Se samples .................................................................................................................. 1 16

F i i e 6.1. Typical I-mode TOF waveforms in a-Se for (a) holes and (b) electrons. The transit times are indicated as tr. [(a) F = 0.98 VIP, (b) F = 5.69 Wpm] ............. ,., ............................................................................................ 120

Figure 6.2. (a) Typical electron TOF photocurrent showing the location of the photocurrent's knee and co~~esponding !4 value point. (b) The photocurrent of (a) is differentiated revealing a local maxima corresponding to the location of the knee in (a). Only a portion of the differentiated waveform (the tail) has been shown for clarity .............................................................. 121

Figure 6.3. Plot of TOF tramit time vs. 1N for (a) holes showing = 0.132 2 2 ....................................... cm N s and (b) electrons showing = 0.003 18 crn Ns. 122

Figure 6.4. (a) Hole and (b) electron mobility plotted as a hc t ion of applied field, The hole mobility shows very little field dependence while the

............. electron mobility shows a slight field dependence of the form * a FO.". 123 F i e 6.5. Typical electron lFTOF waveform in an a-Se film showing il, i2, and

............................................................................................................................... ti 124 Figure 6.6. Fractional recovered hole photocunent as a hction of intamption

time for a-Se. Measured hole lifetime h m the plot is 132 p. The charge packet was halted at a depth of 0263 L where L is the thickness of the film. ....... 125

F i e 6.7. Fractional recovered electron photocurrent as a function of interruption time for a-Se. Measured electron lifetime h m the plot is 657 p. The charge packet was halted at a depth of 0.263 L where L is the

............................................................................................... thickness of the film. I26 F i e 6.8. Fractional recovered photocurrent at a depth of 0.537 L for the same

sample as in Figure 6.7. The higher y-intercept indicates that less charges are tost at this depth than at the shallower halt depth of Figure 6.7. ...................... 127

Figure 63. Two possiiilities for the lost charge observed in hctiond recovered electron photocurrent lFTOF plots: (a) electron diffusion while the packet is halted and (b) large dispersion during driR The latter case is more likely, as

.............................................. ........... supported by the experimental evidence. .... 1 28 F i e 6.10. Effective electron fransit time tr' plotted as a fimction of

interruption time ti in a typical IFTOF experiment. The quantity plotted is the incremental difference in the effective transit time compared to the transit time obtained &om a TOF experiment .................................................................. 130

Figure 6.11. PIots of differentiated electron lFTOF photocurrent tails vs. time for an a-Se sampie. The intermption time was varied fiom 50.8 p (bottom

................................................................................. curve) to 699. I ps (top curve) 13 3 F i i e 6.12. Incremental eIectron dispersion h m an IFTOF experiment relative

to the dispersion h m a TOF experiment, ATOF, pIotted vs. the interruption ....................................................................................................................... time ti 134

Figure 6.13. Measured electron lifetime, normalized to the initial (t = 0) Lifetime, for an a-Se film over the course of a 6 hour period. The measured lifetime did not significantly vary with time, therefore the measurement technique did not alter the trapping characteristics of the a-Se film under study. The electron lifetime was initially determined to be 5 I0 ps. ......................................... 135

F i i e 6.14. (a) Electron and (b) hole TOF transit times, normalized to the initial t~ and plotted as a function of cumulative dose. Irradiation by 58.2 keV beam; sample shorted to ground during irradiation. .............................................. 138

F i i e 6.15. Normalized electron lifetime at two depths within an a-Se film tracked over time. The film was initially irradiated with a 32.8 keV beam giving an absorbed dose of 26.2 mGy, and the sample was shorted to ground during irradiation. [Sample 97 1002 121 ................................................................. 140

Figure 6.16. Normalized electron lifetime at two depths within the same film as that in Figure 6.15. Irradiation specifics: 32.8 keV, 26.2 mGy, F = 1.96 Wpm. ...................................................................................................................... 141

Figure 6.17. The a-Se film of Figures 6.15 and 6.16 is exposed to an unfiltered x- ray beam of 90 kVp, 15 mA and 1 second duration. The sample was shorted during irradiation. .................................................................................................. 142

Figure 6.18. Nonnaiized electron lifetime at two depths tracked over time. Irradiation specifics: 32.8 keV, 28.1 mGy, shorted during irradiation. [Sample 97 1205 621 ................................................................................................ 143

Figure 6.19. Same sample as that of Figure 6.18; irradiation specifics: 32.8 keV, 28.1 mGy, F = 2.16 V / p . ............................................................................. I44

Figure 630. Same sample as that of Figure 6.18; irradiation specifics: 58.2 keV, 25.4 mGy, sample completely open circuit during irradiation ....................... 144

Figure 6.21. Normalized electron lifetime at two depths tracked over time. Irradiation specifics: 58.2 keV, 25.4 mGy, F = 2.16 Wpm. [Sample 971205 621 ........................................................................................................................... 145

Figure 682. Same sample as that of Figure 6.21, except that the absorbed dose is 1.8 mGy ................................................................................................................... 145

Figure 6.23. Normalized hole and electron Lifetime at the same depth tracked over time. M a t i o n specifics: 582 keV, 1.8 mGy, F = 1.96 Vlpn.

................................................................................................ [Sample 97 1002 121 147 Figure 634. Same sample as that of Figure 6.24, except the dose is now 24.9

mGy ......................................................................................................................... 147 F i e 635. Effective electron transit time t-r' vs, ti for the data of Figure 6.23.

(a) Immediately before irradiation, (b) immediately following irradiation, and (c) two hours after irradiation. At this low dose (1.8 mGy), there is no

.................................... significant change in the behaviour of tT' with irradiation. 148 F i i e 626. Effective electron transit rime tT' vs. ti for the data of Figure 6.24.

(a) Immediately before irradiation, (b) immediately folIowing irradiation, and (c) two hours after irradiation At this high dose (24.9 mGy), the behaviour of tr' changes noticeably with irradiation, but recovers within two hours ........................................................................................................................ 149

Figure 637. Changes in the electron lifetime at two depths within an a-Se I lm as a function of absorbed dose. The samples were shorted to ground while they

were being irradiated. (a) 32.8 keV beam [Sample 971205 621. (b) 32.8 keV beam [Sample 971002 121. (c) 58.2 keV beam [Sample 960521 46 SE 151. (d) 58.2 keV beam [Sample 97 1205 621. ................................................................ 15 1

Figure 6.28. Changes in the electron lifetime at two depths within an a-Se fi as a function of absorbed dose. The samples were biased with a field of - 2 V@n while they were being irradiated. (a) 58.2 keV beam, F = 1.96 V l p [Sample 971002 121. (b) 32.8 keV beam, F = 2.16 V l p [Sample 97 1205 621. (c) 58.2 keV beam, F = 2.16 V l p [Sample 971205 621. (d) 58.2 keV beam, F = 2.35 Wpm [Sample 980622-31. ............................................................. 152

Figure 6.29. Changes in the electron lifetime at two depths within an a-Se film as a fimction of absorbed dose. The samples were biased with strong fields while they were being irradiated. (a) 58.2 keV beam, F = 7.84 V l p [Sample 971002 121. (b) 58.2 keV beam, F = 8.66 V / p [Sample 971205 621. (c) 58.2 keV beam, F = 17.1 V1p.u [Sample 980622 - 31. ............................. 153

Figure 630. (a) A schematic diagram of Tanaka's 1980 [I211 model of bistable local bonding geometries and (b) corresponding double well potential. ................ 156

Figure 6.31. Bond twisting model in a-Se. The equilibrium state (a) is altered when a LP electron is excited from atom A (b). A then feels a strong coulombic attraction to B, which twists A into a new position (c). The excited electron then recombines (d) and the structure is "fiozen in". ................... 157

Figure 6.32. Schematic diagram of the transformation of an exciton in Se into an NAP pair accompanied by atomic distortion. The resulting IVAP is sometimes referred to as a "self-trapped exciton". The resulting Se,+ and

Se; defects are commonly referred to as D' and Dm defects, respectively. ............ 1 58 Figure 633. Formation of metastable triply-coordinated defects which serve to

cross-link adjacent Se chains. Antibonding electrons are denoted as "e". The metastable triply-coordinated defects may decay into (I) their ground state, (11) new bonds, or (111) into an NAP. ......................................................... 159

Figwe 63. Traditional view of hole trapping in a-Se. (a) Electrically neutraI a- Se with an isolated VAP defect and a drifting photoinjected hole. (b) Hole trapped by the D' defect resulting in an electtically neutral defect with a dangling bond .......................................................................................................... 1 63

Fignre 635. Traditional view of electron trapping in a-Se. (a) Electrically neutral a-Se with an isolated VAP defect and a drifting photoinjected electron. (b) Electron trapped by the D" defect resulting in an electrically neutral defect with a dangling bond. ....................................................................... 162

Figure 636. (a) The single trapped hole of Figure 6.34 (b) that results in a neutral singly coordinated defect may rid itself of its dangling bond by approaching a nearby chain and forming a neutral threefold defect. (b) The. unpaired electron of the neutral threefold coordinated defect could be

....... immediately neutralized by a nearby photoinjected hole to form a Dt d e f m 163 F i e 637. (a) The single trapped electron of Figure 6.35 (b) that results in a

neutral triply coordinated defect may rid itself of its dangling bond by releasing a bond and forming a neutral twofold bond and a nearby neutral singly coordinated defect. (b) The unpaired electron of the neutral singly

coordinated defect could be immediately paired by a nearby photoinjected electron to form a D- defect. .................................................................................. 164

F i r e 638. Two Se chains in which two twofold Se atoms are physically quite close to one another. (a) A photoinjected hole neutralizes a lone pair electron on one atom, leading to (b) a transient bond that cross-links the two chains as well as a single unpaired electron. (c) A second photoinjected hole neutralizes the unpaired electron, fonning a pair of D' defects .............................. 165

F i r e 639. Two Se chains in which there exist stretched and compressed bonds. (a) A photoinjected electron nears a stretched bond to form (b) a D- defect and a neutral singly coordinated defect that has an unpaired electron. (c) The neutral singly coordinated defect d i m a short distance before meeting a second photoinjected electron to form another D* defect. ...................................... 166

Figure 6.40. The shallow rested electron lifetime (closed circles) of Sample 971002 12 tracked over the course of the work. The absorbed dose on the occasion immediately preceding each experiment is indicated with the open diamonds. The dose preceding experiment 6 is not known, as that was the

........... exposure without any filtering placed in the beam (mentioned previously) 172 Figure 6.41. The shallow rested electron lifetime (closed circles) of Sample

971205 62 tracked over the course of the work. The absorbed dose on the occasion immediately preceding each experiment is indicated with the open diamonds. ............................................................................................................... 172

Figure 6.42. (a) Normalized electron (solid circles) and hole (open diamonds) lifetimes as a function of total time spent at 35 "C. (b) The room temperature recovery of those l i f h e s tracked over a three day period following the heat treatment. Both lifetimes were measured at the same

............................................ depth within the film: 0.263 L. [Sample 97 1002 121 174 F i 6.43. (a) Normalized electron mobility as a function of total time spent at

35 OC. (b) The room temperature recovery of the mobility tracked over a ..................... three day period following the heat treatment. [Sample 97 1002 121 175

Figure 6.44. Normalized electron lifetime at two depths within an a-Se sample before irradiation (stage I), immediately after irradiation (stage 2), and following 3 hours spent at 35 "C (stage 3). Irradiation specifics: 58.2 keV, 35.7 mGy, sample shorted during irradiation. [Sample 96052 1 46 SE 1 53 ............ 176

Figure 6.45, Normalized electron lifetime at two depths within two different a-Se samples. (a) Before irradiation (stage l), immediately after irradiation (stage 2), following 30 seconds of IR soaking (stage 3). Irradiation specifics: 58.2 keV, 96.6 mGy, sample shorted during irradiation and during soaking. [Sample 971002 121 (b) Before irradiation (stage I), immediately after itradiation (stage 2), following 30 seconds of IR soaking (stage 3), foUowing 90 seconds total IR soaking (stage 4). Irradiation specifics: 582 keV, 28.6 mGy, sample shorted during irradiation, but F = 4 V l p during IR soaking.

...................................................................................... [Sample 960521 46 SE15] 177 Figure 6.46. Normalized electron and hole lifetimes at the same depth (0.40 1 L)

within an a-Se sample initially (stage I), following 15 minutes of ultrasonic treatment (stage 2), two hours thereafter (stage 3). [SampIe 960521 46 SE15J ...................................................................................................................... 178

Figure 6.47. A typical x-ray induced photocment in a biased a-Se film, detailing the "spikes" due to the self-rectifying nature of the tube and the "rising baseline". Data obtained with a 58.2 keV beam and Sample 1463-3 biased at 1.77 Vlpn ............................................................................................................... 180

Figure 6.48. The energy required to create a fiee EHP as a function of the reciprocal electric field for four different mean x-ray beam energies spanning the range 32.8 - 58.2 keV. Linear regressions for the four sets of data converge to within 0.5 eV to yield an expected WL = 5.9 eV at infinite field. ........................................................................................................................ 182

Figure 6.49. Dependence of WEHP on the mean x-ray beam energy at a constant field of 10 V l p . .................................................................................................... 183

Figure 6.50. Plot of the energy required to create a collected EHP, Wm, as a function of temperature. The mean beam energy was 58.2 keV and the electric field was held constant at 1.77 V l p . [Sample 1463-31 ........................... 185

Figure 651. Photogeneration efficiency as a hc t ion of temperature. Mean beam energy: 58.2 keV, sample biased at 1.77 V l p . [Sample 1463-31 ............. 186

F i e 6.52. Plot of the charge contained in the baseline and photocurrent spikes vs. temperature. 58.2 keV, 1.77 V l p , Sample 1463-3. The two exhibit markedly different temperature dependencies. ...................................................... 1 88

Figure 6.53. Baseline charge as a function of the reciprocal of the temperature. The amount of charge released in the persistent photocwent becomes thermally activated below approximately -20 O C . The activation energy is 0.16 eV, which does not comespond to the deep hole or electron traps in a-

LIST OF TABLES

Table 4.1 A concise comparison of candidate x-ray photoconductive materials ............ 59 Table 5.1. Filter combinations and tube settings for the four different spectra ........... 103 Table 53. The four diffetent x-ray beams and their Kph values. ................................... 109 Table 6.1. Charge transport properties of the four a-Se films involved in the . * . carner hfebme study. ....................................................................................... 136 Table 6.2. Charge transport properties of the two a-Se films involved in the x-ray

photoconductive experiments. ... .... . . .. . ... .. . .. . .. . . .. . . .. . . . . .. . .. . . . . . . . . . . . . . . . . . . . . . . . . 1 36 Tabb 63. Minimum operating field for the rested (best case) and damaged

(worst case) electron lifetime conditions observed during the study ...................... 154

LIST OF ABBREVIATIONS

a-Se

a-Si:H

Am

AB

CB

CCD

CFO

CZT

DEC

DOS

DSC

EHP

ESR

FET

FPGA

FWHM

HVL

IFTOF

IR

IT0

I-v

IV AP

~ V P LP

MCA

amorphous selenium

hydrogenated amorphous silicon

analog-todigital converter

antiinding

conduction band

charge coupled device

density of states model proposed by Cohen, Fritzsche and Ovshinski

cadmium zinc telluride

deviant electron configurations

density of states

differential scanning calorimetry

electron-hole pair

electmn spin resonance

field effect transistor

field programmable gate array

W-width half-maximum

half value layer

intermpted field timesf-flight

&-red

indium-tin-oxide

merit-to-voltage

intimate valence alternation pair

peak kilovolts

lone pair

multichannel analyzer

MOSFET

NB

NSB

PACS

PC

PPM

PPU

PZT

SNR

SPECT

SSR

TEA

TFT.

TOF

TP

metal oxide semiconductor fieId effect transistor

nonbonding

normal structure bonding

picture archival and communications system

personal computer

parts per million

pulse pileup

lead zirconate titanate

signal-to-noise ratio

single photon emission computed tomography

solid state relay

transversely excited atmospheric

thin film transistor

time-of-flight

transient photoconductivity

transistor-transistor logic

valence alternation pair

valence band

1. Diagnostic Radiography and Amorphous

Semiconductors

1.1 Introduction

To the best of our present knowledge, we live in a quantum universe on a world

governed by the laws of quantum mechanics. This theory, pioneered over 60 years ago

by physicists such as Heisenberg, Shriidinger and Dirac, has helped us to understand the

intricacies of semiconductor behaviour. Indeed, it has catapulted us headlong into our

modem technological age mainly through the development of the transistor. However,

there is one caveat to quantum mechanics; the complex mathematical application of the

theory to a solid may be greatly simplified only if the material is crystalline in nature (i.e.

periodic). The nonperiodic spacing of the atoms in an amorphous material renders the

application of quantum mechanical theory to these materials quite difficult This is due

to a dramatic rise in the mathematical complexity when dealing with these nonperiodic

mctures.

Electronic devices based on amorphous materials did not experience the same

explosive growth that their c q d k e counterparts enjoyed for a number of reasons.

First, it was Iong suspected that amorphous semiconductors did not share the same charge

transport properties of their crystalline forms. When it was discovered that this was not

the case, modeling the behaviour of an amorphous semiconductor became a daunting task

because of the nonperiodic nature of the amorphous solid. Only since the 1960s has

much of the theoretical understanding of amorphous semiconductors been derived,

mainly through experimez~tal observation. Despite this late start, it is quite probable that

amorphous semiconductors will be the basis of the next era of dramatic growth in the

microelectronics i n d w [I]. Amorphous semiconductors are an attractive alternative to

crystalline semiconductors because they possess a greater diversity of physical properties

and the preparation (or growth) of amorphous solids does not usually require the same

carefully c o n t r o l 1 W d s l o ~ c h i q u e s . This amounts to a potentially tremendous

economic savings for the industry.

In addition, amorphous semiconductors are aiready in majar commercial use in

such applications as amorphous silicon thin film transistors (TFTs), photoconductors and

solar cells. They are also starting to make an impact in the field of medical diagnostic

radiography as an x-ray detector material. The development of radiography and modem

radiographic image formation through the use of photographic film will be discussed in

the next section.

1.2 Introduction to Radiographic Imaging

The discovery of x-rays by the German physicist Wilhelm Conrad Roentgen

(1845 - 1923) in 1895 was an accident. In spite of their accidental discovery, he found

that his newly termed "x-rays" could be used to view the internal structures of the human

body and aid in medical diagnosis. This was very quickly followed by the development

of the field of medical radiography.

Figure 1.1 is an x-ray image of the hand of Frau Roentgen. It is one of the 6rst

known x-ray images of the internal structure of the buman body. Although the image is

very fuzy and the contrast is poor by modem standards, the potential practical use of x-

rays was immediately apparent In June 1896, only 6 months after Roentgen announced

his discovery, x-rays were being used by battlefield physicians to locate bullets in

wounded soldiers.

Radiography st i l l counts itseIf as one of the most usefial and widely used tools to

aid physicians in making a patient diagnosis. It should be noted that diagnostic

radiography refers to those processes that form medical images through the use of x-rays

as the infomation carrier [2]. However, for the most part, modem radiography differs

very little fiom the first systems developed over 100 years ago in that it is still a

photographic film based analog technology.

F i e 1.1. An x-ray image of the hand of the wife of Wilhelm Roentgen. The image was obtained with an exposure of more than 30 minutes; an eternity by modern standards. [Image obtained h m htt~:llwww.accessexcellence.orelAEIAEUCC/historicai backmund.html]

Radiographic imaging systems rely on the differential attenuation of ionizing

radiation (x-rays) by the different s t m ~ and tissues in the body to produce a

radiological image. A typical configuration is shown in Figure 1.2, and consists of the

f d a r projection radiographic system with its x-ray source and an x-ray detector that

are placed in h n t of and behind the patient, reqectiveIy. In present systems, the

detector is almost exclusively based on photographic film.

X-ray Detector

n

X-ray S o w

F i e 1.2. Typical projection radiographic medical imaging system.

The x-rays that pass through the patient undergo differential attenuation

(governed by the body tissues through which they must pass) and this modulates the

radiation intensity that reaches the detector. This radiologicai image must then be

detected and stored until a usable optical image can be produced for the medical

practitioner. Conventional detect& consist of a cassette of photographic film

sandwiched between two fluorescent screens; the screens convert the incident x-ray

photons into visible light which then in turn exposes the film. To recover the image, the

film is &ally developed using standard photographic techniques.

This traditional method has numerous drawbacks. First, there is the problem of

how and where to store the x-ray films once they have been developed, as they occupy

considerable space. Further, these images must be developed using conventional

photographic techniques which are tedious and involve hazardous chemicals. Second,

the information on these films is difficult to share; in order for a doctor to consult a

colleague, the films must either be shipped to their destination or they must somehow be

scanned into digital form in order to be shipped electronically. Another much less

desired option is for the patient to travel to the site of the second doctor and have another

set of x-rays taken there. Third, this method suffers fiom image blwing, a natural

consequence of the fiWphosphor screen combination. Fourth, and perhaps most

important, this method of forming a radiological image requires a relatively high x-ray

dosage (intensity) in order to satisfactorily expose the film so that an image may be seen

afier development, This is due to the fact that the phosphor layers are rather insensitive

to x-ray radiation. Since radiation damage in living tissue is cumulative [3], it is strongly

desirabIe to reduce the x-ray intensity required for a given radiological image. Reduction

of x-ray exposure is tantamount to increased patient safety. The interaction between x-

ray radiation and the human body can lead to biological effects that were first noticed

soon after the discovery of x-rays. The hazard of burns due to overexposure soon became

evident; the risk of cancer was realized Iater. The damaging effects of x-ray radiation

have been summarized as follows:

"Unfortunately, x-rays, like other forms of ionizing radiation, affect living tissue, and exposure to x-rays must be limited to minimize tissue damage. At Iow doses, the biological effect is to change the cell metabolism and structure. This effect appears after only some latency period, which decreases as the radiation dose increases. Higher radiation doses result in cell death. The effect of radiation is cumulative, with small doses received at long intervals having an additive effect. Hence it is very important that x-ray imaging systems use the minimum amount of x-ray energy consistent with obtaining a good image [3]."

The idea of a solid state planar detector for use in x-ray imaging is not a new one;

the concept was first investigated in the 1940s and interest has been fervently renewed of

late [4]. The various advantages promised by a solid state detection scheme will be

introduced in the next section.

1.3 Advantages of Solid State Detection

Any effort to reduce the x-ray exposure of patients (and physicians--as in the

case of some fluoroscopic medical procedures like angiography) would increase overall

safety for all concerned. This particular area is where solid state detection promises the

greatest advantage; with proper selection of the detector material it is possiile to reduce

the exposure level relative to fiIm based radiography and still obtain an image of

comparable, if not superior, quality.

Solid state detection also promises to produce sharper images than photographic

film based techniques. Since a solid state detector directly converts incident x-ray

photons to mobile charge carriers, these carriers may be collected through the use of an

applied electric field. The carriers, for the most patt, travel along the straight lines of the

applied electric field before they are collected. An image may, for instance, be

constructed by ''reading" the amount of charge collected for each given area of the

detector. In the case of an active matrix array detector, careful selection of pixel size,

detector thickness and applied bias field will heip to minimize charge carrier dispersion

between adjacent pixels in the detector, contributing to a sharp image. Conversely, the

visl%le light created by an x-ray photon striking a phosphor in a conventional film

cassette cannot be "steered" as in the case of charge carriers in an electric field. The light

has to propagate through the phosphor layer to the film in order to expose it and create a

useable image. That light difbes appreciably due to scattering in the phosphor layer,

and thus reduces the sharpness of the recorded image.

Electronic x-ray image reconstruction holds many other advantages over present-

day film based techniques. First and foremost are the savings in the quite substantial

physical space needed to store conventional x-rays once they are taken. Approximately

80% of the imaging data in modem hospitals are x-ray images recorded through the use

of photographic film [S]. The need for the chemicals and the time to develop the film are

also eliminated. Further, the need fbr film itself is negated with a reusable solid state

detector.

Digital radiography lends itself to such advantages as computer aided diagnosis

161, and also permits the possibiiity of remote access and teleradiology where highly

qualified personnel could provide service to remote regions from a central location.

1.4 Solid State Imaging Systems

There are two broad classes of solid state x-ray imaging systems, delineated by

how they collect the charge created by the x-ray photons incident upon them. In general,

the detector may be used with a perm~ulentIy applied bias (short circuit mode) or in an

open circuit fashion. The open circuit (xerographic) mode is discussed first.

1.4.1 Xerographic Mode Detection

The xerographic mode was the basis of early solid state radiographic systems,

probably because of the similarity with the xerographic process used in commercial

photocopiers. The detector consists of a layer of amorphous selenium (a-Se) deposited

onto an aluminum substrate. The surface of the detector is then electrostatically charged

to a high potential with a scorotron prior to x-ray exposure. This uniformly distriiuted

surface charge generates a strong bias electric field within the a-Se layer. The detector is

then placed behind the patient and an x-ray exposure is performed.

The x-rays incident on the a-Se layer Liierate charge carriers in the bulk of the

layer, which are then swept either to the stlrfilce or to the A1 substrate, depending both on

the species of carrier (electrons or holes) and the polarity of the applied field. The net

effect is that some of the d a c e charge of the a-Se layer is proportionately discharged in

relation to how much x-ray radiation was incident at each location on the !ayer, greater

discharge occurring where exposure was higher (i.e. 'hugh tissue) and lesser discharge

occurring where exposure was lower (i.e. through bone). This leaves a latent eiectrostatic

image on the d a c e of the detector, which can be transformed into a viewable image in a

number of ways.

Again, this particular method of forming an x-ray image is very similar to the

xerographic process, so it should come as no surprise that the first development method

to be used was powder cloud development [5 ] . Powder cloud development is an andog

image formation technique. The deveIoped image could be viewed directly on the

selenium plate or transferred to paper for a permanent record. The selenium plate is

reusable, but must be cleaned of residual powder after each cycle.

A variety of methods to electronically develop the latent electrostatic image have

been developed. One such method involves the use of an array of micro-electrometers to

sequentially scan the charge distniution present on the surface of the selenium plate [7-

91. The general idea is that if a small sensing electrode is held very close to the surface of

the selenium plate, then the current flowing through the probe as the plate is slowly slid

past the probe will be proportional to the latent electrostatic image (charge) on the plate.

The probe current versus position data must be processed to yield an image, and this data

is generally converted to digital form to be processed and recorded by computer. This

method is advantageous in that the image is not altered by the readout process itself, and

repetitive scans of the same image may be made to improve recognition of fine detail and

verify previous measurements. However, this method suffers h m the fact that it must

employ mechanical parts to position the a-Se plate and precisely hold the electrometer

probes just above its surface. Obviously a I00036 electronic solution would be preferred,

especially since the a-Se plate must be transported to and inserted into the

scanning/analysis unit after every exposure; a tedious and an unnecessary task. Philips

has introduced a commercial digital x-ray imaging system called Thoravision that is

based on the electrostatic readout technique in which the latent electrostatic image on a

photoconductor drum is read by using a series of electrostatic probes. The measurement

is then digitized to provide an electronic image.

Another electronic deveIopment technique involves the sequential discharge of

the latent image through the use of a scanning pulsed laser [lo-1 I]. A transparent

electrode is coupled to the surf'ace of the detector and assumes a potential related to the

d a c e charge of the latent image. As the laser is scanned across the d a c e of the

detector, the charge under the electrode is locally discharged, inducing a current to flow

in the electrode. The image may be reconstructed by correlating the change in the

electrode signal to the position of the laser. This method is capable of high resolution

provided that the laser is precisely focused and p o s i t i o n e k immense tecbnologicd

feat in and of itself,

The principal disadvantage of the xerographic mode is that in the absence of a

permanent applied bias field, areas that are exposed to high x-ray fluences (intensities)

will experience a weakening of the static bias field. This is due to the fact that the

surfice charge will be increasingly depleted as more charge is liberated by x-rays. If the

field becomes too small, then charge carriers will no longer be drifted straight to either

the surface or the substrate, resulting in a blurring of the image. This same phenomenon

effectively negates xerographic mode detection systems h m continuous (real time)

acquisition of data, as in the case of fluoroscopy, since there is no practical method to

replenish the surface charge on the detector in real time.

1.4.2 Active Matrix Array Imaging System

A number of similar imaging systems based on externally biased a-Se plates with

active ma& readouts have recently been proposed 112-151. A schematic representation

of these systems is shown in Figure 1.3.

An amorphous selenium layer is used to convert the incident x-ray photons into

eiectron-hole pairs (EHPs) that are then swept apart by the externally applied bias field

and collected on pixel electrodes. As in the xerographic mode of operation, the charge

released in each area of the a-Se layer is proportional to how much x-ray radiation was

incident on that area, which in turn is proportional to the radiographic image. The active

matrix array consists of an array of thin 6lm ttaasistor (TFT) switches. As shown in

Figure 1.3 (b), the charges that reach the bottom of the a-Se layer (holes in this case) are

collected by pixel electrodes and stored by a capacitor Cij. The TFT switches are used to

control the reading of the image charge, one line at a time. The TFTs in each row have

their gates connected and the TFTs in each column have their sources connected. During

readout, a gate line is activated and the charge fiom the individual pixel electrodes in that

line are read in a parallel fashion and multiplexed into a serial data stream which is fed to

Multiplexer - Digitizer - Computer

Data (Source) Lines

. 1 b

E l & '. i Shield v - sia

Figure 1.3. (a) Schematic of a-Se plate with active matrix readout and (b) a cross- section through one pixel (not to scale). [Afta Rowlands J., and Kasap S.O., "Amorphous semiconductors usher in digitd x-ray imaging", Physics Today, 50, 1997, pp* 24-30.]

a computer for processing and display. The next row is then read, and the process

continues sequentially until the whole matrix has been scanned.

The active matrix m y system offers a number of advantages over traditional

film based radiography and xerographic radiographic methods. First, it offers a

potentially high resolution imaging method, governed by the size of the pixel electrodes

and the lateral spread of the charge carriers. The pixel size in current systems is

approximately 150 pm, which translates to an image quality approaching that of

conventional film based systems. However, the pixel size of future systems can be easily

reduced, leading to higher resolution in the radiographic image, Unlike a xerographic

mode system, no handling of the detector is required, and there are no moving parts

involved in the readout of the image. Further, the readout of the image is limited only by

how fast the TFT array can be scanned (several times per second). The x-ray image in

Figure 1.4 was obtained by the experimental active matrix system developed at the

Sunnybrook Health Science Centre in Toronto.

Figare 1.4. An x-ray image of a wrist phantom obtained by an experimental active matrix array imaging system (reduced Erom origid; courtesy of Dr. J. Rowlands, Sunnybrook Health Science Centre).

Another advantage lies in the permanently applied bias which allows for a

radiographic technique called fluoroscopy. Fluoroscopy is an x-ray imaging technique

that produces red time x-ray images. Charges are continuously generated and collected

as long as x-ray photons strike the a-Se plate. By continuously polhg the active matrix

array, a real time image can be generated at every complete scan time. FIuoroscopy finds

extensive use in angiography; a medical procedure to examine the blood vessels of the

heart. Angiography requires that a red time image of the heart be available to the

physician in order to properly guide the necessary instnrment(s) to the site of any

coronary blockage and mpen the artery, as with heart attack victims.

1.5 Research Objectives

The areas of investigation that will be undertaken in this work involve the charge

transport parmeters of biased a-Se pIates and how these are influenced by x-ray

exposure. The basis of this experimental examination will be the Time-of-Flight and

Intermpted Field Time-of-Flight (IFTOF) transient photoconductivity techniques. Both

are pow& ptoven methods for studying the charge transport kinetics in a host of

materials. The following sections will provide a brief outline of the research objectives

for this work.

13.1 Carrier Drift Mobilities and Trapping Times

Pure a-Se is not used as an x-ray photoconductor because pure a-Se crystallizes

over rime. Crystalline selenium is unsuitable as an x-ray photoconductor because it has a

much lower dark resistivity than a-Se, which leads to a dark m e n t that is orders of

magnitude greater than in the amorphous solid. It was found that alloying pure a-Se with

mall amounts of arsenic (0.2 - 0.5% As) greatly improved the stability of the composite

tllm and heiped to prevent crystaliization. However, the addition of arsenic was also

found to adversely affect the hole lifetime p because the arsenic introduces deep hole

traps. If the alloy is doped with 10 - 20 parts per million @pm) of a halogen (such as

chlorine), the hole Hetime is r e s t d to its initial value. Thus, the photoconductive

plates typically in use consist of a-Se that has been alloyed with 02-0.5% As (nominal

03% As) and doped with 10 - 20 ppm Cl.

Charge transport parameters such as electron and hoIe mobiIity (k and h) and

electron and hole lifetime (G and rb) in these alloyed a-Se plates depend quite strongly on

the relative amounts of the doping species, the original purity of selenium, and also on

the sample preparation conditions. The carrier Lifetimes demonstrate a higher sensitivity

to these factors than the mobilities. As mch, each and every caudidate sample that will

be used in the course of this study must be examined to determine these critical charge

transport fictors, Any samples found to possess poor charge transport will be excluded

ftom the balance of the study-

These initial measurements provide a means of comparing the suitability of the a-

Se plates as an x-ray detector with other possiile caudidate maten'als such as cadmium

zinc te1Iuride (CdI-,Zn,Te or more simply, CZT), PbO, CdTe, etc. The comparison of

charge transport parameters may be performed by analyzing the Schubwegs, the product

of the charge carrier mobility, the carrier lifetime and the applied field. The Schubweg is

a measure of the average distance a carrier will travel before becoming trapped.

1.5.2 Changes in Charge Transport with X-ray Exposure

Studies of hole transport in a-Se have shown that when a-Se radiographic plates

are exposed to x-rays, the hole lifetime becomes affected. For example, Kasap et al. [16]

have demonstrated that x-ray exposure of unbiased a-Se xerographic photoconductive

films decreases the hole lifetime.

Electron Wetime, and how it may be affected by x-ray exposure has not been

previoudy reported. A set of experiments using the IFTOF transient photoconductivity

technique to monitor changes in electron lifetime induced by x-ray exposure will be

performed (the photoinjected charge is supplied by a laser pulse). Furthermore, the

difference between exposures made with the photoconductor under bias and no bias

conditions must be investigated, corresponding to the two possl%le modes of operation of

a digital radiographic detector. Three methods to restore the electron lifetime will be

investigated; annealing, Sa - r ed (IR) soaking and ultrasonic treatment. These results

will be monitored to search for any indication of x-ray induced unrecoverable damage to

the a-Se samples. Ultrasonic recovery and permanent x-ray induced damage have not

been previously reported.

1.53 Electron Hole Pair (EHP) Creation Energy

The x-ray sensitivity of a material is the true measure of its worthiness as an x-ray

imaging receptor, and the EHP creation energy is one of the factors that determines the x-

ray sensitivity of a material. The study involves monitoring the x-ray induced

photocurrent in a biased a-Se film. By integrating the x-ray induced photocurrent, the

total amount of charge liberated by the incident x-ray photons in the film can be found. If

the amount of energy that the x-ray beam deposits in the a-Se film is known, then a

determination of the energy to kee an electron-hole pair, WmP, may be made. W ~ P is

the average energy required per collected (freed) electron-hole pair, it is not the average

energy per created electron-hole pair, Wk . The difference arises because some of the

created electron-hole pairs quickly recombine and do not contribute to the photocurrent,

effectively raising the energy required to generate collectable EHPs. This process is well

understood in the optical regime [I71 but is still a point of controversy at x-ray energies.

The competing theories predict different bias field, temperature, and x-ray energy

dependencies. A set of experiments detailing how Wmp varies as a function of these

parameters will be performed and will thus assist in resolving this dispute.

1 S.4 Persistent X-ray Photocurrent Investigation

[mmediately following x-ray irradiation there exists a penistent photocurrent in a-

Se whose origin has traditionally been attniuted to the thermal release of trapped charge

carriers. This persistent photocurrent limits the speed of an a-Se radiographic detector

becase the detector must be allowed to rest between exposures to M y discharge this

persistent photocurrent to prevent image ghosting. An experiment that investigates how

this persistent photocurrent changes with sample temperature will be performed in an

effort to identify the true source of this phenomenon.

1.6 Thesis Outline

This thesis is divided into a total of seven chapters. FoIIowing this introductory

chapter, a brief overview of the various properties of amorphous selenium will be given

in Chapter 2. Chapter 3 will provide the theoretical principle of the TOF and EFTOF

transient photoconductivity technique. A discussion of the properties of x-ray

photoconductors will be presented in Chapter 4, along with a detailed discussion of the

method of calculating the x-ray energy absorbed in an a-Se layer. A description of the

compIete experimental apparatus and a brief description of the sample preparation

procedure will be provided in Chapter 5. Chapter 6 will present the results of the

experimental work. The conclusions drawn fiwm the experimentit1 results will be

presented in Chapter 7, along with some recommendations for future work.

2. Electronic Band Structure and Electrical Behaviour

of Amorphous Selenium

2.1 introduction

To gain some measure of understanding of the ekctronic properties of

amorphous selenium (a-Se), a theoretical grasp of its electronic energy band structure is

required. Unfortunately, the quantum mechanical framework that so effectively predicts

the behaviour of crystalline semiconductors cannot be applied to the atomic arrangement

found in an amorphous material due to the nonperiodicity of the structure. As such, the

behaviour of amorphous semiconductors cannot be ptecisely predicted in the same

manner as crystalline semiconductors. Therefore it must suffice to map their electronic

band structure mainly through observations obtained through rigorous experimentation.

2.2 Atomic Structure of Amorphous Solids

Amorphous materials are best compared to a crystalline solid so as to accentuate

the diffetences in their structures, but also to reflect the similarities. A perfect elemental

crystal consists of a regular spatial arrangement of atoms, with precisely defined

distances (the interatomic spacing) separating adjacent atoms. The bonds of each atom

are also arranged at identical angular intervals. The result of this perfect ordering is a

periodic structure, as shown schematically in Figure 2.1 (a).

At &st glance, the amorphous material pictured in Figure 2.1 (b) bears little

resemblance to its orderly crystalline cormterpart. However, even though long range

order is absent in this solid, there is still a high degree of short range order. This short

range order manifests itself because the atoms of the amorphous solid must still satisfy

their individual valence bonding requirements, which leads to little deviation in the

interatomic spacing of the atoms relative to the crystalline case [If!]. However, there is

some small deviation in the bonding angle between adjacent atoms, leading to a

disruption of periodicity in the material.

Figure 2.1. Two dimensional representation of the structure of (a) a crystalline solid; (b) an amorphous solid. Spheres marked "0" and "U" represent over- coordinated and under-coordinated atoms, respectively.

Figure 2.1 (b) also depicts two possible defects that can be found in amorphous

materials: atoms that are either over- or under-coordinated b r n their normal structure

bonding (NSB). The inherently random nature of amoxphous materials and the existence

of these over- and under-coordinated defects has a profound impact on the electronic

band structure of the material.

2 3 Band Theory of Amorphous Semiconductors

Application of quantum mechanical theory to crystalline solids leads to the band

theory; the regular atomic structure of the crystal manifests in bands of allowable

electmn energies when large numbas of atom (i.e. lo2') are brought into close

proximity to form a solid, as shown in Figure 2.2. These bands describe the number of

electron states per unit energy per electron at energy E, through a function called the

density of states, g(E). In the case of most semiconductors, the two principal bands

(denoted the valence and conduction bands) are separated by a gap in allowable electron

energies, called the bandgap E,. This seemingly simple allowable electron energy

shucture determines the electronic properties of all crystalhe semiconductors.

c Conduction Band 1.

(extended states)

Band Gap t v

Valence Band (extended states) //

Densrty of States, g(E)

Figure 23. Density of states for a crystalline semiconductor.

The band theory was first derived for the case of periodic atomic structures (i-e.

crystalline solids) because the rigid periodicity made possr3le some mathematical

simplifications. Because amorphous materials did not possess any obvious order, it was

long believed that their electronic band structure was vastly different from the crystalline

case. However, when it was discovered that amorphous solids possessed the same basic

electronic and optical properties as their crystalline brethren [19], it was concluded that

only short range order in the atomic structure was necessary for the band theory to be

applicable.

The first attempt to quantify the energy band structure of amorphous materials

was undertaken by Mott [20]. He noted that all semiconductor crystals have two things

in common:

each individual electron within the crystal is descnied by extended Bloch wavefunctions that possess long range order in both phase and magnitude; the allowable electron energies fall into bands which are separated by a well delineated (ie. "sharp") energy gap.

Mott's work was based on the assumption that amorphous materials, despite their

obvious structural difference with aystalhe materials, will have certain similarities in

their band structure. He postulated that the Bloch wavefunctions of the amorphous solid

would have long range order in their amplitudes, but only shoa range order in their

phases; this results in a dissolution of the sharp band edges of the crystal to tails of

highly localized states that extend into the energy gap. This concept is illustrated in

Figure 2.3 (a) where there is an energy E, (the conduction band edge) above which the

eIectmnic states are extended and below which they are localized (and vice-versa for the

valence band edge &).

The first quantitative assessment of disorder and its effect on the solutions of the

Schrijdinger equation was performed by Anderson [21]. In an amorphous solid, the bond

angles between adjacent atoms are slightly distorted h m their ideal value, and long

range order is destroyed. Anderson demonstrated that these conditions of random

variations in the electron potential lead to IocaIized tail states which lie within the

forbidden energy gap, now dubbed Anderson localization.

A Conduction ,' I ,

A Conduction

Band Band

Conduction , /'

Band b

Figure 23 . DOS models as proposed by (a) Mott; (ti) Cohen, Fritzsche and Ovshinski ((30); (c) Marshall and Owen. The hatched regions denote localized energy states.

These tail states in an amorphous semiconductor have a profound effect on h e r

conduction. In a crystal, the carriers travel in the conduction or valence band via

electronic energy states which extend throughout the entire crystal. Due to the transition

between the extended and tail states in an amorphous solid, there is a corresponding

transition in the carrier mobility. Carriers can either tunnel between localized states,

controlled by thermal activation, or, given sufficient energy, they can travel in the

extended states, as in the crystalline case. This change in the mobility of charge carriers

led to the idea of a mobility gap in amorphous materials, much akin to the bandgap of

crystalline solids.

M. H. Cohen, H. Fritzsche and S. Ovshinski suspected that Mott underestimated

the amount of disorder in an amorphous solid. As such, the localized tail states would

extend throughout the energy gap of the material and actually overlap in the region of the

Fermi level, as shown in Figure 2.3 (b) [22]. This hypothesis later became known as the

CFO model. It is important to note that even though there exists a continuum of energy

levels throughout the gap, metallic conduction in this model is not possible because the

widened tail states are still highly localized in space, as in Mott's model.

The DOS model proposed by Marshall and Owen [23] is shown in Figure 2.3 (c).

They noted that all solids, crystalline or amorphous, contain defects such as dangling

bonds, chain ends, interstitials, vacancies, substitutional impurities, etc. These defects

lead to localized energy states within the bandgap of the material, in addition to the

disorder induced tail states. It was previously assumed that the disorder induced states

would be of sufficient number to mask these defect states, but Marshall and Owen

proposed that there would be significant mid gap states caused by these defects. It is

important to note that, in this model, the Fermi level is determined by these defect

statdonor-l ike and acceptor-like in the upper and lower half of the gap, respectively.

The Fermi level remains near the middle of the gap because self compensation adjusts the

concentrations of the donor and acceptor states. This has ramifications with respect to

extrinsic conduction, as doping the amorphous solid with donors or acceptors would have

little effect on the Fermi level; thus extrinsic conduction would be difficult to achieve.

Obviousiy since the electronic properties of a semiconductor are linked--through

the DOS-to any deviations fiom the NSB present in the solid, these deviations hold

particdar interest. In the following section, the various defects that may arise in a-Se are

discussed.

2.4 Bonding Structure of Amorphous Selenium

Selenium is a member of the group VI column of the periodic table; the elements

from this column have the family name of chalcogens. The atomic number (2) of

selenium is 34, and it has six valence (outer shell) electrons. The two valence electrons

in the s state form a lone pair (LP) and do not participate in bonding. Of the remaining

four p state electrons, only two are available for covalent bonding because there is

normally another lone pair in the p state. These lone pairs are sometimes referred to as

nonbonding states. This two-fold coordination bonding configuration for selenium has an

optimal bond angle between the bonds of 105" [34] and represents the lowest energy

configuration of the atom. Selenium therefore exhibits a chainlike structure because of

this divalent bonding scheme. In addition to native defects, elements from groups N or

V of the periodic table may be introduced to cross-link the chains to achieve three

dimensional stability in the solid.

Selenium has two crystalline fonns, monoclinic Se (a-Se) and trigonal Se (y-Se).

a-Se is composed of Ses rings while y-Se consists of parallel spiral Se chains. Given

these two possible ring and spiral chain structures in crystalline Se, it is natural to assume

that amorphous selenium would have ring-like and chain-like structures randomly

distriiuted throughout the solid. However, recent structural studies of a-Se and its alloys

favour a random chain model in which almost all the individual atoms of the solid are

bonded in a twofold coordinated chain structure. The dihedral angle @ of this chain

remains constant in magnitude but changes sign randomly [24,25]. The dihedral angle @,

as shown in Figure 2.4, is defined as the angle separating adjacent bonding planes and

may be seen by aligning the observer on the bond that connects atoms 2 and 3.

In a-Se, the dihedral angle is random so that regions that are ring-We or regions

that are chain-like are possible. If + or - are used to denote the relative phase of the

dihedral angle, then a sequence of +-+- has been tenned ring-like and a sequence of +t+i

or -- chain-like [24]. The local order shown in Figure 2.5 may then be characterized as

tt+-+-+-. This random chain model has successfully been employed to explain the

Figure 2.4. SeIenium chain and the definition of the dihedral angle 4. The dihedral angle is defined as the angle separating the two planes dehed by atoms 123 and 234. It may be observed by looking down the bond connecting atoms 2 and 3 [26].

vlibrational spectra of a-Se to account for Sea-like spectral features in the inbred

absorption and Raman scattering spectra without invoking a mixture of distinct ring-like

and chain-like portions present in the strumre. Other structural studies of a-Se generally

support the random c h i n model [27,28].

The structure of an amorphous solid is not compktely random, and there is, in

fact, a degree of order to the stnrcture-at least between individual atoms. As stated

above, each individual atom in the structure strives to fill its vdence states, thereby

normally defiulting to a random chain structure. However, not ail the atoms can satisfy

their individuai valency requirement due to the lack of periodicity in the structure.

Consequently, some ofthe atoms become over- or under-coordinated.

Figure 2.6 pictures the bonding schemes possiiIe in a-Se; for the purpose of

discussion, only the lowest energy bonding state, Se,O (which represents the twofold

Se, - Fragment

Chain Segments ' L , . , -. x . ,

Figure 2.5. The random chain model of the structure of a-Se showing localized regions that are ring-like and chain-like [26].

coordinated structure discussed above) will be considered to be the "normal" structure;

the other posslible states will forthwith be considered defects.

The lowest energy electrically neutral defect is the trigonally coordinated atom,

~ 4 , as shown in Figure 2.6 (d). Three of the pshell electrons enter into bonding states,

and the fourth electron enters into an antibonding state. Another common electrically

neutraI defect is a chain end, denoted ~4 as in Figure 2.6 (b). These defects possess

three pshell electrons which reside in nonbonding states, and one electron available for

boding.

Notation Structure Bonding States

A,B

s4 A A A ; 1 I , i v .

::-.A- '4 '+. B A

v

Energy

E = -2 E b

E = -Eb

E = -Eb+ Uc

E = - 2 E b + A

Figure 2.6. Structure and energy of simple bonding con6ptions for selenium atoms. Straight lines represent bonding orbitals, lobes represent lone-pair (nonbonding) orbitals, and circles represent antibonding orbitals. The energy of a lone-pair is taken as the zero energy. [Adapted from 381.

a-Se 61ms contain a large number (estimates range as high as 10'" 1Id //an3 of

thermodynamically derived charged structural defects called valence alternation pairs

(VAPs), which correspond to some of the Se atoms being over- or under-coordinated [29-

3 11. The absence of a detectable electron spin resonance (ESR) signal is indicative of no

unpaired electrons (dangling bonds) in the structure [32, 331. This means that there

cannot be a singly bonded neutral Se atom, ~ 4 , or a triply bonded neutral atom, ~ 4 , but

a pair of charged centers of the type Se; and (VAP). If the atoms of the pair are in

close proximity? they are termed an intimate valence alternation pair (NAP).

It is energetically more favourable to form the pair of over- and under-coordinated

atoms than it is to form singly bonded defects, as these singly bonded defects are

somewhat unstable. For example, a chain end, ~ 4 , can lower its energy by approaching

the lone pair on a normdly coordinated ~ e i atom and generate an NAP. The diffusion

of the resulting NAP pair away h m each other can furtfier serve to reduce the energy of

the solid. Thus the reaction + ~e,' + S< + Se; is exothermic because the lone pair

electrons have been absorbed into dative bonding. Figure 2.7 is a schematic illustration

of a typical a-Se structure with an IVAP defect.

Figure 2.7. I11usttation of the a-Se structure detailing an NAP defect.

Many photoelectric properties of a-Se and its alloys can at least be qualitatively

explained by using concepts based on VAP and WAP defects and interconversions

between the normally bonded (twofold coordinated) atoms present in the structure and

these defects. The physics of such processes has been extensively discussed in the

literature [34-363. Their existence and the possible defect reactions that can occur in the

structure have led to many important prerllctions and much insight into the behaviour of

chalcogenide semicoaductors, For example, the linear dependence of the steady state

photoconductivity on the light intensity in a-Se has been interpreted via photoinduced

NAP-type centers [37].

2.5 Band Model of Gmorphous Selenium

The preceding sections may act as the foundation to understand the energy band

structure of amorphous selenium. As stated earlier, the bond lengths of a-Se do not

significantly differ &om the aystalline solid, but the bond angles randomly vary in their

orientation. This, in large part, Ieads to tails of localized energy states that bridge the

energy gap that would not otherwise be present in the energy band of a crystalline

semiconductor. Further, the large concentration of IVAPs in the solid leads to two

localized energy states in the energy gap that are donor- (due to SG defects) and

acceptor-like (due to %- defects), as discussed in section 2.3.

With this in mind, the currently accepted DOS model for a-Se is presented in

Figure 2.8 [39]. It was developed through various transient photoconductivity and

electrophotographic measurements of cycled-up residual and dark discharge [39-421.

Experimental evidence suggests that the localized states (both shallow and deep) in the

energy gap are due to structmd defects of various types that are stable at room

temperature [43]. The near exponentially decaying shallow trap densities with discrete

manifolds at energies of - 0.29 eV above E, and - 0.35 eV below Ec determine the hole

and electron drift mob&@ through a shallow trapconttolled transport mechanism 1441.

These traps are known to be native dekcts, but their exact nature has not been

determined. However, it has recently been proposed that these defects are due to dihedral

angle distortions in the structure of a-Se where the lone-pair orbitals on adjacent Se atoms

approach parallel alignment [45].

. Deep Electron Traps

h 1.0- - - r - - - - - - 5

- ;/CIeep Hole Traps

. EF = 1.06 8V . . - - , Shallow Hole Traps --.

g(E) (cm *3e~'1)

F i 2.8. Experimentally determined density of states function for amorphous selenium 1393.

2.6 Charge Transport in a-Se

In a crystalline semiconductor, conduction occurs mainly due to charge carriers

moving in the extended energy states (the valence and conduction bands). Both the

valence and conduction bands caa carry charges; electrons travel in the conduction band,

while positive holes travel in the valence band. Free electrons and holes are able to drift

in the extended states under the influence of an appIied electric field. In the absence of

some external excitation (i-e. light or x-ray photons), fiee electrons and holes are created

by random thermal viirations of the crystal lattice. These vi%rations can excite electrons

h m the vaIence to the conduction band, provided the vibrations have sufficient energy.

Photoconductivity, on the other hand, depends on incident photons to excite electrons

across the gap-again, if the photons possess sufficient energy. The electron and hole

mobilities in a crystal are limited by the mean time between scattering events (e.g. from

lattice vibrations and defects). These scattering events are rare (relative to the amorphous

case), and as such, mobilities on the order of - lo3 cm2/Vs are commonplace in

crystallie semiconductors.

In an amorphous semiconductor such as a-Se, the localized states ptay a very

important role in conduction. These locdized states that lie in the energy gap act as

trapping centers that can remove drifting charge carriers from conduction, resulting in

effective drift mobilities of - lo-' c&s for holes, and - 10" cm2/Vs for electrons in a-

Se. There are two general classes of traps: shallow and deep, as identified in Figure 2.8.

There is a nearly discrete set of shallow traps at - 0.29 eV above E, and - 0.35 eV beiow

Ec that mainly serve to slow the progress of drifting charge carriers. In addition, Figure

2.8 demonstrates two sets of deep traps that are distributed in energy near the midgap;

these traps are sufliciently energetically deep that a carrier, once trapped, is effectively

forever removed from conduction.

Each trap is described by both a capture and a release lifetime. The capture

lifetime is defined as the mean time that a mobile carrier can drift in the extended states

before beaming trapped. The release lifetime is defined as the mean time that a camer

will remain in a trap before being released back into the emended states (be. a trap dwell

time). The capture lifetime of a trap is inversely proportional to the concentration of

unfilled traps. Obviously, the carrier lifetime will be short if the trap concentration is

high, and vice versa Once a carrier is trapped, it will remain immobile until a lattice

vlihration imparts enough energy to the carrier to excite it back into the extended states,

where it can drift once again. Release h m shallow traps is appropriatety much faster

than release h m deep traps since low energy lattice vibrations are much more EeIy than

the high energy vibrations required to impart enough energy to h e a deeply trapped

carrier. The shallow trap release time is very short, and a typical carrier may experience

many shallow capture and release events while traversing the solid during, say, a timesf-

flight experiment. The deep trap reIease time, however, is very Iong, and a deeply

trapped carrier is essentially permanently removed fiom condution when considering the

timescaie of a typical time-of-flight experiment.

2.7 Optical Properties of a-Se

A photoconductor is simply a transducer for converting incident photons to

mobile charge carriers which can be collected and detected. An important measute of the

worthiness of a photoconductor is the optical absorption coefficient a Optical

absorption in crystalline semiconductors is dictated by the probability that a photon will

excite an electron across the bandgap and generate a fke electron-hole pair (EHP).

Therefore, a depends both on the incident photon energy and the DOS at the band edges.

If the photon energy is less than the bandgap, no absorption will occur.

The optical absorption coefficient of a-Se exhibits an Urbach edge of the form

a = 7.35~10-12e /an, corresponding to excitation of carriers h n the midgap

localized states into the extended states [46]. At higb photon energies, the absorption

coefficient has been found to obey 4hv)-(hv-k) [47], where E, = 2.05 eV is the optical

bandgap at room temperature. This behaviour has been attniuted to a sharp rise in the

density of states at the band edges.

Even though the optical absorption coefficient shows considerable absorption at

photon energies above 2 eV, the quantum efficiency has been found to exhibit a strong

fieId and photon energy dependence, as shown in Figure 2.9. The quantum efficiency

determines the probability that optically generated EHPs will dissociate in the presence of

an applied field to form free electrons and holes. The mechanism behind the fieId

dependent quantum eficiency observed in a-Se bas been explained by the Onsager theory

[17]. The Onsager theory calculates the probability that an EHP will dissociate in the

presence of an appIied field. The quantum ef6cimy is a hct ion of the eIectric field F,

the temperature T, and the initial separation of the EHP r,, the thermalization length.

Thus, the quantum efficiency is dictated by

v =%f (F,T,~), 2.1

where f (F,T,~) is the probability that an EHP will dissociate and q. is the quannun

efficiency of the intrinsic photogeneration process.

Photon Energy h v (eV)

Figure 2.9. Absorption coefficient a (solid h e ) and quantum efficiency q (broken lines) as a hction of the incident photon energy hv for various applied electric fields [17,46].

For crystalline semiconductors, the quantum &ciency is largely detesmined by

recombination kinetics and is generally independent of the electric field As Figure 2.9

shows, this is obviously not the case with a-Se (and many other low mobility solids).

Light exposure has been observed to induce changes in the structure of

amorphous semiconductors such as a-Si:H (amorphous hydrogenated silicon) and a-Se

[36, 37, 48-51]. Specifically, in a-Si:H, light will induce under-coordinated Si atoms

which then act as efficient recombination centers. This causes problems regarding, for

example, solar pane1 efficiency. The nature of the photoinduced structural changes in a-

Se are thought to arise through two different mechanisms: the formation of IVAPs and

the further randomization of the soIid through bond twisting and the resultant relocation

of Se atoms. Photoinduced NAPS wodd affect the lifetime of photoinjected carriers

measured via the ETOF technique, since they would alter the number of native defects

thought to be responsible for the deep traps in the solid. Conversely, the M e r

randomization of the solid wouId alter the tail states in the gap, possibly leading to an

altered W e r mobility or increased carrier dispersion. Any observed changes in the

carrier transport parameters in a-Se upon exposure to x-rays would have to be interpreted

with these mechanisms in mind.

2.8 Summary

This chapter introduced the physical structure and properties of amorphous

semiconductors in general, and amorphous selenium in particular. It is widely thought

that the disorder found in an amorphous semiconductor leads to localized tails of energy

states in the bandgap, and that defects in the material lead to donor- and acceptor-like

states within the gap. These localized gap states govern the electrical conduction

properties of the material.

Charge transport in a-Se is dominated by the presence of shallow traps near the

band edge. This reduces the overall mobility of the charge carriers through multiple

trapping and reIease events fhm these shallow traps. In addition there are distriiutioos

of deep electron and hole traps Iocated near the midgap which can permanently remove a

carrier h m conduction.

While the optical absorption co&cient of a-Se is dependent on the energy of the

incident photons, with an optical gap of - 2.05 eV, the quantum efficiency is dependent

on both the photon energy and the appIied electric field. This field dependence has been

explained by the Onsager theoy, as a consequence, the quantum efficiency only reaches

acceptable levels at high electric fields. In addition, light has proven to induce defects in

a-Se; however, how those defects alter charge transport is not known.

3. The Time-of-Flight Transient Photoconductivity

Technique

3.1 Introduction

As stated earlier, the disordered nature of amorphous materials renders the

thearetical treatment of their charge transport difficult. As such, the study of amorphous

semiconductors heavily relies on measurements of their physicaI and electricd properties.

The Time-of-Flight (TOF) transient photoconductivity technique provides a powerful,

and proven, means of studying the nature of charge transport in low mobility materials in

general, and amorphous semiconductors in particular. This chapter presents the principle

of both the TOF and Intermpted Field Tie-of-Flight (IFTOF) transient

photoconductivity techniques, and the principles and theories involved in the

interpretation of the photocurrent signals for various transport and trapping conditions.

3.2 Principle of the Timesf-Flight Technique

The TOF transient photoconductivity technique simply involves the measutement

of the transient response caused by the drift of injected excess charge carrim through a

high resistivity solid, A simplified representation of the TOF measurement technique is

shown in Figure 3.1 (a). Consider a thin plate of a traphe material of thickness L

which is sandwiched between two metallic electrodes, A and B. Electrode A is

connected to a bias source of voltage V, while electrode B is connected through a

sampling resistor R to ground. In this particular instance, A is kept at a positive potential

with respect to ground. Free carriers are injected into the sample directly under electrode

A by some means of external excitation (i.e. visl%le light, energetic electrons, x-rays,

etc.), and these carriers are then induced to drift across the material by the applied bias.

This transient photocurrent may be monitored by observi~~g the voltage induced across

the sampIing resistor R. It should be noted that if visible light is employed as the

excitation source, then electrode A must be transparent, or semitransparent, if light is to

reach the sample material under test.

Figure 3.1. (a) Simplified schematic and (b) small siguaI ac equivalent for the TOF transient photoconductivity technique. Cs is the sample capacitance.

Analysis of the photocurrent is greatly simplified if only one species of charge

carrier (electrons or holes) are swept across the sample; this may be accomplished by

choosing an excitation source (for instance, visible light) which will be strongly absorbed

near the surface of the material. Proper selection of the photon wavelength to ensure that

the absorption depth 6 is much less than the sample thickness L will prevent bulk

generation of carriers, and hence only one species will be swept across the sample. kt the

present discussion, photogenerated electrons will be immediately removed b m the

sample by the positive potential at electrode A, leaving only photogenerated holes to drift

across the sample to electrode B. Electron transport may be studied by simply reversing

the polarity of the applied bias; in that case, photoinjected holes will be immediately

removed by electrode A while the photoinjected electrons will be fiee to drift across the

sample to eIecErode B.

Just as it is important to limit photogeneration to the region immediately beneath

the surface of the sample, it is equally important to ensure that the duration of the

extmal excitation t, be much Iess than the transit time tr of the injected charge carriers

across the sample. The reason for this particular requirement is that the initial width of

the sheet of photoinjected charge must be kept as narrow as possible in order to facilitate

an acceptable spatial resolution within the sample as the injected cbarge sheet can be

viewed as being akin to a measurement probe whose spatid resolution is limited by its

width w.

The material being studied via the TOF transient photoconductivity technique

must not have a high thermal equilibrium concentration of charge carriers as these could

potentidy recombine with and neutralize the photoinjected charge carriers. This

requirement is quantified by the dielectric relaxation time z d and its relation to the transit

time tr of the charge carriers under study. ~d represents the time required for any excess

(i-e. photogenerated) charge carriers in a material to decay to their thermal eqdi'brium

concentration, and in general rEi must be much longer than rr. By virtue of their high

resistivity, amorphous semiconductors have a low intrinsic charge carrier concentration at

room temperature which g e n d l y means that TA >> t ~ .

Interpretation of the TOF photocmt signal requires that the electric fieId F

within the material under study be constant-not just from one instant to the next-but

also at every location within the sample. Unfortunately, the photoinjected charge carriers

perturb the applied electric field Fa = VdL leading to an enhanced field, F?, in front of the

charge sheet, and a decreased field, Fl, behind the sheet, as shown in Figure 3.1 (a). The

application of Gauss' law and simple eIectrostatics leads to the following relations for Ft

and F2 at any position x' [52] :

and

Here, p, is the concentration of photoinjected charge carriers within the charge sheet, w is

the width of the sheet, and e is the dielectric permittivity of the sample. However, if the

amount of photoinjected charge p,wA (where A is the area of injection) is restrained to be

a small amount such that %wk << Fa, then the internal field at every point within the

sample can be approximated as being equal to the applied field F,, E F, = Fz. This is

known as the small signal condition, and it corresponds to Q, (injected) << V&. This

condition imposes a limit on the number of charge carriers which may be injected into a

sample during a TOF experiment. If there is no adherence to the small signal condition,

then the analysis of the TOF waveform must be modified to take this field e a t i o n

into account 1531.

Ramo's Theorem [54] dictates that the transient signal observed across the

sampling resistor R in the external circuit of Figure 3.1 (a) is due to the photoinjected

charge carriers moving from electrode A to electrode B in the materid. Consider a

charge being induced to flow in a material under the influence of an applied electric fieid

as in Figure 3.2. The work done in moving a positive charge q a &stance & is

dw = Fqdx, 3.3

where dW is the work done in moving the charge and F is the applied electric field. The

energy required to do this work must be supplied by the external source and is

dE = CdQ. 3.4

Therefore, dE must be equal to 6W; it then follows that

qdx m o ' s Theorem]. dQ=- 3.5 L

From Equation 3.5, it follows that for a trapfree solid, the induced photocurrent through

the sampling resistor is

F i e 32. The motion of a charge q through a distance dx in the sample induces a charge dQ to flow in the external circuit.

Figure 3.1 (b) depicts the small signal ac equivalent of the circuit of Figure 3.1

(a), where C, is a combination of the sample capacitance and any other stray capacitance

added by cables, electronics, etc. Obviously, the photocunent signal i&) must flow

through the pardel combination of the sampling resistance R and Cs. Since Cs will

introduce a frequency dependence into the transient response to the induced photocurrent,

there arises two distinct methods of detecting this photocurrent signal, based on the cutoff

fiequency of this RC circuit, since it is, in essence, a Iow pass filter. If V(s) and IpH(s) are

the Laplace transforms of the voltage signal and the p h o t o m t , it can be shown that

In the folIowing discussion, the bandwidth of the photocurrent signal is arbitrarily

defined as the reciprocal of the carrier transit time r ~ .

The first method of measuring the photocurrent signal relies on the bandwidth of

the photocurrent signal being much less than the cutoff fiequency of the parallel

combination of R and C,. This translates to RCs << t ~ , and the inverse LapIace transform

of Equation 3.7 becomes

Y(t ) = ~i~ @) for RC, << rr . 3.8

Equation 3.8 is known as the Emode signal because the observed output signal is directly

proportional to the induced photocurrent. In a trapfree solid, the Emode signaI d l

instantaneously rise to a constant level when the charge carriers are first injected and will

remain at that level until the charge sheet reaches the opposite electrode, at which point

the signal wilI fall to zero.

The second method of measuring the photocurrent signal relies on the cutoff

kequency of R and C., being much less than the bandwidth of the photocurrent signal

(RCs >> tr). Under this condition, the low pass filter formed by R and C, will integrate

the photocurrwt signal, leading to the inverse Laplace transform of Equation 3.7

becoming

Equation 3.9 is known as the V-mode signal, and it is the integral of the I-mode signal.

In a trap-fiee solid, it will rise linearly upon charge injection until the carriers reach the

opposite electrode, at which time it will flatten and remain constant,

The abrupt and obvious change in the I-mode signal when the carriers reach the

opposite electrode renders the determination of the transit time tr in drift mobility

calculations quite easy, and the I-mode technique is thus favoured for this purpose.

Traditionally, the V-mode technique was favoured for a determination of the mount of

charge that was injected into a sample, and it is sometimes referred to as the charge

transient technique for this reason. However, with the advent of digitizing oscilIoscopes

and powerfhl soha re analysis techniques, numerical integration of the I-mode signaI is

both simple and fast-elirninating the need for m m e m e n t of the V-mode signal.

Many semiconductors have deep traps situated in their bandgaps. A carrier, once

trapped, is effectively permanently removed fiom conduction given the large energy

barrier over which the trapped carrier must be thermally excited. The existence of these

deep traps can significantly reduce the number of free carriers in the charge sheet as it

drifts across the sample. For a given species of trap with a mean capture lifetime T~ and

assuming that carrier release firom the trap is negligible during the transit time of the

carriers, the number of h e carriers wiU decrease in an exponential manner. Equation 3.6

t&m becomes i @ ( r ) = e ~ ~ ~ to reflect the time dependence of the total number of tT

charge carriers. The resulting I-mode signal will then be modified to become

Similarly, the corresponding V-mode signal may be found by integrating the photocuxrent

signal according to Equation 3 -9 to yield

Equation 3.1 1 is the well known Hecht relationship which was extensively used to

estimate the trapping time of charge carriers fiom TOF photocurrent signals for many

years [55]. Figure 3.3 presents a comparison of photocurrent signals for a trap ftee solid

and a solid with a set of deep traps. Although the TOF technique may be employed to

measure the deep trapping lifetime of a material, the interrupted fieId TOF (IFTOF)

method is far better suited to the purpose.

Figure 33. Signals h m the TOF experiment where (a) is the Emode signal and (b) is the V-mode signal, Solid lines represent expected signals in a trapfree solid while dashed lines represent signals in a solid with deep traps only.

3.3 Principle of the Interrupted Field Time-of-Flight Technique

The ETOF technique is similar to the TOF transient photoconductivity method

with one obvious difference: in contrast to the TOF technique, where the charge packet

is allowed to traverse the entire sample without interruption or delay, the LFTOF method

will hdt the charge packet somewhere (virtually anywhere) within the bulk of the sample

by disconnecting the applied bias at the appropriate time. After some interval (called the

intermption time t ~ ) the bias is reapplied and this interrupted photocurrent is recorded.

Figure 3.4 depicts a typicaI IFTOF waveform and a representative TOF waveform for

comparison. The photocurrent within the sample is directly proportiond to the

concentration of fiee photoinjected carriers via i@(o = eppFO, where e is the electronic

charge, is the charge carzier mobility, p is the charge carrier concentration (or density),

and F, is the applied electric field. The magnitude of the photocureat immediately

before interruption is denoted il and immediately after intermption, i2. In the presence of

deep traps h m which the reIease time 7, is much Ionger than the transit time r~ the

concentration of free carriers within the packet is decreased by the presence of those deep

traps in the bandgap of the sample. Thus the ratio of i2/il may be written as [56-621

Figure 3.4. (a) A typical TOF waveform and (b) a typical IFTOF waveform. The interruption time is ti; the magnitude of the photocurrent immediately before and after intermption is denoted it and i2.

-'% i2/iI = e ? 3.12

where z is the effective trapping time of that species of deep trap. The determination of .t

is straightfornard: it may be determined h m the slope of a semilogarithmic plot of the

ratio of i2/i1 verms interruption time.

3.4 Transient Trap Limited Theory

Section 3.2 introduced the principie of the TOF experiment, but the expressions

deveIoped for the I- and V-mode transient responses of photoinjected w r i e s do not

reflect those found in amorphous semiconductors. Dispersion (spreading) of the canier

packet is appreciable as charge carrier kinetics in these materials are heavily influenced

by the large concentration of localized states within their bandgap. The transient trap

limited theory is developed in this section for two common situations: a monoenergetic

Level of traps and a distribution of traps.

3.4.1 Monoenergetic Trap Distribution

Consider a semiconducting materid as shown in Figure 3 -5 with a current due to a

photoinjected electron packet flowing within the sample. The number of fiee electrons

within a thin slice of thickness dx may increase due to the flow of electruns into the slice

or due to t h d release of trapped electrons within that slice. Ef the material is assumed

to have a high resistivity (as is the case with a-Se), then the effect of recombination may

be ignored, since the number of intrinsic charge carriers within the sample will be

negligible. This particular situation may be mathematically desm'bed by the following

continuity equation:

Here, n(x,,t) is the concentration of fiee electrons in the slice, J(x,t) is the net current

density flowing into the slice, n&,r) is the density of trapped eIectrons in the she, and e

is the electronic charge.

F i e 3.5. Current flow with trapping and release processes in a thin slice of semiconductive material.

The net current density J(x,t) is comprised of two components: the conduction

current JC{x,t) and the diffusion cumnt Jdx,t). The conduction current is due to the drift

of electrons under the influence of the applied field; this is expressed mathematically as

J,(x,t) = ew(~,f)F(x,t) . Here, is the microscopic mobility of the eIectr~ns and F(x.0 is

the appIied electric field. The diffusion crrrrent is due to spatial variations in the

concentration of charge carriers and is given by Jo(x,,t) = eD6n(x,t)/&, where D is the

diffusion coeficient of the electrons. The total current density is the sum of these two

components,

w x , t ) ~ ( x , t ) = e ~ ~ , n ( x , t ) ~ ( x , r ) + e.- . 3.14 &

The substitution of Equation 3.14 into Equation 3.13 leads to the following one

dimensional continuity equation:

A similar derivation may be made for the case of holes being the charge carriers.

Equation 3. I5 may be solved if the rate of detrapping is known. The rate equation, as the

expression Gndx,t)lSt is known, is determined by the difference in the charge trapping

and release rates. Given that rc and rr are the mean capture and release times for the

carriers hrn the traps, the rate equation becomes

where it is assumed that there is no trap saturation effect.

A number of assumptions may be made so that the simuItaneous solution of

Equations 3.15 and 3.16 for the fiee electron charge density n(x,t) is simplified. For

instance, the electric field for small signal TOF measurements is approximately uniform

at any time t or position x, as discussed in section 3.2. Thus, the electric fieId will not

vary with either time or position, so that the 6F(x.,t)/& term is zero. Another

simplification that may be made is to ignore the diffusion term in the continuity equation

since the magnitude of the diffusion current is usually considerably smaller than the

conduction current.

Initially, an impulse of No eIectrons is injected into the sample at time t = 0.

Mathematically, this initial condition translates to

n(x,0) = N06 (x,~) 3.17

and

%(x,0)=0 fo rx r0 . 3.18

Finally, the boundary conditions which account for the finite length of the sample are

also necessary to solve Equations 3.1 5 and 3.16. These are

n(x,t)=0 f o r x > l 3.19

and

Equations 3.15 and 3.16 have been solved by applying the initial and boundary

conditions above and by using Laplace transform techniques [63, 641. This leads to the

following expression for the k e electron charge density

where z = x l v , I& is the first order hyperbolic Bessel function, U(x) is the unit step

There are two components to Equation 3.21. The first texm quantifies that portion

of the injected charge in the packet that drifts unhindered across the sample. These

carriers do not endure any trapping and release events, but the number of these untrapped

charges decreases exponentially as they drift toward the opposite electrode. Any charges

that have been trapped and then released back into the conduction band at some time t are

represented by the second term in Equation 3.21. These carriers will obviously lag

behind the untrapped carriers (due to at least one trapping and release event), and will

contribute to an appreciable photocurrent beyond the unhindered carrier transit time

corresponding to t~ = LIW.

The concentration of free carriers within the sample at any time t may be found by

integration of Equation 3.21 over the length ofthe sample. There is no simple explicit

expression for the time dependence of the charge carrier concentration, so particular

(limiting) cases are used to evaluate the trapping parameters. The prediction of the

transient response of charge carriers during TOF experiments is not adversely affected by

these limiting cases, since these conditions may be achieved through proper selection of

such parameters as sample thickness, bias voltage and temperature. The I-mode current

response for both low and high field conditions is examined in the following sections.

3.4.1.1 Low FieId Case

The conditions T, << T, < t~ state that the capture time is much shorter than the

time it takes for the fastest (untrapped) wriers to traverse the sample, as given by tr =

WpJ. The release time fiom these traps is also less than tr. These partidar conditions

mean that the charge carriers will likely undergo many trapping and release events before

finally reaching the opposite electrode. It may be presumed that since the release time

h m these traps is less than the transit time of the carriers that these traps are shallow in

nature, and energetically lie very close to the band edge. When appIied to Equations 3.15

and 3.16, these restrictions cause the time dependence of the total fke electron

concentration to become very small (vanish) over a long t h e interval. By virtue of

conservation of charge, the total number of free electrons is given by

The I-mode photocurreat resulting fiom this special case may be found by substitution of

Equation 3.22 into Equation 3.6 to yield

e x =c =-- 't, +=r ips for f C ct err-.

t~ '=c +=r %

Equation 3.23 closely resembles the trap-he I-mode case presented in section

3.2, except that the transit time has beem increased by a factor of (T~ + T,)/G. This

implies a reduction in the effective carrier mobility from p,, to p, where p is defined as

The scalar 8 is referred to as the SWOW trapantroiled transport factor or mobility

reduction fhctor, When carrier mobility is reduced by a dwell time in the traps, the

transport mechanism is texmed shallow trapantrolIed transport. NegIecting the h e

derivatives of the h e and trapped electrons in Equations 3.15 and 3.16 has the

unfortunate r d t that an expression that rdates the relative amount of spreading of the

charge packet as it crosses the sample cannot be obtained. The spread of the packet

(dispersion) is significant (as evidenced by the long tails found in typical TOF

waveforms), and is due to the random nature of the trapping and release events. This

spreading is considerably higher than that due to simple diffusion done [65].

3.4.1.2 High Field Case

If the applied field is sufliciently strong, it is possible for charge carriers to cross

the sample without any trapping and release events taking place (tT < z,). The

photocment signal may then be separated into two t m s ; one for the drift of carriers for

t c tr, and the second for t > rr. The first term has been derived as [66]

The second term represents the response for those carriers that were trapped and later

released back into the conduction band, and most will cross the sample without becoming

trapped again. The photocurcent signal in this case is [66]

r~ e-x. fort > r, . ifi =-- 2 T J ,

3.42 Extended Trap Distribution

Analytically analyzing the motion of charges through a range of traps distributed

kt energy is far more complex than is the case for a monoenergetic set of traps. However,

Rudenko and Arkhipov [65] adcicessed the issue by considering a set of traps,

continuously distriiuted in energy, and characterized by q(E). The total density of traps

in the material is then

The total density of carriers in the system, n(i,t), may be broken into those &ers that

The carrier lifetime is dependent on a number of material factors; the caphrre

cross-section at energy E, a(E), the thermal velocity of the fiee carriers, ;, and the trap

density N, through r = . The capture coefficient is defined as the product of

the capture cross-section and the thermal velocity of the h e carriers, c ( E ) = ~ (E); .

The probability per unit time that a fiee carrier will become captured in a trap which lies

in the energy interval h m E to E + dE is then

If the trap release process is thermally activated, the probability per unit time that

a carrier which is trapped at an energy iE below Ec is released is given by

where 1/~,@) is the probability per unit time of release h m a trap at energy E, N, is the

energy density of conduction states, and AE is the height of the energy barrier over which

the wrier must be excited to become h e . The density of trapped carries may be

d e h d as

where p(x.t,E) is the density of trapped carriers in the energy range from E to E + dE.

The rate equation for the trapped carriers in the energy range from E to E + dE may be

written as

or by substitution of Equations 3.29 - 3.3 1 into Equation 3.32,

The rate of change of the trapped carrier density may be found by rearranging Equation

3.33 to get

If there exists a state of thermal equilibrium between fiee and trapped charge

carriers, the time derivative of Equation 3.34 will vanish (Le dnd = d%t = 0 ).

Therefore, the trapped charge density at energy E may be related to the concentration of

f?ee carriers through

The total trapped charge concentration may be found by substituting Equation 3.35 into

Equation 3.31; if Equation 3.28 is considered, then the total concentration of carriers

may be related to the concentration of free carriers:

If the trap-controlled transport factor 8 is defined as

then Equation 3.36 reduces to n/rx,t) = Bn(x,,r) since 8 cc 1.

One last piece is missing &om the puzzle in order to complete the analysis: the

one dimensional continuity equation. If the mutual coulombic repulsion of the carriers in

the charge packet is ignored (i.e. F is uniform throughout the material), and assuming

thennal equiliIbrium, it may be written as

Substitution of the approximation n h t ) = Bn(x,.t) into Equation 3.38 yields the foilowing

equation which is only dependent on n(xt):

where p = Op,, and D = OD,. Note how the reduction in the carrier mobility is similar to

the low field condition of a monoenergetic trap distribution.

As an example, consider the Gaussian trap distribution shown in Figure 3.6. The

distribution is described by

Figure 3.6. Sketch of a Gaussian distnhtion of shallow energy traps that lay immediately below the conduction band edge.

Consider the case where E, >> kT; the solution of Equation 3.37 Ieads to the

foilowing [65]:

Consequently, the carrier mobility becomes

where the mobility has a temperature dependent activation energy of E, = ExkT. On

the other hand, shallow trap distn'butions with a power law dependence on energy (as

found in a-Se) do not lead to a temperature dependent activation energy.

3.5 Summary

The principle of both the Timesf-Flight (TOF) and Interrupted Field TOF

transient photoconductivity experiments were presented in this chapter. Both the TOF

and IFTOF techniques are powefil methods for studying the charge transport parameters

of low mobility solids. The transient TOF waveforms were predicted for common

trapping conditions. The effect that an energetically shallow set of traps has on a species

of charge carriers is to lengthen the amount of time it takes the carriers to cross the

sample, and this is due to the average time the wriers dwell in the traps before being

released. Energetically deep traps, however, will effectively pexmanently remove charge

carriers from conduction, given the very large energy barriers over which they must be

excited in order to become h e .

4. X-ray Photoconductors

4.1 Introduction

The x-ray photoconductive material of a digitaI x-ray imaging system functions as

an x-ray photon-to-electrical charge transducer, and thus plays the single most important

role in the entire digitai x-ray imaging system. The properties of an ideal x-ray

photoconductor will be introduced in this chapter, and several different candidate

materials will be compared with the idea1 case. FindIy, some concepts necessary to

understand the x-ray sensitivity of a material are discussed.

4.2 Ideal X-ray Photoconductive Material

If one were able to find the perfect x-ray photoconductive material, it would have

a number of unique electrical and mechanical properties. A digital x-ray imaging system

relies on the absorption of x-rays that h'berate free charge carriers that may be collected

and then analyzed by some peripheral electronics. Since charge coUection is the

dominant electrical process being employed to detect x-rays, the x-ray photoconductor

must not allow a large dark current to flow. Ideally, the dark current should be zero in

order to maximize the signal-to-noise ratio ( S N R ) of the device. The dark current has

two components: one arising h m the injection of charge carriers at the electrodes, and

another caused by the thermal generation of free carriers in the bulk of the

photoconductor itself. It is weII established that a metallic electrode deposited directIy

onto the i?ee srzrface of a semiconductor can behave as a blocking Schottky contact

Therefore, carefd selection of eIectrode material may aid in reducing the dark current. A

relatively wide bandgap in the photoconductor will aid in reducing thermal generation of

free carriers, since thermal generation of h e carriers occurs from defect states in the

middle of the bandgap.

Again, sin= charge collection is the dominant electrical process being employed,

the x-ray photoconductor shodd liberate as many free charge carriers (electron-hole

pairs, EHPs) as possiile per unit energy incident on the material as x-rays in order to

maximize the detected signal. This translates to a low EHP creation energy in the

photoconductor itself.

Once the free EHPs are created within the x-ray photoconductor, none should be

lost; this ensures that the detected signal remains strong. Free E m s may be lost by two

methods: bulk recombination or deep trapping. Bullc recombination occurs when a

drifting electron and hoie meet and recombine with each other. Free charge carriers may

also become deeply trapped as described in detail in Chapters 2 and 3. Therefore, the

electron and hole Schubweg must each be much longer than the thickness of the

photoconductor itself ( ie. ptF >> L, where p is the drift mobility, r is the trapping time,

F is the electric field and L is the layer thickness) so that deep trapping of charges

becomes unlikely.

During fluoroscopic procedures the detector is continually irradiated and polled

by the accompanying electronics to form a real time image. Therefore, the longest carrier

transit time must be sufficiently short so that all charges in a pixeI are collected by the

time that pixel is next accessed.

In addition to the ideal electrical characteristics above, it would be desirous of an

x-ray photoconductor to strongly absorb x-ray photons within as small a detector

thickness as possibIe. The x-ray absorption of a material increases with increasing

atomic number, therefore, a materia1 with a high atomic number (Z) would be preferred

for this purpose. Strong x-ray absorption is necessary to make use of the maximum

number of x-ray photons that pass through the patient; the higher the absorption, the

lower the x-ray exposure for the patient consistent with an image of high quality. This

characteristic also has ramifications as regards to detector speed; the thinner the detector,

the faster charges are dected, and thus the faster its response.

Deterioration in the properties of the photoconductor with repeated x-ray

exposures should be absent or small so that overall detector performance is not adversely

affected.

Finally, the ideal x-ray photoconductor should be easily grown on or coated on to

any underlying electronics to form the detector itself. This must be done in large areas

(typically 30 x 30 cm and larger) and without damaging the underlying electronics (i.e.

an active matrix array).

To summarize, an ideal x-ray photoconductor should have the folIowing qualities:

Dark current should be absent or at a very low level. Low EHP creation energy in order to maximize detectable signaI per unit exposure (but not so low as to cause problems with thermal excitation). Carrier losses due to recombination or deep trapping should be absent or very low. Carrier transit times must be faster than the polling time to maximize detector speed. Material should be highly x-ray "absorbent"-nearly all x-ray photons incident on the detector should be absorbed within a practical detector thickness. X-ray induced material fatigue should be absent or be so s m d as ro have no effect on detector performance- X-ray photoconductive material must be easily grownlcoated over large areas without damaging any underlying electronics.

4 3 Practical X-ray Photoconductors

In the field of digital radiography, there is, unfortunately, no x-ray

photoconductive material that meets every idealized characteristic as set out in section

4.2. Some materids may be attractive for some properties, but davourable for others.

This section examines a number of different candidate materials for their suitability as a

practical x-ray photoconductor.

4.3.1 Amorphous Selenium (a-Se)

The reason why amorphous selenium finds quite widespread use as an x-ray

photoconductor is that, in many respects, it approaches the characteristics of an ideal x-

ray photoconductor. a-Se is perhaps the most developed of the possible x-ray

photoconductors; it has been extensiveIy studied since it was first used as a xerographic

photoreceptor in the 1940s through to the 1970s. The stabilized a-Se alloys in use today

have excellent charge transport properties, with typical hole and electron ranges (p

products) being 30x10' c m 2 ~ and 5x10' cm2/V rqectively for device grade a-Se

alloys [67]. At typical operating fidds (Le. 1 10 VIP), the hole Schubweg is 1 30 mrn

and the electron Schubweg is 1 5 mm. Since most a-Se detectors are at most - 500 pm

thick, these large Schubwegs will ensure that virhralIy no flee charges that are created by

x-ray irradiation will be lost to trapping.

a-Se is also favourable because of its very low dark current, usually on the order

of -1 nNcm2 at low elecaic fields with conventional metal electrodes. Recent work at

Noran& Advanced Materials 1681, has provided a method for producing a multilayer pi-

n diode-like a-Se detector s t r u m that r e d s in dark currents c 100 p ~ , ~ ' at fields as

high as 20 V l p .

X-ray induced fatigue of a-Se has not been investigated in detail prior to this

work, and electron lifetime was found to exhibit no apparent decrease with repeated x-ray

exposure.

The deposition of a-Se alloys onto substrates is fast and easy to perform using

simple vacuum deposition techniques [69], and deposition rates of - 2 @minute are

typical. The ease and speed with which large Iayers are deposited are distinct advantages

in favour of a-Se as an attractive x-ray photoconductor over competing materials.

Another factor in favour of a-Se is the fact that the substrate temperature during

deposition is kept low (- 60°C) and this will not damage any underlying detector

electronics (cg. an active matrix m y ) .

Where a-Se suffers somewhat in comparison to other materials is in two areas: x-

ray absorption and x-ray sensitivity (EHP creation energy). First, the atomic number (Z)

of selenium is 34; compared with other materials (cadmium zinc telluride Zff - 50, lead

oxide Z,E- 82), a-Se is a rather poor absorber of x-rays and a thick detector must be used

to absorb the same amount of x-ray radiation as compared to a thin layer of a material

with a higher atomic number. Second, the EHP creation energy of a-Se is highly field

dependent; it decreases with increasing field, but at typical operating fields (- 10 V / p )

the energy required to create a collected EHP is reported to be approximately 35 - 55 eV

over the diagnostic beam energy range [70-751. When compared to other competing

materials with EHP creation energies in the range - 1 - 10 eV, a-Se is not particularly

sensitive to x-ray radiation,

4.3.2 Hydrogenated Amorphous Silicon (a-Si:EI)

The attractive qualities of a-Si:H are that it possesses excellent charge transport

properties; it may be doped so that multilayer devices (like p-i-n structures) may be

fabricated, and it may be deposited in large areas. However, it also has a very Iow atomic

number which makes it a very poor x-ray absorber. This material is fi,uther hindered by

the fact that it requires high wbstrate and meding temperatures which would surely

cause damage to any underlying electronics that would be necessary in the detector. Add

to this the very slow deposition rate of a-Si:H, and it becomes a very unfavourable

alternative to a-Se.

433 Cadmium Telluride (CdTe)

CdTe has been widely used as a radiation detector for many years. It is attractive

because it is highly efficient at absorbing and converting x- and gamma-radiation to

mobile charge -a; a co~l~e~uence of its moderate atomic number (& - 50) and low

electron-hole pair creation energy of 4.5 eV [76]. Although bombardment with high

energy protons has been found to induce fatigue through charge trappiag in CdTe

detectors [77], it appears that exposure to diagnostic x-rays does CdTe little or no harm.

As opposed to a-Se, where both holes and electrons contribute to the

photocurrent, CdTe has very poor hole transport. This means that the Schubweg of holes

in CdTe is very small which makes it unlikely for them to cross even thin CdTe layers

without becoming trapped. Another issue is the rather high dark current, which is on the

order of - 10 nA/cm2. Further, the deposition of large area polycrystallim CdTe layers is

usually done by vacuum deposition techniques; they may be sputtered (which is very

slow) or thermally evaporated. As is the case with a-Si:H, the high substrate and

annealing temperatures which are necessary would undoubtedly damage any underlying

electronics present on the substrate.

43.4 Lead Oxide (PbO)

PbO is a polycrystalline semiconductor that has found use for a number of years

in both visiile Light and x-ray sensitive television tubes [78]. Since the & of PbO is

approximately 82, it is an excellent absorber of x-ray photons, but PbO is porous in

structure which offsets its high effective atomic number. Dark currents are capabIe of

being < 1 nNcm2 since PbO may be doped to form pi-n structures. The electron-hole

pair creation energy is reported to be 8 eV [76], which means that PbO is quite sensitive

to x-rays.

Growth of PbO layers tends to be difficult; they are d y vacuum deposited,

but relatively easy to manufacture PbO crystallites suspended in a resin binder have also

been successfblly investigated [79]. However, PbO will adversely react with air, causing

an increase in dark current and a decrease in x-ray sensitivity. Funhermore, thick PbO

layers will degrade h m prolonged x-ray exposure; this effect has not been noticed in

thin layers, but thin layers do not absorb d c i e n t x-ray radiation to make them feasibie

in medical imaging applications.

4.3.5 Crystalline Materials

There are a number of crystalline semiconductors which are popular materials for

use in radiation detectors and pulse height spectroscopy applications, notable among

them being germanium (Ge) and cadmium zinc telluride (Cdl-,Zn,Te or CZT). They are

highly sensitive to x-ray radiation, and in general have good charge transport properties;

however, a Ge detector must be cooled with liquid nitrogen before its charge transport

becomes acceptable. Their main drawback lies in the fact that they are crystalline and it

has so far proven not feasible to grow the very large crystals that are needed for flat panel

x-ray image detectors. However, there has been much recent work in development of

gamma-ray medical imaging systems based on many discrete CZT detectors or arrays of

detectors [80], and there exist commercially available SPECT (Single Photon Emission

Computed Tomography) imaging units by Siemens and Diguad, to name two.

Of all the materials discussed thus far, a-Se is the best choice at present since it

possesses many of the qualities of an ideal x-ray photoconductor, even if it is slightly

lacking in its x-ray absorption and sensitivity. Table 4.1 summarizes the properties of

these materials and compares them with the ideal case.

Table 4.1 A concise comparison of candidate x-ray photoconductive materials. I Desired I Ideal I aSe I aSkH I CdTe I PMI I

Property

X-ray smsitivity Carrier losses X-ray absorption X-ray

Material Low High

Low

ktigue Deposition

Hi&

None

Low Highonlyat high fields

Low

Easy

Modcraft

Small

Low High

Low

Vacuum Coating (Easy)

Low

Unknown

High High

Hi&

Easy, but high substrate

temperatures

Low Hi&

Low

Hi&

None

High

High

Easy, but high substrate

tempetanrres

Difficult

4.4 X-ray Sensitivity

The sensitivity of a material to x-rays is of major concern in the field of

diagnostic radiography since, in general, the more sensitive the detector, the less radiation

to which the patient is expased. Specifically, the x-ray sensitivity of a material is a

measure of the amount of energy required to Liberate mobile charge carriers in that

material, and depends on two interreIated processes: the absorption of x-ray photons, and

the creation of charge carriers by those photons. In order to calculate the x-ray sensitivity

of a given material, the amount of energy that an x-ray beam deposits in that material

must be known.

4.4.1 The Energy Absorption Coefficient

The linear attenuation coefficient of a material is used when dealing with the

absorption of visible (or near vislMe) light (usually by a semiconductor), and should be

familiar to the reader. However, at high photon energies, a term called the energy

absorption coefficient must be used to calculate the amount of energy that a beam of

photons (e.g x-ray photons) wiIl impart to a material.

The linear attenuation coefficient is a measure of the &action of photons that

interact per unit thickness of an attenuator (a photoconductive material in this case). For

example, if an attenuator is placed in an x-ray beam as in Figure 4.1, then the number of

photons transmitted by the atteauator.is given by

N = N,e-*, 4.1

meaning that

AN= N , ( L - ~ - " ) 4.2

photons interacted with it. No is the number of primary photons incident on the

attenuator, N is the number of photons transmitted by the attenuator, AT is the thickness

of the attenuator, and p is the Iinear attenuation coefficient of the material (the field of

nuclear science generally employs the symbol p as opposed to a with which most

engineers are familiar).

-c S J X .-

Scattered Photon S b _--- /

Incident Primary Beam I Attenoator Scattered Photon S

Figure 4.1 An attenuator is placed in an x-ray beam.

When an x-ray beam passes into an absorbing medium, several different processes

may occur, as detailed in Figure 4.2- The entire process is random; for instance the

photon scattered fhm the primary interaction at A in Figure 4.2 may or may not interact

Radiation enters attenuator in the form of a beam of x-rays

X Primary interaction occurs with A an electron

High speed electron giving

Ionization, excitation, breaking molecular bonds, heat

More like A & B

Figure 4 3 A number of different interactions are possiile when an x-ray photon enters a material.

with the attenuator again before leaving it, Also, the recoiling high speed electron from

the interaction at A in Figure 4.2 is not Iikely to transfer all of its kinetic energy to the

attenuator, as some energy may be lost (radiated) as bremsstrahtung (braking) radiation.

Whereas the linear attenuation coefficient is a measwe of the fraction of photons

that interact per unit tfiichas of attenuator, the energy absorption coefficient is a

measure of how much energy is absorbed by the attenuator through those photons that

interact with it (also per unit thickness of attenuator, as with the hear attenuation

coefficient). The attenuation of a photon beam produced by a layer of a given material

depends on the number of electrons and atoms present in that layer. It is for this reason

that the hear attenuation coefficient of a material is divided by its density, so that clear

comparisons may be made between the attenuation introduced by differat materials,

independent of density. This coefficient, represented by (p/p), is called the mass

attenuation coefficient and has units of m2kg, SimilarIy, the energy absorption

coefficient, W p ) , is also normally divided by the density of the material and also has

the unit mt/kg.

The energy absorption coefficient may be related to the linear attenuation

coefficient; imagine a beam of photons incident on an attenuator in which N photons

reach the tayer Ax. The number of interactions that occur in this layer is given by

n = p N A x . 4.3

If the average energy absorbed pcr interaction is c, then the energy absorbed in Ax

through interactions with photons of energy hv is

To summarize, the bear attenuation coefficient is a measure of the fraction of

photons that will interact with a given thickness of material. A number of diffnent

interactions are possible, and may or may not impart energy to the material. For

example, it is possible that a scattered photon resulting h m the primary interaction with

an electron in the material may exit the material without fUtther interacting with the

material. It is also possible (though not likely) that the high speed electron that recoils

fiom the primary interaction may Iose all of its energy through radiation (bremsstrahlung)

without imparting any energy to the material. Thus it becomes quite obvious that the

linear attenuation coefficient of a material cannot be used to caIcuIate the amount of

energy that an x-ray beam deposits in that material.

4.4.2 Energy Absorption and Detector Thickness

As in section 4.4.1, the absorption of energy depends on the atomic number Z and

the density of the material. However, the energy of the photons incident upon the

materid also strongly determines the energy absorption. Figure 4.3 shows the energy

dependence of the energy absorption coefficient of a number of common

photownductors.

Most of the energy absorption coefficients in Figure 4.3 exhibit sharp vertical

edges where the energy absorbed by the material suddenly increases sharply as the

energy of the incident photons climbs. This is caused by a strong onset of absorption

when an x-ray photon ejects an inner core electron, such as from the K shell, into the

conduction band. The incident x-ray photon imparts all of its energy to the electron in

this instance. This is known as the photoelectric effect, and is the dominant absorption

process for photons with energy below approximately 50 keV. Another possible

interaction between an x-ray photon and an electron will result in both an energetic recoil

electron and a scattered photon of less energy than the incident photon. This absorption

process is known as Compton scatteriug, and is the dominant process above - 50 keV

(and into the MeV range). The probability of Compton scattering decreases as the photon

energy increases, thus the decrease in the energy absorption weEcients in Figure 4.3 as

the photon energy climbs. Compton scattering is almost independent of the atomic

number of a material, but it is strongly dependent on the electron density of that material.

1 00

Photon Energy (keV)

Figure 4 3 Energy absorption coacient vs. photon energy for various photoconductive materials [8 11.

X-ray photoconductors are evaluated through comparison of their K and L

photoelectric edges and their absorption over ceaain photon energy ranges as to whether

they arr suitable for uses such as mammography (mean photon energy - 20 keV) or chest

radiology (mean photon energy - 40 keV). In general, higher absorption is preferred

since the minimization of patient dose requires that most of the x-ray radiation incident

on the detector be absorbed within it, and materiais which highly absorb x-rays at a given

energy are preferred over those which are less absorbing at that same energy. A rule of

thumb is that the absorption depth 6 = I l k must be less than the thickness of the

detector, L. Heace the required detector thiclcness depends on the photon energy and the

particular radiographic imaging application, If the required detector thickness is taken as

3~ then an a-Se detector wodd have to be about 80 pn for mammography and about

660 pm for chest radiology. The same two applications would require a CdTe detector to

be 160 and 180 pm respectively. Obviously it would be preferable to be able to use the

same detector for a number of imaging applications, and for this purpose CdTe would be

more attractive than a-Se as the x-ray photoconductor.

It should be emphasized that the detector cannot be made to be very thick;

whereas a thick detector would absorb more x-ray radiation, the fiee wriers generated

within it would have a correspondingly longer distance to travel in order to be colIected

and thus detected. Therefore, there is a compromise to be made when choosing an

appropriate detector thickness: one must balance the requirement for maximum energy

absorption which dictates a thick detector with the charge transport properties (the

Schubwegs) of the materid, which favour thinner detectors. The response speed of the

detector must also be considered if it is meant to operate in the fluoroscopic mode, which

requires that the detector be thin.

4.43 Electron-Hole Pair Creation Energy: WWIP

Since an x-ray photoconductor is essentially an x-ray photon-to-electrical charge

transducer, it is desirous that the photoconductor be as ef6cient as possible in converting

x-ray photons to mobile charges. Therefore, the amount of x-ray energy needed to create

a single, collected, electron-hole p a i r - W m u s t be as low as possible in order to

maximize the amount of detected charge AQ produced by incident radiation of energy

M.

When an x-ray photon enters a photoconductor, an energetic primary electron is

either ejected from an inner core shell via the photoeiectric effect, or is set in motion

through the result of Compton scattering (as discussed in section 4.4.2). This energetic

electron travels within the solid and will cause many additional collision ionizations

along its path (or track). Thw the initial interaction with an x-ray photon of energy of

several keV is capable of producing thousands of EHPs; a potentially very large signal if

all the charges are collected.

Klein [82] first showed that, for many semiconductors, the energy Ww to cTeate

an EHP depends on the energy bandgap E, of that material via what has become known

as Klein's rule: WWp = 2.8 Eg + Ephonon. The phonon energy term is expected to be small

(1 0.5 eV) so that WEHP is typically - 2.8 E,. Figure 4.4 shows this correlation between

WEHP and the bandgap energy E,; the solid line represents the WUIP = 2-8 Eg + 0.5 eV

behaviour f?om Klein's rule. There are many solids that are accurately represented by

&in's rule, but there are also a number of solids such as a-Si:H, PbIz and AgCI that

have WUrp values substantially less than that predicted by Klein's rule. It should be noted

that many crystalline semiconductors also exhibit an electric field independence in their

w,, value.

Alig and Bloom [83] were able to intuitively explain Klein's WEHP = 2-8 Eg rule

as follows. First they assumed that the masses of the primary energetic electron and the

masses of the secondary electron and hole that it creates are all equal. Just before the

collision, the primary electron has a momentum P and the kinetic energy threshold for

ionization is Er. From conservation of linear momentum, each particle has a momentum

of PI3 immediately after the collision. Since the kinetic energy of a particle is

proportional to the square of its momentum, each particle must have an energy of Ed9;

therefore, the minimum total kinetic energy of the EHP is then 2E#. Since the creation

of an EHP involves the excitation of an electron across a bandgap of energy E,

conservation of energy dictates that the total energy of the EHP, E, + 2Ed9, and the

recoiling primary electron, Ei9, must be equal to the kinetic energy El of the incident

primary electron just prior to the collision. Therefore,

Diamond 1 . 4 /

/,

y'

Klein's Rule / /'

\ Que and Rowlands

0 1 2 3 4 5 6

Bandgap (eV)

F i e 4.4. EHP creation energy vs. bandgap for a selection of photoconductive materials [8 11.

In tenns of the bandgap of the material, the minimum kinetic energy 2Ed9 of the

EHP is then E#3. The average energy to create an EHP, WEHp, must be equal to the

bandgap energy E, plus the average kinetic energy of the EHP. Alig and Bloom assumed

that the kinetic energy of the EHP may vary from 0 to EI; assuming that the density of

states varies as E:'* where Ek is the kinetic energy of the secondary electron or hole, then

where the factor of 2 accounts for the generation of two particles-the EHP. Substitution

of Equation 4.5 in Equation 4.6 leads to

Obviously a more rigorous derivation should include the effective masses of the

different particles and the effect of the generation of phonons in the impact ionization

process, but this fematkably simple derivation and expression accurately predicts byEHP

for a vast amber of (crystalline) semiconductors, as attested to by Figure 4.4. However,

some materials exhibit an electric field dependent WWp whose origin is still a point of

controversy.

Since amorphous materials are inherently disordered, Que and Rowlands (701

have argued that conservation of linear momentum in the ionization process may be

somewhat relaxed. This line of reasoning then Ieads to the lowest or saturated WmP,

W&, at very high electric fields: Wk = 2.2E, + Ea, where Epk is again a small

phonon energy term This is illustrated as the dashed line of Figure 4.4. Therefore, for a-

Se with E, = 2 2 eV, application of Que and Rowlands' theory would lead to wLP = 5

eV. The field dependence of Wm for a-Se and many other low mobiIity solids has

proven difficult to understand.

It is well known that the primary electron will generate many EHPs, but only a

tiaction of those EHPs will be collected. In general, the higher the electric field, the

more charge is collected. If it is assumed that practically no carriers are lost due to

trapping, as is the case for device quality photoconductive materid, then the losses can be

attrtiuted to onIy three sources: bulk or bimolecular recombination between drifting

holes and electrons, geminate (Onsager) recombination, or columnar recombination.

These three cases are illustrated in Figure 4.5.

' ' : m q n a r - A ', '\'\ \ Recombination

/--1 3 \ h L \ '>,

Electric Field

Figure 4.5. Schematic representation of the different types of recombination that are possible in an a-Se photoconductor. The cylinders represent the tracks of primary electrons. Bulk recombination occurs outside the tracks between charges that originated in different tracks. Geminate recombination occurs between the original hole and electron. Columnar recombination takes place between electrons and holes h m diffetent pairs, but within the same track.

In bulk recombination, the recombination rate is proportional to the concentration

of both species of charge carrier, therefore, the collected charge would not increase

linearly with the intensity of the incident x-ray photons-the collected charge would

exhibit a square root dependence on the x-ray intensity. However, since experiments

show that the amount of collected charge increases linearly with the x-ray photon

intensity, bulk recombination cannot be a factor in the charge loss mechanism in a-Se-

The simultaneously generated electron and hole it leaves behind have a strong

codombic attraction and may eventually recombine; hence the term geminate

recombination. This is the accepted model for the optical quantum efficiency of a-Se,

and the number of EHPs that escape geminate recombination is governed by the Onsager

model.

Columnar recombination involves the recombination of nongeminate electrons

and holes from within the columnar track of a primary electron. As the intensity of the x-

ray photons increases, the number of tracks also increases but these tracks rarely overlap

so that recombination within a track remains unaffected by the intensity of the radiation.

This means that the collected charge will increase linearly with the x-ray photon

intensity, in agreement with observations.

4.5 Summary

This chapter introduced the concept of an ideal or perfect x-ray photoconductor so

that a comparison of a number of different candidate materials could be performed to

assess their worthiness as an x-ray photoconductive material. a-Se is an attractive

candidate except for its rather poor x-ray absorption and field-dependent Wmp.

Although a-Se is not an ideal x-ray photoconductor, it is the best choice at present.

X-ray sensitivity was defined, and the various factors that affect the x-ray

sensitivity of a material were discussed. At present, there is no consensus as to how

EHPs, created by x-ray photons, are removed from conduction in a-Se. The two

competing charge loss theories, geminate and columnar recombination, predict different

temperature and x-ray beam energy dependencies.

5. Experimental Apparatus and Procedure

5.1 Introduction

A complete description of the equipment and procedures employed in the

measurement of the various charge transport and x-ray photoconductive properties of a-

Se are given in this chapter. The preparation of the a-Se samples is descriied first,

followed by the TOF/IFTOF apparatus and then the x-ray photoconductive measurements

are finally discussed. Some miscellaneous experimental tools are introduced last.

5.2 a-Se Thin Film Preparation

As mentioned in Chapter 4, a-Se is an attractive x-ray photoconductive material

because it may be quickly and easily deposited as a uniform film over large areas. The

film is grown by thermally evaporating seIenium pellets onto a conductive substrate

(electrode); the substrate is usually either oxidized aluminum or conventional glass

coated with a transparent indium-tin-oxide (lTO) conductive layer. The main difference

between a glass/lTO substrate and an aluminum substrate is in the amount of charge

injection that each wiU i n t m d u c ~ IT0 layer will inject considerably less charge into

the a-Se film under reverse bias than will an aluminum substrate. This point becomes

important when electrons are being studied, as the a-Se films must be reverse biased, as

first introduced in Chapter 3, This Iarge reverse bias dark w e n t made the study of

electron transport in samples with an aluminum substrate very difficult, and this point

will be explained in more detail in section 5.3.

Noranda Advanced Materials of Saint-Laurent, Quebec provided the necessary

materids for the films, including the electronic grade liquid quenched vitreous selenium

pellets. Optical emission spectroscopy was employed prior to delivery of the source

material to m e its purity, which is specified as being 99.999%. To coat a typical film

(a square of 3 cm x 3 cxn and - 60 pn in thickness) would require - 230 mg of selenium,

with more materid required to create a thicker film.

5.2.1 Substrate Preparation

Aluminum will spontaneously react with oxygen to form an electrically insulating

oxide layer - A1203. Tbis oxide layer is necessary to act as an insulator between the

substrate and overlying a-Se film to help prevent charge injection tiom the A1 electrode

into the film [84]. This oxide layer is also required because it f o m an amorphous base

for the a-Se f k . However, if the growth of this oxide layer is -sed, it will not

be uniform and its electrical characteristics will be unreliable. To provide some

protection against scratches, the duminum is coveted by a thin poiper coating. This

coating must be removed by ullrasonically cleaning the aluminum in a sequence of

acetone, distilled water, methanol, and W l y distilled water. A heated (65°C) caustic

etch solution of sodium carbonate, sodium phosphate, and distilled water is used to

partially remove any preexisting oxide layer before the sample is cleaned for a final time.

This 6nal cleaning consists of a nitric acid dip and repeated washings in distilled water

and detergent solutions. The oxide layer was rhea regrown under controUed conditions to

ensure its uniformity; this simply involved placing the substrate in a 300°C finnace for 4

- 5 hours. The preparation of the iTOighss substrates only required repeated washings,

as glass may be purchased with the IT0 conductive layer already in place. Once the

substrates were properly prepared, they were then ready for a-Se deposition.

5.2.2 Vacuum Deposition System

It has been previously reported [85] that the hole range ( p ~ product) of a-Se films

is strongly dependent on the substrate temperature during the sampIe

preparationfevaporation process. If the substrate temperature is held above the gIass

transition temperature of the a-Se alloy being deposited, the hole range in the a-Se film is

then maximized. Electron ranges are not particularly sensitive to the substrate

temperature and are mainly dependent on the purity of the source material. To find the

glass transition temperature of the selenium source material, Differentia1 Scanning

Calorimetry @SC) measurements were performed prior to evaporation. For typical a-Se

alloys used in the course of this work, glass transition temperatures varied fiom 40 -

5S°C, depending on the heating andlor cooling rates used during the DSC analysis.

The a-Se films were evaporated through the use of an NRC 31 t 7 vacuum

deposition system, as illustrated in Figure 5.1. The chamber is evacuated first by a

mechanical vacuum pump, followed by a diffbsion pump to a high vacuum of - 10' Torr. The substrate@) are heated and maintained above the glass transition temperature

of the a-Se alloy being deposited. A thermocouple provides a means of monitoring the

substrate temperature. A molybdenum boat contains the selenium pellets, which are

melted by passing a large (100 - 150 A) ac current through the boat. The boat

temperature is maintained at - 270°C, which, like the substrate temperature, is monitored

by a thermocouple. A deposition rate of - 2 @minute is achieved at this boat

temperature. When the h ( s ) reached the desired thickness, both the boat and substrate

heaters were turned off and the h ( s ) were allowed to slowly cool under vacuum. The

completed h ( s ) were then allowed to age in the dark at room temperature (no Ianger

under vacuum) to allow their physical properties to stabilize. A few days is all that is

required for the aging period.

Substrate Heater i - - < M I Jar

,, ,' ,- , ,, Substrate ,

Moveable Shutter I

' , </' f' ,/ , , , -. ' - - . d

Gas Inlet ----A-

Thermocouple - Molten Selenium r -

/' 1 ,*

' , MolyWenum Boat / '

.' /

' , Canying lSOA

v Diision Pump

Fie 5.1. Schematic diagram of vacuum deposition system.

5.23 Transparent Electrode Deposition

As previously stated in Chapter 3, TOF/l'FTOF studies necessitate a transparent

electrode through which some external excitation (visible light in this study) may be

eransmitted to the a-Se layer underneath the electrode. A Hummer VT sputtering system,

depicted in Figure 5.2, was used to sputter transparent metallic electrodes onto the

surface of the a-Se films. TOF studies have been proven to be independent of electrode

material [8q, so the choice of contact materid was not critical to the work. Gold (Au)

was chosen as the electrode material because the transparency of the electrode is

relatively easy to control; in addition, a suitably transparent Au electrode is also

acceptably conductive (meaning that the resistance of the electrode itself is quite iow)

and may be mated in a relatively short time.

Wmum Chamber /----- Insulation

/----- Cathode

CahodeShisld ,/ -- - Au A t m s - '.

' . . . '* Gold Target

- Anode

Figure 5.2. Schematic diagram of the electrode sputtering systesn.

To place a transpareat Au eIatmde on a sample, an aluminum mask having a

firevlar apmrre with an area of eithe~ 0.5 or 1.0 cm2 was first placed in contact with the

sample and secured with ordinary househoId scotch tape. The sample and mask were

then loaded into the vacuum chamber of the Hummer VI sputtering system. Evacuation

of the chamber could take upwards of - 45 minutes, during which time the vibration of

the mechanical vacuum pump would shift the position ofthe mask were it not secured to

the sample with scotch tape. Once evacuated of air (to a pressure of - 50 mTor), the

chamber was flushed with argon (As) for several minutes to ensure that little air remained

in the chamber. The Ar needle valve was then adjusted to maintain a chamber pressure

of I00 mTorr, at which point a large dc bias (3 - 4 kV) was applied between the anode

and cathode. This causes the Ar to ionize and form a plasma; the positively charged Ar

ions are accelerated toward the cathode where the Au target is also [mated. The collision

between these Ar ions and the target causes Au atoms to be dislodged; once disIodged

they settle on every exposed surface in the chamber. The unmasked portion of the a-Se

sample is coated in this fashion and thus forms the electrode. An acceptable electrode

would require a deposition of - 12 minutes at an Ar pressure of - I00 mTorr and a

plasma current of - 13 rnA. The side-to-side resistance of an acceptable contact ranged

from - 30 - 300 R.

Electrical connections to the substrate and top electrode for either TOFAFTOF

studies or for x-ray photoconductivity studies were made in one of two equivalent ways.

Contact could be established through a thin wire bonded with high purity silver paint (SPI

#5001), or by pressure contacts. Although either method is acceptable, the contacts made

with silver paint were the easiest to use, as the pressure contacts were at times difficult to

establish an acceptable electrical connection.

During TOFIIFTOF measurements, the photoinjected charge packet must be

confined to the area directly beneath the transparent electrode. If charge is photoinjected

in a region adjacent to the transparent electrode, it will drift under the influence of the

weak fXnging field at the edge of the electrode, leading to problems in interpreting the

TOFATOF waveform. During the course of this work, heavy black paper with a small

circular aperture was used to expose only the transparent electrode itself to the laser

pulse.

53 TOFAFTOF Apparatus

The TOF transient photoconductivity technique was pioneered in the late 1950s

and early 60s by Brown [87], Spear [88], and Kepler [89]-who used it to study the

electrical characteristics of a number of different materials. The TOF transient

photoconductivity technique is attractive because it allows the direct measurement of

either the electron or hole drift mobility of the material under scrutiny. As first

mentioned in Chapter 3, the TOF technique consists of measuring the transient

photocurrent that results when photoinjected charge carriers are induced to drift across a

high resistivity solid under the influence of an externally applied electric field-

A pulse of light of very short duration is the most common method of injecting

charge carriers into the sample; as such, care must be taken when choosing the

wavelength of the light pulse so that it is strongly absorbed by the material under study.

A number of different light sources have been used by different authors, including xenon

1901 and N2 [91] flash lamps to study a-Se, nitrogen pumped dye Iasers to study silicon

backbone polymers [92] and Q-switched ruby lasers to study organic polymers [93].

Variations of the versatile TOF technique have been employed to study a number

of different charge transport characteristics. For example, advance application of the bias

bas been used to study the effect of negative bulk space charge on the transport

characteristics in a-Se films [94]. Delaying application of the bias until a h charges are

photoinjected allows the study of charge carrier surface recombination [95]. Finally,

double injection (of holes on one side of the sample and electrons on the other) has been

used by Dolezalek and Spear [96] and by Haugen and Kasap [97] to study bulk

recombination in orthorhombic sulphur crystals and a-Se respectively.

The TOF/IFTOF apparatus employed throughout this work is illustrated in Figure

5.3. MOSFET high voltage switches apply the bias to the sample and connect the

floating voltage follower to ground and to the oscilloscopes; photoinjection is achieved

by a nitrogen laser. The transient photocurrent is captured and displayed by two

osciIloscopes: the andog scope captures the first portion of the IFTOF waveform (prior

to intemption) and the digital scope captures the second portion (immediately foUowing

the reapplication of the bias field). The entire process is contded by an FPGA-based

(Field Programmable Gate Array) timing generator which itself is controlled by a

computer. Transient photoconductivity measurements must be performed in the dark (for

obvious reasons) and the Pb cage thus served a dual purpose; to prevent operator x-ray

exposure and to isolate the a-Se sample being studied from any stray light in the room.

I X-ray Unit

Figure 53. Simplified view of the TOFFTOF apparatus. Two PCs were used, the 8088 PC connected to the CCD camera (analog oscilloscope) was necessary because the antiquated video b e storage card could not be used in the modern pentiurn PC.

5.3.1 Nitrogen Laser

A Laser Photonics LN 103C Transversely Excited Atmospheric (TEA) nitrogen

Iaser provided the light pulse that was used to photoinject charge carriers in the a-Se

samples under study. This laser provides a pulse with a duration of 300 ps at a

wavelength of 337.1 nm and a peak power of 250 kW. The light pulse from the laser was

coupled to an optical 6ber pigtail (Newport Optical Fiber adapter MM-2A) which was

fed into the Pb cage and held in place above the a-Se sample by a clamp.

As specified in Chapter 3, one requirement for the light source is that its duration

be much less than the transit time of the carriers across the sample. The thinnest sample

examined in this work had a thickness of 170 E.L~; this translates to a minimum possl3Ie

hole transit time of - 0.74 p (this minimum transit time is limited by the maximum

possible bias voltage of 3 kV h m the high voltage supply). Indeed, 300 ps is much less

than this minimum transit time. Another requirement for the light source is that it be of a

wavelength that is strongly absorbed near the surface of the a-Se sample. Light photons

of wavelength 337.1 nm have an energy of 3.68 eV; from Figure 2.9 the absorption

coefficient of a-Se at this energy is - 5.5 x 10' Icm which corresponds to an absorption

depth of - 0.02 p. Therefore, the nitrogen laser easily meets the requirements of the

light source for the TOFmOF measurements.

The TEA N2 laser is based on a 3-level laser system where the laser transitions

occur between the &Iu (C-state) and the B%, (B-state) electronic states of the Nz

molecule, as shown in Figure 5.4 [98, 991. A gas discharge excites electrons h m the

ground to the C-state by extmal electron impact collisions. The lifetime at the C-state is

relatively short (- 2 ns) at atmospheric pressures, while the B-state is metastable. This B-

state metastability prevents a nitrogen laser from continuous operation; as a result, TEA

N2 lasers may only be operated in a pulsed mode. An electrical discharge is employed to

achieve the rapid population inversion between the C- and B-states.

15 -

Electron Collisions - ' 337nm

Nuclear Separation (Angstroms)

F i e 5.4. Potential energy culves for the lowest triplet states in the N2 molecule 1981.

The Blumlein excitation method, pictured in Figure 5.5, is used to excite the Nz

molecules and produce a laser discharge. Initially, CI and Cz are charged to a high

potential (- 10 - 11 kV for this work). The laser channel electrodes are shorted by an

inductor. When the spark gep is triggered, C1 is rapidly discharged through the gap and a

damped oscillation occurs in the right hand loop of the circuit, causing a voltage reversal

to appear on Cr. This causes an overvoltage-and electrical breakdown-in the laser

channel. Discharge occurs very quickIy after the gap is triggered (on the order of -

Charged to High Voltage w

I' '., Spark

' . G a p laser v -: Head <.

A 4'

Figure 55. B I d e i n circuit used for rapid excitation of N2 laser.

The LN 103C Iaser is configured to be triggered by two separate logic high 'TTL

pulses. The firs& pulse (of at least 100 ns duration) to the Trigger Reg input will charge

the excitation circuit of Figure 5.5. A second pulse (of at least 100 ns duration) to the

Trigger Low input must fallow the first pulse between 30 and 50 ps later to fire the laser.

If the second puke does not arrive within 50 p of the ht, the laser will self fire. If the

spark g;ap is optimally adjusted, h e LN 103 C's operating manual claims that the jitter

between the arrival of the Trigger Low pulse and the firing of the laser is - 2 ns. Figure

5.6 depicts the Trigger Reg and Trigger Low timing requirements of the laser.

53.2 MOSFET High Voltage Switches

The single lagest impediment to performing UTOF measurements is the Iarge

transients that appear across the sampling resistor when the bias is applied to and

removed h m the sample under study, as pichued ia Figure 5.7. These transients are

problematic because they could potentially be sufficient to harm the voltage follower

amplifier and/or the osciUoscop, and they can obscure the small photocurrent si@.

Past work [58, 62, 100, 1011 usad various means to null the effect of these transients;

among them a Schering4ype bridge, a complimentary pulsed bias technique, and a

resistance ratio-type bridge. These methods had numerous drawbacks; the

comphentary pulsed bias technique was only useM for low bias voltages and the

bridge techniques required carell and tedious balancing of the bridge. The act of

baIancing the bridge could take several hours to accomplish; very often, some samples

simply could not be balanced.

Laset Self Fires --- Firing Windm -

Trigger Reg

Trigger Lm

Figure 5.6. Trigger timing requirements of the LN t 03 C nitrogen laser.

This work employed a MOSFET high vaitage switch to simply avoid the

switching transients induced in the sampling resistor. The high side switch applied and

removed the high voltage bias to the sample, and the low side switch

comecteddiswmected the floating reference of the voltage follower to ground and its

High Side Switch

F i e 5.7. Application and removal of the high voltage bias during the IFTOF experiment induces large switching transients across the sampling resistor.

output to the oscilloscopes. Slightly delaying (usually on the order of a few p) the

reconnection of the voltage follower after reapplication of the bias allowed the transient

on the sampling resistor to decay to a level that would no longer threaten the voltage

follower or oscilloscopes. Although a slight switching transient was still superimposed

on the photocurrent signal, this could be digitally subtracted through software: a

waveform without photoexcitation would be recorded and then subtracted fiom the

recovered photocurrent to yield a transient-fiee waveform. This method introduced a

sIight "dead time1' into the measurement of the second part of the IFTOF photocurrent; a

period of time following reapplication of the bias to the sample when the photocurrent

signal could not be recorded. However? this dead time effect wuld be minimized by

choosing a relatively low bias field so that the carrier transit time would be relatively

long compared to this dead time. For example, electron transit times for the samples

analyzed in this work were typically chosen to be - 500 p, while typical dead times were

- 10 - 30 p, depending on the sample, the polarity of the applied bias, and the value of

the sampling resistor. With the dead time much less than the carrier transit time,

interpolation of the magnitude of the photocurrent (i3 at the reapplication of the bias is

straighdbrward.

There were two requirements for the MOSFET high voltage switches: they had to

be able to switch 3 kV (the maximum available from the EG&G Ortec 556 high voltage

supply) in less than 100 ns. MOSFETs rated at up to 1.2 kV were available when the

switches were constructed, but 1 kV models were more readily available at lower wst. In

order to switch 3 kV, at least three 1 kV MOSFETs co~ected in series were required.

However, this minimal configuration was rather dangerous: if one FET became

damaged, it would trigger a cascade failure of the other two and would likely damage the

rest of the test equipment. To provide some redundancy should a FET fail, six I kV

MOSFETs connected in series were chosen for each switching chain.

The switching speed of a FET is mainly controlled by one factor. the speed with

which the gate capacitance may be charged or discharged. Therefore, to achieve the high

switching speeds required, the gate of each FET had to be driven by a high current, low

impedance circuit. Optoisolators with an integrated npdpnp transistor pair were chosen

to drive each FET to facilitate this requirement. A detailed schematic of one MOSFET

with its optoisolator gate driver is found in Figure 5.8. This FETfdriver pair f o m the

basic building block of the two switches built for this work. The role of the optoisolator

is twofold: it provides the high drive current necessary to quickly switch the FETs, and it

also provides electrical isolation between the high voltages being switched and the timing

generator driving the switches themselves.

+6!! - - x o . 1 pF

-5 - - 9 v + -

/ Drain -,

-1 , - MOSFEr +-- I

0-- -=.. 4

nn ',4 A

Source

Figure 5.8. The MOSFEWoptoisolator gate driver pair that forms the basic building block of the switches built for this work. MOSFET: Motorola MTP3NlOOE n- channel enhancement-mode rated at 1000 V, 3 A, bn = 4 Q, Q* = 32.5 nC. Optoisolator: Hewlett Packard HCPL-3101 Power MOSFETflGBT Gate Drive Optocoupler rated at 5000 Vac isolation and 0.4 A peak output current. A calcuiated switching time of 8 1 ns is achievable.

In addition to connecting or disconnecting the bias voltage to tbe a-Se samples,

the high side switch also had to ground the sample during those intervals when the bias

was disconnected fiom it. Grounding the sample was necessary so that there would be no

externd bias field during the interruption time ti of the IFTOF experiment. If a bias

remained across the sampIe during this intermption time, the charge packet would

continue to drift in the sample under the influence of the external field and would not be

haIted. It is for this reason that the high side switch has two switching chains of six FETs

each to alternately apply the bias to the sample and to short the sampIe to ground when

the bias is removed. A schematic of the complete high side switch is presented in Figure

5.9. To simplify the operation of the switch, only one TTL trigger is used for control;

the trigger is used to control the state of one chain while an inverted copy of the trigger

controls the state of the other chain. The TTL trigger to SimultaneousIy connect the bias

to the sample and disconnect the sample h m ground is active low. When the trigger is

high, the bias is disconnected h m the sample and the sample is internally shorted to

ground. The high side switch had one additional layer of optoisolation to protect the

timing generator and pentium PC just in case of an equipment failure.

!TngprlLL*.CPI I I , h r p g r ~ r a l

~oprorsolstod I , m d ! opaPisorarod 61FEfPalr - - siFEf Pair & s : F l T Pair A -

To Sample *-2

Figure 53. Schematic of the high side switch.

A schematic of the Iow side switch is given in Figure 5.10. Like the high side

switch, it had to be buiIt to withstand transients that could potentidly reach 3 kV, but its

two chains--one to connect the floating ground of the voltage folbwer to true ground

and another to connect the output of the follower to the oscilloscopes-had to be

switched in unison. Like the high side switch, this was triggered by a single active low

TTL signal and it also had one additional layer of optoisolation in case of an equipment

failure.

D lopoibolatoc . .. LOp(oisolatod Z - 1 2 - Loptoisolator/ 2

FET Pair 'FET Pair FET Pair

Amplifier Fleeting Ground

V a l (4 V)

Figare 5.10. Schematic ofthe low side switch,

A chain of six FETs has an appreciable stray capacitance to ground of - 400 pF

when turned on. This is the main reason that a floating amplifier arrangement was used

in conjunction with the low side switch to feed the transient photocment signal to the

oscilloscopes. By floating the amplitier, tbe effect of this large stray capacitance was

minimized. The stray capacitance of the chain connecting the floating ground of the

amplifier to true ground could be charged quickly because the only limiting resistance is

introduced by the chain itself (- 40 R when on), The stray capacitance of the chain

connecting the output of the amplifier to the oscilloscopes was limited only by the output

resistance of the amplifier (- 300 Q), which meant that it couId also be charged quickly.

The high side switch has easily withstood its rated switching voltage of 3 kV on

numerous occasions throughout the work with no failures to date. The speed of the

switches is also faster than the specified switching speed of 100 ns. Figure 5.11

illustrates the unloaded "on" and "off' voltage transients of a 100 V dc bias. 10 - 90%

and 90 - 10% switching speeds are 52 ns and 28 ns respectively; well within the

specified limit. Due to the inductive leads of the switch, there is a small amount of

ringing present when the switch is turned on/ofK This ringing could conceivably cause

problems when the bias is reapplied to the sample during an IFTOF procedure. However,

this ringing extinguishes in - 300 ns, and this is negligiile when compared to the cartier

transit time of a typical IFTOF experiment, The switching times of the low side switch

are identical to that of the high side switch. Figure 5.12 is a photograph of the low side

switch.

5.33 Voltage Follower

The voltage folIower is necessary to drive the relatively large capacitance

introduced by the coaxial cables that comect the oscilloscopes to the sampling resistor.

A very simple battery driven follower based on the Harris HA-5033-5 Video Buffer is

pictured in Figure 5-13. The batteries, power switch, 0.1 pF bypass capacitors and f 12 V

voltage regulators have been omitted for clarity. The HA-5033-5 has a low output

0 200 400 600 800 1000

Time (ns)

F i e 5.11. Unloaded switching transients of the high side switch.

resistance of 5 GI and a wide small signal bandwidth of 250 MHz. However, since the

buffer was being used to drive the 500 kR input impedance of the two oscilloscopes (two

paralleled I MQ resistances to ground) instead of the low impedance matched load for

which it was designed, stability b&e a problem. The series 270 R output resistor

made the amplifier unconditiodly stable, but limited its bandwidth to 20 MHz. This 20

MHz bandwidth was d c i e n t for the work, especially when considering that previous

projects [loo, 1021 used voltage followers with bandwidths of less than 10 MHz. Figure

5.14 depicts the gain vs, Ereqwncy response of the voltage follower.

Figure 5.12. Photograph of the low side switch.

F i r e 5.13. Circuit schematic of the voltage follower.

5.3.4 FPGA Timing Generator

The success11 completion of a TOF or IFTOF measurement requires strict

control over the timing of the switches, the triggering of the oscilloscopes, and the fixing

of the laser. To control these devices, a timing generator was constructed fiom an Altera

UP 1 Education Board and a National Instruments DIO-3WS Digital InputlOutput card

mounted inside the pentium PC.

The Altera UP1 Education Board interfaces to the pentium PC via the parallel

port and may be programmed via Altera's Max+Plus II student edition software. The

evaluation board contains an EPFlOK20 FPGA and a 25.175 MHz crystal oscillator.

Figure 5.14. Gain vs. frequency response of the voltage follower.

Other components are present, but are not relevant to the following discussion. External

connections via headers on the UP1 Board were made to the DIO-32HS card (inputs for

the timer functions) and to the test equipment (?TL timing outputs). A simplified

diagram of the connections to the UP1 Board are given in Figure 5.15.

Anera UP1 Education Board

25.175 MHz Cfystal Oscillator

EPFlOKM FPGA

Configuration Information via Parahel Port

(M~X+PIUS II Software)

F i e 5.15. A simplified diagram of the connections to the Altera UP1 Education Board showing the flow of data into and out of the board.

90

The EPFlOK20 FPGA was configured via the Max+Plus [I software to function

as a liming generator based on a number of chained single shot counters. These counters

were loaded and initially triggered via the pentium PC by the DIO-32HS card. The

25.175 MHz crystal oscillator on the UP 1 Board functioned as the timing reference. The

chained counter structure that the FPGA is programmed to emulate is shown in Figure

5.16. The input data and clock signals have been omitted from each counter to relieve

clutter. Operation of the timing generator was very simple. LabView software running

on the pentium PC would conveniently load each counter in the FPGA via the DIO-32HS

card according to the TOFIFTOF timing requirements set by the operator. Once aH

counters were loaded with their appropriate values, the software would trigger (via the

DIO-32HS) the first counter to begin counting down; once it completed its countdown, it

- . . A-

Laset Rag - Laser R e g - Scow 1 Laser LOW -- k" , (4

scopa 1 Lassr Low - Amp I

TrigBer (dl Amp uf (I) L Amp-Bias

(0) L m - B i a s

a Bias On (h) 'I

A m ~ o f f O

A m - Bias

5.16. A block diagram of the chained counters responsibIe for generating the timing waveforms necessary to control the TOF/IFTOF apparatus, and the timing waveforms thus created (not to scale).

would simultaneously disable itself and trigger the next counter in the chain to begin its

countdown via an active high signal, and so on. These signals that each counter passed to

the next in the chain were used to trigger the switches, osciIIoscopes and the laser, as

shown in Figure 5-16.

5.3.5 Data Acquisition

Data acquisition was performed via two oscilloscopes; one analog, one digital. A

Tektronix 2467B 400 MHz analog oscilloscope was used to capture the fht part of the

IFTOF waveform. Those waveforms were then captured and stored in the 8088 PC by a

Tektronix DCSO 1 CCD Camera Data Acquisition System. The DCSO I system consists

of a Tektronix C1002 charge coupled device (CCD) camera mounted to the fiont of the

oscilloscope and connected to a PC DXOl video frame storage card which resided in the

8088 PC. The digitized waveforms were transferred to the pentium PC via floppy disk

for subsequent analysis.

A Tektronix TDS210 1GSaIs (1 gigasample per second) 60 MHz digitizing

oscilloscope was used to capture the second part of the lFTOF waveform. The

waveforms were then directly transfened to the pentiurn. PC via a GPIB bus connection to

the oscilloscope and a N a t i d Instruments AT-GPlBtTNT card mounted in the pentium

PC.

The andog oscilloscope was used in this work strictly out of necessity. The CCD

camera waveform acquisition system is both slow and inaccurate, and the 2467B was the

cause of a sizeable (-2 mV) noise signal in all the recovered waveforms. Further, the use

of the 2467B negated the porn'bility of measuring the electron transport properties of

samples with duminum substrates, as first mentioned in section 5.2. The reason for this

is as fcllows. For this work, electron transport was measured not by revetsing the bias of

the high voltage supply, but by reversing the leads on the a-Se samples, as illustrated in

Figure 5.17. This method is functionally equivalent to reversing the polarity of the

applied bias.

Figure 5.17. Sample connections for measurement of (a) hole transport and (b) electron transport.

Since the sampling resistor and the oscilloscopes are dc coupled by the voltage

follower when the low side switch is closed, any dc ofiet across the sampling resistor

arising due to a dark current in the a-Se sample will be passed to the oscilloscopes as

well. In order to not perturb the applied bias field, the injected charge packet must be

small, as first discussed in Chapter 3. Under these coaditions, the maximum size of a

photocurrent wavefonn is at most 10 - 20 mV, and must be displayed on a small

voltageJdivision setting (i.e. 2 mV/div) so that the overall resolution of the photocurrent

remains high However, at 2 mV/div, the adjustable vertical position (dc offset) of the

waveform on the 24678 is very limited. The electron photocurrent of a sample with an

aluminum substrate fkquently could not be displayed by the 2467B oscilloscope because

of this limitation.

5.4 X-ray Photocurrent (Wm) Measurements

The measurement of Ww is straightforward, provided that the amount of energy

that the x-ray beam deposits in the sample is known. In very basic terms, the procedure

involves biasing an eIectroded a-Se detector, irradiating it with x-rays, and monitoring the

resuItant photoamrent Wmp is then simply the ratio of the x-ray energy deposited in the

sample to the amount of charge collected in the photocurrent. However, the energy that

the x-ray photons deposit in the sample is rather difficult to determine because the output

spectrum of an x-ray tube is not monoenergetic, and is difficult to measure.

5.4.1 X-ray Exposure System

X-radiation is electromagnetic radiation produced either when hi@ speed

electrons are rapidly decelerated, which results in the production of bremsstrahlung

(braking) radiation, or when an electron transitions from a high to a low energy state in

an atom, which results in characteristic (monoenergetic) radiation. An x-ray beam

produced by an x-ray tube contains both types of radiation.

A typical x-ray tube is pictured in Figure 5.18. An x-ray tube is a temperature-

limited vacuum tube diode; electrons are emitted by a hot filament cathode and are

accelerated by a high (- 100 kV or more) voltage to strike a small focused area on an

anode. The anode is typically fabricated from tungsten or a tungsten alloy because only

about 1% of the energy of the electrons is emitted as x-ray photons (1031. The rest of the

Vacuum Tube \ ,

I \

Tungsten Anode j 1.. Farget)

i-----'

- - -

- -

\ Fdcusing Cup !,'

Figure 5.18. Schematic diagram illustrating the major components of a rotating anode x-ray tube.

energy is dissipated as heat in the anode-thus the need for a high melting point materid

such as tungsten. Heat dissipation may be aided by a rotating anode assembly such as

that pictured in Figure 5.18, so that the electron beam does not burn one spot on the

anode.

X-ray photons are emitted fiom an x-ray tube when electrons are absorbed in the

anode; their energy depends on the kinetic energy of the incident electrons. Like the

visl%le light photons produced by an ordinary incandescent light bulb, the x-ray photons

produced by an x-ray tube are not monoenergetic and consists of a continuum of photons

with different energies. Bremsstrahlung or braking radiation is produced by the

deflection of an energetic electron by the nuclei of the anode (target) atoms.

Bremsstrahlung radiation has a wide distniution with an upper limit equal to the energy

of the incident electrons. Characteristic radiation results when the vacancies in the inner

shells of an atom (produced by the collision of an incident energetic electron) are filled

by outer shell electrons. The energy that the outer shell electron loses when it fills the

inner shell position forms well defined characteristic peaks in the output energy spectrum

of the x-ray tube. The various mechanisms leading to x-ray photon production are

illustrated in Figure 5.19.

The various x-ray spectra employed during the course of this study were produced

by a Gendex GX-1000 dental x-ray unit. This particular model had an adjustable tube

voltage, variable h m 50 - 100 kVp. The tube current had two settings: 10 or 15 mA.

The output x-ray intensity of this unit is not constant with time; the output occurs in

1/120h second bcspikes" corresponding to the self-rectifying nature of the tube and the 60

Hz suppiy voltage. The unit is equipped with an impulse timer which may be set to

output anywhere fiom 3 spikes to an output train of spikes of 5 second duration. The x-

ray head itself is equipped with 2.7 mrn of inherent A1 fltering and a threaded collimator

which could be removed if needed. The x-ray head was mounted inside a coxnmercidly

produced lead-lined cabinet for safety.

Characteristic Incident Electrons

F i e 5.19. Typical electron interactions with a target. (a) Electron suffers ionizationid losses, giving rise to delta rays and heatt (b) Electron ejects K shell electron leading to characteristic radiation. (c) Collision between nucleus and eiectron of energy E leads to bremsstrahlung radiation of energy hv. The electron recedes tiom the collision with energy E - hv. (d) Collision where electron is completely stopped by a collision with the nucleus. The 111 energy of the electron is released as bremsstrahlung radiation.

The output x-ray photon spectral distribution is affected by three factors: tube

c m t , tube voltage, and filtration. The number of energetic electmns emitted tiom the

cathode is dependent on the filament current; the higher the filament current, the higher

the number of electrons ejected from the cathode and accelerated to the anode (i-e. the

tube current). The higher the number of electrons that strike the target, the more x-ray

photons emitted. As the tube voltage is increased, the maximum energy of the output x-

ray spectral distniution is also increased. This is due to the increased kinetic energy of

the electrons impinging the anode. The resulting beam will also have a higher mean

energy than one produced with a lower tube voltage (all other factors being equaI). The

spectral components of an x-ray beam may also be changed by adding filtration- to the

beam. In general, higher energy photons of an x-ray beam will be less affkcted by a

given thickness of filter material (eg. Al, Cu, Pb, etc.) than lower energy photons. Thus

filtration will preferentialIy pass higher energy photons and block lower energy photons

and will have the overall effect of increasing the mean energy of the beam (again, all

other factors being equal).

5.4.2 Pulse Height Spectroscopy

A major chdenge in determining the energy deposited in the a-Se detector layer

was the determination of the exact spectral distnition of the x-ray beam. One approach

is to formulate an estimate of the effective energy of the beam through a measurement of

the halfvalue layer (HVL) of the beam [73,102]. The estimate of the mean energy of the

beam, in conjunction with a measurement of the exposue (defined as the amount of

ionizing radiation required to Iiberate 2 . 5 8 ~ 1 0 ~ C of charge in 1 kg of dry air) of the

beam is used to calculate the energy that the beam deposits in the a-Se samples. This is a

very crude method and is susceptible to error. Another very involved approach is to

measure the spectral components of an x-ray beam by employing an x-ray diffraction

device [104]. This method does offer an accurate means of measuring the spectrum of an

x-ray unit, but was unsuitable for this work because of a lack of equipment, the energies

involved, and the contined space inside the x-ray cabinet.

The method of measuring the spectral components present in the beam of the x-

ray unit utilized in this work involved the use of a commercially available pulse height

spectroscopy measurement unit. The basic premise of pulse height spectroscopy is very

straightforward: a crystalline material with a well defined, field independent, EHP

creation energy is used as the detector. This crystal is biased at some field, and the

photocurreat resulting fiom any incident radiation is fed to an analog-todigital converter

(AID), as seen in Figure 520. Since the detector material has a well defined field

independent Ww, ,the amount of fiee charge carriers h i e d by an incident photon will

be directly proportional to the energy of that photon. The amplitude (or height) of the

resulting photocurrent "spike" will in turn be directly proportional to the energy of the

incident photon, and the AID channel number to which the pulse height corresponds will

also be directIy proportional to the energy of that photon. A typical measurement

involves a summing of the number of pulses that fall into the various A/D channels.

-, -.. - Radiation Source

Intermediate Energy Photon /' .

- Lowest Energy Photon

\ / Highest Energy Photon \

Detector -- -

b

To Amplifier@) 8 AID Photocurrent Pulses

F i 530, The principle of pulse height spectroscopy. The photon with the highest energy is emitted from the radiation source k t and strikes the detector first, generating the highest photocurrent pulse. The process is repeated, in turn, for the lowest and intermediate energy photons, which create the lowest and intermediate photocment pdses, respectively.

Therefore, a determination of the energy (spectral distriiution) of the radiation striking

the detector simply involves counting the number of photocurrent spikes with a given

height. By graphing the cumulative number of pulses in each A/D channel vs. the A/D

channel number, a graphical representation of the spectral distriiution of the radiation

impinging the detector may be formed.

The pdse height spectroscopy unit used during the course of this work consisted

of an eV Products Model 180 CZT detector, an EG&G Ortec Model 297 CZT Detector

Probe Amplifier, and an EG&G Ortec DART MCA (muitichannel analyzer) Portable

Gamma-Ray Spectrometer equipped with MAESTRO for Windows control software. A

photograph of the unit is shown in Figure 5.2 1. The amplifier and DART unit both had

to be wrapped in - 1 cm of Pb sheeting (as seen on the photograph) to prevent any

unforeseen and undesired effects arising from any stray x-ray photons interacting with the

sensitive electronics inside the units. The only component which was exposed to x-ray

radiation during the measurements was the CZT detector itself.

Figure 531, A photograph of the pulse height spectroscopy measurement unit used in the course of this work. The photograph on the left shows the DART A/D unit and the Pb shielded amplifier and detector. The photograph on the right is a top view of the amplifier and detector alone, with the top portion of the Pb shielding removed to reveal the cylindrical detector atop the ampfifier.

Pulse height spectroscopy was originally developed to be used in "low count"

situations-situations in which the x-ray or gamma radiation striking the detector is

rather infrequent so that the resulting photocunent pulses are well separated in time and

may be easily ddineated. Such applications usually involve the detection and

identification of radioactive isotopes in various materids or locales. When the arrival

time between each photon that strikes the detector becomes smalI, the photocurrent pulse

due to each photon will then add; a phenomenon known as pulse pileup (PPU). These

additive pulses can appear to have been caused by a single photon with an energy equal

to the sum of the energies of the individual photons. Even though the measurement unit

was equipped with circuitry to detect and reject severe PPU conditions, this one

phenomenon proved difficult to overcome in the spectral measurements.

5.43 Spectral Measurements-Procedure

Because the pulse height spectroscopy unit is specifically tailored for low photon

fluences (intensities), using it to measure the output spectrum of the x-ray unit was very

difficult. The x-ray unit is designed to output a great number of x-ray photons, and the

number of photons that reached the detector had to be significantly decreased so as to

reduce PPU. Figure 522 is a plot of the measured spectrum of a single 10 impulse burst

of the x-ray unit without any filtering or apertures placed between the tube head and the

CZT detector. The tube settings were 50 kVp and 15 mA; the lower energy cutoff of the

detector was set to approximately 15 keV. As can be seen h m the plot, the measured

spectrum approximately exponentially decays with increasing energy, but the spectrum

has components that extend all the way up to about 440 keV (the upper limit of the

detector). Since the tube voltage was set to 50 kVp, it is physically impossible that the

tube emitted photons of energy greater than 50 keV. As was discovered, extensive A1

filtering, occasional f b filtering and the use of very small apertures to limit the number of

x-ray photons that reached the CZT detector were necessary to limit (but not completely

eliminate) the effect of PPU.

It was first necessary to determine how much Pb shielding was necessary to

completely block ail x-ray photons h m reaching the detector. Trial and error indicated

that - 9 mrn of Pb was necessary to block all x-ray photons fiom reaching the detector

practically all of the h e . This step was crucial to the measurement of the spectrum of

the x-ray beam-if my x-ray photons reached the detector by any other path other than

through the allowed aperture, then the measured and actual spectra would not be the

same. This is also the reason why the entire pulse height spectroscopy unit had to be

wrapped in - 1 cm o f Pb shielding.

300 400 500

Energy (keV)

F i e 532. Measured spectrum of the x-ray tube at 50 kVp and 15 mA. The effect of PPU completely obscures the real output spectrum of the x-ray unit when no filtering is present.

Once the necessary shielding had been determined and installed, apertures of

various sizes were drilled through the top Pb shield directly above the CZT detector. The

h a 1 aperture that was employed during the measurements had a diameter of just 0.016"

(406.4 p), corresponding to a #78 drill bit. Even with this very small aperture severely

restricting the number of x-ray photons reaching the detector, PPU still obscured the

spectrum of the x-ray nit.

The number of photons reaching the detector was fuaher cut by pIacing extensive

A1 and occasiond Pb fiIters in the beam. Trial and error resulted in the discovery of four

combinations of different iilter thicknesses and material combinations, and tube kVp and

mA settings that gave satisfactory spectral measurements and minimal PPU. The filters

were placed directly into the collimator of the x-ray unit, where a slot was cut for their

insertion.

The measurement of each spectrum occupied the better part of one day because x-

ray tube heating necessitated cooling intervals. To gather enough counts to produce a

satisfactorily smooth spectrum requires many exposures of the x-ray unit. The x-ray head

used in this work was purchased new at the beginning of 1999 because the pulse height

spectrometer revealed that the previous head was defective, most Likely due to

overheating caused by overuse. To limit tube heating, a short exposure of 116 second ( I 0

impulses) was selected as the "x-ray quanta" that would be used throughout the work. At

most, 30 116 second bursts, each 20 seconds apart, would be performed before the tube

was allowed to cool for 15 - 20 minutes. The measured spectra were the accumulated

counts resulting fiom as many as 360 of these individual 10 impulse bursts.

5.4.4 Measured X-ray Spectra

The four combinations of A1 and Pb filters and tube kVp and mA settings that

resulted in satisfactory spectra are summarized in Table 5.1. It was desired to obtain

mean beam energies that spanned a range typical of mammography, chest, etc.

radiographic energies. It was desirable to have a beam with a mean energy - 20 - 25 keV

(i.e. mammography), but the lower energy limit of the pulse height spectroscopy unit was

- 15 keV. Any attempt to set the lower energy limit of the unit < 15 keV resulted in

severe noise corruption of the spectrum and the complete characterization of a Iow

energy beam would have proven difficult, if not impossible. The uncertainty in the tube

voItage setting for the 32.8 keV beam in Table 5.1 arises h m the x-ray unit itself There

is a common tube voltage control knob with two scales: one for a tube current of 10 mA

and another for a tube current of 15 mA. Neither scale extends below 50 kVp, but 50

kVp on the 10 mA scale would correspond to - 42 kVp on the 15 mA scale, shodd it

extend that low. To obtain the 32.8 keV beam, the voItage was set to 50 kVp on the LO

mA scale, even though the tube cuxrent was set to 15 mk

Again, PPU could not be completely eliminated, but its presence served as a

helpful c o ~ a t i o n of the measured spectra. For example, consider the raw spectrum of

the 39.2 keV beam as shown in Figure 5.23. An interesting aspect to note fiom Iooking

at the raw data is the nature of the pulse pileup "tail" that extends to higher energies. For

example, photons of energy -38 - 42 keV are much more plentill or likely than photons

at other energies. Therefore, it would be reasonable to expect that a number of these

photons would likely arrive at the detector at the same time leading to puke pileup.

Indeed, the high energy tail has a plateau that extends fiom about 70 keV to about 85 keV

or so (- 2x the energy of the incident photons). Also, since the maximum energy of the

beam is 50 keV, it would be reasonable to expect that the high energy tail should

"extinguish"at around 100 keV. This is because two photons, each with energy 50 keV,

are not likely to arrive at the detector at exactly the same time siuce 50 keV photons are

not in abundance relative to photons of other energies. That is exactly what is observed

in the plot; the tail extinguishes at about 100 keV. Another conclusion that can be drawn

h m this analysis is that the energy response of the CZT crystal is indeed linear, that is,

a pulse caused by an 80 keV photon is twice the height of a pulse caused by a 40 keV

photon. Further supporting evidence is seen in the form of the very subtle, tiny peak

observed in the data at about 120 keV- This peak cannot be seen on the graph, but is

discernable if the raw data itself is examined. Again, -40 keV photons are the most

plentill, and therefore, more likely to arrive at the detector at the same time causing

pulse pileup. The instance where three 40 keV photons arrive at once is not overly likely,

but the miniature peak at 120 keV attests to its existence.

Table 5.1. Filter combinations and tube settings for the four different spectra.

The presence of spectra1 components with energy above the maximum tube

voltage of the x-ray unit in the measured spectra is a sure sign of PPU, and to accurately

caicdate the energy that the beam deposits in an a-Se detector, these erroneous

Mean Beam . Envg~y(keV)

32.8 39.2 47.1 5 8 2 *

Tube Voltage (kvp) - 42

50 60 80

Tube C.urrent ( m ~ )

15 10 10 10

A1 Filtering (mm)

12.7 22.05 18.7

31.75

Pb Filtering @m)

250 250

0 20 40 60 80 100 120 140

Photon Energy (keV)

F i i 5.23. Raw spectral data obtained from the pulse height spectmscopy unit for the 39.2 keV beam. PPU is present, but it may be used as a check on the data,

components had to be discarded. There was also a fair amount of false low energy

photon counts in the data, but their presence may be easily explained. The pulse height

spectroscopy unit as a whole was not noise-free and any low amplitude noise would

wunt as false occurrences of low energy photons. Therefore, the raw data accumulated

fiom the puke height spectroscopy unit had to be manually filtered. Before and after

pictures of the 39.2 keV beam are presented in Figure 5.24. Asymptotes were simply

estabhshed in the raw data to reflect the fact that the x-ray spectrum did not, in fact,

extend above 50 keV nor extend below - 27 keV.

25 30 35 40 45 50 55

Photon Energy (keV)

25 30 35 40 45 50 55

Photon Energy (keV)

F i i e 5.24. (a) Raw data and (b) t i l t e d spectra of the 39.2 keV beam.

The final filtered spectra of the four beams with mean energies of 32.8,39.Z, 47-1

and 58.2 keV are presented in Figure 525. The cwves were normalized to aI1 have the

same peak count for comparison. It should be noted that the mean energies of the beams

were evaluated through

where g(E) is the measured spectrum which extends fiom energy El to energy E?.

As a final note, the issue of accuracy in the pulse height spectrometer should be

addressed. A pulse processing system, when operated over a long period of time7 will

exhibit some drift that broadens pulses of a single amplitude into a distribution of

diffmt heights. By definition, the resolution R of a system is defied as the ratio of the

quadrature sum FWHM (MI width half maximum) energy drift from all somes in the

system to the energy of interest. The reported total drift of the MCA I 75 ppm. The

reported drift of the CZT detector is 4.3% @ 59.5 keV 241~m peak, 3.5% @ 122 keV

n ~ o peak, and 1.2% $662 keV %s peak. Therefore, the warst resolution over the

energies measured in this work is Rwmt I &75x 1 od)'+ (0.~3)' = 4.3% . It should be

noted that pulse drift and resolution are not a Iarge concern in this work, since the x-ray

5000 - - 39.2 keV Beam 47.1 keV Beam

10 20 30 40 50 60 70 80

Photon Energy (keV)

Figure 525. The manually fdtered spectra of the four beams used throughout the course of this work with mean energies of 32.8,39.2,47.l and 58.2 keV.

spectra being measured were continuous; therefore, pulse spreading is not a significaut

issue. Pulse spreading really only becomes important when examining the characteristic

emission peaks of radioactive i s o t o ~ w h i c h again reIates to the intended use of the

pulse height spectroscopy unit: radioactive isotope detection and identification.

5.4.5 Energy Absorbed in a-Se Layer and WEHP

To calculate the energy absorbed in the a-Se layer being irmdiated, one had to

know how many x-ray photons were present in the beam. e.g.

Here, E is the photon energy, d@(E)ldE is the number of photons with energy between E

and E + dE, A is the area of the detector, L s is the detector thickness, and A ~ L is the

energy absorption coefficient of selenium for photons of energy E.

That information was not possible to obtain born the pulse height spectroscopy

unit since the efficiency curve of the system was not known; this point will be discussed

below. The concept of "counts" vs. energy does not directly relate to the actual number

of photons that struck the detector without the intrinsic efficiency correction. Therefore,

the need for a calibrator x-ray photoconductor became apparent. The general idea of this

approach is that a detector material with a well deibed WW and energy absorption

coefficient could be irradiated by the four different beams characterized thus far. If the

resulting photocurrent was monitored to find the charge liberated in the calibrator

material, then the true amount of energy that the x-ray beam deposited in it could be

found via

Here e is the electronic charge and Q is the charge collected through integration of the

photocurrent (coulombs). Equations 52 and 5.3 codd then be used together to relate the

measured "counts" vs. energy spectrum to the actual number of photons vs. energy h m

the x-ray unit

Therefore,

where Kph is a dimensionless constant of proportionality that relates the measured

spectrum of CN counts at energy E in energy intmal dE to the actual energy absorbed in

the calibrator material. q(E) is the intriosic &ciency of the detector for photons with

energy between E and E + dE and will be discussed in detail beIow. The measured

spectrum is in units of keV, thus the need for the fictor of 1000. If Kfi could be found

for each of the four different beams, then the energy absorbed in the a-Se layer could be

easily calculated.

The intrinsic efficiency, q(E), of a radiation detector is a measure of the

probability that the detector will release charge in response to being struck with a photon

of energy E. Thus the measured spectra, which consists of counts vs. photon energy,

must be divided by the intrinsic efficiency of the detector in order to arrive at the true

spectra. This is required to accurately calculate the energy absorbed in the detector, as in

Equation 5.4. Since a CZT crystal was readily available fiom the pulse height

spectroscopy unit, it was chosen as the caIiirator material. However, the intrinsic

efficiency of the CZT detector employed in this work was not known and was not readily

measurable. It was expected that the intrinsic efficiency of a CZT detector should at least

resemble that of a CdTe detector, and such data is available [105]. Since the intrinsic

efficiency of CdTe varies at most - 10% over the range of photon energies employed in

this work (-80% at 30 keV, rising to 90% at 60 keV, and falling to - 82% at 80 keV), the

impact of this term on the measured spectra would be small. Indeed, a good

approximation would consider the efficiency to be constant over the energy range seen in

this work. This was the approach haHy taken; q was set to be unity and the constant

Kph would then account for geometric efficiency variations. The validity of this

technique was tested through the comparison of previously reported WEHP values with

those obtained by this method. At a constant bias field of 10 Vlpm, Wmp has been

reported to be - 35 - 55 eV over the diagnostic x-ray energy range [70 - 751; this study

found that WEHP was approximately 50 eV for the four different beams descri i earlier.

In addition, a simple check was performed. The measured spectra were corrected by the

efficiency curve of a CdTe detector (from [1051) and the Wmp values (with and without

the correction) for a sample irradiated by the 58.2 keV beam at F = 9.4 V / p differed by

only 0.41 %. This agreement with published results and the small difference in WUIP with

and without an efficiency correction were taken as Mer support that approximating the

intrinsic eEciency of the detector as constant was valid. The complete results are found

in Chapter 6.

The Wm for the Cdo.&.rTe detector tiom eV Products is 5.0 eV f 0.1 eV

[106], and the energy absorption coefficient was obtained from the Nationd Institute of

Standards and Technology [81]. The density of C&&,Te is given as 5.9 g/cm3 [107].

The CZT crystal itself is a 5 mm cube. The CZT detector was biased with a lOOOV dc

source, and the photocurrent resulting fiom x-ray irradiation was displayed and captured

by a Tektronix TDSZlO 1GSals (I gigasample per second) 60 MHz digitizing

oscilloscope. The photocurrent was transferred to computer and numerically integrated

to reveal the total charge h i t e d by x-ray irradiation. The complete experimental

apparatus and procedure involved in this task will be discussed in fiuther detail in the

following section.

Table 5 2 presents the Kph values of each of the four x-ray beams. Since the

output of the x-ray tube inevitably varies slightly fkom exposure to exposure, the charge

collected from 30 ten impulse '%burstsw was averaged to arrive at an average charge,

Qrnmge, liberated in the CZT crystal when irradiated by the different beams. Each Kph is calculated using this 30 measurement average. It should be noted that when performing

Wmp measurements on the a-Se layers, the distance between the x-ray unit and the a-Se

layers had to be equal to the distance between the x-ray unit and the CZT crystal when

the Kfi values were measured and calculated. Care was taken in this regard throughout

the measurements; if the two distances were not the same, the x-ray intensities seen by

the CZT crystal and a-Se layer would not be the same, and the Kph values would no

longer be accurate.

Once the Kfi values were found, the energy absorbed in the a-Se layer was then

trivial to accurately calculate. The energy absorbed in the a-Se layer is found by

Table 5.2. The four different x-ray beams and their Kh values.

and the electron-hole pair creation energy is then

M u n Beam Energy (IrcV) 32.8 39.2 47.1 58.2

6, (dhe~ionless) 55792 248.94 145.5 1 267.63

where Q is tfie charge collected from the a-Se layer when irradiated by an x-ray beam-

5.4.6 EV Converter

The layout of the x-ray induced photocurrent measurement system is shown in

Figure 5.26. These measurements were the most straightforward undertaken in this work.

An a-Se sample is b i d by a dc supply and the photocment resulting h m x-ray

irradiation is amplified by an I-V converter and captured by the TDS21O digital

oscilloscope. The captured waveforms are transferred to computer and numerically

integrated to iind the charge Liberated in the a-Se sample by the x-rays that strike it. The

energy that the x-ray beam deposits in the sample may be calculated as described above,

meaning that the energy to mate a coI1ected EHP, WMP, may be easily calculated.

X-ray Unit

Figure 5.26. Diagram of the apparatus used to measure the WEHP of a-Se.

StrictIy speaking, a Astor couId be considered to be a ~lrrent-to-voltage (I-V)

converter, and in many respects a simple resistor was the major component of the I-V

converter used in the come of this work. The I-V converter was necessary to amplify

the very Iow (- nA) x-ray induced photocurrents in the a-Se samples to a readily

detectable voltage. It was also necessary for it to provide a virtual ground at its input to

prevent the effect of sample loading. A schematic of the I-V converter built for this work

is pictured in Figure 5.27. The batteries, power switch, 0.1 pF bypass capacitors and f

15 V voltage regulators have been omitted for clarity.

Figure 527. Circuit schematic of the I-V converter built for this work with a variable conversion gain selectable in decades h m lo5 to 10' VIA.

The principle of operation of the I-V converter is straightforward. When an

operational amplifier (opamp) has a negative feedback path from its output to its

inverting input, it will strive to keep the two inputs at the same potential. Therefore,

since the noninverting (+) input of the first stage is grounded, the opamp will keep the

inverting input (-) at ground potential as well (a virtual ground input). The very low

input bias current (30 PA) of the TLO8 1 opamp ensures that the opamp itself will not load

the small (- nA) photocurrent signal, and its gain-bandwidth product of 3 MHz ensures

that all significant spectral components of the x-ray induced photocurrent will be passed.

Once the amplified photocurrent emerges h m the h t stage, it is inverted, that

is, the x-ray induced photocurrent pulses are negative. To flip the photocurrent pulses

positive, an inverting stage (the 3" stage) was added A voltage follower 2d stage is

present to isolate the 1" and 3d stages. An ac coupling capacitor is available in case the

dark current in the a-Se sample being studied is unacceptably high. The I-V converter

was calibrated through the use of a Keithley 616 Digital Electrometer. The maximum

conversion gain available from the I-V converter was lo9 VIA.

Low pass filtering of the x-ray induced photocurrent is performed by the 3" stage

of the I-V converter, with a cutoff frequency of 5 kHz. This filtering is necessary to

reduce high fkquency noise present in the high field dark current of an a-Se sample, and

to help force the I-V converter to be unconditionally stable. A passband of 5 kHz was

sufficient to adequately pass all significant components of the photocurrent pulses.

5.4.7 Low Temperature Protective Chamber

A previous study into the effect of temperature on the EHP creation energy of a-

Se [73, 1021 was extremely limited in its achievable temperatures because water vapour

would condense on the cooled sample and form a conductive path h m the electrode to

ground with disastrous results.

To prevent water vapour h m reaching the sample, a simple protective chamber

was fashioned h m the remnants of a 1 liter plastic soft drink container and an aluminum

"flwf' as pictured in Figure 5.28. The a-Se sample was held securely against the

aluminum "floor" with pressure contacts so that good thermal contact with the alumhum

would be maintained Connections to the electrode and substrate of the sample were

made with silver paint and to coaxid connectors mounted in the aluminum. A

thermocouple was placed in direct contact with the top of the sample near the electrode

so that the temperature of the sample codd be monitored. AU joints and holes in the

container, except one bleed hole, were then sealed with caulking. During the

experiments, the chamber was continually flooded with argon gas via the cap of the

container so that the Ar, being more dense than air, would displace any and all moisture

laden air present in the container.

Bleed Hole

Figure 538. Diagram of the low temperature protective chamber flooded with Ar.

The temperature dependence of the EHP creation energy of a-Se was investigated

by initially cooling the sample (inside its protective chamber) with dry ice (solid COz at - 195 K or - 7g°C). The WEHP of the sample was then measured as the sample warmed to

near room temperature.

5.4.8 X-ray Collimator/Fiiter Housing

Aluminum a d lead fltm were extensively used to create x-ray beams with

certain energy disttibutions, as described previously. However, x-ray photons that are

scattered by these filters can cause problems in that these low energy scattered photons

can increase the apparent dose seen by the detector [lo81 (the a-Se samples in this case).

To reduce the effect of scattered radiation, collimators are frequently employed to limit

the beam to a narrow cross section. Disks of Pb sheeting with a circular aperture of 2.5

cm diameter were placed inside the collimator of the x-ray unit as shown in Figure 529.

The collimator had a slot cut in its side for the insertion of up to 6 mrn of AI sheet filters,

but the majority of the filters were secured to the bottom of the collimator using duct

tape.

Threads into X-ray Unit 7 - Slot (for up to 6 mm /- &--- of additional filters)

I 5 mrn

I

_I 5 mm

-r-

Pb Disks

Pb Disks

F i e 5.29. A cutaway view of the modified collimator attached to the x-ray unit to restrict scattered radiation fiom reaching the samples.

With the collimator in place, care had to be taken to ensure that the a-Se samples

were aligned beneath the aperture in the collimator. Alignment was not absolutely

critical, as the x-ray unit had a sizeable (approx. 2 cm x 3 crn) spot where the measured

x-ray photon intensity was uniform. The a-Se samples were placed at a distance of - 25.5 cm from the focal spot of the x-ray unit during all measurements.

5.5 Miscellaneous Experimental Tools

The memement of the electron lifetime re and how it was affected by annealing

at elevated temperatures, IR soaking and exposure to uItrasonic vi'bration employed three

additional experimental tools, as discussed below.

5.5.1 Sample Oven

To elevate and control the temperature of the a-Se samples during annealing, a

small simple oven was built as shown in Figure 5.30. An Omega CN2012 Programmable

Temperature Controller used a thermocouple to measure the temperature of an aluminum

block with a corded heating element threaded through it. A solid state reIay (SSR) inside

the temperature controller would connect the heating element to h e voltage (120 V, 60

Hz) in order to raise the temperature of the aluminum block to the operator specified

Corded Heater Element ' ,' /'- , - /,. ' . -

,'- I-

/

Foam Insulation

120 Vac Wi State Relay

F i e 530. Simple oven for sample annealing using a temperature controller and a heating eiement. A thermocouple connected to the temperature controller provides a means to measure the temperature of the duminum block

setpoint. To prevent stray light in the room from reaching a sample while it was being

annealed, the unit was fitted with a Iid.

5.53 IR Light Source

The IR light source used in this work was identical to that used by Haugen [102],

and it consists of an ordinary household lamp with a standard incandescent tungsten Light

bulb. A thin silicon wafer was secured to the bell shade of the lamp with electrid tape;

the wafer completely covered the opening of the lamp. With the lamp turned on, only

photons of energy I 1 .I2 eV (the bandgap of silicon) wodd be transmitted by the silicon

wafer, forming a good IR light source.

5.53 Ultrasonic Waves

Ultrasonic waves were created by the apparatus shown in Figure 5.3 1 [I 091. A

Philips PM5134 Function Generator created a 4 MHz sine wave which was amplified and

fed to a Matec ICS-SR PZT (lead zirconate titanate) ultrasonic transducer with a nominal

resonant frequency of 4 MHz. Viscous automotive grease served as an acoustic matching

fluid so that the acoustic waves created by the traducer would be adequately coupled

into the a-Se sample being tested. The unit was fitted with a removable Iid to shield the

a-Se sample undergoing ultrasonic treatment fkom any stray room light.

Signal Generator 552 \ --.

4-00 MHz ' .z 6 6

Ultrasonic Waves \

L'

- :\>/ - PZT Transducer

"

L' ArnpTir

F i 531. Equipment employed to create ultrasonic waves in the a-Se samples.

5.6 Summary

The experimental systems used in the course of this work were introduced; a

brief description of the sample preparation procedure led the discussion. After selenium

pellets are thermally evaporated onto conductive substrates, semitransparent Au

electrodes are sputtered onto the samples to facilitate transient photoconductivity and

Wmp measurements.

A TOF/IFTOF transient photoconductivity apparatus was descnied; this system

allows the charge transport properties of high resistivity solids to be examined.

irradiation of the samples by x-rays allows the study of x-ray induced properties such as

changes in electron lifetime.

A system to measure the x-ray induced photocunent in an a-Se film was

introduced. This system measures the number of free carrim produced by irradiating an

a-Se sample with x-rays, and provides a means to calculate the EHP creation energy in a-

Se. The sample must be isolated from atmospheric water vapour when performing low

temperature measurements to prevent the buildup of condensation and thus the

destruction of the sample.

Finally, some miscellaneous experimental tools used in the course of this work to

investigate the electron lifetime dependence of a-Se on annealiug, IR soaking and

dtrasonic treatment were discussed

6. Results and Discussion

6.1 Introduction

Chapter 5 presented the experimental details of the study undertaken in this work,

and a comprehensive summary of the results are discussed in this chapter. The samples

used throughout the course of this work were chlorinated a-Se:0.2-O.S%As; otherwise

known as stabilized amorphous selenium, and henceforth simply called a-St.

This chapter first presents a description of the TOF and IFTOF experiments

performed on the samples to determine their suitability as an x-ray detector. h general, a

detector must possess suitably good charge transport such that a charge carrier that is

created by x-ray irradiation will not become trapped as it traverses the sample. The

results of these preliminary measurements are presented £irst, followed by extensive

IFTOF measurements to gauge any changes in charge transport upon exposure to x-rays.

This subject is dealt with in detail in section 6.3. Section 6.4 discusses the measurement

of WUp and how it varies with temperahue, bias field and mean x-ray beam energy. The

penistent x-ray photocufient is examined in section 6.5 in an effort to M e r understand

this phenomenon.

6.2 Charge Transport Measurements

6.2.1 TOF Measurements

The small signal condition, introduced in Chapter 3, must be met when

performing TOF or IFTOF measurements. Under small signal conditions (ie. when the

photoinjected charge is much less than C,V'), the electric field F = VJL inside the sample

may be approximated as being uniform at alI points within it. This may be sew

experimentally by an almost rectangular photocurrent pulse in samples with relativeIy

long camier lifetimes. The transition h m the small signd condition to charge

overinjection is easy to determine, as the photocurrent pulse will suddenly change h m

being rectangular to almost triangular in shape. Charge overinjection was avoided during

all measurements through the use of neutral density optical attenuators placed in the path

of the laser pulse.

It has been shown that the internal bias field will remain constant and uniform

following application of the bias until a transition time tsc is reached f 1101. A h this

transition, a space charge will build in the bulk of the sample due to charge injection from

the contacts. The internal field then becomes non-uniform as dictated by the Poisson

equation dF/& = pk, where p is the buik charge density and E is the permittivity of the

materid. If this bulk space charge is allowed to build in the sample prior to

photoexcitation, then the photocurrent will either increase or decay depending on the

species of bulk charge [94]. The transition time of a-Se is typically on the order of - 100

ms, and in the present measurements photoexcitation took place within 20 ms of bias

application to avoid these field pexturbations.

Figure 6.1 presents typical TOF photocurrent waveforms of holes and electrons

(the amount of photoinjected charge is not equal for both cases). Examination of both

p h o t o ~ ~ e n t s reveals that minimal deep trapping occurs during the transit time, as

evidenced by the reIatively flat plateaus of the waveforms. The eIectron photocurrent

displays a rather sharply decaying Ieading edge and it is thought that this behviour may

be due to carrier thermhtion (the time necessary for the photoinjected carriers to reach

equiliium with the shallow traps). This behaviour wdd also be caused by

nonuniformities within the sampIe itsec and it is not conclusiveIy known which may be

the cause. It should be noted that all samples employed in the study exhiiited this

electron transport behaviour,

Time (ps)

Time (ps)

F i i e 6.1. Typical I-made TOF waveforms in a-Se for (a) holes and (b) electrons. The m i t times are indicated as t ~ . [(a) F = 0.98 V/CL~, (b) F = 5.69 V l p ]

For this study, the transit time of the charge caniers was defined as the time when

the photocurrent drops to half its value at the knee of the waveform, as shown in Figure

6.2 below. This definition was necessary because electron TOF waveforms can undergo

quite significant trapping during their trip across the sample, making a determination of

the nominal photocurrent during drift difficult. It was found that there is a maxima in the

Wefetztiated photocurrent corresponding to the knee of the photocurrent (as shown in

Figure 6.2), making automated location of this point through software very easy. This

maxima is a mathematical consequence of the cliffamtiation algorithm.

The charge carrier mobility p and the measured transit time are related through

the following equation

Where L is the sampIe thickness and V is the appIied voltage. Thus a plot of the

F i e 63. (a) Typical electron TOF photocurrent showing the location of the photocurrent's knee and corresponding '/4 value point (b) The photocurrent of (a) is differentiated reveaIing a local maxima corresponding to the Iocation of the knee in (a). M y a portion of the diffimitiated waveform (the tail) has been shown for clarity.

measured transit time t~ VS. 1/V for various applied voltages will yield a straight h e

whose slope is the mobility of that charge carrier. Figure 6.3 (a) and (b) are plots of the

transit time vs. 1/V for holes and electrons respectively. The calculated hole mobility

from Figure 6.3 (a) is p+, = 0.132 c m h s and the calculated electron mobility from Figure

6.3 (b) is ~ l , = 0.003 18 cm2Ns.

F i e 63. Plot of TOF transit time vs. IIV for (a) holes showing = 0.132 c m 2 ~ s and (b) electrons showing 14 = 0.003 18 an2Ns.

When Equation 6.1 is used to calculate the drift mobilities of both holes and

electrons and these mobilities are plotted as a hc t ion of applied eIectric field, the plor

of Figure 6.4 (a) and (b) are obsenred. As can be seen, the hole mobility shows tittle

observable field dependence, while the electron mobility has a very slight power law field

dependence of the type = F where n was experimentally determined to be - 0.17,

which is quite small. This small field dependence is not I l l y understood at present, but

for the Iimited field ranges employed in this work, both the hok and electron drift

mobilities were assumed to be constant and field independent.

108 i 07 IOB 1 o7 Field (Vim) Field (Vlm)

[a) (b)

Figure 6.4. (a) Hole and (b) etectron mobiIity plotted as a function of applied field. The hole mobility shows very little field dependence while the electron mobility shows a slight field dependence of the form ~4 = l?",

6.2.2 IFTOF Measurements

The charge transport propexties of an x-ray photoconductor are not completeIy

characterized by the drift mobilities of its charge carriers. Some carriers may be Iost due

to deep trapping and will thus decrease the effective sensitivity of the detector.

As described earlier, the FTOF experiment provides a convenient means of

measuring the mean canier lifetime z by measuring the hctional recovered photocurrent

as a hction of the interruption period. Figure 6.5 shows a typical FTOF waveform for

the case of drifting eIectrons. The field is removed at $1 and reapptied at t2, corresponding

to an intenuption time q = t2 - tl. Since the photocurrent at any particular instant is

pro@onaI to the number of drifting charge carriers at that same instant, the slope of a

semilogarithmic plot of i j i r vs. ti yieIds the charge carrier trapping lifetime 7.

During the IFTOF experiment, charge carriers are trapped under zero applied

field, and the measured carrier lifetime should thus be independent of applied voltage,

provided that the charge packet is halted at the same depth in the sample. A recent study

bas c o d b e d this fact [6 11.

Figure 6.5. Typical electron IFTOF waveform in an a-Se film showing i,, i2, and ti.

By measuring ir and i2 for several different interruption times ti, the plots found in

Figures 6.6 and 6.7 for the case of holes and electrons respectively were obtained. The

hole lifetime was determined to be T,J, = 132 ps and the corresponding electron lifetime

was T= = 657 ps. These lifetimes seem reasonable when compared to the TOF waveforms

of Figure 6.1; neither photocurrent exhibits any discemable decay since f~ << r. The

exponentid decay of the fhctional recovered photocurrent for both cases (holes and

eiectrons) may be used as evidence that there is no release of trapped carriers fiom deep

traps during the timescale of the experiment.

0 20 40 60 80 100 120 140 160

Intemption Time (ps)

Figure 6.6. Fractional recovered hole photocurrent as a function of interruption time for a-Se. Measured hole lifetime b m the plot is 132 p. The charge packet was halted at a depth of 0.263 L where L is the thickness of the fi.

One immediately obvious feature of Figure 6.7 is the fact that the plot of

hctional recovered electron photocurrent does not pass through the origin; in other

words, a zero interruption time does not correspond to all injected charges being

subsequently collected. This peculiar feature of electrons was noted for all samples

examined during the course of the work and for all depths at which the charge packet was

halted. Since there is no applied field while the charge packet is halted, any motion of

charge carriers must be due solely to the self-field drift alone, caused in part by the

mutual wulombic repulsion of the charge carriers themselves. This motion of electrons

then resuIts in a hction of the charge packet which reaches the top electrode and is

removed h m the sample, resulting in the behaviour seen in Figure 6.7. Figure 6.8 is a

plot of the M o n a 1 recovered electroil photocurrent as in Figure 6.7, except that the

charge packet was halted at a depth of 0.537 L. Since the y-intercept of this plot shows

that less charge was lost during intemption at 0.537 L than at 0.263 L that serves as

confirmation that !he charges are indeed being lost through the top contact. A recent

Interruption Time (ps)

Figure 6.7. Fractional recovered eIectron photocurrent as a hct ion of intemption time for a-Se. Measured eiectron lifetime from the plot is 657 p. The charge packet was halted at a depth of 0.263 L where L is the thickness of the fi.

study [I 111 also discovered that electron dispersion in a-Se films is anomalous in that the

high amount of dispersion evidenced by an electron charge packet in a TOF measurement

cannot be reconciled through conventional diffusion, multiple trap and release events

from a monoenergetic set of traps, or hopping transport.

The exact mechanism for this behaviour is unknown, but two theories are

presented schematically in Figure 6.9 below. The first possibility is that the zero field

diffusion of electrons in a-Se is considerably higher than previously thought, leading to

the considerable spreading of what was once a narrow charge packet upon its halting.

Some charge may eventually reach the top electrode and be lost from the sample, as in

Figure 6.9 (a). The second possibility is that the electron charge packet undergoes a very

large amount of dispersion as it drifts across the sample. This may leave an appreciable

number of electrons behind the main ''fotmati0n7' as it is swept across the sample; a large

kction of these sIow carriers may Iie within a diffusion length of the top contact and can

be removed from the sample while the packet is halted, as shown in Figure 6.9 (b). The

0 100 200 300 400 500 600 700 800 900

Interruption Time (p)

F i e 6.8. Fractional recovered photocurrent at a depth of 0.537 L for the same sample as in Figure 6.7. The higher y-interce~t indicates that less charges are lost at this depth than at the shallower halt depth of Figure 6.7.

fact that more charges are lost when the packet is halted at a relatively shallow depth in

contrast to when the charge packet is haIted relatively deeply suggests that the latter case

is the cause. If pure diffusion were responsible, some charges could reach the bottom

contact as well as the top, and here should be relatively equal numbers of charges lost at

both shallow and deep haking depths. The shape of the differentiated photocurrent tail

(see Figure 6.2 (b)) corresponds to the shape of the charge carrier packet [loo, 1 1 11 and it

shows a very long tail region corresponding to ''slow" carriers, also in agreement with a

large amount of dispersion. The width of the tail of a typical TOF electron photocurrent is

also suggestive of dispersion being the cause, as the total spread of the tail is comparable

to the transit time of the carriers.

An alternative and convenient method of determining the charge carrier deep

trapping lifetime was also used throughout the course of this study. The ratio qTlql,

comsponding to the amount of charge present in the photocment pulse immediately

prior to and following interruption will also yield the same carrier Lifetime when plotted

semilogarithmidly versus the interruption time [6O, LOO]. This method was preferred

because it could be performed quickIy and automatically via software. This method dso

offered the advantage of being relatively immune to any noise present on the recovered

photocurrent pulses [60, loo], and it offered a "neutd' analysis of the data in that it did

not have any preconceived notions of how the data should appear, as opposed to human

eyes.

Photoexcitation

a-Se Sample -

During Interruption Packet Spreads /7- . ,

, , , i '. .

, I i .. . , " %, +b

,:Packet When ,, _ - - - / . Halted -

> Direction of Drift

+ Direction of Drift

Figure 6.9. Two possibilities for tbe lost charge observed in fractional recovered electron photocurrent IFTOF pbts: (a) ekctmn diffusion while the packet is halted and (b) large dispersion during driR Tbe latter case is more likely, as supported by the experimental evidence.

The charge carrier Hetime is found via the slope of a linear regression applied to

the various vdues of fractional recovered photowent (or charge) when plotted

semilogarithmically against the interruption time. Hence the error in the carrier lifetime

is directly related to the uncertainty in the slope of the linear regression. For a given set

of N ordered pairs of data, y&), the equation of a least squared error line that best fits

that data is given by [I 12- 1 141

y=mx+b, 6.2

where m is the slope and b is the y-intercept of the line. m is found through

where

and

where ; is the average of all xi and ; is the average of all fi The standard deviation of

the slope is

where

Thus the error in the charge carrier lifetime is simply + a,. This relation was used

throughout the course of the work to determine the error in the calculated charge carrier

lifetimes.

As mentioned previously, there is ample evidence to suggest that the electron

charge packet undergoes very significant dispersion during its drift across the a-Se

sample. This is aIso reflected in a pIot of the effective carrier transit time vs. intenuption

time (ti) for the case of electrons, as in Figure 6.10. The effective transit time tr' is

defined as the time it takes for the photoinjected carriers to be swept out of the sample

during an IFTOF experiment, minus the interruption time itself. As can be seen in Figure

6.10, initially the effective transit time increases linearly with increasing ti, with a

pronounced corner at ti - 200 p after which point the linear increase in tr' with t i is not

as strong. This phenomenon is unique to electrons in a-Se, as hole transport does not

display this behaviour [IOO]. This seemingly odd behaviour-that the eIectron transit

time is affected by and depends on an intmption in the field-may be explained in

Interruption Time (ps)

Figure 6.10. Eff'ective electron transit time tT' plotted as a function of intmption time ti in a typical IFTOF experiment. The quantity plotted is the incremental difference in the effective transit time compared to the transit time obtained from a TOF experiment.

terms of carrier dispersion and the non-symmetric shape of the electron charge packet.

The eIectron charge carrier packet is highly non-symmetric (refer to Figure 6 2

(b)), with a region near the "head" of the packet in which most of the carriers reside.

There is also a long tail region which contains a significant number of carriers that tags

behind the main 'Tormation". For this study, the transit time (or effective transit time)

was defined as the point in time at which the tail of the photocurrent decayed to !4 its

value at the %nee'' of the waveform. This may be equivalently stated as the time at

which half the photoinjected carriers left the a-Se sample. Therefore, since the

photoinjected electron charge packet is highly non-symmetric, self-fieid drift while the

packet is halted will significantly alter the overall shape and electron distribution within

the packet. This would also alter the location of the half charge point within the carrier

packet, leading to a change in the observed effective transit time which depended on the

intenuption time as seen in Figure 6.10. This also explains why this behaviour is not

observed for the case of bole transport in a-Se, since the hole packet is highly symmetric

[IOO] and its half charge point would be relatively immune to self-field drift while the

packet is halted.

The apparent coma at - 200 p seen in Figure 6.10 may be due to the mutual

coulombic repulsion of ike carriers. As time progresses, the packet will spread more and

more, but the spread should slow somewhat as ibe carriers become deeply trapped. The

electron Lifetime corresponding to the data of Figure 6.10 was measured to be 707 p.

Therefore, at 200 p, the number of charge carriers remaining in the packet will be

-200 e -0.1 1 = -64 where the 0.1 1 is the hction of charge lost at zero interruption time

because a plot of hctional recovered photocurrent vs. intemption time does not pass

through the origin for electrons, as discussed d e r . It is quite remarkable that the

corner of Figure 6.10 corresponds to 64% of the charge remaining in the packet; in other

words, to 36% ofthe charge lost from the packet Since = 0.36, it may be surmised

that the spread of the carrier packet initially has an exponential dependence on the

number of free Wers in the charge pack% with a characteristic T = 200 p.

The observed dependence of the effective transit time for electrons on the

intermption time during an IFTOF experiment coincided with a similar dependence of the

measured electron dispersion on the interruption time. For this study, the dispersion of

charge caniers was measured by examining the differentiated tail of the TOF or IFTOF

waveforms. The precise method has been discussed elsewhere [86, 100, 1021, so only a

very brief outline of the procedure will be presented here.

In general, differentiation tends to be a very noisy process which can be prone to

large errors depending on the initial "smoothness'* of the waveform to be differentiated.

In an effort to smooth any noise present in TOF or IFTOF waveforms, a least squared

error parabolic curve is fit to a portion of the data, hereby r e f d to as the window.

That parabola is then diffaentiated, yielding one data point corresponding to the slope of

the original TOF or lFTOF waveform at the middle of the window. The window is then

"slid" one point along in time and the process is repeated until the entire waveform has

been analyzed.

The measured dispersion for this study corresponded to the FWHM of the

differentiated tail; in other words, to the 95 magnitude width of the differentiated tail,

consisteat with other studies [86, I#, 102, 11 11. Figure 6.1 1 is a plot of the

differentiated tails of recovered electron IFTOF waveforms, with ti = 50.8, 99.1, 150,

200.8, 300, 399.1, 500.8, 600 and 699.1 p. It can be seen that dispersion generally

increases with increasing interruption time ti-

The measured FWHM dispersion vs. intermption time corresponding to the

cwes of Figure 6.1 1 is presented in Figure 6.12. The data is presented in Figure 6. I2 as

an incrementd dispersion relative to the dispersion present in a TOF waveform, ATOF.

Just as with the case of the effective transit time (Figure 6.10), there is a comer in the

curve at an interruption time of - 200 p. Again, the mutual wdombic repulsion of free

photoinjected charge carriers would tend to strongly spread the carrier packet for small

interruption times; as more free charges become deeply trapped, the observed increase in

I t I L I L

0 200 400 600 800 1000

Time (ps)

Figure 6.1 1. PIots of differentiated electron IFTOF photocunent tails vs. time for an a-Se sample. The intemption time was varied from 50.8 ps (bottom curve) to 699.1 P (top m e ) .

dispersion would tend to slow for large interruption times. The comer at - 200 p lends

firrther evidence that the spread of the packet is strongly dependent on the concentration

of free photoinjected carriers.

The bulk of the work done in this study concerned any x-ray induced changes in

the electron deep trapping lifetime r, and it was therefore essentiai to etlfllre that the

measurement of & itself had no effect on the charge transport properttes of the material

under study. Of particular concern was the pssiiility of trap filling, whereby the

measurement of the trapping lifetime itseIf would reduce the number of avaiIabIe d i e d

traps, ieading to an increase in h e apparent lifetime. To determine if the measurement of

the trapping lifetime itself was af5ecting the materid, a set of four measurements at two

0 100 200 300 400 500 600 700 800

Intemption Time (ps)

Figure 6.12. Incremental electron dispersion h m an EFTOF experiment relative to the dispersion h m a TOF experiment, ATOF, plotted vs. the interruption time ti.

hour intervals was performed. The results of this study are presented in Figure 6.13. The

charge packet was halted at a depth of 0.287 L for every measurement. As can be seen,

the measured electron Hetime does not significandy vary from measurement to

measurement. It was therefore assumed that the method of measuring the electron

lifetime did not alter the material itself, and was herefore a valid meamremat tool.

6.23 a-Se Film Quality

As stated before, the average distance that a charge carrier will travel before

becoming deeply trapped is an important measure of the suitability of an a-Se fiIm as an

x-ray detector. This distance, denoted the Schubweg, is equd to the product of the

0 1 2 3 4 5 6 7

Time (h)

Figure 6.13. Measured electron lifetime, normalized to the initial (t = 0) lifetime, for an a-Se fiIm over the course of a 6 hour period. The measured lifetime did not significantly vary with time, therefore the measurement technique did not alter the trapping characteristics of the a-Se film under study. The electron lifetime was initially determined to be 5 10 p.

charge carrier mobility, deep trapping lifetime and applied electric field. In general, the

Schubwegs (both electron and hoIe) of a film must be greater than the film thickness in

order to ensure that all charges that are Ir'berated by x-rays will be collected before they

become trapped.

The a-Se samples used in the course of this work were selected on the basis of

their excellent charge transport properties so that a study of films simiIar to those in use

in commercially available systems could be made. The study of charge carrier Iifetime

and how it was affected by exposure to x-rays involved the use of four samples, each

detailed in Table 6.1 below. Every sample exhiiited a variance in its charge carrier

lifetime which was dependent on the depth at which the lFTOF analysis was performed.

For this reason, the carrier lifetimes presented below represent the lowest measured

lifetimes in the sample. The concept of the minimum operating field is introduced so that

a clear comparison among the samples may be made; it is defined (for this study) as the

smallest fieId which will result in a Schubweg equal to twice the Nm thickness for the

charge carriers with the smallest ~ L T product (the range of the carrier). This ensures that

the carriers with the lowest range will likely be able to traverse the sample without

becoming trapped. In general, the sample with the lowest minimum operating field will

have the best charge transport.

Table 6.1. Charge transport properties of the four a-Se films involved in the carrier

The two samples used in the measurement of WEHP and the persistent

lifetime study.

photocurrent study are each detailed in Table 6.2.

Table 6.2. Charge transport properties of the two a-Se films involved in the x-ray

Minimum Operating

Field (Vim) 5.56 7.18 4.05 537

~ ( p s )

504 447 697 185

Typical operating fields in a commercial digital x-ray imaging system employing

z b w )

167 186 178 - 100

a-Se as the detector exceed 10 V l p - As can be seen in Tables 6.1 and 6.2, the necessary

(cm2ffs)

0.00357 0.003 18 0.00327 0.00342

Sample

960521 46 SE 15 971002 12 971205 62 980622 - 3

photoconductive ex-ents.

minimum operating field for good charge collection is below this threshold for every

Sample

~un#30

f 463-3

sample. Thus it may be stated that the samples employed in this study were each

L (Clm)

500 5 10 462 170

Minh- Operating

Field (Vim) 1.n 3.0 1

L W)

64

565

commercial device quality.

CIL (cm2ffs)

0.132 0.132 0.133 0.126

% W)

- uo 952

zb(Cu)

- 15 - 300

(cm2ffs)

0.1 I

0.129

~ ( c m 2 f f s )

0.0029

0.00394

63 X-ray Induced Changes in Charge Transport

This section will first introduce the experimental results of the study into the

effect that x-ray irradiation has on the charge transport properties of a-Se. The subject of

Light induced structural changes in amorphous semiconductors in general, and a-Se in

particular were briefly introduced in Chapter 2. This topic will be discussed in detail

folIowing the experimental results, and the applicability of these light induced effects to

the present study involving x-rays will be covered next. How annealing, IR soaking and

ultrasonic treatment affect the charge transport of a-Se will be introduced last.

All of the measurements that follow are presented as a function of absorbed dose;

a quantity in common use in the field of radiography. Absorbed dose is simply defined

as the energy absorbed by an object (as x-rays for example) per unit mass of the object.

Dose has units of Grays, with one Gray (Gy) equal to one Joule per kilogram.

In all but one special case, the a-Se samples were inadiated by the heavily filtered

x-ray beams described in Chapter 5. Since the spectra of the x-ray unit was known for

those four beams, the amount of energy that the beam deposited in each sample was

straightforward to calculate. Once the energy deposited in the sample was found through

Equation 5.5, the dose was then calculated by dividing that energy by the mass of the

electroded sample.

The a-Se samples used during the course of this work were usually subjected to a

maximum cumulative absorbed dose of - 30 mGy at any one time before the samples

were allowed to rest in the dark for several days to recover. For comparison, a patient

undergoing a traditional film-based diagnostic x-ray procedure absorbs a dose which is

typically less than 2 mGy, depending mainly on the area of the body being examined.

6.3.1 Charge Carrier Transit Time tr

Since the charge carrier mobility is controlled by the shallow states present in the

bandgap of a-Se, any x-ray induced changes in the carrier mobility would be due to the

alteration of either the concentration or the capture cross-section of these states. To

determine if x-ray exposure had any effect on the charge carrier mobility, two

experiments were perfonn&ne for the case of electron transport and one for the case

of hole transpod These straightforward experiments simply consisted of monitoring the

TOF transit time tr of both electrons and holes as an a-Se sample was irradiated with x-

rays. The results are found in Figure 6.14, and are expressed as a normalized transit time.

The measured electron transit time was found to change negligibly (< 1.5%) as the dose

was varied from 0 to 25.4 mGy. This change fell within the t~ measurement error of 2%;

therefore, it was concluded that x-ray irradiation has no effect on electron mobility. Hole

transit was also found to be independent of x-ray irradiation, as shown in Figure 6.14 (b).

Cumulative Dose (mGy)

0 5 10 15 20 25 30

Cumulative Dose (mGy)

F i e 6.14. (a) Electron and (b) hole TOF transit times, normalized to the initial t~ and plotted as a function of cumulative dose. Irradiation by 58.2 keV beam; sample shorted to ground during irradiation.

6.3.2 Charge Carrier Lifetimes

The bulk of the measurements performed during the course of this work involved

the measurement of the charge carrier lifetimes of a-Se fiIms and how those lifetimes

change upon exposure to x-rays. Most measurements were of the electron Lifetime, but a

number of hole lifetimes were also obtained. The data that follows is the cuImination of

over 3500 FTOF measurements resulting in almost 500 individual lifetimes.

The measurements were performed under a number of different conditions,

including different bias levels during irradiation, mean energy of the x-ray beam used for

irradiation, and absorbed dose. These parameters were varied so that the reaction of the

carrier lifetimes to these irradiation conditions could be observed. A single lifetime

usually involved 8 - 10 individual IFTOF measurements and took approximately 15 - 20

minutes to complete. In all thefigures that follow, carrier lifetimes are nomlized to the

initial lifetime obtainedjust prior to x-ray irradiation (i.e. the t = 0 point).

It was initially expected that irradiating an a-Se film with x-rays would drastically

lower the electron lifetime within that film; hthermore it was expected that a recovery

would be observed as the material relaxed back to its initial state. With this in mind, a

nmbex of experiments were performed whereby the electron lifetimes at two depths

within an a-Se £iim were initially measured. The film was then irradiated with x-rays and

the Lifetimes at those same two depths were again immediately measured Quite

surprisingly, the measured lifetimes immediately before and after irradiation usually did

not differ by more than lo%, as seen in Figures 6.15 and 6.16. Even more surprising was

the fact that as time went on, the measured lifetimes continued to change-in some cases

rising and others falling.

This u n d behaviour--that the measured electron lifetimes initially hardly

changed upon irradiation by x-rays, but then continued to change as time progressed-

was completely unexpected. It was thought that perhaps the heavily f3tered nature of the

x-ray beams used in this work could be the cause of this phenomenon. Figure 6.17 is a

32.8 keV 26.2 mGy Shorted during irradiation -

0.5 1 I I I I I /A 0 2 4 6 8 10 72

Time (h)

Figure 6.15. Normalized electron lifetime at two depths within an a-Se film tracked over time. The film was initially irradiated with a 32.8 keV beam giving an absorbed dose of 26.2 mGy, and the sample was shorted to ground during irradiation. [Sample 97 1002 121

plot of the measured electron lifetimes of the same film as that in Figures 6.15 and 6.16,

except that all filtering was removed from the beam when the sample was exposed. The

x-ray unit was set to a 90 kVp, 15 mA, 1 second exposure. The exact absorbed dose due

to this exposure is not known, but it can be safely said that it was much higher than that

in Figures 6.1 5 and 6.16. As with the previous two figures, the electron lifetimes before

and immediately after irradiation hardly changed; however, as time progressed, the

measured lifetimes continued to change. Therefore, these observed changes in the

electron lifetime within these films were assumed to be unrelated to the specifics of the

experimentd procedure itself (i-e. the heavily filtered beams).

o 6 i 32.8 keV 26.2 mGy F = 1.96 Vlpm during irradiation

Time (h)

Figure 6.16. Normalized electron lifetime at two depths within the same film as that in Figure 6.15. Irradiation specifics: 32.8 keV, 26.2 mGy, F = 1.96 Wpm.

These changes in the measured electron lifetime have been observed in all

samples examined in this work. Furthermore, the changes show no discernable

dependence on the irradiation wnditio*i.e. electric field strength during irradiation or

beam energy. Essentially, these changes do not appear to follow a recognizable pattern

and the fact that the lifetimes at two different depths act independently (they do not

necessady "track" each other) testifies that they can be quite localized within the film.

Figures 6-18 - 6.20 are further plots illustrating how the measured electron lifetime

within an a-Se film varies with time after the initid x-ray irradiation.

Another interesting feature of the eIectron lifetime is that its behaviour with time

after i d a t i o n showed no observable dependence on the absorbed dose over the range

i No Filtering 90 kVp, 15 mA, 1 second exposure Shorted during irradiation

0 2 4 6 8 10 144

Time (h)

Figure 6.17. The a-Se 6lm of Figures 6.15 and 6.16 is exposed to an u t e r e d x-ray beam of 90 kVp, 15 mA and 1 second duration. The sample was shorted during irradiation.

examined in this study. Figures 6.21 and 6.22 depict how the measured electron lifetime

changed with time after irradiation. A11 variables for these two plots were identical

except for the absorbed dose. In Figure 6.21, the absorbed dose was 25.4 mGy whereas it

was 1.8 mGy for Figure 6.22. In both plots it may be seen that the electron lifetimes

immediately before and after inadiation hardly change (< - 10°?), but as time progresses,

the measured lifetimes vary somewhat. Although the absorbed dose of Figure 622 was

l a than 1110~ that of Figure 6.21, the measured lifetimes evince greater variance as time

progresses. At ht glance, this may seem to suggest that a low dose induces a bigger

change in the electron iifetime, but an examination of the other figures presented thus far

proves that the changes lie within the general range observed under other widely varying

irradiation conditions.

"4 32.8 Ice" , , , 28.1 mGy Shorted during irradiation

0.5

0 2 4 6 8 10 96

l7me (h)

Figure 6.18. Normalized electron lifetime at two depths tracked over time. Irradiation specifics: 32.8 keV, 28.1 mGy, shorted during irradiation. [Sample 971205 621

The samples, once irradiated, were allowed to rest in the dark for at least two days

before they were again used. This allowed the samples to recover fiom any x-ray

induced changes. Sometimes the original dark-rested electron lifetimes would be

recovered, as in Figures 6.15, 6.19 and 6.20; however, sometimes the datk-rested

electron lifetimes would be as much as 20% higher than that of the previous study, as in

Figures 6.16 - 6.18 and 6.2 1. As with the changes in electron lifetime as time progressed

after the sample was irradiated, these dark-rested lifetimes seem to be random in that

there is no discernable pattern between the irradiation conditions and the resulting rested

lifetime a few days later.

This odd electron lifetime behaviour'hat the lifetime would hardly change

immediately following irradiation, but could change quite dramatically hours Iater-was

quite unexpected, as stated earlier. Since the electron lifetime was not affected by the

IFTOF measurement itself (Figure 6.13), therefore, initially-upon irradiation--some

change within the a-Se films had to be driving the later changes in electron lifetime.

0.6 4 28.1 mGy F = 2.16 Vlpm during irradiation

Time (h)

F i e 6.19. Same sample as that of Figure 6.18; irradiation specifics: 32.8

58.2 keV 25.4 mGy Open circuit during irradiation

0 2 4 6 8 1 44

Time (h)

Figure 6.20. Same sample as that of Figure 6.18; irradiation specifics: 58.2 keV, 25.4 mGy, sample completely open circuit during irradiation.

0 2 4 6 8 1 44

Time (h)

/+ 0.287L

0 0.573 L 1.1 -

r" 1.0

= 0.9

P I t2

Figure 6.21. Normalized electron lifetime at two depths tracked over time. Irradiation specifics: 58.2 keV, 25.4 mGy, F = 2.16 V l p . [Sample 971205 621

0.8 - z

0.7 - OS6 - 0.5

58.2 keV 25.4 mGy F = 2.16 Vlpm during irradiation

I I I I /A

Time (h)

1.3

Figure 6.22. Same sample as that of Figure 6.21, except that the absorbed dose is 1.8 mG y.

l2 - 1 .I =-

r'

In an effort to find some cause for these changes, both the hole and electron

lifetime at the same depth within a sample were examined as a fimction of time following

irradiation. As can be seen in Figures 6.23 and 6.24, the electron lifetime behaves in the

58.2 keV l.8rnGy F = 2.16 Vlpm during irradiation

0 0.573 L

p 1.0

z 0.7 - 0.6 - P

same manner as seen earlier. Immediately before and after irradiation, it is hardly

affiected, but goes on to change unpredictably hours later. However, the hole lifetime

dramatically fdls 20 - 30% immediately after irradiation; as with the observed changes

in electron lifetime, it proceeds to change unpredictably as time progresses. These

observations allayed some of the previous concern; irradiation by x-rays did induce

immediate changes in the a-Se films, but those changes had more of an effect on hole

transport than electron transport, As time progressed and the a-Se samples relaxed, other

changes occurred, changes which affected the electron lifetime.

As Figures 6.23 and 6.24 attest, there is no significant difference between the two

plots in their electron and hole lifetime behaviour over time, even though the absorbed

dose in Figure 6.24 was in excess of ten times that in Figure 6.23. However, if the

behaviour of the effective electron transit time tr' as a function of IFTOF intermption

time ti is examined for these two cases, a difference clearly emerges, Figure 6.25 depicts

the dependence of the electron effective transit time on the interruption time for the data

of Figure 6.23 just prior to irradiation (a), just after irradiation (b), and two hours after

irradiation (c). At this low dose (1.8 mGy), the effective transit time exhibits no

significant change, with aU the plots exhibiting the same characteristic comer at ti - 200

ps as seen previously. Figure 6.26 shows the dependence of the effective electron transit

time on the intermption time for the data of Figure 6.24. Obviously, this higher dose

(24.9 mGy) has m effect as seen in Figure 6.26 (b) immediately following irradiation, as

the corner disappears and the dependence of tr' on ti becomes more-or-less linear. The

characteristic corner is restored two hours later.

It was stated earlier that the existence of this comer is evidence that the self-fieId

Qift of the photoinjected electron charge packet is strongly dependent on the number of

fke charges remaining in the packet. As some are removed due to deep trapping, the

spread of the packet (and the increase in tT3 slows accordingly. Therefore, the

disappearance of the comer immediately following irradiation could be due to one of two

things: either the irradiation altered the concentration or capture cross-section of the

shallow traps, or a significant bulk space charge interfefed with the mutual codombic

F = 1.96 V/mm during irradiation 1.8 mGy 0.263 L

0 2 4 6 8 10 12 120

Time (h)

Figure 6.23. Normalized hole and electron lifetime at the same depth tracked over time. Irradiation specifics: 58.2 keV, 1.8 mGy, F = 1.96 Wpm. [Sample 97 1002 121

- 582 keV F = 1 96 Vlmm during irradiation /'+

24.9 mGy 0.263 L T

Time (h)

Figure 6.24. Same sample as that of Figure 6.24, except the dose is now 24.9 mGy.

repulsion of the photoinjected electrons. Since the TOF electron transit time was

147

measured to be independent of x-ray irradiation, that rules out the possibility of a change

in the shallow traps somehow being the cause.

lnterruption Time (ps) lnterruption Time (ps)

0 200 400 600 800

Interruption Time (p)

Figure 6.25. Effetive electron transit time f ~ ' vs. ti for the data of Figure 6.23. (a) Immediately before irradiation, (b) immediately following irradiation, and (c) two hours after irradiation. At this low dose (1.8 mGy), there is no significant change in the behaviour of tr' with irradiation.

A bulk space charge of a single sign (positive or negative) will alter the electric

field within the a-Se sample, leading to a region where the photoinjected charge packet

will travel under the influence of a slightIy enhanced field, and another region with a

slightly decreased field. This condition results in photocments that either increase or

decay significantly, depending on the sign of the photoinjected carriers and the sign of

the bulk charge itself [102, 1151. Since neither the electron nor hole IFTOF

photocurrats noticeably changed shape during the course of the measurements, that d e s

Interruption Time (ps)

Interruption Time (p)

Interruption Time (p)

F i e 626. Effective electron transit time rT' us. ti for the data of Figure 6.24. (a) Immediately before irradiation, (b) immediately foIlowing irradiation, and (c) two hours after irradiation. At this high dose (24.9 mGy), the behaviour of rr' changes noticeably with irradiation, but recovers within two hours.

out the possibility of a bulk space charge of a single sign.

Nevertheless, the change in the effective transit time behaviour from being

noticeably dependent on the fiee photoinjected canier concentration before irradiation to

being relatively independent of it as in Figure 6.26 (a) and (b) would seem to indicate that

a bulk space charge is present to interfere in the zero-field diffusion of the photoinjected

electrons. The absence of a shift in the shape of the LFTOF waveforms rules out a bulk

charge of a single sign. Therefore, it is likely that x-ray irradiation induces a bulk space

charge within the a-Se film consisting of an approximately equal number of both positive

and negative charges. The experimental evidence shows that the concentration of this

bulk charge is proportional to the absorbed dose obtained fiom the x-ray beam, and that it

dissipates within two hours after irradiation.

Knowing how the charge carrier lifetimes within an a-Se radiographic detector

change over time after being irradiated with x-rays has scientific merit, but little practical

merit since a detector will likely be used many times throughout the course of a day in a

typical diagnostic setting. Therefore, if the carrier lifetimes of the a-Se samples are

continually polled as the samples are irradiated, the plots in Figures 627 - 6.29 are

obtained. Figure 6.27 shows how the electron lifetime within the samples varied as a

function of dose for two different beam energies. The data of Figure 6.27 was obtained

for the case of the samples being short circuited to ground while they were irradiated-

The plots of Figure 6.28 similarly detail how the electron lifetimes vary as a bct ion of

dose for two beam energies, except the samples were biased with an electric field of - 2

V l p while they were irradiated. The data found in Figure 6.29 was obtained while the

samples were biased with relatively strong electric fields.

The individual plots of Figures 6.27 - 6.29 do not substantially differ in their

behavior, in general it can be seen that the electron lifetime can be erratic at absorbed

doses < 5 mGy. At higher absorbed doses, - > 5 - 10 mGy, the electron lifetime

generally settles to a relatively constant level which is generally within the range of 70 - 100% of the lifetime before irradiation. Also, factors such as mean x-ray beam energy

and bias level during irradiation generally have little effect on the behaviour of the

electron lifetime vs. absorbed dose.

(a) Absorbed Dose (mGy) (b) Absorbed Dose (mGy)

0 5 10 15 20 25 30 0 5 10 25 20 25 30 35

Absorbed Dose (mGy) (dl

Absorbed Dose (mGy) (c)

Figure 627. Changes in the electron lifetime at two depths within an a-Se film as a function of absorbed dose. The samples were shorted to ground whiIe they were being irradiated. (a) 32.8 keV beam [Sample 971205 621. (b) 32.8 keV beam [Sample 971002 121. (c) 58.2 keV beam [SampIe 960521 46 SE15j. (d) 58.2 keV beam [Sample 971205 621.

58.2 keV F = 1-96 Vlpm

0.5

32.8 keV F = 2.16 Vlpm

0.5 0 5 10 15 20 25 30 0 10 20 30 40 50

Absorbed Dose (mGy) Absorbed Dose (mGy) (a) (b)

Absorbed Dose (mGy) (a

0.6 -( 52.8 keV - I F = 2.35 Vlpm

Absorbed Dose (mGy) (a

Figure 6.28. Changes in the electron lifetime at two depths within an a-Se film as a fimction of absorbed dose. The samples were biased with a field of - 2 V/pm while they were being irradiated. (a) 58.2 keV beam, F = 1-96 V/pm [Sample 971002 121- (b) 32.8 keV beam, F = 2-16 Vlpm [Sample 971205 621. (c) 58.2 keV beam, F = 2.16 V@n [Sample 971205 621. (d) 5 8 2 keV beam, F = 2.35 Vlpm [Sample 980622-31.

As stated earlier, the measurement of one l i f i i e took approximateIy 15 - 20

minutes; therefore, to M a t e a sample and measure the lifietimes at two depths within it

generally occupied the better part of an hour. The plots of Figures 627 - 6.29 therefore

each took about one 8 hour day to accumulate, and the changes in electron lifetimes over

time were undoubtedly also contributing to the observed changes.

0 2 4 6 8 1 0

Absorbed Dose ( m a ) (c)

F i i e 6.29. Changes in the electron lifetine at two depths within an a-Se film as a function of absorbed dose. The samples were biased with strong fields while they were being irradiated. (a) 58.2 keV beam, F = 7.84 Vim [SampIe 971002 121. (b) 582 keV beam, F = 8.66 Wpm [Sample 97 1205 621. (c) 5 8 2 keV beam, F = 17.1 V@n [Sample 980622 - 31.

How these changes in electron lifetimes would affect the performance of a

radiographic detector would be minimal at worst. The electron lifetimes observed in this

study never fell below - 60% of their rested value; therefore, the Schubweg would also

fall by - 60% in the worst case. This translates to a change in the minimum operating

field (introduced in Table 6.1) of 110.6 = 1.67; therefore, the minimum operating field of

the four samples studied would have to rise 67% to ensure adequate charge colIection.

These new minimum operating fields under the worst case electron lifetime condition are

presented in Table 6.3.

Table 63. Minimum operating field for the rested (best case) and damaged (worst case) electron lifetime conditions obierved during the study.

-

Sample I Best Cue Min. Openting ( Worst Cue Min. I

Again, since typical operating fields in a commercial digital x-ray imaging system

employing a-Se as the detector exceed 10 VICL~, the expected impact of the observed

decrease in electron l i f h e on charge collection would be minimal.

96052 1 46 SE 15 97 LO02 12 97 1205 62 980622-3

6.33 Light Induced Structural Changes in a-Se

Photo-induced effects in chalcogenide glasses (and a-Se in particular) are of

considerable technological importance, having applications in optical imaging, hologram

recording and optical mass memories. A number of different physical manifestations

resulting &om light exposure have been documented, among them light induced

crystallization [SO, 1161, suppression of photocrystallization by simdtaneous use of two

different lasers [117], polarization dependent photocrystallization [118], and revmi le

photo-amorphization [119,120].

Field (Vlw) 5.56 7.18 4.05 5.37

Perhaps the first photo-induced effect to be noticed was the phenomenon of

reversiile photodarkening, peculiar to the amorphous state of a material. Photodarkening

Operatha Field (V/pm) . 9.27 12.0 6.75 8.95

is the photoinduced increase in the low energy opticai absorption of a materid, induced

by light with an energy approximately equal to that of the bandgap of the material. It is

most often characterized by a nearly paraLleI shift of the low energy absorption edge (the

Urbach edge) to lower energy. Photodarkening can be reversed with time, annealing, or

by exposure to light of a different energy (photobleaching). This shift of the Urbach edge

must therefore be accompanied by an extension in the disorder-induced tail states into the

energy gap, thereby reducing the bandgap of the material.

These photostructural changes were originally presumed to be caused primarily

by the creation of IVAPs [49,51]; however when Tanaka (in 1980) [I211 examined all

available experimental evidence, he concluded that these structural changes could not be

explained by NAPS. He noted that the observed changes in the optical absorption

(which also coincided with changes in the volume and x-ray difhction pattern of the

material) must then be due to a bulk oriented feature of the disordered network; a

quantitative disorder such as fluctuations in bond length, angle or closed shell distance.

He introduced the concept of the double well potential to explain the phenomenon of

atom displacement, as pictured in Figure 630. He theorized that through some as-yet

unknown process, an atom originally in position A could be transformed to a new

metastable position A' in the solid. A relativeIy Iarge number of atoms being moved into

new positions in the solid would then significantly alter the overall randomness of the

material, thereby accounting for the observed changes in sample volume, x-ray

diffiction, and IR spectra.

Tanaka subsequently expanded his 1980 theory [122, 1231 with a possible

mechanism that would result in the movement of atoms to new positions through bond

twisting. Refer to Figure 6.31. Suppose the equili'brium configuration of the atoms

cotresponds to configuration (a). If a photon having an energy comparable to the

bandgap excites an electron fiom the lone pair (LP) of atom A (it is generally accepted

that the LP electrons fonn the top of the valence band while the anttinding orbitals

(AB) form the bottom of the conduction band [123]) to an electronic configuration

corresponding to configuration (b), there will exist a strong coulombic attractive force

F i r e 630. (a) A schematic diagram of Tanaka's 1980 [12 t ] model of bistable local bonding geometries and (b) corresponding double well potential.

between atom A and the LP electrons of atom B. This force has been estimated as being

- 1 eV [123], which may be enough to surmount the energy barrier faced by A in moving

to a new position as pictured in wdguration (c). This twisting motion can be completed

within lo-" xc [l23]. At that point, the excited electron recombines snd the resulting

twisted structure becomes 'Ybzen in" (d). This new structure is distorted in its

intermolecular bonding distance, giving rise to an increase in the randomness of the solid,

and a corresponding narrowing of the bandgap. It is very interesting to note that this

model relies on the presence of geminate EHPs to cause these bond flipping

photostructural changes.

A photoinduced EHP does not necessarily have only two posslile fates:

dissolution or recombination. An excited electron and the hole it leaves behind can

sometimes travel in unison; bound by their mutual wulombic attraction and contributing

nothing to the net dc current When an electron and hole act in this manner, tbey are

referred to as an exciton [124], and have hydrogenic-like energies. Street hypothesized

that an exciton could cause an WAP pair, as shown in Figure 6.32--he calIed this

situation a ''self-happed exciton" [125]. Biegelsm and Street [126] subsequently

supplied experimental evidence to support this theory.

Figure 631. Bond twisting model in a-Se. The equilibrium state (a) is altered when a LP electron is excited Erom atom A (b). A then feels a strong coulombic attraction to B, which twists A into a new position (c). The excited electron then recombines (d) and the structure is "hzen in".

It is important to realize that the shift in the optical absorption coefficient

coincident with photodarkening is not of sufficient magnitude for the phenomenon to be

caused by charges excited from the deep midgap traps associated with isolated VAP

defects, thus shallower energy states must be the cause. There is evidence to suggest,

however, that the close proximity of the S4 and S< defects in an IVAP wodd lead to

energy levels that lay shallower in the bandgap of a-Se than the deep traps due to isdated

VAP defects [126, 1271, Charges excited from these shallow traps would help to account

for the photodarkening phenomenon.

Se Chain

Before Self-Trapping

After Self-Trapping

F i e 632. Schematic diagram of the transformation of an exciton in Se into an IVAP pair accompanied by atomic distortion. The resulting NAP is sometimes referred to as a "self-trapped exciton". The resulting Sg and Ski- defects are commonly referred to as D' and D' defects, respectively.

Other photoinduced defects, stable at low temperature, have been reported. These

are primarily the fonnation of a pair of neutral threefold coordinated defm [128-13 11

which serve to cross-link Se chains, as shown in Figure 6.33. These defects are

metastable and tend to disappear quickly after the light is turned off and at elevated

temperatures. They may decay into either their original (ground) state, into stable new

twofoId bonds, or into an NAP as in Figure 6.33. Observations reported by Roy et al.

[132] support the second case (path I1 in Figure 6.33).

Other models for the photoinduced changes have been suggested, among them a

symmetry reversal of the pyramid centered at an overcoordinated atom to explain the

photoinduced optical anisotropy found in chalcogenide glasses [1333.

r(

Dark .4 \

tight Off

Figure 6.33. Formation of metastable triply-coordinated defects which serve to cross- Link adjacent Se chains. Antibonding electrons are denoted as "e". The metastable triplyaordinated defects may decay into (I) their ground state, (11) new bonds, or (m) into an NAP.

Oscillatory behaviour in the optical properties of amorphous GeSez films under

cw illumination h m a He-Ne laser has even been reported [134-1361. It was found that

under strict conditions, depending mainly on the incident power density of the laser, the

amount of light reflezted and transmitted by the a-GeSq films would oscillate at a rate

between - 3 - 50 Hz. These oscillations in the optical properties of the films were

accompanied by a simultaneous oscillation in the induced photocurrent. The oscillations

were subsequently explained as follows [137]: incident light fiom the He-Ne laser

induces photostructural changes leading to photodarkening. Once the transition to the

dark state has been achieved, the amorphous film absorbs considerable energy h m the

laser beam, causing it to undergo localized heating. This heating causes thermal

relaxation of the light induced defects, leading to a bleached state. Once the film is

bleached, the process begins anew. The photoinduced changes were assumed to be of the

type proposed by Tanaka [12 1-1231.

In 1993, Fritzsche [I383 examined all the evidence available at that time and

finally concluded that there is ample evidence that amorphous chalcogenides possess a

degree of medium range order (MRO). illumination by light tends to induce a large

amount of disorder, in agreement with Tanaka [I21 -1231, but also, to a lesser extent,

induces VAP or VAP-like defects. However, despite all the effort that has gone into

trying to understand this phenomenon over the last 20 years, it is likely that the true

origin of the photoinduced defects will remain unclear for some time.

Even though x-rays are far more energetic than the visible light usually employed

to induce structural changes in a-Se, it is stdl conceivable that they could induce those

same changes. As descriied in Chapter 4, an x-ray h t interacts with a solid by

producing an energetic '"hot" electron. This hot electron then travels in the solid, ionizing

many other e lemns before coming to rest. The ionizations produced by that hot

electron will undoubtedly have a range of energies, with some falling roughly in the same

range as those that induce photostructural changes-that is, energies on the order of the

bandgap. Thus x-ray induced photostructural changes in a-Se should be possible, as

confirmed by Tai et al. [139] (but with an eye toward employing these effects for VLSI

mimlithography). Relatively weak ionizations wodd likely not be solely resportstile for

the observed changes; energetic electrons or x-ray photons could also induce changes

through bond rupturing. It is unfortunate that the two broad fields of optics and

electronics do not mix for there is no published infomation as to how these

photostructural changes (ie. changes in the optical properties) of a material will affect the

charge transport properties of that material; this aspect of a-Se has not been

systematically investigated prior to this work.

633.1 Trapping Mechanisms and the Observed Results

In order to begin to grasp the structural mechanisms at the heart of the observed

changes in electron and hole trapping lifetimes upon x-ray irradiation, it is critical that the

trapping mechanisms themselves first be examined.

The first fact that mwt be stressed is that in the dark and at room temperature,

there exists no detectable ESR signal in a-Se [32, 33, 5 1, 121, 126, 140-1421. Since

dangling bonds (unpaired electrons) are the cause of an ESR signal, there therefore are

negIigible dangling bonds present in a-Se at room temperature and in the dark. Optically

induced ESR signals can be observed in a-Se at room temperature [32, 331, but they

disappear quickly after the light is turned off. Unfortunately, exactly how quickly they

disappear has been omitted fbm the literature. However, it can be estimated that the

relaxation would be on the order of seconds at most since Kolobov et al. [MO- 1421 found

that an optically induced ESR signal in a-Se at 20 K completely disappeared within 20

minutes when the sample was warmed to 150 K..

The traditional view is that in a-Se, VAPs or NAPS would be a possible cause of

deep charge trapping. This situation is presented in Figure 6.34 for the case of hote

trapping and Figure 6.35 for the case of electron trapping. Suppose that initially a

particular region of the a-Se material contains an isolated D' and D' defect. A

photoinjected hole would be electrostatically attracted to the singly coordinated D- defect,

creating an electrically neutral Se,O atom with an unpaired electron. Silarly, a

photoinjected electron would be electrostatically attracted to the triply coordinated D'

d e f a creating an electrically neutral ~e,' atom with an unpaired electron.

The immediately obvious problem with this traditional view of charge trapping is

that a single trapped charge will result in a smgle unpaired electron, contrary to dI

published observations involving ESR studies [32,33,51, 12 1, 126, 140-1421. It may be

argued that the concentration of photoinjected charge in a typical TOF or IFTOF

F i e 6.34. One possible view of hole trapping in a-Se. (a) Electrically neutraI a-Se with an isolated VAP defect and a drifting photoinjected hole. (b) Hole trapped by the D' defect resulting in an eIectrically neutral defect with a danghg bond.

experiment is very small and the resulting concentration of unpaired electrons would then

be correspondingly difficult to measure via an ESR test. However, it is very dear h m

the published data that these unpaired electrons are extremely unstable at elevated

temperatures, and it would be expected that very few unpaired electrons would exist at

room temperature, if any at all.

Figure 635. One possl'bIe view of electron trapping in a&. (a) Electrically neutral a-Se with an isolated VAP defect and a drifhg photoinjected eIectron. (b) Electron trapped by the D* defect resulting in an ekctrically neutral defkct with a dangling bond.

In order to resolve the observed absence of unpaired electrons with the traditiod

charge trapping model, it is necessary that it be modifled slightly. For instance, the

singly coordinated neutral defect resulting from a single trapped hole in Figure 6.34 (b)

could rid itself of its dangling bond by approaching a nearby chain, as in Figure 6.36.

The resulting neutral threefold coordimted defect has an unpaired electron which could

be immediatefy neutralized by a second photoinjected hoke in the vicinity, as in Figure

6.36 (b).

Figure 636. (a) The single trapped hole of Figure 6.34 (b) that results in a neutral singly coordinated defect may rid itself of its dangling bond by approaching a nearby chain and forming a neutral threefold defect. (b) The unpaired electron of the neutral threefold coordinated defect could be immediately neutratized by a nearby photoinjected hole to form a D' defect.

Similarly for electron trapping, the neutral threefold coordinated defect of Figure

6.35 (b) could rid itself of its dangling bond by releasing one of its bonds, resulting in a

neutral twofold coordinated atom at the site of the original D+ defect, and a nearby

electrically neutral singly coordinated defect as in Figure 637. The resulting neutral

singly coordinated defect has a single unpaired electron which couid be immediately

paired by a second photoinjected electron in the vicinity, as in Figure 6.37 (b).

This model is consistent with the published experimental evidence in that it

explains tbe absence of unpaired electrons in a-Se at room temperature. One can

hypothesize the foIlowing model: that charge trapping in a-Se does not involve a singIe

charge carrier, but involves a pair of carriers in order to maintain '8ond neutrality".

F i e 637. (a) The single trapped electron of Figure 6.35 (b) that results in a neutral triply coordinated defect may rid itself of its dangling bond by releasing a bond and forming a neutral twofold bond and a nearby neutral singly coordinated defect. (b) The unpaired electron of the neutral singly coordinated defect could be immediately paired by a nearby photoinjected electron to form a D* defect.

From xerographic and IFTOF experiments, Kasap et al. found that the capture

radii of deep hole traps in a-Se are about 2 - 3 A; on the onda of the Se-Se bond length

[143]. Since the capture radius of an IVAP or isolated VAP defect is expected to be at

least 2 - 3 times that of the bond length [143], deep hole trapping into these defects

would appear unlikely. indeed, it has long been suspected by some researchers that

trapping into VAPs or IVAPs was unlikely and that low cross-section electrically neutral

traps of unknown origin were the cause [5 1 1.

A possible hok trapping mechanism not involving a VAP or N A P defect is

illustrated in Figure 6.38. This new model builds on the observations of Kolobov et al.

[128-13 11 that a-Se tends to form a large number of neutral, paired, mpIy coordinated

defects when illuminated by light, Since illurninzition ha^ :-.- nhserved to create these

bonds, then perhaps hoIe trapping could accomplish the same thing. If a lone pair

electron on one of two relatively close twofold coordinated atoms on adjacent chains

were to be neutralized by a photoinjected hole as in Figure 6.38 (a), then those two Se

atoms could form a bond that Iinks the two chains, as in Figure 6.38 (b). This transient

situation would Ieave one atom with an unpaired electron which wouId have to be

neutralized by a second photoinjected hole to create a D' defect, as in Figure 6.38 (c).

This model is consistent with the observation that the hole trap cross-section is of the

order of a Se-Se bond length.

Figure 6.38. Two Se chains in which two twofold Se atoms are physically quite close to one another. (a) A photoinjected hole neutralizes a lone pair electron on one atom, leading to (b) a transient bond that cross-links the two chains as well as a single unpaired electron. (c) A second photoinjected hoIe neutralizes the unpaired electron, forming a pair of D' defects.

The deep trapping cross-section of electrons in a-Se has not been measured, so

there are no published results on which to base a rigorous theory. However, if it is

assumed that electrons are trapped not by VAPs or NAPS, but by traps of a small cross-

section as with holes, then the situation illustrated in Figure 6.39 couid be a posslhIe deep

electron trapping mechanism. In a-Se, the bond lengths between adjacent atoms is not

uniform; as a result, some bonds will be shorter than the average, and some will be

longer. Consider the case of two adjacent Se chains, as in Figure 6.39 (a). Some bonds

are relatively short while others are relatively long. If a photoinjected electron is in the

vicinity of one of these stretched bonds, it could be detained by one of the previously

twofold coordinated Se atoms to form a D' defect and a neutral singly coordinated defect

with one unpaired electron, as in Figure 6.39 (b). Then a second photoinjected electron

could be trapped by the neutral defect to form another D- defect with no unpaired

electrons.

F i i e 639. Two Se chains in which there exist stretched and compressed bonds. (a) A photoinjected electron nears a stretched bond to form (b) a D- defect and a neutral singly coordinated defect that has an unpaired electron. (c) The neutral singly coordinated defect diff ies a short distance before meeting a second photoinjected electron to form another D- d e f a

It should be noted that this new charge trapping theory--that trapping involves

the capture of two carriers-is simply the reverse of a previously reported emission

process. Both Melnyk [I441 and Baxendde and Jubasz [I453 speculated that an a-Se D*

defect could decompose into a D' defect through the emission of two h e holes.

With the above possible electron and hole trapping mechanisms in mind, the

results of the present study into how charge carrier lifetimes are affected by x-ray

irradiation may be summarized as follows:

neither electron nor hole mobility is affected by x-ray irradiation; hole lifetimes are immediately and dramatically affected by x-ray irradiation, independent of the irradiation conditions; electron lifetimes exhibit very little change immediately following irradiation by x-rays, independent of the irradiation conditions; both electron and hole lifetimes continue to change hours after being irradiated; these changes appear to be unpredictable in nature (sometimes rising, sometimes falling); the rested electron and hole lifetimes days after being irradiated are not necessarily equal to their pre-irradiated values, in some cases being 20% hisher, electron lifetimes sometimes behave erratically if the sample is continually irradiated, but generally exhibit little change with continued irradiation; behaviour of the IFTOF effective transit time (and thus dispersion) is disrupted by high doses of itradiation but unaffected by low doses-likely cause is a bulk space charge of equal numbers of positive and negative charges, and this charge disappears within two hours after irradiation.

There are several clues as to the trapping mechanisms at work in the a-Se samples

when subjected to x-rays. The most interesting is the observation that the FTOF

effective electron transit time changes from being highly dependent on the number of tiee

photoinjected carriers remaining in the packet to being independent of it when the

samples are irradiated with high x-ray doses (Figures 6.25 & 6.26). Since the carrier

mobilities were found to be unaffected by irradiation, the most likely mechanism for this

dependence is that the x-rays form NAPS witbin the a-Se film that interfkre with the

mutual coulombic repulsion of the free photoinjected carriers; the concentration of

NAPS is strongly dependent on the absorbed dose, as evidenced by the observation that

low doses do not change the behaviour of the effective IFTOF electron transit time upon

irradiatioa The formation of IVAPs is thought to be dependent on geminate EHPs [12S]

and the number of EHPs that escape geminate recombination is very low in a-Se, even at

moderate electric fields [73].

Therefore, the IFTOF effective electron transit time behaviour points to x-ray

irradiation initially creating a high concentration of lVAPs in the a-Se flhs, and this

concentration increases with the absorbed x-ray dose. Intuitively, this point makes sense

in that more x-ray irradiation would be expected to cause a larger change in the a-Se film.

The restoration of the original effective transit time behaviour within two hours is

evidence that these NAPS decay back to their equilibrium concentration fairly quickly

and must therefore be quite unstable at room temperature.

The next observation is that hole trapping is very dramatically and immediately

affected by irradiation, and this change is independent of dose for the limited range

employed in this study (refer to Figures 6.23 & 6.24). Electron lifetime was initially

found to change very little (C lOa/a), if at all, upon irradiation. The deep trapping

lifetimes of both electrons and holes were observed to continue to change over a period

of several hours after irradiation, and were approximately restored to their original values

several days later.

Since the hole lifetime immediately f d s upon irradiation, it could be possible that

the x-ray induced lVAPs detected through the electron IFTOF e f f d v e transit time

experiment would be the cause of this dramatic decline in lifetime. However, the hole

lifetime f d s just as dramatidly for low doses as for high, and the presence of NAPS

was detected only at high doses. It is important to remember that these NAPS vanish

within two hours, but the observed hole lifetime continues to fall well beyond two hours

later.

These facts support the theory that deep charge trapping does not occur at charged

defect centers. If charged defects were the cause, then both the hole and electron

lifetimes should dramatically fall upon irradiation, since irradiation produces a high

concentration of IVAPs. These NAPS disappear within two hours after irradiation, while

the electron and hole lifetimes continue to change; in some cases rising, in others falling.

Tanaka's bond twisting model [ 121 -1231 relies on geminate EHPs to achieve the

relocation of atoms to new metastable locations in the solid, and as stated previously,

geminate EHPs are in abundance in a-Se. Therefore, it is reasonable to expect that a

large number of atoms will be twisted into new positions by x-ray irradiation, and that as

time progresses some may be able to overcome the potential barrier and relax back to

their ground state position.

If, as a result of bond twisting, some atoms find themselves in relatively close

proximity to a neighbour, then the bond formation hole trapping mechanism of Figure

6.38 would likely become dominant. Indeed, since the hole lifetime dramatically falls

upon irradiation, this trapping mechanism wodd seem to be the cause. Another

interesting point is the fact that the hole Iifetime continues to change hours after the

IVAPs have relaxed. This would point to these metastable twisted bonds being more

stable (relatively speaking) than WAPs at room temperature.

Since the electron Iifetime does not drarnaticaIIy fall immediately upon

irradiation, electron trapping by charged defects (NAPS) cannot be the dominant

trapping mechanism, for the reasons cited above. If the stretched bond mechanism of

Figure 6.39 is dominant, then the bond twisting model of Tanaka [121-1231 cannot

significantly increase the number of stretched bonds in the solid. The reason for this

statement is the fact that the hole Lifetime significantly falls upon irradiation, likely due to

these twisted bonds, but the cotresponding electron lifetime is hardly affected.

As time progresses after the sample has been irradiated, both the electron and hole

lifetimes continue to change; sometimes rising, sometimes falling. This is indicative of a

mdom structural relaxation as the atoms of the a-Se film strive to tearrange themselves

into an energetically agreeable arrangement, but not necessarily their original

configuration; Le. those very same flipped atoms flipping back. At any one instant the

concentration of "close neighbod atoms may increase--Ieading to an increase in the

deep hole traps and a decrease in the hoIe Wedme, and at any other instant this

concentration may decreaseleading to an increase in the hole lifetime.

Correspondingly, the electron lifetime initially is hardly affected by irradiation itself, but

goes on to change hours later. At any one instant, the concentration of stretched bonds

may increas-leading to a decrease in the electron lifetime, and at any other instant this

concentration may decreas-leading to an increase in the electron Lifetime. Again, this

relaxation is a random process which depends on the local atomic arrangement of the

atoms, and that explains the observed behaviour of the electron and hole lifetimes over

time--sometimes falling, sometimes rising.

This theory also explains why the electron lifetime of a sample can sometimes

behave erratically as the sample is continually irradiated. M a t e the sample once, and

the atomic arrangements are disrupted. Irradiate it again, and the arrangements are

"shaken up" once more. The resulting atomic arrangements are random, depending on

which atoms were twisted where, and the corresponding lifetime wodd then also be

unpredictable since the concentration of traps would be highly dependent on the local

atomic arrangement.

Two very limited studies into the effect of x-rays on the hole lifetime in a-Se

tilms were done prior to this work. In the first [16], xerographic measurements of the

cycled up and first residual of a-Se plates were performed and it was concluded that the

number of hole traps increased as a result of exposure to x-rays. It was speculated that

the accompanying decrease in the hole lifetime was most Iikely due to trapped electroas

which could recombine with the photoinjected holes, thus leading to the reduced lifetime.

It shodd be pointed out that these xerographic measurements do not minimally disturb

the very traps they are meant to detect; instead, these measurements work by filling d l

available traps by repeated cycling until no change in the amount of charge retained by

the a-Se flh can be detected.

The second published study [60, 1001 observed that the hole lifetime in an a-Se

film, measured via the lFTOF technique, decreased by a factor of - 5 as a result of

exposure to x-rays. The large displacement currents during switching were eliminated

via balanced bridge techniques [58, 6062, 1001. These measurements can take a long

time to perfonn since the bridge is difficult to properly balance, as mentioned in Chapter

3. No indication of when the measurements were obtained relative to the initial

irradiation of the a-Se film are found in the references.

These previous results are consistent with this study. In both, it was found that

the hole lifetime decreases upon exposure to x-rays, consistent with the findings of this

study. However, it is known that minimal (if any) filtering was employed in the x-ray

beam of these studies, which could explain why the observed hole lifetime decreased by a

factor of - 5 in the second study (an 80% decrease), whereas the maximum decrease in

hole lifetime noted in this study was - 60%.

6.33.2 Irreversible X-ray Induced Damage

It is desirable that a radiographic x-ray detector not be adversely affected by

continual x-ray irradiation over time; in other words, that it not be fatigued by the x-rays

themselves. This requirement is desirable for obvious reasons. The rested electron

trapping lifetimes measured in this work were monitored over time to gauge the

posslhility of irreversible x-ray induced damage. Figures 6.40 and 6.41 illustrate how the

rested electron lifetimes at a depth of 0.263 L for Sample 971002 12 and 0.287 L for

Sample 971205 62 respectively varied h r n expaiment to experiment. The total

absorbed dose deposited in each sample just prior to each rested lifetime is also listed (the

open diamonds in the plots). The samples were allowed to rest in the dark at room

temperature for at least two days before being firrther irradiated. It is obvious fhm the

figures that there is no downward trend indicative of permanent damage. Instead, these

plots lend further credence to the earlier theory that the charge carrier lifetimes in an a-Se

6lm are dependent only on their local atomic arrangement. As such, both the behaviour

of the carrier lifetimes with time after irradiation and the rested lifetimes days later would

not be correlated and would be difficult to predict.

63.4 Electron Lifetime Recovery

One of the original gods of this study was to not only investigate how x-ray

irradiation affected the charge transport of a-Se films, but aIso if annealing, IR soaking

and ultrasonic treatment could be employed to recover the original properties of the film.

Experiment

800

h rn a. 750 - w

E .- 700 - 1

t 2 u

650 - iil u Q) u

600 -

550

Figure 6.40. The rested electron lifetime at a depth of 0.263 L (closed circles) of Sample 971002 12 tracked over the course of the work. The absorbed dose on the occasion immediately preceding each experiment is indicated with the open diamonds. The dose preceding experiment 6 is not known, as that was the exposure without any filtering placed in the beam (mentioned previously).

2

I 0 Dose

f 0 0

0 I 1 I I 1 I 1

1 2 3 4 5 6 7 8

Experiment

Figure 6.41. The rested electron lifetime at a depth of 0.287 L (closed circles) of Sample 971205 62 tracked over the course of the work. The absorbed dose on the occasion immediately preceding each experiment is indicated with the open diamonds.

6.3.4.1 Annealing

Annealing the samples at 35 OC had a dramatic effect on their charge transport. In

general, electron transport deteriorated greatly and displayed a decline in both the deep

trapping lifetime and the mobility. The hole mobility displayed no discemable change,

but the hoIe lifetime was affected to a greater degree than the electron lifetime. This data

is presented in Figures 6.42 and 6.43.

All three qualities-the electron and hole lifetimes and the electron mobility-do

not totally recover back to their original values. This would be indicative of irreversible

changes induced by the time spent at an elevated temperature. The elevated temperature

could conceivably alter the local atomic arrangement of the a-Se film, leading to the

observed changes in the carrier lifetimes. These observations are in agreement with

previous studies which reported that the drift mobility in a-Se is strongly temperature

dependent [I461 and that the bandgap decreases at elevated temperatures [147]. This

latter study implies that the decrease in the bandgap is due to an increase in disorder

within the d i d , consistent with the bond-flipping theory of Tanaka [12 1 - 1231. Thus the

obsmed decrease in the hole and electron Hetimes upon anneding lends hther

credence to the carrier trapping theory introduced earlier.

When a sample was first irradiated by x-rays and then annealed at 35 OC, the plot

of Figure 6.44 was obtained. The electron lifetimes at two depths within the sample were

initially measured (stage 1 of Figure 6.44) and then the sample was irradiated with x-rays.

The lifetimes at those same depths were again measured (stage 2) before the sample was

annealed for 3 hours at 35 "C (stage 3). It appears that annealing does not aid electron

lifetime recovery after a sample is irradiated with x-rays.

Total Time Spent at 35 C (h) I 4

0.6 ' I I

24 48 72

(b) Recovery at Room Temperature (h)

Figure 6A2. (a) Normalized electron (solid circles) and hole (open diamonds) lifetimes as a hction of total the spent at 35 "C. (b) The room temperature recovery of those lifetimes tracked over a three day period following the heat treatment. Both Iifetimes were measured at the same depth within the film 0.263 L. [Sample 971002 121

Total Time Spent at 35 C (h)

(b) Recovery at Room Temperature (h)

Figure 6.43. (a) Normalized electron mobility as a hction of total time spent at 35 "C. (b) The mom temperature recovery of the mobility tracked over a three day period following the heat treatment. [Sample 971002 121

6.3.4.2 IR Soaking

Soaking an a-Se sample with IR light has proven usefid in discharging x-ray

induced bulk space charges [102, IZTJ, so an investigation into whether IR soaking

would aid electron lifetime recovery was wananted.

Rested d Exposed P Anneal

Stage

Figure 6.44. Nomdized electron lifetime at two depths within an a-Se sample before irradiation (stage i), immediately after irradiation (stage 2), and following 3 hours spent at 35 O C (stage 3). Irradiation specifics: 58.2 keV, 35.7 mGy, sample shorted during irradiation. [Sample 96052 1 46 SE 151

Even short periods (- 30 seconds) of IR soaking would noticeably raise the

temperature of the sample through radiant heat transfer. It is therefore quite difficult to

discern if the observed effects were due to the action of the IR photons alone, or a

combination of the IR photons and sampIe heating. In general, the electron lifetimes

decreased as a result of IR soaking, but this was expected given that other researchers

have reported that sub-bandgap light decreased the deep trapping lifetime of both

electrons and holes in a-Se [51]. The d t s are presented in Figure 6.45.

63.43 Ultrasonic Treatment

Very early in the study, ultrasonic treatment (at 4 MHz as mentioned in Chapter

5) was conceived as a very simple prucedure to perform on an a-Se sample to investigate

whether or not the ultrasonic treatment did indeed have any effect on the charge transport

properties of the film. As with IR soaking, it was difficult to establish whether the

observed changes were caused by the ultrasonic waves themselves, or due to sample

heating. Unfortunatelyy during the 15 minute ultrasonic matment, the ultrasonic

P pa 0.9 Rested

Exposed

30 sec Soak

0.5 - (a)

0.4 r I

1 2 3

Stage

1 .l

1.0 - !! Exposed 0.401 L

0 0.603 L 0s9 - Rested w

8 0.8 - .-

E l -

0.7 - 90 sec Soak 5 Z 0.6 - @

0.5 - (b) P

Stage

45. Normalized electron lifetime at two depths within two differen samples. (a) Before hradiation (stage I), immediately after irradiation (stage 2), following 30 seconds of IR soaking (stage 3). Irradiation specifics: 58.2 keV, 96.6 mGy, sample shorted during irradiation and during soaking. [Sample 97 1002 121 (b) Before irradiation (stage l), immediately after irradiation (stage 2), following 30 seconds of IR soaking (stage 3), following 90 seconds total IR soaking (stage 4). Irradiation specifics: 582 keV, 28.6 mGy, sample shorted during irradiation, but F = 4 Vljm during IR soaking. [Sanq.de 960521 46 SE I S]

transducer noticeably warmed (to - 40 "C), and this undoubtedly had an effect on the a-

Se sample.

The electron and hole lifetimes of an a-Se film at a single depth in the sample

were first measured, and then the sampIe was subjected to a 15 minute ultrasonic

treatment. The electron and hole lifetimes were again measured at the same depth in the

sample immediately foIIowing the ultrasonic treatment, and two hours thereafter. The

results are found in Figure 6.46.

F i r e 6 Normalized electron and hole lifetimes at the same depth (0.401 L) within an a-Se sample initially (stage l), following 15 minutes of ultrasonic treatment (stage 2), two hours thereafter (stage 3). [Sample 96052 1 46 SE I S]

P 2 h Later

It is clear fiom the figure that the electron and hole lifetimes were adversely

affened by the treatment, but it is difficult to discern whether this directly resuIted f?om

the dtrasonic treatment itseIf or the inadvertent heating of the sample during treatment.

However, it is felt that the observed changes are likely caused by sample heating. This

part idar aspect of the behaviour of aSe should be investigated W a if an apparatus

could be designed that maintained the sample at a constant temperature while it was

being subjected to uItrasouic waves.

a 0.9- P" o 0.8 - 8

Rested Z

= 0.7 (P

0.6 pi 0

0.5 - P 15 min Ultrasonic

6.4 Electron Hole Pair (EBP) Creation Energy - WEHp

The mechanism(s) underlying the liberation of mobile charge carriers in a-Se by

x-rays has both a practical and a scientific significance. Practically, it is desired to

achieve the maximum possible collected charge when irradiating a-Se with x-rays in

order to achieve high quality radiographic images with low x-ray doses. This particular

quality is arguably the biggest controlling factor in the sensitivity of a-Se radiographic

detectors. Scientifically, one desires to understand exactly how x-rays come to liberate

free charges within a-Se, and wbat changes take place in the solid when this occurs.

The integration of the x-ray induced photocurrent when a biased a-Se film is

irradiated with x-rays reveals the number of carriers keed by the incident x-rays. Figure

6.47 pictures a typicd x-ray photocurrent produced by the ten impulse x-ray "quanta"

used throughout the work. There are two fatures of Figure 6.47 that are immediately

apparent: the photocurrent "spikes" caused by the self-rectifying x-ray tube, and a subtle

''rising baseline" which increases slightly with every x-ray burst. This rising baseline

corresponds to a persistent x-ray photocurrent. The exact cause behind the rising baseline

is not known, though it is unlikely to simply be carriers released &om traps, as previously

proposed [67, 681. There may be a contribution arising from increased contact injection

[73], although it is unlikely that this is the primary hctor. WEHP was evaluated by

excluding the charge in this rising baseline, consistent with other studies [67, 681. The

rising baseline will be examined in detail in section 6.5.

Another feature of Figure 6.47 are the nmt spikes at tie beginning of the

pulsetrain. This aspect of the x-ray photoments was a phenomenon unique to the x-ray

unit employed in the work, and not the r d t of the heavily filtered beams used

throughout the study. It is suspected that these smaller leading spikes are the

consequence of the x-ray tube somehow ''warming up" because these spikes were still

observed even if the filtering were removed* These small spikes were also observed in

the x-ray photocurrents induced in the CZT calkator detector, so their existence being

unique to a-Se is also ruled out.

Photocurrent "Spikes"

Rising Baseline

0.10 0.15 0.20 0.25

Time (s)

F i e 6.47. A typical x-ray induced photocurrent in a biased a-Se film, detailing the "spikes" due to the self-rectifying nature of the tube and the "rising basehe*. Data obtained with a 58.2 keV beam and Sample 1463-3 biased at 1.77 V l w .

The Gendex x-ray unit employed in this work is not the highest quality x-ray unit

available; as such, its output varied slightiy as evidenced by the non-uniform spikes

comprising the latter part of the puIsetrain shown in Figure 6.47. To reduce the effect

this variance in the beam output had on the charge collected h m the x-ray photocurrent

(and thus, the error in WEHP), all measurements of WmP were based on the average of 10

individual measurements, each of a 10-impulse burst Once this average collected charge

was found, the energy deposited in the a-Se layer was found via the method described in

Chapter 5 and the electron hole pair creation energy, Wmp, was calculated.

a-Se is well known for its field dependent photogeneration. A large fraction of

photogenerated carriers are lost via various recombination processes thought to exist in a-

Se, which means that the measured WEHP is an effective value needed to create collected

EHPs. The intrinsic energy to create an EHP, wLp, corresponds to no recombination

losses which is expected to be the case at very high fields. The theoretical intrinsic

energy bYO, via Klein's rule [82, 831 may be used as a starting point to establish a value

for w&. As desmied in Chapter 4, this energy will be about 2.8 E, plus a small (0.5

eV) phonon texm. With E, = 2.2 eV for a-Se, whP = 6.7 eV. If the conservation of

momentum rule is relaxed in a-Se, as argued by Que and Rowlands [70], wLp = 2.2 E,

plus a small phonon texm. This leads to wLP = 5.3 eV in a-Se. An estimate of WL in

a-Se may be found by extrapolating the WUIP VS. l lF behaviour to 11F = 0.

The energy to create a collected EHP as a hct ion of the reciprocal electric field

for four different beam energies is presented in Figure 6.45. The strong field dependence

is very evident fiom the plot, and the number of collected charges increases with

increasing field strength. The linear regressions for each data set all remarkably converge

to within 0.5 eV at infinite field, or the expected value of wL,, . This convergence in the

data gives an average WL = 5.9 eV, in reasonable agreement with the theoretical values

of 5.3 - 6.7 eV introduced above, This value is also in close agreement with the previous

reported result of 5.9 eV [73, 1023. Asswning an operating field of 10 Vlpn, Wmp has

been reported to be in the rage of - 35 - 55 eV over the diagnostic beam energy range

[70-751, also in close agreement with the present results of - 50 eV.

It was found that Wm did not si&cantly vary as a hct ion of the mean x-ray

beam energy over the limited range employed in this work. Figure 6.49 is a plot of Wm

at a constant electric field of I0 VIpm (0.1 CMN) vs. the mean x-ray beam energy.

There is a very slight energy dependence which tends to lower values of WEHP at higher

beam energies. If a hea r regression is applied to the data (as shown), the slope of the

h e yields a WUIP dependence upon the mean beam energy of -0.05 eVkeV. Given the

Figure 6.48. The energy required to create a 6ee EHP as a function of the reciprocal electric field for four different mean x-ray beam energies spanning the range 32.8 - 58.2 keV. Linear regressions for the four sets of data converge to within 0.5 eV to yield an expected W&,, = 5.9 eV at infinite field. [Sample Run #30]

limited energy range of this study, this value is comparable to Fieidler's reported

dependence of 4.24 eVkeV which was obtained over a range spanning h m - 30 keV

to 200 keV [71]. It should be noted that a similar analysis was performed on the

remainder of the data found in Figure 6-48. It revealed that all dopes of the linear

regressions were negative. The average d u e of this energy dependence was found to be

4 - 0 2 1 eV/keV.

I Slope: -0.05 eV/keV

30 35 40 45 50 55 60

Mean X-ray Beam Energy (keV)

Figure 6.49. Dependence of Wmp on the mean x-ray beam energy at a constant field of 10 V / p .

The a-Se sample which was used to obtain the WwP vs. reciprocal field plot of

Figure 6.48 has a minimum operating field (Table 6.2) of 1.77 Wpm. FieIds above this

minimum value will generally ensure that no charges are lost to trapping, therefore,

trapping may be neglected as a charge loss mechanism.

There are essentially two expected recombination processes that may be

responsible for the charge Ioss and would thus explain the strong field dependence of

WW. These are geminate and columnar recombination, mentioned previously in Chapter 4. In geminate recombination the twin excited eIectron and the hole it leaves

behind recombine with each other [17,70,73, 1483, whereas in columnar recombination,

electrons and holes within the track of a primary electron drift and recombine within that

m k [70, 1493.

Que and Rowlands set forth a number of requirements for both geminate and

columnar recombination in order to determine which is the likely controlling process for

x-ray photogeneration in a-Se [70]. Geminate recombination is strongly temperature

dependent because the largest controlling factor which determines whether the

photogenerated electron and hole dissociate is the initial separation of the two charges, r,.

This initial separation increases with increasing temperature, meaning that the probability

of dissociation, and thus the yield, will decrease with decreasing temperature. On the

other hand, columnar recombination is relatively insensitive to temperature.

Figure 6.50 is a plot of WUIP US. temperature which was measured at a constant

electric field of F = 1.77 Vlpn and a mean beam energy of 58.2 keV. The measurements

span a temperature raage from - -60 "C to room temperature. It is very obvious that

WEHP exhibits a very strong temperature dependence that decreases the yield (and thus

increases WEHP) as the temperature is lowered. These results are generally indicative of

geminate recombination being the dominant recombination mechanism in a-Se when

irradiated by x-rays. Recently, Haugen et ti/. [73] reported no observable WEHp

temperature dependence in a-Se, but the temperature range of the study was extremely

limited (-10 OC to room temperature). However, their results are consistent with the

behaviour observed in this study, as WEHP shows only a weak temperature dependence

down to - -30 OC, after which the association becomes much stronger.

It is common to refer instead to the photogeneration efficiency as opposed to the

EHP creation energy, Wm. The photogeneration efficiency, q, can be defined as the

ratio of generated EHPs that escape recombination relative to the total number of created

EHPs. This may be expressed as a ratio of intrinsic and apparent EHP creation energies

200 220 240 260 280 300 320

Temperature (K)

Figure 630. Plot of the energy required to create a collected EHP, WW, as a function of temperature. The mean beam energy was 58.2 keV and the electric field was held constant at I.77 Vlpn. [Sample 1463-31

The large value of Wm and its field dependence lead to a field dependent

photogeneration efficiency with a value less than unity. Figure 6.51 plots the

photogeneration efficiency vs. temperature for the data of Figure 6.50. The

photogeneration efficiency is very small at low temperature, about 0.2% at - -60 "C,

rising to - 4% at room temperature.

200 220 240 260 280 300 320

Temperature (K)

Figure 6.51. Photogeneration eFticiency as a function of temperature. Mean beam energy: 58.2 keV, sample biased at 1.77 V/w. [Sample 1463-31

As stated earlier, the initid separation of the EHP controls the probability that

they will escape their mutual wulombic attraction and become free. An optical photon

can only produce a single EHP, and this initial separation will therefore be dependent on

the photon energy. In other words, the initial separation will be the same for each EHP.

When an x-ray photon interacts with a sotid, thousands of EHPs can be created and their

initial separations will span a range. The distriiution of these separations shouId have

little or no dependence on the x-ray photon energy, as the EHPs are created randomly.

Therefore, WmP should be independent of the mean x-ray beam energy if geminate

recombination were the dominant recombination mechanism. As was seen in Figure

6.49, WmP shows very M e , if any, dependence on the x-ray beam energy. Again, this

fact may be indicative of geminate recombination being the dominant recombination

mechanism in a-Se.

Columnar recombination should exhiiit a pronounced decrease in Wmp with

increasing photon energy. Columnar recombination in a-Se was modeled by S a h p

[l SO], and he found that the charge density within the primary electron track decreased as

the x-ray photon energy increased, and was due to an increase in the mean separation of

EHPs within the column. This reduction in the concentration of h e carriers within the

column then directly results in a decrease in the recombination which in turn reduces

WmP. A direct comparison between the present results and Sahyun's predicted values

cannot be made since Sahyun's values extend only up to fields of - I V i m and this

study started at a field of 10 V i p . However, it is readily obvious that Sahyun's

calculated Wmp at - 1 V/pn of - 13 - 27 eV for beam energies in the range of 26 - 42

keV is far too low; this does not agree with the reported measurements of WEHP at 10

Vlpm of - 35 - 55 eV [70-751, nor with the results of this study.

There has existed a controversy over whether geminate or columnar

recombination is dominant in a-Se, but the results of this study seem to favour geminate

over columnar recombination. The very pronounced temperature dependence of Wmp

and the lack of a strong dependence on the x-ray beam energy is strongIy suggestive that

geminate recombination is the controlling charge loss mechanism in a-Se when irradiated

by x-rays, consistent with the accepted mechanism at optical energies [17].

6.5 Persistent X-ray Induced Photocurrent

Immediately fbIlowing x-ray irradiation, there exists a persistent photocurrent in

a-Se whose origin has been attributed to the thermal retease of deeply trapped holes [67,

681. This persistent photocurrent is problematic in a practicaf setting because it places a

limit on how quickly the a-Se detector may be again used following an exposure. If the

detector is not allowed to rest for a sufficient length of t h e (about three minutes [67,

68]), image ghosting may occur, whereby a remnant of the previous image will appear in

the present image.

This study of the persistent photocurrent was triggered quite by accident when it

was discovered that the magnitude of the rising baseline mentioned earlier remained quite

large at low temperatures, even though the x-ray induced photocurrent spikes decreased

dramatically. Figure 6.52 is a plot of the charge contained in the rising baseline vs. the

sample temperature. The details of the irradiation are the same as the WEHP VS.

temperature study presented in section 6.4: 58.2 keV, 1.77 V/pm, and Sample 1463-3. It

is immediately apparent that the basetine exhi'bits a temperature dependence which is

quite different &om that displayed by the photocurrent spikes, which have also been

plotted for comparison.

Baseline 0 Photocurrent Spikes

200 220 240 260 280 300

Temperature (K)

Figure 6.52. Plot of the charge contained in the baseline and photocurrent spikes vs. temperature. 58.2 keV, 1.77 Wpm, Sample 1463-3. The two exhibit markedly different temperature dependencies.

It was subsequently discovered that the charge contained in the rising baseline

was thermally activated. Figure 6.53 is a semi-Iog pIot of the rising baseline charge as a

fimction of 1/T. The baseline charge saturates at higher temperatures, but shows an

activated type of behaviour at temperatures below approximately -20 OC. This activation

energy is - 0.16 eV; since the deep hole traps in a-Se lie at - 0.8 eV, the persistent

photocurrent cannot be caused by the thermal release of deeply trapped holes (or

electrons for that matter).

Figure 6.53. Baseiine charge as a fimction of the reciprocal of the temperature. The amount of charge released in the persistent photoment becomes thmal ly activated below approximately -20 O C . The activation energy is 0.16 eV, which does not correspond to the deep hole or electron traps in a-Se.

The rather surprising result that the persistent photocurrent is thermally activated

fiom a previously unknown level in the bandgap lends fbrther credence to the theory of

photoinduced structural changes and that charge trapping is not into charged defects as

put forward in section 6.3. Intuitively, however, this makes sense. Irradiation by x-rays

creates transient metastable bonds or can rupture previously existing bonds leading to

metastable d e f m with unpaired electrons [128-132, 140-1423. Those unpaired electrons

(dangling bonds) are highly unstable and are released as these metastable defects

decompose into more stable forms: either an Se,' or an Se; defect. It is likely that the

release of these charges is the origin of the persistent photocunent, and is a temperature

activated process, in agreement with the observation that an ESR signal may be “frozen"

into a-Se at very low temperatures by Light [ 140-1 421.

6.6 Summary

The charge transport properties of various stabilized a-Se i3ms were evaluated via

the TOF and ETOF techniques to ensure that they had acceptably long carrier

Schubwegs so that charge trapping would not be a factor in the studies that followed.

It was found that x-ray irradiation has no effect on the TOF transit time of holes

or elmons, meaning that x-rays do not affect the shallow gap states in a-Se. CFTOF

studies of both electrons and holes suggest that irradiation produces NAPS, and their

concentration increases with the absorbed dose of the Miation. These NAPS disappear

quickly (within two hours after irradiation) and do not play a part in the deep !rapping of

holes or eiectrons, which was proposed to take place through bond rearrangements. The

electron and hole lifetime of an a-Se film continues to change hours after being irradiated

by x-rays, and this can only occur by way of a structural rearranging of the soIid as it tries

to assume a configuration consistent with a low stmctmd energy. This behaviour was

previously observed as a result of optical excitation, but the corresponding cbarge

transport properties of a-Se with these photostructural changes in pIace had not been

investigated prior to this work. La general, hoIe tramport deteriorates much more with x-

ray irradiation than electron transport. The observed changes in carrier lifetimes were not

of a sul3cient magnitude to affect the performance of a commercial radiographic

detector.

Both hole and electron transport was found to deteriorate as a result of amding,

IR soaking and ultrasonic treatment. Except for the case of ultmonic treatmeat (which

had not been investigated previously), these observations were in agreement with

reported results.

The e f f d v e EHP creation energy, WEHP, was determined by measuring the

number of free carriers generated through irradiation of a biased a-Se film and dividing

that number by the energy deposited in the a-Se layer by the x-ray beam. WEHP has a

pronounced field and temperature dependence which suggests geminate recombination as

being the dominant charge loss mechanism. The intrinsic EHP creation energy, w&, was found to be 5.9 eV, in close agreement with theoretical and previously published

values.

The persistent x-ray induced photocurrent was examined over a wide temperature

range, and it was found to exhibit a thermally activated behaviour, with an activation

energy of 0.16 eV. This energy does not correspond to a level of any known Craps in the

bandgap of a-Se. It is probably associated with the x-ray induced photostructural changes

that were used to explain the behaviour of the charge lifetimes upon exposure to x-rays,

or perhaps changes in the metaVa-Se contact behaviour.

7. Conclusions and Recommendations

7.1 Introduction

The objective of this work was to study the charge generation, transport and

trapping propexties of stabilized a-Se films as they pertain to its use as an x-ray

photoconductor. The TOF and IFTOF transient photoconductivity techniques were

initially employed to study the carrier drift mobility and deep trapping lifetimes of the

films. The x-ray induced photocurrent was also investigated using a high gain cunent-to-

voItage (I-V) converter.

The implementation of practical selenium-based solid-state x-ray imaging systems

requires a sound understanding of the charge carrier generation and transport behaviour

in the presence of x-rays. The optical properties and behaviour of opticalIy generated

charge carriers in a-Se films have been extensively studied over the last four decades,

primarily due to the use of a-Se in xerography in the 1960s and 70s. This study strove to

answer some of the questions raised by the use of selenium in the presence of x-rays.

7.2 Charge Transport Study

The a-Se samples were investigated to ensure that their charge transport

properties were acceptable for use in x-ray radiographic image detectors. Electron

transport was found to be the main controlling fictor in the determination of the

Schubweg of the sample. Each sample employed in the study had charge transport

properties comparable to that of a commercial device quality film.

Electrons were discovered to undergo very significant dispersion during either

TOF or IFTOF studies. The total spatial width of the electron charge packet is

comparable to the thickness of the film itself. Consequently, a plot of the hctionai

recovered electron photocurrent vs. LFTOF intemption time did not pass through the

origin for all samples used in the study. The dispersion of the electron charge packet

during an IFTOF procedure was found to depend very strongly on the concentration of

free carriers remaining in the packet. This suggests that the self-field of the photoinjected

carrier packet contniuted substantially to the dispersion.

7.3 Changes in Charge Transport Upon Exposure to X-rays

It was found that x-ray irradiation has no effect on the TOF transit time of holes

or electrons, meaning that x-rays do not affect the shallow gap states in a-Se. IFTOF

studies of both electrons and holes suggest that irradiation produces IVAPs, and their

concentration increases with the absorbed dose of the irradiation. These IVAPs disappear

quickly (within two hours after irradiation) and do not play a part in the deep trapping of

holes or electrons, which was proposed to take place through structural rearrangements.

The electron and hole lifetime of an a-Se 6lm continues to change hours after being

irradiated by x-rays, and this can only occur by way of a structural remangement of the

solid as it tries to assume a configuration consistent with a low structural energy. This

behaviour was previously observed as a result of optical excitation, but the corresponding

charge transport properties of a-Se yith these photostructural changes in place had not

been investigated prior to this work. En geueraI, hole transport deteriorates much more

with x-ray irradiation than electron transport. The observed changes in carrier lifetimes

were not of a sufficient magnitude to significantiy affect the performance of a

commercial a-Se based radiographic detector.

Both hole and eIectron ttansport were found to degrade as a result of annealing,

IR soaking and ultrasonic treatment. Except for the case of ultrasonic treatment (which

was not previously investigated), these observations were in agreement with reported

results.

7.4 EBP Creation Energy

The effective EHP creation energy, Wm, was determined by measuring the

number of fiee carriers generated through irradiation of a biased a-Se film and dividing

that charge by the energy deposited in the a-Se layer by the x-ray beam. WW has a

pronounced field dependence consistent with previously reported Watp vs. l/F

behaviour. Below - 240 K, WEHP was observed to exhibit a strong temperature

dependence which seems to suggest geminate recombination as being the dominant

charge loss mechanism. The intrinsic E W creation energy, w & ~ , was found by

extrapolation to be about 5.9 eV, in close agreement with theoretical and previously

published values.

7.5 Persistent X-ray Induced Photocurrent

The persistent x-ray induced photocurrent was examined over a wide temperature

range, and it was found to exhiiit a thermally activated behaviour below - 240 K, with an

activation energy of 0.16 eV. This energy does not correspond to a known level of traps

in the bandgap of a-Se. It is possl%Ie that it is related to the nature of charge trapping

proposed to be coincident with x-ray induced structural changes in a-Se, or x-ray induced

changes in the metda-Se interface (e.g. enhanced contact injection).

7.6 Suggestions for Future Work

It was discovered during the course of this work that the ekctron and hole

lifetimes depended very strongly on the temperature of the a-Se fib itself. To exclude

any lifetime variations imposed by the varying ambient temperature of the laboratory, a

proper cryostat should be employed which could be used in conjunction with the existing

lead-lined x-ray cabinet. This cryostat would also make the measurement of WEHP and

the baseline charge (persistent photocurrent) over a wide temperature range much easier

to perform, These particular measurements were the most difficult to perform during this

work, as condensation from atmospheric water vapour made the measurements very

challenging.

Although the MOSFET switches performed quite satisfactorily, their stray

capacitances slightly limited the recovery speed of the IFTOF waveforms. CommercialIy

available MOSFET switches have lower stray capacitances and should improve

performance of the system. Commercial switches are also attractive for the simple fact

that they do not have 12 individual power switches and they do not require time-

consuming battery changes every three weeks of continuous operation.

In order to vet+@ that the release of unpaired electrons is the true cause of the

persistent x-ray photocurrent, an ESR study must be performed. However, these ESR

measurements would be difficult to perform given that the persistent photocurrent, and

thus the unpaired efectrons, disappear very quickly after irradiation. This means that tbe

ESR study will have to be performed immediately following irradiation (within seconds).

Essentially, an apparatus that would allow the simultaneous x-ray irradiation of the

sample and an ESR measurement is needed.

The theory underlying photostructural changes in a-Se is quite well developed and

the results of this study would seem to indicate that the same structural changes can also

be induced by x-rays. In order to have a direct comparative study, it would be instructive

to irradiate not just the thick a-Se samples used for the charge transport studies, but aIso

some very thin samples so that the optical properties of a-Se could be studied as well.

This would be useful to explore if x-ray irradiation induces such phenomena as

photodarkening. When used in conjunction, these two studies could provide clarification

as to the cause of both photodarkening and charge trapping in a-Se.

It is apparent that although a-Se was extensively studied by researchers at Xerox

during the 1960's and 7OYs, that there are still many aspects of its properties that are not

understood. Most theories concerning amorphous semiconductors were developed

mainly during the 1980's through to the present day. During this time a-Si:H has usurped

most of the attention, but this study would indicate that a reexamination of a-Se is

warranted.

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