High Resistivity Amorphous Selenium Alloy Semiconductors For Radiation Detection Applications

92 Pages·2014·3.02 MB·English

by  Abhinav Mehta

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Theses and Dissertations
1-1-2013
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Abhinav Mehta
University of South Carolina
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Mehta, A.(2013). High Resistivity Amorphous Selenium Alloy Semiconductors For Radiation Detection
Applications. (Master's thesis). Retrieved from https://scholarcommons.sc.edu/etd/2412
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HIGH RESISTIVITY AMORPHOUS SELENIUM ALLOY SEMICONDUCTORS FOR
RADIATION DETECTION APPLICATIONS

by
Abhinav Mehta
Bachelor of Technology
Amity University, 2011

Submitted in Partial Fulfillment of the Requirements
For the Degree of Master of Science in
Electrical Engineering
College of Engineering and Computing
University of South Carolina
2013

Accepted by:
Krishna C. Mandal, Director of Thesis
Guoan Wang, Reader
Lacy Ford, Vice Provost and Dean of Graduate Studies
© Copyright by Abhinav Mehta, 2013
All Rights Reserved.
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DEDICATION

I would like to dedicate this work to my parents and to my friend for their
encouragement and support.
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ACKNOWLEDGEMENTS

I am thankful to my advising professor and supervisor, Prof. Krishna C. Mandal,
who supported and taught me with great enthusiasm. Under his guidance I have learned
new skills and acquired knowledge which will help me throughout my career as an
electrical engineer. I would like to thank Prof. Guoan Wang, Electrical Engineering
Department, for motivating and supporting my research and also for taking out time to
oversee my work as a reader for this thesis.
I would like to express my gratitude to my fellow lab members Dr. Sandeep K.
Chaudhuri, Mr. Sandip Das, Dr. Ramesh M. Krishna, Mr. Kelvin M. Zavalla and Mr.
Mohamad A. Mannan who have been a source of nonstop guidance and support.

I am thankful to Prof. Arnold Burger and Mr. Michael Groza of Fisk University,
Nashville, TN, for characterizing our a-Se alloy materials. I would also like to thank the
Electron Microscopy Center of University of South Carolina for SEM studies and EIC
Laboratories, Inc. Norwood, MA, USA, for their collaboration.

I would like to thank DOE Office of Nuclear Energy’s Nuclear Energy University
Programs (Grant No. DE-AC07-05ID14517) for providing partial funding for this
research work.

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ABSTRACT

High resolution noninvasive tools of diagnosis has always derived and enabled
scientific and medical research to probe and better understand subtleties of matter,
intangible to the human eye. Radiation detection systems are highly dependent on
advancements in materials and devices with front-end electronics. There are various
discrete applications of these radiation detectors and each application imposes certain
requirements so there is no single optimum radiation detector.
Flat panel x-ray imagers have gained high demand in the past decade because of
exponential improvement in readout electronics. We have synthesized and investigated
stabilized amorphous selenium (a-Se) alloys suitable for high resolution flat panel x-ray
imagers for medical diagnostic applications, primarily in digital mammography and chest
radiography systems. Also, in our pursuit to develop high energy physics radiation
detectors, we present a novel method to detect neutrons by exploring alpha detection
capabilities of isotopic boron doped a-Se alloys.
The synthesis of well-defined a-Se (As, Cl) alloys and boron doped a-Se alloys
have been carried out using a specially designed alloying reactor. The alloy composition
has been optimized to ensure good charge transport properties and opto-electronic device
performance. The synthesis of a-Se (As, Cl) alloys has been carried out by thoroughly
mixing zone-refined (ZR) Se (~7N) with previously synthesized Se-As and Se-Cl master
alloys. These synthesized alloys were then used as precursor materials for synthesizing
isotopically enriched boron doped alloys. To study and verify the desired physical,
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electrical, and opto-electronic properties of the synthesized alloys, they have been
characterized by various characterization techniques such as scanning electron
microscopy (SEM), x-ray diffraction (XRD), glow discharge mass spectroscopy
(GDMS), differential scanning calorimetry (DSC), Raman spectroscopy, x-ray
photoelectron spectroscopy (XPS), and current-voltage (I-V) characteristics. We have
achieved our goal, and have successfully grown stable highly-resistive a-Se alloys and
fabricated single layer planar detectors which can perform under high voltage bias with
low electronic noise. Results of high energy alpha particle response with a specific
signature of thermal neutron detection using boron doped a-Se alloy detector has been
successfully investigated and demonstrated.

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TABLE OF CONTENTS

DEDICATION.................................................................................................................. iii
ACKNOWLEDGEMENTS ............................................................................................ iv
ABSTRACT ....................................................................................................................... v
LIST OF TABLES ............................................................................................................ x
LIST OF FIGURES ......................................................................................................... xi
CHAPTER 1: INTRODUCTION .................................................................................... 1
1.1 BACKGROUND .......................................................................................................... 1
1.2 THESIS OVERVIEW ................................................................................................... 2
1.3 BACKGROUND OF X-RAY DETECTORS ..................................................................... 3
1.4 INTRODUCTION TO FPXIS BASED ON AMORPHOUS SELENIUM ................................ 6
1.5 MATERIAL AND ELECTRICAL PROPERTIES OF AMORPHOUS SELENIUM .................... 8
1.6 ALLOYING CONSIDERATIONS FOR STABILIZED AMORPHOUS SELENIUM ................ 13
1.7 INTRODUCTION TO BORON DOPED AMORPHOUS SELENIUM ALLOYS ...................... 15
CHAPTER 2: PREPARATION OF AMORPHOUS SELENIUM ............................ 18
2.1 OVERVIEW ............................................................................................................. 18
2.2 PURIFICATION OF SELENIUM PRECURSOR MATERIAL ............................................ 19
2.3 AMORPHOUS SE (AS, CL) ALLOY SYNTHESIS ........................................................ 26
2.4 FABRICATION OF A-SE (AS 0.52%, CL 5PPM) ALLOY FILMS .................................. 33
2.5 CONCLUSION .......................................................................................................... 37
CHAPTER 3: CHARACTERIZATION OF AMORPHOUS SELENIUM ALLOYS
AND ALLOY FILMS ..................................................................................................... 38
3.1 OVERVIEW ............................................................................................................. 38
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3.2 X-RAY DIFFRACTION ............................................................................................. 38
3.3 SURFACE MORPHOLOGY STUDIES .......................................................................... 41
3.4 RAMAN SPECTRSCOPY ........................................................................................... 43
3.5 DIFFERENTIAL SCANNING CALORIMETRY .............................................................. 47
3.6 OPTICAL ABSORPTION STUDIES ON A-SE (AS,CL) ALLOY FILMS ........................... 49
3.7 X-RAY PHOTOELECTRON SPECTROSCOPY SURVEY SCAN ...................................... 50
3.8 ELECTRICAL CHARACTERIZATION ......................................................................... 51
3.9 CONCLUSION .......................................................................................................... 56
CHAPTER 4: BORON-DOPED SELENIUM ALLOY SYNTHESIS AND
CHARACTERIZATION FOR NEUTRON DETECTION APPLICATION ........... 57
4.1 OVERVIEW ............................................................................................................. 57
4.2 PHASE DIAGRAM OF THE BORON-SELENIUM SYSTEM ............................................ 57
4.3 SYNTHESIS OF BORON DOPED A-SE (AS, CL) ALLOYS ............................................ 58
4.4 SYNTHESIS OF BORON DOPED A-SE (AS, CL) ALLOY FILMS ................................... 60
4.5 X-RAY PHOTOELECTRON SPECTROSCOPY .............................................................. 61
4.6 PHOTOCURRENT CHARACTERISTICS....................................................................... 63
4.7 CURRENT-VOLTAGE MEASUREMENTS ................................................................... 64
4.8 CONCLUSIONS ........................................................................................................ 66
CHAPTER 5: DETECTOR TESTING TO DEMONSTRATE THERMAL
NEUTRON DETECTION CAPABILITIES ................................................................ 67
5.1 BACKGROUND ........................................................................................................ 67
5.2 EXPERIMENTS WITH ALPHA PARTICLES ................................................................. 68
5.3 RESULTS ................................................................................................................ 69
CHAPTER 6: CONCLUSION AND FUTURE WORKS ........................................... 71
6.1 CONCLUSION .......................................................................................................... 71
6.2 FUTURE WORKS ..................................................................................................... 72
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REFERENCES ................................................................................................................ 74

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