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STUDIES OF TRAPPING AND LUMINESCENCE PHENOMENA IN YTTRIUM ALUMINUM GARNETS By CHRISTOPHER RICHARD VARNEY A thesis submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY WASHINGTON STATE UNIVERSITY Department of Physics and Astronomy December 2012 © Copyright by CHRISTOPHER RICHARD VARNEY, 2012 All Rights Reserved © Copyright by CHRISTOPHER RICHARD VARNEY, 2012 All Rights Reserved To the Faculty of Washington State University: The members of the Committee appointed to examine the thesis of CHRISTOPHER RICHARD VARNEY find it satisfactory and recommend that it be accepted. ____________________________________ Farida A. Selim, Ph.D., Co-Chair ____________________________________ Matthew D. McCluskey, Ph.D., Co-Chair ____________________________________ Gary S. Collins, Ph.D. ii ACKNOWLEDGEMENT I first would like to thank my parents for their love and support. I am grateful for all the years they spent pushing me to work hard and prioritize my studies. Without their guidance, I could have never become the physicist I am today. I am fortunate to have had such an upbringing. I am grateful for the efforts of my advisor, Dr. Farida Selim, under whose guidance I was able to find myself as a physicist and a researcher. I am indebted to the time and patience she afforded me during this dissertation. My committee: Drs. Farida Selim, Matt McCluskey, and Gary Collins, for their time and trouble in reviewing this manuscript. Each member played a crucial role in my development during my graduate career and it is fitting that they comprise my committee. My co-workers I worked with in this lab: Autumn Pratt, Jianfeng Ji, David Mackay, Sherif Reda, Mohammad Khamenchi, John Buscher, Frederick Chen, and Kimberly Heiner. I am grateful for their willingness to help and often do the more mundane tasks. I also thank Mike Rowe and Marianne Tarun for the measurements they conducted. There were countless members of the faculty and staff of the physics and astronomy department at Washington State University who have helped me along the way and I could thank every one of them for the unique way they have affected my graduate experience. I would like to thank Dr. Fred Gittes, for taking me on to work on the bloodstain pattern analysis research project I did under him and helping me prepare for the preliminary examination; Dr. Gordon Johnson, for helping me teach physics to others which in turn made me a more capable physicist; Dr. Yogendra Gupta, for iii developing me into the scientist I am today and helping me realize my goals; Tom Johnson, for all the assistance he provided and all the hours lost discussing Cougar basketball; Dr. Mark Kuzyk, for providing floor hockey as an escape from work and studies; Dr. Mike Allen, for being a good friend; and the wonderful ladies in the physics office, in particular Sabreen Dodson, Laura Krueger, and Mary Guenther, who kept me on track throughout my graduate career. I am grateful for the friends I made along the way while working toward my doctorate. Time spent with them seemed to make all the hard work worth it, especially when we could all commiserate together about struggles with research or classes. I wish to thank my wonderful girlfriend, Elora DeGreef, and her family for all the support and encouragement they have shown me for as long as I have known them. Elora is my love and inspiration, and it is to her that this thesis is dedicated. This work was supported by the National Science Foundation, who funded me and my work under Grant No. DMR 10-06772. iv STUDIES OF TRAPPING AND LUMINESCENCE PHENOMENA IN YTTRIUM ALUMINUM GARNETS Abstract by Christopher Richard Varney, Ph.D. Washington State University December 2012 Chair: Farida A. Selim Rare-earth-doped yttrium aluminum garnet (YAG) crystals are important photonic materials with a wide range of applications. The optical properties and performance of these crystals are largely governed by exciton dynamics, which is greatly affected by the presence of defects. In this work, the optical and scintillation properties of undoped and rare-earth-doped YAG crystals were studied in conjunction with thorough characterization of defects. First, a comprehensive study of optical properties, including absorption and luminescence, was carried out. Color centers were found to be present in both undoped and doped crystals. A new x-ray luminescence spectrometer was developed and installed to investigate both luminescence and scintillation properties in a new way. Defects that trap excitons were studied by thermoluminescence spectroscopy. Deep and shallow traps were identified by high and low thermoluminescence measurements and their energy levels in the band gap were calculated. Positron annihilation lifetime spectroscopy was carried out for the first time on YAG to provide v information about defect types, structures, and concentrations. The positron measurements revealed the presence of isolated aluminum vacancies and defect complexes of aluminum and oxygen vacancies. They also revealed the dependence of defect structure on growth atmosphere and post-growth treatments. This knowledge gained from thermoluminescence and positron lifetime measurements elucidated the effects various defects have on scintillation properties and suggested ways to control them. Lastly, a new fast scintillator with good energy resolution is discussed. vi TABLE OF CONTENTS ACKNOWLEDGEMENTS…………………………………………………………...iii-iv ABSTRACT……………………………………………………………………………v-vi TABLE OF CONTENTS……………………………………………………………vii-viii LIST OF TABLES……………………………………………………………………..…ix LIST OF FIGURES……………………………………………………………………...xii CHAPTER 1. INTRODUCTION……………………….………………………………………..1 1.1. REFERENCES….……………………………………………………………8 2. EXPERIMENTAL METHODS……………………………...…………………..15 2.1. SAMPLES…………………………………...……………………………...15 2.2. NEWLY DEVELOPED EXPERIMENTAL TECHNIQUE TO STUDY LUMINESCENCE…………………………....…………………………….21 2.3. REFERENCES……………………………………………………………...23 3. OPTICAL PROPERTIES OF YAG…………………………………………..…24 3.1. ABSORPTION……………………………………………………………...25 3.1.1. RESULTS……………………………………………………………...29 3.2. PHOTOLUMINESCENCE…………………………………………………42 3.2.1. RESULTS……………………………………………………………...46 3.3. COLOR CENTERS…………………………………………………………66 3.3.1. RESULTS……………………………………………………………...73 3.4. RADIOLUMINESCENCE.…………………………………………………83 vii 3.4.1. RESULTS……………………………………………………………...87 3.5. REFERENCES…………………………………………………………….102 4. DEFECT CHARACTERIZATION BY TRAP LEVEL MEASUREMENTS....117 4.1. THEORY…………………..….…………………………………………...123 4.2. RESULTS………………………………………………………………….135 4.3. REFERENCES…………………………………………………………….165 5. DEFECT IDENTIFICATION BY POSITRON LIFETIME MEASUREMENTS…………………………………………………………….170 5.1. METHODS………………………………………………………………...174 5.2. RESULTS………………………………………………………………….185 5.3. REFERENCES…………………………………………………………….210 6. SCINTILLATION PROPERTIES……………………………………………...216 6.1. METHODS………………………………………………………………...218 6.2. RESULTS………………………………………………………………….224 6.3. REFERENCES…………………………………………………………….249 7. CONCLUSION…………………………………………………………………253 8. APPENDIX A…………………………………………………………………..258 viii LIST OF TABLES 2.1.1 List of samples studied in this thesis…………..…...………………………………17 4.2.1 Thermoluminescence activation energies of Ar-grown undoped YAG (integration range: 340-570 nm)……..………………………………………………………………151 4.2.2 Thermoluminescence activation energies of Ar-grown undoped YAG (integration range: 570-800 nm)…………………….…………………….…………………………151 4.2.3 Thermoluminescence activation energies of O 2-grown undoped YAG (integration range: 340-570 nm)……………………………………………………………………..152 4.2.4 Thermoluminescence activation energies of O 2-grown undoped YAG (integration range: 570-800 nm)……………………………………………………………………..153 4.2.5 Thermoluminescence activation energies of Ce:YAG 0.1%, 0.15%, and 0.2%.....162 4.2.6 Thermoluminescence activation energies of Ce:YAG 0.2% calculated by the initial rise and corrected initial rise methods………………………………………………….162 4.2.7 Thermoluminescence activation energies of Ce:YAG 0.3% ………………….....163 4.2.8 Thermoluminescence activation energies of Ce:YAG 0.14%................................163 5.1.1 Summary of equipment settings used for PALS experiments……………………178 5.2.1 Positron lifetimes in polished Ar-grown undoped YAG 5x5x1………………….186 5.2.2 Positron lifetimes in unpolished (fine ground) Ar-grown undoped YAG 10x10x1…………………………………………………………………………………186 5.2.3 Positron lifetimes in unpolished (fine ground) Ar-grown undoped YAG 5x5x1...186 5.2.4 Positron lifetimes in H 2-grown undoped YAG 10 mm dia. x ~1 mm……………189 5.2.5 Positron lifetimes in O 2-grown undoped YAG 10 mm dia. x ~1 mm……………189 ix 5.2.6 Positron lifetimes in Ce:YAG 0.1%........................................................................196 5.2.7 Positron lifetimes in Ce:YAG 0.15%......................................................................199 5.2.8 Positron lifetimes in Ce:YAG 0.2%........................................................................200 5.2.9 Positron lifetimes in Ce:YAG 0.14% 10 mm dia. x ~1 mm……………………...202 5.2.10 Positron lifetimes in polished Ce:YAG 0.3%.......................................................204 5.2.11 Positron lifetimes in unpolished (fine ground) Ce:YAG 0.3%.............................204 5.2.12 Positron lifetimes in Nd:YAG 1%........................................................................206 5.2.13 Positron lifetimes in Tm:YAG 0.8%....................................................................206 5.2.14 Positron lifetimes in Yb:YAG 5%........................................................................208
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