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OPTICAL and DEFECT STUDIES of WIDE BAND GAP MATERIALS By OPTICAL AND DEFECT STUDIES OF WIDE BAND GAP MATERIALS By CHARLES RICHARD SHAWLEY A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY WASHINGTON STATE UNIVERSITY School of Mechanical and Materials Engineering DECEMBER 2008 © Copyright by CHARLES RICHARD SHAWLEY, 2008 All Rights Reserved © Copyright by CHARLES RICHARD SHAWLEY, 2008 All Rights Reserved To the Faculty of Washington State University The members of the Committee appointed to examine the dissertation of CHARLES RICHARD SHAWLEYfind it satisfactory and recommend that it be accepted. ______________________________ Kelvin Lynn, Chair ______________________________ M. Grant Norton ______________________________ David Field ______________________________ Matthew McCluskey ii ACKNOWLEDGMENT The author is truly indebted to Dr. Kelvin G. Lynn for his invaluable assistance and guidance and for sponsoring this work. Many thanks are due to all members of the Center for Materials Research at Washington State University. Also, much gratitude is extended to Dr. M.G. Norton, Dr. D.P. Field, and Dr. M.D. McCluskey for their guidance and evaluation. Several of the photographs and plots used in this dissertation were provided by graduate students Denys Solodovnikov and Jalal Nawash as well as by Dr. Marc Weber. Their help is greatly appreciated. Many thanks are extended to Dr. Jim Elliston at the WSU Radiation Center for their help with the neutron activation analysis measurements The majority of this work was performed under the auspices of the Department of Defense, Joint Electromagnetics Technology Program Office, under the Assistant Secretary of Defense for Command, Control, Communications, and Intelligence (ASD/C3I), under contract N66001-00-C-6008. This work was supported under a subcontract from VLOC Inc. The author acknowledges and appreciates the discussions with the other YAG and YVO4 team members at VLOC and at the Advanced Materials Development Center (AMDC) at II-VI Inc. A portion of the photoluminescence research described in this paper was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Many thanks are extended to Dr. Zheming Wang for his assistance with the photoluminescence measurements. iii OPTICAL AND DEFECT STUDIES OF WIDE BAND GAP MATERIALS Abstract by Charles Richard Shawley, Ph.D. Washington State University December 2008 Chair: Kelvin G. Lynn Non-destructive, optical techniques can be used to find and identify defects in solid-state laser materials that can inhibit the material’s laser performance. With accurate detection and identification, these defects can be eliminated or their effects minimized through an array of methods. Two materials, YVO4 and YAG (Y3Al5O12) are studied, and their most common and problematic defects are optically identified. The presence of grain boundaries in a-axis grown YVO4 and their absence in c-axis grown YVO4 as well as the ability of c-axis grown YVO4 to accommodate a higher concentration level of Nd3+ ion suggest that the change of growth-axis direction from a-axis to c-axis may improve optical quality and laser performance of Nd:YVO4 laser elements. In Nd:YAG, the improvement in laser performance through aluminum diffusion and oxidizing heat treatment may point to the improvement in stoichiometry by the elimination of both oxygen and aluminum vacancies. Also, the multiple valence state impurity ion, iridium, is identified in YAG through optical measurement techniques and neutron activation analysis, and the minimization of its affect on laser performance through a process of aluminum diffusion and oxidation heat treatment is discussed. iv TABLE OF CONTENTS Page ACKNOWLEDGMENT....................................................................................................... iii ABSTRACT.......................................................................................................................... iv LIST OF TABLES................................................................................................................. x LIST OF FIGURES............................................................................................................... .xiii CHAPTER 1. INTRODUCTION AND BACKGROUND....................................................... 1 1.1 History of Laser Development........................................................... 1 1.2 Laser Operation..................................................................................3 2. CRYSTAL GROWTH OF SOLID-STATE LASER MATERIALS.................. 9 2.1 Czochralski Crystal Growth.............................................................. 9 2.2 Growth Issues for YVO4 and YAG................................................... 16 2.2.1 Pulling Rates......................................................................... 16 2.2.2 Crucible Material.................................................................. 17 2.2.3 Growth Atmosphere.............................................................. 17 2.2.4 The Addition of Dopants...................................................... 19 2.2.5 The Crystal/Melt Interface................................................... 21 2.2.6 Heating and Cooling............................................................. 22 2.2.7 Non-Stoichiometry and Defects........................................... 22 3. LITERATURE REVIEW OF YTTRIUM ORTHOVANADATE...................... 30 3.1 The Growth of YVO4......................................................................... 30 3.2 Physical Properties of YVO4............................................................. 36 v 3.3 Effects of Increasing Nd3+ Ion Concentration................................... 44 3.4 Thermal Issues for Nd:YVO4 Laser Crystals.................................... 50 3.5 Length, Doping, and Cut Direction Considerations for Nd:YVO4 Laser Elements................................................................. 53 3.6 Major Defects in Nd:YVO4.............................................................. 58 3.6.1 Non-Radiative Processes in Nd:YVO4................................ 60 3.6.2 Inclusions and Non-Stoichiometry in Nd:YVO4................. 61 3.6.3 Silicon Impurities in Nd:YVO4............................................ 66 4. DEFECT STUDIES OF YTTRIUM ORTHOVANADATE.............................. 73 4.1 Experimental Motivation................................................................... 73 4.2 Dependence of Segregation Coefficient on Growth Axis................. 74 4.3 The Laser Cavity and Cooling Unit for Laser Power Measurements......................................................................... 75 4.4 Oxygen Vacancy Studies of Nd:YVO4............................................. 78 4.4.1 Solarization Measurements of Undoped YVO4................... 78 4.4.2 Fluorescence and Absorption Studies of Undoped YVO4.................................................................... 80 4.5 Dislocations, Grain Boundaries, and Polygonization in Nd:YVO4....................................................................................... 83 4.5.1 Cross-Polarization Studies of Nd:YVO4.............................. 85 4.5.2 Laser Power Studies of a- and c-axis Grown Nd:YVO4...... 88 4.6 Measurements of Hydroxyl Impurities in Nd:YVO4........................ 90 4.7 Annealing Considerations in Nd:YVO4............................................ 92 4.8 Summary for Improving the Laser Performance of Nd:YVO4...................................................................................... 94 4.9 Suggestions for Future Work............................................................. 95 vi 5. LITERATURE REVIEW OF YTTRIUM ALUMINUM GARNET................. 99 5.1 The Growth of YAG......................................................................... 99 5.2 Physical Properties of YAG.............................................................. 100 5.3 Laser Performance of Nd:YAG........................................................ 107 5.4 Temperature Effects in Nd:YAG...................................................... 109 5.5 Thermal Lensing and Thermal Fracture............................................ 112 5.6 Major Defects in Nd:YAG................................................................ 114 5.6.1 Concentration Quenching and Non-Radiative Relaxations............................................................................ 115 5.6.2 Stress and Strain Issues in Nd:YAG.................................... 121 5.6.3 Color Centers in Nd:YAG.................................................... 125 5.6.3.1 The F-centers in Nd:YAG........................................ 128 -- 5.6.3.2 The Oh -center in Nd:YAG...................................... 131 5.6.3.3 Exciton Defects in Nd:YAG.................................... 132 5.6.3.4 Hydroxyl Defects in Nd:YAG................................. 133 5.6.3.5 Chromium Impurities in Nd:YAG........................... 135 5.6.3.6 Cerium Impurities in Nd:YAG................................. 136 5.6.3.7 Erbium Impurities in Nd:YAG................................. 141 5.6.3.8 The Antisite Defect in Nd:YAG............................... 142 5.6.3.9 Other Minor Impurities in Nd:YAG......................... 145 5.6.3.10 Iron and Iridium Impurities in Nd:YAG................ 149 5.6.3.11 Properties of the 256nm Absorption Peak.............. 152 5.7 Improving
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