SURFACE and BULK DEFECTS in CADMIUM ZINC TELLURIDE and CADMIUM MANGANESE TELLURIDE CRYSTALS by Oluseyi Stephen Babalola Disserta
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SURFACE AND BULK DEFECTS IN CADMIUM ZINC TELLURIDE AND CADMIUM MANGANESE TELLURIDE CRYSTALS By Oluseyi Stephen Babalola Dissertation Submitted to the Faculty of the Graduate School of Vanderbilt University In partial fulfillment of the requirements For the degree of DOCTOR OF PHILOSOPHY In Interdisciplinary Materials Science December, 2009 Nashville, TN Approved: Professor Leonard C. Feldman Professor Arnold Burger Professor Timothy Hanusa Professor Jim Wittig Professor Bridget Rogers Doctor Ralph James ©Copyright by Oluseyi Stephen Babalola December, 2009. ii Ope ni fun Olodumare To Mary, Tolu, eleven siblings iii ACKNOWLEDGEMENT I owe my deepest gratitude to Professor Leonard Feldman, my advisor and chair of my Ph.D. committee for his encouragement, supervision and support from my early years in Vanderbilt through my preliminary and qualifying examinations to the concluding level of this thesis. I am very grateful to Professor Arnold Burger, my co-advisor, for mentoring and supporting me through my advanced degree programs. I am also grateful to Doctor Ralph James for providing me with the opportunity to learn and do research with the brightest minds and world-class equipment. I specifically thank these three aforementioned mentors for creating the atypical inter-institutional collaboration involving Vanderbilt University, Fisk University and Brookhaven National Laboratory towards advancing the science of room temperature nuclear detector materials, and in so doing, expanding my horizon and providing me with an extensive list of scientific equipment and the golden opportunity of working with the best scientists in this field. I would also like to acknowledge the other members of my committee; Professor Timothy Hanusa, Professor Bridget Rogers and Professor James Wittig for their tutoring and availability to help. I especially thank Prof. James Wittig for providing me with teaching assistantship during my early years at Vanderbilt. I especially thank my collaborators, colleagues and friends, who have worked countless hours with me, and in so doing, shared laboratories, equipment, office spaces, lecture rooms and stories. I am grateful to Eugene, who introduced me to the iv Materials Physics laboratories and the Van-De-Graff accelerator; Dr. Anthony Hmelo, who introduced me to, and helped me in, the VINSE clean room; Mike Groza, who shared his valuable knowledge of CZT detector fabrication and testing, Vladimir Buliga and Constantin Coca, who helped with detector fabrication steps of photolithography and gold sputtering, and to all other scientists at Fisk University’s Advanced Materials Science and Applications group. I am grateful to Dr. Stephen Egarievwe for his mentorship and support. I express my gratitude to Dr. Aleksey Bolotnikov for introducing me to and tutoring me on the Interactive Data Language (IDL) as well as his mentorship in other research areas. I also thank the entire Detector testing group at Nonproliferation and National Security Department of Brookhaven National Laboratory; Anwar, Giuseppe, Ge, Yonggang, Kim, Rubi and my fellow students. I am very grateful to Shirley Kendall and Rosa Palmore of the Diversity office of BNL for their continued help and support. I thank Shola, Yemi, Funmi and Toyin as well as daddy and mommy for their unwavering and continued camaraderie. Finally, I would like to thank my loves, Mary and Tolu, who help give meaning to all of these. Thank you all. To God be the glory. Oluseyi Stephen Babalola. v TABLE OF CONTENTS Page DEDICATION . iii ACKNOWLEDGEMENTS . iv LIST OF FIGURES . x LIST OF TABLES . .xiv Introduction . 1 PART ONE: A REVIEW . 5 Chapter I History and Motivation . 6 Overview . 6 1.1 History of Nuclear Detectors . 6 1.1.1 Early Radiation Detectors . 7 1.1.2 Recent detectors: Semiconductors . .10 1.2 Motivation for CZT Research . .13 1.3 Present Status and Challenges . 17 1.4 CdMnTe, a new material for room temperature nuclear detector applications . 19 References . 21 II CZT detector properties, fabrication and operation . 25 Overview . .25 2.1 CZT properties . 26 vi 2.1.1 Crystal Structure . 26 2.1.2 Resistivity . 28 2.1.3 Charge Transport Properties: . 29 2.2 CZT crystal growth and detector fabrication . 30 2.2.1 Crystal growth . .30 2.3 CZT detector fabrication . 33 2.4 CZT Device operation . 34 2.5 Nuclear Radiation . .35 2.6 Radiation detection . .36 2.6.1 Photoelectric absorption . 36 2.6.2 Compton scattering . 38 2.6.3 Pair Production . 39 2.7 Operational Characteristics of Semiconductors . 39 2.7.1 Band Structure . 39 2.7.2 Charge Carriers . 40 2.7.3 Impurities and dopants . 41 2.7.4 Trapping and recombination . .42 2.7.5 Electrical contacts . 43 2.7.6 Leakage current . 43 2.7.7 Reverse biasing . 44 References . 48 PART TWO: ORIGINAL STUDIES . 51 III Studies of Surface Condition . .52 Overview . 52 3.1 Motivation . 53 vii 3.2 Experiment . 55 3.3 Results and discussion . 57 3.3.1 Micro-characterization of surface roughness . .57 3.3.2 Effect of surface roughness on leakage current . 59 3.3.3 Effect of surface roughness on leakage current . 60 3.3.4 Rough surface as cathode . .63 3.3.5 Rough surface as anode . 66 3.4 Conclusion . 68 References . 69 IV Chemical etching effects on CZT and CMT nuclear detectors . 72 Overview . 72 4.1 Motivation . 73 4.2 Process of chemical etching . 74 4.3 Experiment . 76 4.4 Results and discussion . 77 4.5 Conclusion . 84 References . 85 V Study of bulk defects in CZT and CMT crystals . 88 Overview . 88 5.1 Motivation . 89 5.2 Defects . ..