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Study of Impact Excitation Processes in Boron Nitride for Deep Ultra-Violet

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Study of Impact Excitation Processes in Boron Nitride for Deep Ultra-Violet Study of Impact Excitation Processes in Boron Nitride for Deep Ultra-Violet Electroluminescence Photonic Devices A thesis presented to the faculty of the Russ College of Engineering and Technology of Ohio University In partial fulfillment of the requirements for the degree Master of Science Thushan E. Wickramasinghe August 2019 © 2019 Thushan E. Wickramasinghe. All Rights Reserved. 2 This thesis titled Study of Impact Excitation Processes in Boron Nitride for Deep Ultra-Violet Electroluminescence Photonic Devices by THUSHAN E. WICKRAMASINGHE has been approved for the School of Electrical Engineering and Computer Science and the Russ College of Engineering and Technology by Wojciech M. Jadwisienczak Associate Professor of Electrical Engineering and Computer Science Mei Wei Dean, Russ College of Engineering and Technology 3 ABSTRACT THUSHAN E. WICKRAMASINGHE, M.S., August 2019, Electrical Engineering and Computer Science Study of Impact Excitation Processes in Boron Nitride for Deep Ultra-Violet Electroluminescence Photonic Devices Director of Thesis: Wojciech M. Jadwisienczak Studies and contemporary technology have shown the feasibility of developing direct current (dc) driven III-nitride deep ultra-violet (UV) photonic devices through band gap engineering of epitaxially grown hetero-structures. Alternatively, one can consider developing deep ultraviolet (UV-C) light sources operating on the principles of hot electrons impact excitation processes in a boron nitride (BN) phosphor. It was shown that high quality BN nanosheets (BNNSs) can generate excitonic emission at 225 nm under electron excitation of 6 kV and thus can be considered as a potential material for developing alternating current (ac) driven thin electroluminescence (ACTEL) devices. In this work we consider a theoretical approach based on the Bringuier model [J. Appl. Phys. 70, 8 (1991), pp. 4505-4512.] for generating luminescence in the UV-C region from hexagonal BN (h-BN) through impact excitation under a high electric field. Applying the Lucky Drift Model and Born approximation to high field electronic transport in h-BN we took into account ballistic and drift mode models to optimize a prospective device performance. The original model concerning Mn luminescent centers embedded in a ZnS host was adopted for an un-doped h-BN host. We used the lucky drift approach to study the probability of primary electrons encountering a collision within the 4 lattice and thereby arrive at an efficiency of secondary electrons being excited to generate the desired near band edge (NBE) transmissions. It was found that in ACTEL device biased at 8.5 × 105 푉푐푚−1 a primary electron encountering an impact excitation would travel ~20 μm in a single h-BN layer before gaining sufficient kinetic energy to undergo a second collision which significantly reduces the device efficiency. Furthermore, we have also considered the efficiency of electroluminescence (EL) in h-BN by using the impact excitation rate theory developed by Neumark [Phys. Rev. 116, 6, (1959), pp. 1425-1432.] for a ZnS lattice. While our model has good agreement with the literature on ZnS based ACTEL devices (i.e. 17(푉푏⁄푉0)% where Vb is the barrier voltage of the of the device and V0 is the voltage drop the electrons pass through as defined by Neumark), we found that the EL efficiency for h-BN is much lower 0.3(푉푏⁄푉0)%. Using an estimate for 푉푏⁄푉0 at a 110 V applied voltage we found the external efficiency of the h-BN to be 0.04%. Finally, we have simulated the ACTEL device’s efficiency by considering different h-BN layer thickness and the applied field in order to optimize the device. 5 DEDICATION To Fatherhood! 6 ACKNOWLEDGMENTS First and foremost, I would like to thank my life coaches, my parents, for guiding me to where I am today. Their love and caring support helped me at every step of my life and has allowed me to prepare to face whatever challenges that may come my way. Second, I would like to thank my loving wife, thank you for sticking by me through all the tough times and believing in me. I know it can’t be easy to put up with my long nights, but you continue to be a pillar of strength that I depend on time and time again. I also want to thank my best mate, my brother, for his motivation and moral support. I am ever grateful to my thesis advisor, Prof. Wojciech Jadwisienczak. You are a great mentor and have led me through to great success. I would also like to thank the rest of my committee members, Prof. Savas Kaya, and Prof. Jeffrey Dill, Prof. Justin Frantz. Thank you for your guidance and encouragement. A very special gratitude goes out to all other professors from the department of Electrical Engineering, including but not limited to, Prof. Starzyk and Dr. Rahman. I want to say a special thank you to my dear friends and colleagues, Kasun Amarasinghe, Tharindu, Paranathanthri, Chamika Hippola, Kosala Yapabandara, Perry Corbett, Tyler Danley, Nick Compton, and Ramana Thota. I have come to enjoy and depend on our conversations and discussions for clarity. I would be remised if I didn’t mention the wonderful Denise Cribben from the office of Electrical Engineering. I would be lost without you. And a special mention to the National Science Foundation for providing funding for my work. Last but by no means least, thank you to everyone from the Russ College of Engineering, it has been an eventful and exiting three years. 7 We would also like to express our gratitude towards Dr. Joshi, Dr. Joshipura of Sardar Patel University for the assistance provided in the calculation of the hexagonal BN target cross section which was pivotal in our calculation. 8 TABLE OF CONTENTS Page Abstract ............................................................................................................................... 3 Dedication ........................................................................................................................... 5 Acknowledgments............................................................................................................... 6 List of Tables .................................................................................................................... 10 List of Figures ................................................................................................................... 11 1 Introduction .................................................................................................................. 13 1.1 History of Ultraviolet Light ............................................................................ 16 1.2 Boron Nitride .................................................................................................. 18 1.3 Growth of Boron Nitride ................................................................................. 21 1.4 Deep UV ACTEL Devices .............................................................................. 24 1.5 Basics of luminescence ................................................................................... 28 1.5.1 Impact Ionization ...................................................................................... 29 1.5.2 Impact Excitation ...................................................................................... 29 1.6 Deep UV Light Emitting Devices ................................................................... 30 2 Experimental Method ................................................................................................... 33 2.1 The ZnS Model ............................................................................................... 33 3 Calcualtion .................................................................................................................... 41 3.1 Breakdown Voltage ........................................................................................ 41 3.2 The Lucky Drift Approach .............................................................................. 42 3.3 High Field Transport ....................................................................................... 46 3.4 The Cross Section of h-BN Target.................................................................. 47 3.5 The Impact Excitation Rate ............................................................................ 49 3.6 Efficiency of Electroluminescence ................................................................. 49 4 Results and Discussion ................................................................................................. 52 4.1 Comparison between h-BN and ZnS .............................................................. 52 4.2 Comparison between cubic BN and hexagonal BN ........................................ 53 4.3 Probability of Primary Electrons Reaching and Energy E from Rest ............. 54 5 Future Work ................................................................................................................. 64 References ......................................................................................................................... 65 Appendix A: Presentations and Awards ........................................................................... 82 9 Appendix B: Codes ........................................................................................................... 83 Permissions and Authorizations ........................................................................................ 93 10 LIST OF TABLES Page Table 1 Properties of hexagonal boron nitride ................................................................
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