Growth and Behaviors of InN/GaN Multiple Quantum Wells by Plasma-Assisted Molecular Beam Epitaxy

Fully realizing the potential of InGaN semiconductors requires high quality materials with arbitrary In-content. To this date the growth of In-rich InGaN films is still challenging since it suffers from the low growth temperatures and many detrimental alloying problems. InN/GaN multiple quantum wells (MQWs) and superlattices (SLs) are expected to be promising alternatives to random InGaN alloys since in principle they can achieve the equivalent band gap of InGaN random alloys with arbitrarily high In-content and at the same time bypass many growth difficulties. This dissertation focuses on studying the growth mechanisms, structural properties and energy structures of InN/GaN MQWs. Molecular beam epitaxy (MBE) growth of InN/GaN MQWs were carried out at 550 C and 680 C, which are close to the low and high ends of the allowed growth temperature window. Reflection high energy electron diffraction (RHEED) was demonstrated to be a valuable tool for understanding the MQW growth. By associating the RHEED intensity transient features with surface atomic processes such as the adsorption/desorption of metal species, the growth process was successfully monitored in situ. Also, at the high growth temperature, RHEED was successfully used to study the adsorption/desorption kinetics of indium surface coverage to gain knowledge of how to control InN deposition. The MQW growth at 680 C show that indium surface coverage over 2 MLs before GaN capping is a key factor for consistent quantum well formation. The consistent PL emissions at ~375 nm were attributed to the insertion of 1-ML thick QWs. At 550 C, both PL emission and QW thickness showed a selfregulating behavior. The redshift of PL emissions with the InN deposition saturated at ~423 nm while the QW apparent thickness were no more than 2 MLs. The residual indium accumulation identified by RHEED suggests that QWs are generally InGaN layers instead of coherent InN layers, which is supported by k.p calculations. Finally, a growth mechanism was proposed to explain the preservation, structural and optical properties of the quantum wells. Acknowledgements First and foremost, I would like to thank my Ph.D. advisor, Dr. Salamo, for accepting me in his amazing research group and supporting my study for all these years. He is a knowledgeable scientist, great team leader, and fantastic educator. Working with him made the long hard Ph.D. journey much less painful. His passion for science and professional attitude set a high bar for being a scientist and inspired me in many ways. I would like to thank Dr. Morgan Ware for being a great mentor and friend. He taught me how to play with the nitride MBE machine. This dissertation would be impossible without his help and encouragement. I am always impressed by how he handles instruments with confidence. I am also impressed by his willingness to help and his patience to listen. I am deeply grateful to my comrade, Dr. Yuriy Mazur. He is the kind of person who makes things happen, in a lot of cases by pushing others. Without him this dissertation would take much longer to finish. I would also like to thank him for caring about my future career. I would like to thank Dr. Andrian Kuchuk for the XRD measurements of my samples, his “secret list” of useful publications, and many fruitful discussions. I would like to thank Yurii for his quality work of PL and TEM investigations on my samples. I would also like to thank him for the discussion of k.p calculations. I would like to thank Dr. Zhiming Wang for his trust and mentoring. He introduced me into Dr. Salamo’s group and guided me into research quickly. I would like to thank Dr. Zhaoquan Zeng for taking care of me in my early years of Ph.D. study. He is a good friend and mentor of me. I would like to thank Dr. Vasyl Kunets for his coaching in the electrical characterization lab. I would like to thank Dr. Mourad Benamara for his technical support of TEM measurements and also for keeping the coffee club running. I would like to thank David Monk for keeping the MBE lab running smoothly. I like to thank Rick, Ken, Renee for running the amazing graduate program, MicroEP. I would like to thank the current and former colleagues in our research groups, Tim, Xian, Yang, Thomas, Paul, Sabina, Pijush, Satish, Paul, Rob, Collin, Mike, Yusuke, Chris, Vitaliy, Jiang, Mohammad, Qigeng, Najala, Samir, Hryhorii, and Samir for their help during my Ph.D. study. This work was supported by the USA National Science Foundation Engineering Research Center for Power Optimization of Electro Thermal Systems (POETS) with cooperative agreement EEC-1449548. Finally, my deepest thanks go to my family. I would like to thank my parents for their unconditional love and support. I would like to thank my wife, a beautiful woman who has always been loving me and supporting me. She brought my sons Matthew and Isaac to this world and makes our tiny rented apartment look like a home through tireless work. This work could not have been done without you. Table of contents Chapter 1: Introduction . 1 1.1. Background . 1 1.2. Basic material properties . 2 1.3. The historical development of nitride materials. 8 1.4. Growth of InN and InGaN . 13 1.5. The InN/GaN quantum wells . 19 Chapter 2: Research methodology . 23 2.1. Background of MBE . 23 2.2. MBE system . 24 2.3. RHEED fundamentals . 27 2.4. III-nitride growth by PAMBE . 30 2.5. The eight-band k.p method . 35 2.5. 1. Bulk materials . 35 2.5. 2. Heterostructures . 39 Chapter 3: InN/GaN MQW growth at 680 ○C . 44 3.