INTEGRATION OF INDIUM GALLIUM NITRIDE WITH NANOSTRUCTURES ON SILICON SUBSTRATES FOR POTENTIAL PHOTOVOLTAIC APPLICATIONS HO JIAN WEI NATIONAL UNIVERSITY OF SINGAPORE 2014 INTEGRATION OF INDIUM GALLIUM NITRIDE WITH NANOSTRUCTURES ON SILICON SUBSTRATES FOR POTENTIAL PHOTOVOLTAIC APPLICATIONS HO JIAN WEI (B.Eng.(1st Hons), NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY NUS GRADUATE SCHOOL FOR INTEGRATIVE SCIENCES AND ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2014 ACKNOWLEDGEMENTS There are many people who have given me invaluable aid in the course of my Ph.D. journey and made this much more palatable. I would like to take this opportunity to express my sincerest gratitude to them. First, I would like to thank my supervisors, Professor Chua Soo-Jin and Professor Andrew Tay, for their guidance, encouragement and support which were instrumental in making this work possible. I have gained much from the fruitful discussions I had with them, not only within the realms of my research work, but also in terms of personal development. They have provided many opportunities in enhancing both the depth and breadth of my research. I also greatly appreciate the help from the other members of my Thesis Advisory Committee (TAC). Professor Choi Wee Kiong, who is the TAC Chairman, has provided a much needed perspective and played a significant role in steering my research direction. I am truly humbled by his attitude towards life. Dr Zang Keyan has imparted valuable knowledge on MOCVD to me, supported my research and shared her experience in navigating research life. Dr Liu Hong Fei has inspired me greatly in my work. His dedication to research and academic finesse is admirable. I benefitted greatly from the many technical discussions I had with him. Next, I would like to thank the staff at the Center of Optoelectronics (COE) in NUS, namely, Ms Musni Bte Hussain and Mr Tan Beng Hwee, for their help in administrative matters. I also greatly appreciate the friendship and support of my fellow students in COE. Special mention goes to Dr Wee Qixun who has mentored me and taught me much about the growth and characterization of III-nitrides. ii I am grateful for the opportunity to perform part of my research work at the Institute of Materials Research and Engineering (IMRE), A*STAR and would like to thank many of the IMRE staff who have helped me in the training and operation of equipment there. This includes Mr Jarrett Dumond, Dr Tanu Suryadi Kustandi, Dr Liu Hong, Ms Tan Hui Ru, Ms Teo Siew Lang, Ms Doreen Lai, Mr Lim Poh Chong and Mr Eric Tang. I am also indebted to my ex-colleagues and ex-laboratory mates at Lab 10 who have provided much needed support in the course of my work. In addition, I would also like to acknowledge the help from the Singapore-MIT Alliance of Research and Technology (SMART) for providing me temporary access to its high-resolution X-ray diffraction (HR-XRD) equipment. I would like to thank Dr Abdul Kadir and Dr Kohen David Alexandre for operation and meaningful discussions of the machine. Next, I would like to thank Dr Michael Heuken from AIXTRON SE for providing me substrates for MOCVD growth. I am immensely grateful to the NUS Graduate School for Integrative Sciences and Engineering (NGS) for providing me with a Scholarship and support for this Ph.D. work. NGS and her staff have been extremely helpful in ensuring the well-being of students. I truly appreciate their support. Last but not least, I would like to thank my family, fiancée and friends for their love, unwavering support and understanding while I was both physically and/or mentally absent during my Ph.D. journey. iii TABLE OF CONTENTS DECLARATION ......................................................................................................... i ACKNOWLEDGEMENTS ....................................................................................... ii TABLE OF CONTENTS .......................................................................................... iv SUMMARY ................................................................................................................ ix LIST OF TABLES ..................................................................................................... xi LIST OF FIGURES .................................................................................................. xii LIST OF SYMBOLS ............................................................................................. xxiii Chapter 1 Introduction............................................................................................... 1 1.1. Current Status of Photovoltaics (PV) for Solar Energy Harvesting ..................... 1 1.2. Motivation for Integration of InGaN with Nanostructures on Si in PV ............... 3 1.2.1. Advantages of InGaN for PV Applications ............................................... 3 1.2.2. Merits of Si as a Growth Substrate for InGaN PV Applications ............... 5 1.2.3. Potential and Technical Barriers of InGaN Solar Cells ............................. 8 1.2.4. Relevance of Nanostructuring and its Benefits ....................................... 10 1.2.4.1. Nano Selective Area Growth (Nano-SAG or Scheme A) ......... 11 1.2.4.2. Nanoheteroepitaxy on Nanopillar Substrates (Scheme B) ........ 12 1.2.4.3. Benefits of Nanostructures ........................................................ 13 1.2.4.4. Plausible InGaN/Si Tandem PV Device Structures................... 19 1.3. Scope and Thesis Organization .......................................................................... 23 Chapter 2 Background and Review of InGaN Growth ......................................... 25 2.1. Introduction ........................................................................................................ 25 2.2. Structure and Characteristics of Group III-Nitrides............................................ 25 2.3. Challenges in InGaN Growth and their Conventional Mitigation ...................... 30 2.3.1. Gallium Meltback Etching and Unintentional Nitridation of Silicon ...... 30 2.3.2. Thermal Expansion and Lattice Mismatch .............................................. 31 2.3.3. Composition Inhomogeneity and Phase Separation ................................ 34 2.3.4. Temperature Tradeoff Between Good Structural Quality and High Indium Content ........................................................................................ 36 2.4. Novel Growth Strategies ..................................................................................... 39 iv 2.4.1. Development of New Growth Methods .................................................. 39 2.4.2. In-situ Silicon Nitride Masking ............................................................... 40 2.4.3. Selective Area Growth ............................................................................ 41 2.4.4. Epitaxial Lateral Overgrowth (ELO) ....................................................... 44 2.4.5. Nanostructured Growth ........................................................................... 45 2.4.5.1. Non-templated Nanostructure Growth ...................................... 46 2.4.5.2. Templated Nanostructure Growth ............................................. 49 2.4.5.2.1. Nano Selective Area Growth (Nano-SAG or Scheme A) .... 49 2.4.5.2.2. Nanoheteroepitaxy on Nanopillar Substrates (Scheme B) ... 53 2.5. Chapter Summary ............................................................................................... 55 Chapter 3 Experimental Methods: Patterning, Growth & Characterization ..... 57 3.1. Introduction ........................................................................................................ 57 3.2. Nanoimprint Lithography ................................................................................... 57 3.2.1. Background ............................................................................................. 57 3.2.2. Step and FlashTM Imprint Lithography (S-FILTM) ................................... 58 3.3. Metalorganic Chemical Vapour Deposition (MOCVD) ..................................... 60 3.3.1. Background ............................................................................................. 60 3.3.2. EMCORE/Veeco D125 MOCVD System............................................... 61 3.3.3. Thermodynamics Consideration .............................................................. 66 3.3.4. Kinetics Considerations ........................................................................... 67 3.3.5. Hydrodynamics and Mass Transport ....................................................... 68 3.4. Characterization Techniques .............................................................................. 69 3.4.1. Scanning Electron Microscopy (SEM) .................................................... 69 3.4.2. Atomic Force Microscopy (AFM)........................................................... 72 3.4.3. Transmission Electron Microscopy (TEM) ............................................. 74 3.4.4. X-ray Diffraction (XRD) ......................................................................... 77 3.4.5. Photoluminescence (PL) Spectroscopy ................................................... 83 3.4.6. Reflectance Spectroscopy ........................................................................ 86 3.5. Chapter Summary ............................................................................................... 88 Chapter 4 Nanopatterning Techniques on Si Substrates .....................................