Heterogeneous Integration of III-V Multijunction Solar Cells on Si Substrate: Cell Design & Modeling, Epitaxial Growth & Fabrication Nikhil Jain Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Electrical Engineering Mantu K. Hudait, Chair Luke F. Lester Marius K. Orlowski Jean J. Heremans Paul E. Plassmann March 27, 2015 Blacksburg, VA Keywords: III–V-on-Si solar cells, multijunction solar cells, heterogeneous integration, solar cell modeling, GaAs-on-Si epitaxy, Ge-on-Si epitaxy. Copyright 2015, Nikhil Jain Heterogeneous Integration of III-V Multijunction Solar Cells on Si Substrate: Cell Design & Modeling, Epitaxial Growth & Fabrication Nikhil Jain ABSTRACT Achieving high efficiency solar cells and concurrently driving down the cell cost has been among the key objectives for photovoltaic researchers to attain a lower levelized cost of energy (LCOE). While the performance of silicon (Si) based solar cells have almost saturated at an efficiency of ~25%, III-V compound semiconductor based solar cells have steadily shown performance improvement at approximately 1% (absolute) increase per year, with a recent record efficiency of 46%. However, the expensive cost has made it challenging for the high efficiency III-V solar cells to compete with the mainstream Si technology. Novel approaches to lower down the cost per watt for III-V solar cells will position them to be among the key contenders in the renewable energy sector. Integration of such high- efficiency III-V multijunction solar cells on significantly cheaper and large area Si substrate has the potential to address the future LCOE roadmaps by unifying the high-efficiency merits of III-V materials with low-cost and abundance of Si. However, the 4% lattice mismatch, thermal mismatch polar-on- nonpolar epitaxy makes the direct growth of GaAs on Si challenging, rendering the metamorphic cell sensitive to dislocations. The focus of this dissertation is to systematically investigate heterogeneously integrated III-V multijunction solar cells on Si substrate. Utilizing a combination of comprehensive solar cell modeling and experimental techniques, we seek to better understand the material properties and correlate them to improve the device performance, with simulation providing a very valuable feedback loop. Key technical design considerations and optimal performance projections are discussed for integrating metamorphic III-V multijunction solar cells on Si substrates for 1-sun and concentrated photovoltaics. Key factors limiting the “GaAs-on-Si” cell performance are identified, and novel approaches focused on minimizing threading dislocation density are discussed. Finally, we discuss a novel epitaxial growth path utilizing high-quality and thin epitaxial Ge layers directly grown on Si substrate to create virtual “Ge-on-Si” substrate for III-V-on-Si multijunction photovoltaics. With the plummeting price of Si solar cells accompanied with the tremendous headroom available for improving the III-V solar cell efficiencies, the future prospects for successful integration of III-V solar cell technology with Si substrate looks very promising to unlock an era of next generation of high-efficiency and low-cost photovoltaics. ii Dedication: 1. My always inspiring and loving parents (Monica and Saurabh Jain) and my grandparents (Usha & R.B. Jain, Rani & Jagdish Jain). 2. My caring and beautiful wife, Apoorva Vasan, my brother and sister-in-law (Vinay & Megha Jain). 3. To my future children - I wish they can find the same curiosity, passion and love for a field as I had the fortune to experience. 4. To all the amazing people who share the dream for a better, cleaner, safer and a more peaceful tomorrow. iii ACKNOWLEDGMENT As I start to write my PhD dissertation, I look back in time and I’m reminded about my truly amazing experience at Virginia Tech over the last four and a half years. The graduate school journey has not only given me a solid academic foundation, but has also prepared me to face challenges across all the facets of life. The ability to think critically, working in teams on collaborative projects and dealing with experimental failure on numerous occasions have taught me the important skills of perseverance and determination. The journey has been truly worthwhile, both at the professional and personal front. First and foremost, I would like to sincerely thank my adviser, Dr. Mantu Hudait for giving me the opportunity to pursue my PhD under his mentorship. We have been extremely fortunate as members of the ADSEL group to have Dr. Hudait available almost 24/7 to discuss our doubts and brainstorm new ideas. I am really thankful to him for giving me the freedom to explore and pursue new research directions. I would also like to thank the members on my committee: Prof. Luke Lester, Prof. Marius Orlowski, Prof. Jean Heremans and Prof. Paul Plassmann for their guidance and research direction. Joining the ADSEL group as the first student and seeing the group grow and advance has been exciting. The challenges and learning that came being among the first students in the group has been really satisfying. The experience gained during setting-up the MBE growth facility, developing a dislocation-dependent modeling capability, installing the I-V/QE equipment and developing cleanroom fabrication process from scratch has been really rewarding. I want to express my sincerest thanks to all the ADSEL group members – Yan Zhu, Michael Clavel, Patrick Goley, Jheng-Sin Liu and Peter Nguyen for their help and support during this research. I specially thank Michael Clavel for giving me company during wafer fabrication and later for being my MBE growth companion and for putting in endless hours towards the end of my PhD to make sure my work is finished on time. Also, a special thanks to Yan for being my growth mentor in the initial days and Patrick Goley for helping out with TEM. I would also like to extend my sincere thanks to the following people - 1) Don Leber for his enormous help during MBE chamber installation and for cleanroom training. 2) The team at Nanoscale Characterization and Fabrication Lab - Jerry Hunter, Stephen McCartney, Andrew Giordani and Christopher Winkler for help with material characterization studies. 3) The Crosslight team for all their help and support during this research. 4) The MBE team at Veeeco Instruments, Inc. - John, McDonough, Rich Bresnahan and Dana Johnson. iv 5) Andy Jackson at MBE control solution. 6) Kathy, Melanie, Kim, Cindy and Becky at the ECE department for all the administrative help. 7) A special thanks to Matthew Meitl and John Wilson at Semprius for being excellent mentors during my internship in Fall 2012. 8) Prof. K.C. Hsieh at University of Illinois at Urbana Champaign for instilling the passion for semiconductor research and Prof. J.J. Coleman for giving me the opportunity to conduct hands-on research during my undergrad. I gratefully acknowledge the funding support from the Institute for Critical Technology and Applied Sciences, Virginia Tech and Naval Air Warfare Center Weapons Division. I would also like to thank all my teachers, family and friends who have supported me all through these years and helped me grow into a better individual. Thanks to Yash, Rahul, Sagar, Harjas, Saransh, Sudharshan, Adrian, Pallavi, Karl, Natasha, Raman, Naureen, Jaspreet, Aditya, Ankit, Rikin, Arun, Akshay, Gurtej, Siddhant, Akrit, Harris, Teju and Sean and the Virginia Tech Cricket Club for allowing me to have a life in Blacksburg outside Whittemore Hall. My friends from high school at DPS Mathura Road and my undergrad friends at Manipal & UIUC also played an important role towards shaping my career. A special thanks to Kay Luttrel and the pathfinders group at Blacksburg Baptish Church for providing an extremely important dimension to my life during the time at Virginia Tech. Lastly, I would like to extend my sincere thanks to my entire family for their unconditional support, love and encouragement. During the PhD years at Virginia Tech year, my family lost a very dear and close member, my grandmother, Usha Jain. When I had first made the decision to pursue a PhD, she was the most delighted person in my family and always encouraged me to dream big. I thank my father, Saurabh Jain for being extremely supportive and for his guidance along every facet of life and my mother, Monica Jain for teaching me to be humble and content, but at the same be always hungry to seek improvement. A special thanks to my brother, Vinay Jain and his wife, Megha Jain for their love and support and for keeping the family knitted together. I am also very grateful to my wife, Apoorva and the new Vasan family. I thank Apoorva for being my pillar of support and strength and for all the countless sacrifices she’s made for our relationship. I have been fortunate to have her by my side as a constant source of encouragement. v TABLE OF CONTENTS Abstract ......................................................................................................................................................... ii Dedication.................................................................................................................................................... iii Acknowledgement ....................................................................................................................................... iv Table of Content .........................................................................................................................................