III-V METAMORPHIC MATERIALS AND DEVICES FOR MULTIJUNCTION SOLAR CELLS GROWN VIA MBE AND MOCVD Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Daniel Joseph Chmielewski, B.S., M.S. Graduate Program of Electrical and Computer Engineering The Ohio State University 2018 Dissertation Committee: Professor Steven A. Ringel, Advisor Professor Tyler J. Grassman Professor Sanjay Krishna Professor Lei Raymond Cao Copyright by Daniel Joseph Chmielewski 2018 ABSTRACT III-V multijunction solar cells (MJSC) are capable of the highest conversion efficiencies among all solar cell classifications. These devices are thus of major interest for both terrestrial and space applications. However, the economics of the terrestrial and space markets leads to significantly different design requirements for III-V MJSCs to become more economically viable in each market. In the terrestrial market, despite their high efficiency, the high manufacturing cost of III-V MJSCs currently limits their applicability in a market that is currently dominated by crystalline silicon. Thus, lower cost III-V MJSC approaches must be developed for them to become more competitive. This intuitively leads to the concept of merging III-V MJSCs with Si solar cells to demonstrate III-V/Si MJSCs. Such an approach simultaneously takes advantage of the high conversion efficiency of III-V MJSCs and the low-cost manufacturing of Si. In the space market, III-V MJSCs are already the dominant technology due to their high efficiency, radiation hardness, and reliability in extreme conditions. However, new III-V MJSC approaches must be developed if they are to push the boundary of conversion efficiency even further. An approach to improve the efficiency and thus economic viability is through the use of additional high-performance sub-cells at optimal bandgaps to more ideally partition the solar spectrum. i Although the design requirements for improving the economic viability of III-V MJSCs in the terrestrial and space markets differ drastically, the design of III-V MJSCs can be altered to meet the design requirements for both markets by using the versatile technique of III-V metamorphic epitaxy. This is the growth of relaxed (i.e. unstrained) III-V compounds at a lattice constant that differs from that of the substrate. The major advantage of III-V metamorphic epitaxy is that it provides an additional degree of freedom for III-V MJSC device design. Traditional lattice-matched growth limits the number of materials that are available to integrate with the substrate material, which in turn limits the available bandgaps that can be achieved for a given III-V MJSC design. This dissertation aims to leverage III-V metamorphic epitaxy to develop various critical components of III-V MJSCs grown by both molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). This includes the development of metamorphic tunnel junctions to enable III-V/Si MJSC approaches for future terrestrial applications and the development of wide bandgap (AlzGa1-z)xIn1-xP top cells (lattice- matched vs. metamorphic) to push the efficiency limits of III-V MJSC approaches for future space applications. Tunnel junctions serve as low-resistance, optically transparent interconnects between adjacent sub-cells within MJSCs. For III-V/Si MJSCs, these tunnel junctions are ideally grown at the same lattice constant as the metamorphic III-V sub-cells. Therefore, metamorphic tunnel junctions with relatively unexplored lattice constants are necessary. Development of these metamorphic tunnel junctions initially began with MBE-grown materials and devices. At this early stage of research, the III-V/Si MJSC approach primarily focused on the MBE-grown triple-junction solar cell designed for operation ii under high concentration. This in turn specified a variety of requirements for the necessary lower and upper tunnel junctions of the triple-junction including the necessary peak tunneling current (JP), resistance-area product (RA), and bandgap to minimize parasitic losses within the tunnel junction. After initial development of an MBE-grown metamorphic GaAs0.9P0.1 homojunction tunnel junction, efforts culminated in the demonstration of a high- performance metamorphic double heterostructure tunnel junction. This device achieved -2 -4 2 JP = 510 A·cm and RA = 2.0×10 Ω·cm ; due to these excellent electronic properties, as well as its high optical transparency, this device is suitable for both the lower and upper tunnel junction in the triple-junction. Upon integration into a Ga0.57In0.43P/GaAs0.9P0.1 dual-junction solar cell (a subset of the triple-junction containing only the III-V sub-cells and upper tunnel junction) the tunnel junction operated successfully. Thus, such a design is very promising for future III-V/Si triple-junction solar cells. III-V metamorphic epitaxy was used to explore new III-V MJSC approaches for future space applications. Current research trends are pushing to increase the efficiency of III-V MJSCs via the use of more sub-cells compared to the traditional triple-junction solar cell design. As the number of sub-cell increase from 4 to 6, the ideal bandgap profile of the MJSC shifts the bandgap of the top cell from ~2.05 eV to ~2.3 eV, respectively. Although the ideal bandgap required for the top cell can be achieved via lattice-matched (AlzGa1-z)0.52In0.48P, the necessary Al content tends to reduce device performance due to increased oxygen content. Thus, an alternative solution was explored for achieving a 2.05 eV top cell via the use of III-V metamorphic epitaxy. iii Al content versus misfit was compared in MOCVD-grown lattice-matched (Al0.32Ga0.68)0.52In0.48P and metamorphic Ga0.66In0.34P solar cells. Results demonstrated that the metamorphic Ga0.66In0.34P solar cell possessed substantially higher short wavelength current collection. This was due to the wider-bandgap, internally lattice- matched window layer of the metamorphic Ga0.66In0.34P cell, as well as a longer emitter diffusion length. Device modeling and characterization results suggested similar base diffusion lengths in each cell, and ultimately merits to both approaches were demonstrated. iv DEDICATION To my Mom, Dad, Eric, and Callie v ACKNOWLEDGEMENTS The person that has had the greatest influence on my life has been my Mom. She has supported me unconditionally in more ways than I can possibly list here. She was the one who taught me to appreciate and pursue a higher education. I cannot be more thankful for everything that she has done for me. I would like to thank my Dad for always being there on the drop of a dime and for teaching me the importance of family. My brother Eric has taught me how to live in the present and enjoy life. The amount of love and support that Callie has provided these past several years has been amazing. I am very grateful for her patience as I completed graduate school and look forward to our future adventures together. Prior to graduate school, I received my B.S. in Materials Science and Engineering at The Ohio State University. My undergraduate experience included a variety of undergraduate research and I would like to thank Professor Robert Wagoner, Professor Katharine Flores, and Professor Patricia Morris for supporting and preparing me for graduate school. During my undergraduate experience, I became seriously interested in semiconductors my junior year when taking a course on the processing of electronic materials with Professor Roberto Myers. He sparked this interest, which cascaded into a very eventful senior year taking as many electronic materials classes as I could. This included a course on semiconductor devices with Professor Siddharth Rajan. After vi expressing my interest to him about graduate school in Electrical and Computer Engineering, he told me I should contact Steve Ringel due to his research group’s unique blend of materials and device experience. I immediately stopped by his office to see if he was available. He was. He was also free to talk for what seemed like half an hour. Looking back and now knowing how often Steve is traveling, I feel like I was extremely lucky that day! After joining Steve’s group and transitioning from MSE to ECE, it did seem a bit overwhelming at first – I went from feeling like I knew everything after graduating, to an environment that was very new to me. However, I learned quickly and am thankful for all the support from the students in the group. Andrew Carlin was a great mentor and was the one who taught me how to measure my first tunnel junction. I learned a wealth of information from Chris Ratcliff and Javier Grandal in the MBE lab. Austin Speelman was also very helpful with ECV. As the years progressed, Drew Cardwell and Santino Carnevale joined the group as postdoctoral students. I remember various conversations with Drew and appreciate his ability to present ideas in new and refreshing ways. I had known Santino since working on my senior design project, so I was glad to be working with him again. During my transition from MBE to MOCVD, he played an instrumental role in operating the MOCVD to realize my preliminary tunnel junction designs. As I became one of the senior students in the group, I had the opportunity to mentor various students including Nathan Vaughn, Jacob Boyer, and Daniel Lepkowski. All were very helpful with assisting in the lab and I am glad to have worked with them. The analytical IQE model that Jacob and Daniel created enabled a deeper understanding vii of the AlGaInP top cell work. I also cannot thank Daniel enough for all the help during a month-long tunnel junction growth campaign last summer.