Hybrid molecular beam epitaxy of ultra-wide band gap semiconductors

Abstract

Perovskite oxides exhibit a seemingly universal set of functionalities, blessing them with an alluring mystique that draws unending attention from academic researchers. And yet, one previously unrealized functionality in perovskite oxides is that of an ultra-wide band gap semiconductor: high room-temperature carrier mobilities with a band gap in the ultraviolet. Far from being an academic curiosity, the development of better ultra-wide band gap semiconductors is the key to the downsizing and cost improvement of power conversion devices that are becoming increasingly ubiquitous, and increasingly central to electrification and decarbonization. This thesis is an exploration into the semiconducting properties of perovskites with exceptionally wide band gaps, with the eventual goal of developing a semiconductor that is better for power switching than any existing one. To study these perovskites, I use the modular thin film deposition technique called molecular beam epitaxy (MBE). The exploration starts with the study of SrSnO3, a noncubic perovskite with a band gap substantially larger than that of the more commonly studied BaSnO3. First, I present a systematic study of how film thickness drives a complex interplay among crystal symmetry, strain, structural quality, and transport, with the conclusion that surface effects eclipse all other sources of disorder. With this lesson in mind, I next explore doping of SrSnO3 films exceeding 200-nm thickness, constituting the most extensive doping study of SrSnO3 to date. I achieve a record-high mobility of 72 cm2V-1s-1, and, with the help of collaborators, predict the phonon-limited mobility to be 75~140 cm2V-1s-1 suggesting limited but perhaps substantial room for improvement. Motivated by these encouraging results in SrSnO3, I venture deeper into the ultraviolet with CaSnO3, a material with an even wider band gap. I present not only the first successful growth of CaSnO3 with MBE, but also the first successful doping of CaSnO3 in any form, demonstrating, for the first time, that CaSnO3 is a semiconductor rather than an insulator. I conclude my experimental results with the growth of rutile Sn1-xGexO2 with metalorganic MBE, motivated by the goal of growing the alkaline earth germanates, a family of metastable perovskites that share many properties with the stannates.