This thesis explores the molecular beam epitaxy (MBE) of semiconducting ultra-wide bandgap (UWBG) group IV oxides, with a particular focus on GeO2, SnO2, and their ternary alloy (SnxGe1−x)O2. Through a combination of in-situ characterization techniques and ex- situ characterization, we establish a comprehensive framework that integrates growth kinetics, surface reaction thermodynamics, and etching mechanisms to advance the synthesis and processing of these semiconducting oxides. This thesis first presents a detailed in-situ investigation of GeO2 growth kinetics under conventional MBE using an elemental Ge source and plasma oxygen. We demonstrate that volatile GeO suboxide formation at the growth front introduces a significant kinetic limitation by competing with full oxide formation. A quantitative model is shown to describe the growth behavior, identifying critical thresholds of oxygen flux and substrate temperature that define the GeO2 growth window. This model serves as a guide for optimizing thin-film deposition. Next, a universal in-situ cleaning technique is introduced, based on the oxidation by molecular O2 and subsequent desorption of elemental layers via their volatile suboxides. We experimentally demonstrate this method for Ga and Ge and apply it to remove residual elemental contaminants from oxide-based device surfaces. This process significantly reduces the thermal budget and risk of damage compared to conventional ex-situ etching methods. It enables high-throughput oxide processing by allowing reuse of substrates and rapid recovery from failed growth attempts. In the final part, the kinetic insights are extended to suboxide-source MBE (S-MBE) of binary and ternary oxides, with further exploration of the thermodynamics. Comparative analysis between GeO2 and SnO2 growth reveals the superior oxidation efficiency of SnO and lower volatility, making SnO-incorporation kinetically favorable in binary growth. However, during ternary alloy growth, GeO is preferentially incorporated despite its less favorable kinetics. This behavior indicates complex cation exchange dynamics at the growth front, which we interpret as a form of SnO-catalyzed GeO incorporation. Besides the SnO-to- GeO flux ratio, a precise control of substrate temperature and oxygen flux is shown to be essential for achieving the desired alloy composition. Together, the findings presented in this thesis offer a coherent framework for under- standing and controlling the non-equilibrium growth of oxide semiconductors in MBE system. The methodologies developed—particularly the use of volatile suboxide chemistry for both etching and growth—are broadly transferable to other vapor-phase deposition techniques. This work provides the foundation for future exploration of crystalline (SnxGe1−x)O2-based thin films, heterostructures, and devices, addressing key challenges in epitaxy, interface engineering, compositional tuning, and scalable fabrication in UWBG oxide electronics.
Wenshan Chen (Thu,) studied this question.