Nucleation and faceting in (001) r-GeO2 heteroepitaxy on r-TiO2 by metalorganic vapor phase epitaxy

The ultra-wide bandgap semiconductor rutile germanium oxide (r-GeO2, Eg ≈ 4.6 eV) is gaining momentum in the quest for novel materials for power electronics. In this work, we experimentally and theoretically investigate the physical mechanisms behind the nucleation and growth of epitaxial (001) r-GeO2 on isostructural r-TiO2 substrates via metalorganic vapor phase epitaxy (MOVPE) using isobutylgermane and O2 precursors. In the identified deposition window, the thin film growth seems to be affected by partial GeO suboxide desorption, and we observe that the layers are always composed of r-GeO2 islands embedded and/or surrounded by amorphous material. Ge/Ti interdiffusion at the epilayer-substrate interface is found at the base of each r-GeO2 island; combining experimental analysis and multiscale theoretical simulations we discuss how such a process is fundamental to achieve partial strain mitigation allowing for the nucleation of epitaxial r-GeO2 and suggest in this regard a limiting threshold to avoid the formation of amorphous material. Moreover, we shed light on the formation of different facets in r-GeO2 at early stages of growth and after merging of islands.