Epitaxial stabilization of rutile germanium oxide thin film by molecular beam epitaxy

Ultrawide-band-gap (UWBG) semiconductors have tantalizing advantages for power electronics. Materials such as AlN/AlGaN, β-Ga₂O₃, and diamondhave been developed for UWBG semiconducting devices, however, they are still facing challenges, such as doping asymmetry and/or inefficient thermal conduction. Rutile GeO₂ (r-GeO₂) has been theoretically established to have an UWBG (4.68 eV), high electron and hole mobility (289 cm²V^-1s^-1 and 28cm² V^-1s^-1), high thermal conductivity (51 W m^–1K^–1) and ambipolar dopability. The synthesis of r-GeO₂ thin films has not been reported but is critical to enable microelectronics applications. Here, we report the growth of single-crystalline r-GeO₂ thin films on R-plane sapphire substrates using molecular beam epitaxy. We control the competing reactions between the deeply metastable glass phase formation and rutile phase formation as well as absorption and desorption by utilizing (1) a buffer layer with reduced lattice misfit, and (2) the growth condition that allows the condensation of the preoxidized molecular precursor yet provides sufficient adatom mobility. The findings advance the synthesis of single-crystalline films of materials prone to glass formation and provide opportunities to realize promising UWBG semiconductors.