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

Abstract

Ultrawide bandgap (UWBG) semiconductors (Eg >3 eV) have tremendous potential for power-electronic applications. The current state-of-the-art UWBG materials such as β-Ga2O3, diamond, and AlN/AlGaN, however, show fundamental doping and thermal conductivity limitations that complicate technological adaption and motivate the search for alternative materials with superior properties. Rutile GeO2 (r-GeO2) has been theoretically established to have an ultrawide bandgap (4.64 eV), high electron mobility, high thermal conductivity (51 W m-1 K-1), and ambipolar dopability. While single-crystal r-GeO2 has been synthesized in bulk, the synthesis of r-GeO2 thin films has not been previously reported but is critical to enable microelectronics applications. Here, we report the growth of single-crystalline r-GeO2 thin films on commercially available R-plane sapphire substrates using molecular beam epitaxy. Due to a deeply metastable glass phase and high vapor pressure of GeO, the growth reaction involves the competition between absorption and desorption as well as rutile and amorphous formation. We control the competing reactions and stabilize the rutile-phase growth by utilizing (1) a buffer layer with reduced lattice misfit to reduce epitaxial strain 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 ultra-wide-bandgap semiconductors.