Nucleation Window of Ga₂O₃ and In₂O₃ for Molecular Beam Epitaxy on (0001) Al₂O₃

Abstract

The epitaxial growth of Ga 2 O 3 and In 2 O 3 on (0001)-oriented Al 2 O 3 by plasma-assisted molecular beam epitaxy is investigated. The synthesis of both materials underlies similar growth kinetics, with the formation of volatile suboxides limiting their growth in the metal-rich regime. Quantitative analysis of β-Ga 2 O 3 and c-In 2 O 3 growth kinetics has been performed on appropriate buffer layers mimicking homoepitaxial growth conditions. In particular, we investigate the influence of the amount of supplied atomic oxygen on the growth kinetics of β-Ga 2 O 3 . Considering the heteroepitaxial growth on bare (0001) Al 2 O 3, we found that the observed nucleation window is much more limited than the expected growth window derived from homoepitaxial growth conditions. For Ga 2 O 3 this was studied in detail and the upper limit of the nucleation window was determined in terms of the gallium to oxygen ratio on the growth surface and the growth temperature. Outside the nucleation window, we report that three monolayers α-(Al 0.75±0.04 Ga 0.25∓0.04 ) 2 O 3 formed in the initial stage of growth passivate the surface with respect to further β-Ga 2 O 3 nucleation. We attribute the formation of this layer to the intermixing of a Ga-rich adlayer and Al atoms from the substrate until the solubility limit in the α-phase is reached. The observation is accompanied by a specific RHEED pattern with satellite reflexes assigned to a surface reconstruction in the metal-rich growth regime.