Etching of elemental layers in oxide molecular beam epitaxy by O2-assisted formation and evaporation of their volatile (sub)oxide: The examples of Ga and Ge

Abstract

The delivery of an elemental cation flux to the substrate surface in the oxide molecular beam epitaxy (MBE) chamber has been utilized not only for the epitaxial growth of oxide thin films in the presence of oxygen but also in the absence of oxygen for the growth temperature calibration (by determining the adsorption temperature of the elements) and in situ etching of oxide layers (e.g., Ga2O3 etched by Ga). These elemental fluxes may, however, leave unwanted cation adsorbates or droplets on the surface, which traditionally require removal by in situ superheating or ex situ wet-chemical etching with potentially surface-degrading effects. This study demonstrates a universal in situ approach to remove the residual cation elements from the surface via conversion into a volatile suboxide by a molecular O2-flux in an MBE system followed by suboxide evaporation at temperatures significantly below the elemental evaporation temperature. We experimentally investigate the in situ etching of Ga and Ge cation layers and their etching efficiency using in situ line-of-sight quadrupole mass spectrometry and reflection high-energy electron diffraction. The application of this process is demonstrated by the in situ removal of residual Ga droplets from a SiO2 mask after structuring a Ga2O3 layer by in situ Ga-etching. We predict this approach to be generally applicable in MBE and metalorganic chemical vapor deposition to remove residual elements with vapor pressure lower than that of their volatile suboxides or oxides, such as B, In, La, Si, Sn, Sb, Mo, Nb, Ru, Ta, V, and W.