  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 (QMS) and reflection high-energy
electron diffraction (RHEED). 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. This approach can be generally
applied in MBE and MOCVD to remove residual elements with vapor pressure lower
than that of their suboxides, such as B, In, La, Si, Sn, Sb, Mo, Nb, Ru, Ta, V,
and W.
