OXIDATION OF THE GAAS(110) SURFACE

The principal absorption site of oxygen on the GaAs(110) surface has been deduced from scanning tunneling microscopy (STM) measurements (for low oxygen coverages of the surface) to be in the interchain trough and above the surface. In contrast, photoemission measurements of Ga and As core-level shifts and of oxygen-derived energy levels led to the conclusion that adsorption occurs on or below the surface (for high oxygen coverages), with Ga and As atoms about equally oxidised. To understand these observations, we have performed total-energy calculations using the first-principles pseudofunction method for a model with a periodic half-monolayer of oxygen, adsorbed to a few selected sites in the surface unit cell. Our results, although computed for models with half-monolayer coverages of oxygen, strongly suggest that the oxygen image observed in STM measurements indeed is due to an oxygen atom in the interchain trough, but with the oxygen atom located slightly below the surface and bonded to a Ga atom. For high coverages, our results imply in-surface-plane multi-coordinated adsorption of oxygen, which accounts for the observed oxygen-derived spectral features about 5 eV below the valence band maximum.