Study of oxygen chemisorption on the GaN(0001)-(1×1) surface

Abstract

Clean, ordered GaN(0001)-(1×1) surfaces are prepared by sputtering with nitrogen ions followed by annealing in ultrahigh vacuum. The surfaces are subsequently exposed at room temperature to O2 and the chemisorption process studied using Auger, valence and core-level photoemission and electron energy loss spectroscopies, low-energy electron diffraction, and work function measurements. Saturation occurs at a coverage of Θox=0.4 ML and is accompanied by the removal of surface states near the band edges. The continued presence of a clear (1×1) diffraction pattern, together with other data, indicates a well-defined adsorption site, but the relative importance of Ga–O and N–O bonding remains undetermined. The realization that surface states exist near the valence-band maximum has led to a more accurate determination of the surface Fermi-level pinning position, and of dependent quantities, than given previously. Clean-surface data are also compared with those for surfaces prepared by in situ deposition of Ga metal followed by thermal desorption. No significant differences are seen, which suggests that nitrogen-ion sputtering and annealing is suitable for preparing clean, ordered GaN(0001)-(1×1) surfaces. The results for O chemisorption on atomically clean surfaces have been applied to evaluating the passivation of surfaces prepared by ex situ wet-chemical cleaning. The band bending is found to be ∼0.5 eV less than on atomically clean surfaces.