XPS study of oxygen adsorption on (3x3) reconstructed MBE grown GaN surfaces

The incorporation of oxygen onto the (3x3) reconstructed surface of GaN(000 (1) under bar) has been studied using X-ray Photoelectron Spectroscopy (XPS). It was found that the (3x3) reconstruction corresponds to a fractional Ga adlayer atop a Ga terminated GaN surface. Our measurements indicate a surface coverage of 1.15 +/- 0.2 monolayers of relaxed Ga on the surface. The binding energy separation between the relaxed surface Ga3d core level and bulk Ga3d level was measured to be 1.1 +/- 0.1 eV. A metallic component extending from the bulk GaN valence band maximum out to 0 eV was also present in the XPS spectrum. The separation between the bulk valence band maximum and the Fermi level of the metallic component was found to be 2.1 +/- 0.1 eV. The relaxation of the surface Ga was found to decrease with oxygen exposure indicating Ga-O bonding, with oxygen adsorption terminating at 1.3 +/- 0.2 monolayers. The O1s core level was found to have a FWHM of 2.0 +/- 0.1 eV.