Photoemission study of sulfur and oxygen adsorption on GaN(0001¯)

The surface electronic structure of thin film n-type GaN(0001¯) has been studied using high resolution angle resolved photoemission spectroscopy. Clean surfaces, sulfur-exposed surfaces, and oxygen-exposed surfaces were investigated. Spectra were recorded for the Ga 3d shallow core states and for the valence band states. The sulfur covered surface was found to be inert with respect to subsequent oxygen exposure. For the clean Ga-adlayer terminated (0001¯) surface, the Fermi level is pinned by surface states at least 1.4 eV below the bottom of the conduction band. Sulfur and oxygen covered surfaces exhibit smaller values for surface band bending, with the Fermi edge lying 0.7-1.0 eV below the conduction band minimum. Finally, we discuss a possible new interpretation for the asymmetry in the shape of Ga 3d photoemission feature.