SULFUR PASSIVATION OF GAAS-SURFACES

A first-principles study of the structural and electronic properties of the GaAs(111) surface with an adsorbed monolayer of sulfur atoms is presented in comparison with the previous results for the GaAs(001) surface. We determine the optimal adsorption sites of sulfur atoms and show that S atoms are more tightly bonded to Ga atoms than to As atoms. It is found that the midgap-surface-state density is remarkably reduced and the Fermi level is shifted toward the valence-band maximum by the formation of stable Ga-S bonds. The sulfur passivation of both GaAs(111) and GaAs(001) surfaces can be explained quite well in terms of the formation of Ga-S bonds on the sulfur-treated surface, without introducing any disorder or defects near the surface.