Total-energy density-functional calculations were performed by using first-principles pseudopotentials to determine the atomic and electronic structures of neutral surface and sub-surface vacancies at the (110) surface. The cation, as well as the anion, surface vacancy exhibited a marked inward relaxation of the 3 nearest-neighbor atoms towards the vacancy, while the surface point-group symmetry was maintained. A singly-occupied level was found at the mid-gap for both types of vacancy. Sub-surface vacancies below the second layer exhibited essentially the same properties as bulk defects. The results for vacancies in the second layer revealed features which were not observed for either surface or bulk vacancies. Large relaxations occurred, and both defects were unstable with respect to the formation of antisite vacancy complexes. By simulating scanning tunnelling microscope images for the various vacancies, excellent agreement was found with experimental data for the surface vacancies, and it was possible to predict the signatures of sub-surface vacancies.

Electronic and structural properties of vacancies on and below the GaP(110) surface G.Schwarz, A.Kley, J.Neugebauer, M.Scheffler: Physical Review B, 1998, 58[3], 1392-400