ELECTRONIC AND STRUCTURAL-PROPERTIES OF VACANCIES ON AND BELOW THE GAP(110) SURFACE

We have performed total-energy density-functional calculations using f irst-principles pseudopotentials to determine the atomic and electroni c structure of neutral surface and subsurface vacancies at the Gap (11 0) surface. The cation as well as the anion surface vacancy show a pro nounced inward relaxation of the three nearest-neighbor atoms towards the vacancy while the surface point-group symmetry is maintained. For both types of vacancies we find a singly occupied level at midgap. Sub surface vacancies below the second layer display essentially the same properties as bulk defects. Our results for vacancies in the second la yer show features not observed for either surface or bulk vacancies: L arge relaxations occur and both defects are unstable against the forma tion of antisite vacancy complexes. Simulating scanning tunneling micr oscope pictures of the different vacancies, we find excellent agreemen t with experimental data for the surface vacancies and predict the sig natures of subsurface vacancies. [S0163-1829(98)02827-6].