Electronic and structural properties of the As vacancy on the (110) surface of GaAs

The isolated As vacancy on the GaAs (110) surface is investigated using ab initio pseudopotentials. The relaxed atomic structure reveals an inward relaxation of the neighboring surface Ga atoms regardless of the vacancy charge state. The stable charge state is determined to be (+1) in p-type material from the formation energies. Three vacancy states, two located within the band gap, are identified. The character of the vacancy wavefunctions is described in detail and discussed in connection with the charge-state dependent relaxation. Despite the inward relaxation of surface Ga neighbors, the theoretical scanning tunneling microscopy topography shows enhanced tunneling into these sites, which is in agreement with existing experiments. The origin of this apparent contradiction is attributed to the nature of the wavefunction for one of the unoccupied vacancy states.