Structure of the As Vacancies on GaAs(110) Surfaces

We report a comprehensive study of the As vacancies ( VAs) in the GaAs(110) surface via ab initio total energy minimization. Previous scanning tunneling microscopy (STM) images of the VAs in p-type GaAs(110) were interpreted with a structure with outward movement of the Ga next to the vacancy. While our simulation of the STM images, using ab initio wave functions, agrees with experiment, our total energy minimization suggests, however, inward movement of Ga. We explain this apparent conflict as a charge induced band bending effect. As a consequence, we predicted that the STM images will depend on the applied bias voltage. We show that the atomic geometry of the surface VAs(q) depends critically on the charge state q in sharp contrast with bulk vacancy.