Geometry and electronic structure of the arsenic vacancy on GaAs(110)

Tunneling microscopy and spectroscopy, in conjunction with tight-binding molecular dynamics, provide compelling evidence that the ``missing As'' defect on GaAs(110) is indeed an As vacancy. Neighboring Ga atoms relax upward by about 0.7 {}, but do not rebond. The defect is positively charged and most likely in a +2 state. Both the relaxation and the preponderance of As vacancies on p-GaAs are explained by the energetics of the defect levels. The essential features of the observations can be understood from qualitative arguments based on hybrid orbitals.