  The electronic structure around a single As antisite in GaAs is investigated
in bulk and near the surface both in the stable and the metastable atomic
configurations. The most characteristic electronic structures of As antisite is
the existence of the localized p-orbitals extending from the As antisite. The
major component of the highest occupied state on As antisite in the stable
configuration is s-orbital connecting with neighboring As atoms with nodes
whereas that in the metastable configuration is p-orbital connecting without
nodes. Localized p-orbitals on the surrounding As atoms around the As antisite
exist in every configuration of As antisite. Such features are retained except
the case of the As antisite located just in the surface layer in which the
midgap level is smeared into the conduction band and no localized states exist
near the top of the valence band. Scanning tunneling microscopic images of
defects observed in low-temperature grown GaAs, possibly assigned as As
antisite, the origin of the metastability, and the peculiarity of the defects
in the surface layer are discussed.
