Using an nb initio pseudopotential approach, we have investigated the exist
ence of localized interface states in epitaxial Al/caAs and Au/GaAs(100) ju
nctions. In spite of the fact that the Al/GaAs and Au/GaAs(100) contacts po
ssess relatively similar Schottky barrier heights, their interface-band str
uctures differ significantly in the region of the GaAs fundamental band gap
. Our results indicate that truly localized interface states can exist near
the Fermi energy in Au/GaAs(100) junctions, even at defect-free interfaces
, whereas no such states are allowed in Al/GaAs(100) junctions. For the abr
upt As-terminated Au/GaAs(100) junctions, in particular, we find As-bridge-
bond interface states located near the Fermi energy, which derive from frus
trated covalent bonds at the interface. The presence of such states could e
xplain the recent observation, by nonlinear spectroscopy, of a sharp midgap
interface-state peak in As-rich Au/GaAs(100) junctions.