Midgap interface states at epitaxial Al/AlAs(001) heterojunctions

Using ab initio pseudopotential calculations, we have investigated the nature of the electronic states with energies within the semiconductor band gap in abrupt, defect-free As-terminated Al/AlAs(001) contacts. Resonant interface states, not accounted for by commonly accepted models, occur at the J-point of the interface Brillouin zone near the Fermi energy in the semiconductor midgap region. They correspond to intermetallic bonds between Al atoms of the semiconductor and those of the metal. The new interface states derive from an interaction between localized states of the Al(001) surface and AlAs conduction band states, mediated by localized states of the non-reconstructed As-terminated AlAs(001) surface.