Interface-Induced Gap States

Heine explained the interface states in Schottky contacts proposed by Bardeen with the tails of the wave-functions of the metal electrons into the semiconductor in the energy range between the valence-band top of the semiconductor and the Fermi level. These metal- or, more generally, interface-induced gap states result from the complex band structure of the semiconductor. Their character changes at their branch point from predominantly valence-band-like to mostly conduction-band-like. The net charge transfer between metal and semiconductor and thus the size and sign of the interface dipole are given by the energy difference between Fermi-level and the branch point. On the other hand, the charge transfer varies linearly as the difference of the electronegativities of the metal and the semiconductor. The respective slope parameters and the branch-point energies thus determine the Schottky barrier heights. In this chapter, the calculated values of these quantities are discussed.