FERMI-LEVEL PINNING IN AN AL-GE METAL-SEMICONDUCTOR JUNCTION

We studied the energetics and electronic structure of the lattice-matched Al-Ge heterostructure using the self-consistent-field pseudopotential method. We showed that interactions at the interface are on the atomic scale and lead to directional metal-semiconductor bonds subsequent to the elimination of dangling bonds by the adsorption of metal atoms. As previously established, these bonds elongate and become less localized upon metallization of the overlayer, but their directional character is shown to be maintained. Buckling of the metal layers at the interface is favored even though metal and semiconductor lattices are almost commensurate. An analysis of the state and charge density leads to the conclusion that the Fermi level is determined by the metal states that decay into the semiconductor. It is ruled out that the dangling-bond surface states (which become resonance states in the conduction-band continua of metal) determine the Fermi level for the Al-Ge hetero-structure.