Theory of metal-semiconductor interfaces

We present a microscopic model for the formation of Schottky barriers at metal-semiconductor contacts. The theory proposes that Schottky barriers are determined by "metal-induced gap states" at the semiconductor surface, which are dangling-bond derived resonances. The dangling-bond character of these states implies that the energies of surface states at clean semiconductor surfaces are very important in Schottky-barrier formation. This work introduces an ionicity parameter, obtained from atomic-term values, to quantitatively characterize the well-known transition from covalent to ionic behavior at metal-semiconductor interfaces. This model directly associates this transition with a truly fundamental change in the electronic structure of the semiconductor substrate and provides a natural interpretation of strong Fermilevel pinning at metal contacts to covalent materials with large optical gaps and low bond polarizabilities.