Model of the electronic structure of amorphous germanium tin alloys

The electronic structure of an amorphous Ge-Sn alloy is modelled by an extension of the coherent potential approximation (CPA), which takes into account the chemical order and the absence of Sn dangling bonds found experimentally. In accordance with experiment, the present calculations predict a smooth linear decrease of the band gap with tin concentration. With respect to transport properties, however, an important difference appears. No electronic levels are theoretically found in the band-gap, which seem to be present in deposited samples, as deduced from conductivity vs temperature measurements. Therefore, these defect centers should arise from local bonding configurations other than the pure tetrahedral substitutional one.