Density of states near the band edges in amorphous germanium

Abstract

We investigate the density of states near the edges of the band gap in tetrahedrally bonded amorphous semiconductors for a number of proposed structural models. This is made possible by using a cluster calculation method previously developed by us. Several techniques have been introduced to eliminate surface effects. The density of states near the band edge is shown to be sensitive to the characteristics of the model structure. Contrary to the usual concept that disorder introduces exponential tailing of the density of states into the energy gap, the presence of odd number rings (especially 5-member rings) makes the upper edge of the valence band drop faster with only a small tailing region. Large exponential tailing into the gap is induced by strain, i.e., stretching of bond lengths and distortion of bond angles.