Optical Properties of Polytypes of Germanium

Abstract

The atomic arrangement of the ideal amorphous form of germanium is thought to be a tetrahedrally coordinated random network. Bearing in mind that the diamond structure is only one crystalline example of such a network, it is instructive to compare the density of states and the optical spectrum of amorphous Ge not only with these same quantities calculated for cubic Ge, but also with those for such simple polytypes as the wurtzite-like 2H structure with four atoms per unit cell and the 4H structure with eight atoms per unit cell. All three structures have identical first and second nearest neighbors, but differ at the third nearest neighbors and beyond. We can gain insight into both Brillouin zone and short range order effects by studying these three theoretically possible polytypes, assigning to each the same atomic density and nearest neighbor separation.