Electronic density of states and optical properties of polytypes of germanium and silicon

The atomic arrangement of the ideal amorphous form of germanium or silicon is thought to be a tetrahedrally coordinated random network. Since the diamond structure is only one crystalline limit of such a network with the appropriate short range order, it is instructive to compare the density of states and the optical spectrum of amorphous Ge and Si not only with those quantities calculated for cubic Ge and Si, but also with those for such simple polytypes as 2H and 4H. Our computations indicate that while N(E) for these polytypes becomes progressively more like presently available experimental N(E) as one continues to add atoms to the unit cell, the gross features of the optical spectra of these polytypes are all very nearly alike, and are consistently different from the amorphous material. © 1972.