Optical-absorption bands in the 1-3 eV range in n-type SiC polytypes

Measured optical-absorption bands in the 1-3 eV range in fairly heavily dop ed n-type SIC polytypes 3C, 6H, 4H, 8H, and 15R are shown to arise from opt ical transitions between the lowest conduction band, which is to some exten t perturbed by impurity effects, to higher conduction bands. The energies o f the transitions are in good agreement with the differences between unpert urbed low-lying energy bands calculated using the full-potential linear muf fin-tin orbital method in the local-density approximation. The polarization dependence is explained by selection rules deriving from the symmetry of t he bands involved. This indicates that the states involved in the transitio ns must to a good extent retain the symmetry characters of the unperturbed bands. On the other hand, the calculated absorption peaks from a pure band- to-band model are much narrower, and slightly lower in energy than the expe rimental ones. Calculations of the density of states over a restricted rang e of k space for the final states indicate that a partial breakdown of peri odicity and hence the Delta k=0 selection rule can account for a major part of the broadening. This explanation is consistent with the degenerate carr ier concentration, associated with the overlap of the impurity band rail wi th the bottom of the conduction band. In 6H-SiC, one feature in the absorpt ion spectrum appears nevertheless to be associated with a more purely band- to-band-like transition. It is a sharp one-dimensional van Hove singularity in the joint density of states at the M point associated with the camel's- back structure of the lowest conduction band. At a lower carrier concentrat ion, this feature is not present, and the transitions appear to have a more localized impurity-to-band character. [S0163-1829(99)05420-X].