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 doped n-type SiC polytypes 3C, 6H, 4H, 8H, and 15R are shown to arise from optical transitions between the lowest conduction band, which is to some extent perturbed by impurity effects, to higher conduction bands. The energies of the transitions are in good agreement with the differences between unperturbed low-lying energy bands calculated using the full-potential linear muffin-tin orbital method in the local-density approximation. The polarization dependence is explained by selection rules deriving from the symmetry of the bands involved. This indicates that the states involved in the transitions 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 experimental ones. Calculations of the density of states over a restricted range of k space for the final states indicate that a partial breakdown of periodicity and hence the Δk = 0 selection rule can account for a major part of the broadening. This explanation is consistent with the degenerate carrier concentration, associated with the overlap of the impurity band tail with the bottom of the conduction band. In 6H-SiC, one feature in the absorption 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 concentration, this feature is not present, and the transitions appear to have a more localized impurity-to-band character.