Infrared Absorption in SiC Polytypes

A strong residual ray absorption is observed in SiC. In most polytypes, additional one-phonon absorption lines are permitted by group-theoretical selection rules, but such lines have not been reported. A procedure is now outlined that makes it possible to classify all the additional weak lines, and to identify three of them reported in previously published work. Two are lines at 62 and 110 meV in $6H$ SiC; the third is a 70-meV line in $15R$. The classification procedure can also be used for Raman lines. On making what appear to be reasonable assumptions, one can estimate the energies of infrared and Raman lines in any SiC polytype. The high anisotropy of the weak modes in these uniaxial crystals is discussed.