We have measured the first- and second-order Raman spectra from semi-insulating $4H-\mathrm{SiC}$ at room temperature under nonresonant excitation. The results are compared with Raman spectra of n-type doped $4H$- and $6H-\mathrm{SiC}.$ The second-order spectra for both $4H$- and $6H-\mathrm{SiC}$ contain an acoustic branch and an optical branch, which is clearly dependent on SiC polytype. The measured two-phonon spectra are assigned by comparison with theoretical calculations.

• Received 11 May 1998

DOI:https://doi.org/10.1103/PhysRevB.59.7282