Raman intensity profiles of folded longitudinal phonon modes in SiC polytypes

Raman spectral profiles of folded longitudinal phonon modes in $4H-,$ $6H-,$ and $21R-$ SiC polytypes which form natural superlattices have been studied experimentally and theoretically. The Raman intensity of the folded longitudinal modes is very weak except for the mode corresponding to a zone-center phonon mode in the $3C$ polytype. The intensity profiles of these modes are interpreted on the basis of the bond polarizability concept when one takes into account difference of the bond Raman polarizability between the cubic and hexagonal environments. The agreement of the measured and calculated intensities indicates that the bond polarizability model can be applied to Raman spectral profiles of the longitudinal folded modes as well as transverse folded modes in SiC.