Selective resonance effect of the folded longitudinal phonon modes in the Raman spectra of SiC

The resonant Raman spectra of 4H-SiC, 6H-SiC, and 15R-SiC were measured fro m visible excitation to ultraviolet excitation. A selective enhancement of the intensities of the folded longitudinal phonon modes relative to the non folded longitudinal optical phonon mode was found. To explain this, we used a bond-Raman-polarizability model with a one-dimensional lattice dynamics model. It uses bond Raman polarizabilities of three bond arrangements, whic h we assumed to have different excitation energy dependences. By using this model, we successfully reproduced the excitation energy dependence of the relative intensities of the folded longitudinal phonon modes. The bond Rama n polarizabilities and force constants used in our calculations are common for all polytypes. We found that the bond Raman polarizabilities of the thr ee bond arrangements had different resonance energies.