Raman Study of Isotope Effects and Phonon Eigenvectors in SiC

We have measured Raman spectra of several SiC polytypes ( $3C$, $6H$, $15R$) made from natural silicon $\left({\approx }^{28}\mathrm{Si}\right)$ and ${}^{30}\mathrm{Si}$. The isotope shifts of the phonon frequencies show characteristic variations with their effective wave vector in the zinc blende $\left(3C\right)$ modification which arises from Brillouin zone backfolding. This allows us to determine the phonon eigenvectors of $3C$ SiC for the dispersion branches along the [111] direction. The observed magnitudes of the Si and C ion displacements, as well as their relative phase, confirm bond charge model and ab initio calculations.