THEORY OF PHONON DISPERSION CURVES IN SILICON CARBIDE POLYTYPES

A theory is presented to explain fine-structure-dependent details in the phonon spectra obtained by Raman scattering measurements on silicon carbide polytypes, using the method of interactions between planes or atoms. Doublets of small energy splitting represent discontinuities in the dispersion curves within the large zone and are analogous to the band gaps appearing in nearly free-electron band structures. They give a measure of the range of interplanar interactions and of small energy differences between polytypes. These results are used to comment qualitatively on the relative stability of silicon carbide polytypes. The theory also accounts satisfactorily for the infrared (ir) strengths of the weak modes observed in these structures. © 1969 The American Physical Society.