Phonon folding and transport in SiC polytypes from first principles

Abstract

Silicon carbide (SiC) crystallizes in more than 200 polytypes with various crystal classes, including cubic, hexagonal, and rhombohedral structures. They differ from each other mainly by the stacking order of hexagonal SiC atomic layers. Such superlattice-like stacking modulates the phonon dispersion by folding the single SiC-layer Brillouin zone into smaller ones along the axial direction normal to the atomic layer, resulting in an increased number of phonon branches. Earlier works suggest that these polytypes share a common unfolded phonon dispersion along the axial direction. We show from first principles that this is only true for an axial high-symmetry line. For other general k points such as in the planar k path, strong hybridization between longitudinal and transverse phonons and large avoided crossings are observed. The difference is a direct consequence of a distinctive symmetry decrease for these k points explained by group theory. The strong hybridization and avoided crossing soften the velocity and enhance the scattering of folded acoustic phonons, leading to significant thermal conductivity reduction for complex SiC polytypes, as confirmed by Boltzmann transport calculations with phonon–phonon scattering from first principles. This work provides a unified description and comparison of phonons in SiC polytypes on equal footing and provides physical perspectives into the impact of stacking order on the phonon transport behaviors.