Unraveling crystal symmetry and strain effects on electronic band structures of SiC polytypes

The modulations of the electronic band structures of hexagonal (2H, 4H, and 6H) and cubic (3C) SiC under biaxial (0001) and (111) in-plane strain are investigated by using first-principles calculations including spin-orbit coupling effects. We have clarified that the strain dependency of the valence bands is closely related to the crystal symmetry and hexagonality. Specifically, tensile strain induces hybridization and crossover between the heavy-hole and light-hole bands in the hexagonal polytypes. On the other hand, the degeneracy between the heavy-hole and light-hole bands breaks in the cubic polytype under tensile strain. Consequently, the hole effective masses change significantly under certain tensile strain in all four polytypes. The values of the critical tensile strain are approximately proportional to the energy differences between the heavy-hole and crystal-field splitting bands under no strain and, in turn, show a correlation with the hexagonality. In contrast to the case of the valence bands, the band structures around the conduction band minima and, therefore, the electron effective masses are insensitive to the strain, except for the ML direction in 6H-SiC. The present study provides principles for elucidating and designing the crystal structure and strain dependency of the electronic band structures and transport properties of SiC.