Polarization, band lineups, and stability of SiC polytypes

We calculate the spontaneous polarization of wurtzite-structure SiC, using the recently proposed supercell technique of Posternak, Baldereschi, Catellani, and Resta [Phys. Rev. Lett. 64, 1777 (1990)] and a first-principles pseudopotential approach. The macroscopic polarization is found to be 4.32×${10}^{\mathrm{-}2}$ C/${\mathrm{m}}^2$, and is mainly due to the electronic-charge-density redistribution rather than a relaxation of the positions of the ions. The valence-band offset at the interface between the wurtzite and cubic forms is determined using the same supercell calculations, giving 0.13 eV, with the wurtzite-structure SiC valence-band edge being higher in energy. Our calculations also predict the crystal-field splitting of the top of the valence band of wurtzite-structure SiC to be 0.12 eV, and give some insight into the nature of the dipole created by a single stacking fault. Finally, the effect of the macroscopic electric fields on the relative stability of SiC polytypes is discussed and found to be negligible.