Vacancies in SiC: Influence of Jahn-Teller distortions, spin effects, and crystal structure

We present results of first-principles calculations for the neutral and charged Si and C monovacancies in cubic (3C) and hexagonal (4H) SiC. The calculations are based on the density functional theory in the local-density approximation as well as local spin density approximation. Explicitly a plane-wave-supercell approach is combined with ultrasoft Vanderbilt pseudopotentials to allow converged calculations. We study the atomic structure, the energetics, and the charge- and spin-dependent vacancy states. The generation of the C-site vacancy is generally accompanied by a remarkable Jahn-Teller distortion. For the Si-site vacancy only an outward breathing relaxation occurs due to the strong localization of the C dangling bonds at the neighboring C atoms. Consequently, high-spin configurations are predicted for Si vacancies, whereas the low-spin states of C vacancies exhibit a negative-$U$ behavior. In the case of hexagonal polytypes, the crystal-field splitting of the upper vacancy levels does not principally modify the properties of the vacancies. The inequivalent lattice sites, however, give rise to site-related shifts of the electronic states.