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

We present results of first-principles calculations for the neutral and cha rged Si and C monovacancies in cubic (3C) and hexagonal (4H) SiC. The calcu lations are based on the density functional theory in the local-density app roximation as well as local spin density approximation. Explicitly a plane- wave-supercell approach is combined with ultrasoft Vanderbilt pseudopotenti als to allow converged calculations. We study the atomic structure, the ene rgetics, 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 oc curs due to the strong localization of the C dangling bonds at the neighbor ing C atoms. Consequently, high-spin configurations are predicted for Si va cancies, whereas the low-spin states of C vacancies exhibit a negative-U be havior. 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-relat ed shifts of the electronic states. [S0163-1829(99)07323-3].