Monovacancies in 3C and 4H SiC

We present first-principles calculations of the atomic and electronic structure of neutral and charged C and Si vacancies in 3C and 4H SiC. We demonstrate that the principal properties of the vacancies depend only weakly on the polytype. In both polytypes the formation of C vacancies is accompanied by a significant symmetry-breaking distortion of the neighbouring Si atoms. Furthermore, a negative-U behaviour is predicted for the C vacancy. For Si vacancies only an outward breathing relaxation of the neighbouring C atoms occurs. Whereas for C vacancies low-spin states are predicted, we find high-spin states for the Si-vacancies.