Monovacancies in 3C and 4H SiC

We present first-principles calculations of the atomic and electronic struc ture of neutral and charged C and Si vacancies in 3C and 4H SiC. We demonst rate that the principal properties of the vacancies depend only weakly on t he 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 hig h-spin states for the Si-vacancies. (C) 1999 Elsevier Science S.A. All righ ts reserved.