First-principles calculations were made of the neutral and charged Si and C monovacancies in cubic and hexagonal material. The calculations were based upon density functional theory in the local-density approximation, as well as upon the local spin-density approximation. A plane-wave super-cell approach was combined with ultra-soft Vanderbilt pseudopotentials in order to permit convergent calculations. A study was made of the atomic structure, energetics, and charge-dependent and spin-dependent vacancy states. Generation of the C-site vacancy was usually associated with a marked Jahn-Teller distortion. In the case of the Si-site vacancy, only an outward breathing relaxation occurred; due to a strong localization of the C dangling bonds at the neighboring C atoms. Thus, high-spin configurations were predicted for Si vacancies whereas the low-spin states of C vacancies exhibited a negative-U behavior. In the case of hexagonal polytypes, crystal-field splitting of the upper vacancy levels did not primarily modify the properties of the vacancies. On the other hand, the inequivalent lattice sites gave rise to site-related shifts in the electronic states.

Vacancies in SiC: Influence of Jahn-Teller Distortions, Spin Effects and Crystal Structure. A.Zywietz, J.Furthmüller, F.Bechstedt: Physical Review B, 1999, 59[23], 15166-80