First-principles calculations were made of the neutral and charged Si and C monovacancies in the cubic or hexagonal phases. 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 obtain convergent solutions. A study was made of the atomic structure, energetics, and charge- and spin-dependent vacancy states. Generation of the C-site vacancy was usually accompanied by 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 neighbouring C atoms. High-spin configurations were therefore predicted for Si vacancies, while low-spin states of the C vacancies exhibited negative-U behavior. In the case of hexagonal polytypes, crystal-field splitting of the upper vacancy levels did not modify the properties of the vacancies. However, the inequivalent lattice sites gave rise to site-related shifts of 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