Theory of the thermal stability of silicon vacancies and interstitials in 4h–sic

Abstract

This paper presents a theoretical study of the electronic and dynamic properties of silicon vacancies and self-interstitials in 4H–SiC using hybrid density functional methods. Several pending issues, mostly related to the thermal stability of this defect, are addressed. The silicon site vacancy and the carbon-related antisite-vacancy (CAV) pair are interpreted as a unique and bistable defect. It possesses a metastable negative-U neutral state, which “disproportionates” into V+Si or V−Si, depending on the location of the Fermi level. The vacancy introduces a (−/+) transition, calculated at Ec − 1.25 eV, which determines a temperature threshold for the annealing of VSi into CAV in n-type material due to a Fermi level crossing effect. Analysis of a configuration coordinate diagram allows us to conclude that VSi anneals out in two stages—at low temperatures (T ≲ 600◦C) via capture of a mobile species (e.g., self-interstitials) and at higher temperatures (T ≳ 1200◦C) via dissociation into VC and CSi defects. The Si interstitial (Sii) is also a negative-U defect, with metastable q = +1 and q = +3 states. These are the only paramagnetic states of the defect, and maybe that explains why it escaped detection, even in p-type material where the migration barriers are at least 2.7 eV high.