Divacancy in3C−and4H−SiC:An extremely stable defect

Abstract

Using first-principles calculations for divacancy defects in $3C\ensuremath{-}$ and $4H\ensuremath{-}\mathrm{SiC},$ we determine their formation energies and stability, their ionization levels, and relaxed geometries (symmetry point groups) for neutral as well as for charged states. For $4H\ensuremath{-}\mathrm{SiC}$ all four possible nearest-neighbor divacancy configurations are considered. We find not only a remarkable high binding energy of about $4 \mathrm{eV},$ but also a strong site dependence (cubic or hexagonal lattice sites) of the formation energies. Applying a Madelung-type correction to deal with the electrostatic interactions between charged supercells, our results indicate a negative-$U$ behavior at ${E}_{\mathrm{V}}+0.7 \mathrm{eV}$ between the charge states $1+/1\ensuremath{-}$ only for nearest-neighbor divacancies on different lattice sites (mixed cubic and hexagonal) in $4H\ensuremath{-}\mathrm{SiC},$ but not for all the other cases (pure cubic or pure hexagonal) in $4H\ensuremath{-}$ or for the cubic divacancy in $3C\ensuremath{-}\mathrm{SiC}.$