Divacancy in3C−and4H−SiC:An extremely stable defect

Using first-principles calculations for divacancy defects in $3C-$ and $4H-\mathrm{SiC},$ 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 $4H-\mathrm{SiC}$ all four possible nearest-neighbor divacancy configurations are considered. We find not only a remarkable high binding energy of about $4\hspace{0.5em}\mathrm{eV},$ 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 $E_{\mathrm{V}}+0.7\mathrm{}\hspace{0.5em}\mathrm{eV}$ between the charge states $1+/1-$ only for nearest-neighbor divacancies on different lattice sites (mixed cubic and hexagonal) in $4H-\mathrm{SiC},$ but not for all the other cases (pure cubic or pure hexagonal) in $4H-$ or for the cubic divacancy in $3C-\mathrm{SiC}.$