By using first-principles calculations of divacancy defects in 3C- and 4H-SiC, formation energies and stability, ionization levels and relaxed geometries (symmetry point groups) were determined for neutral and charged states. In 4H-SiC, all 4 possible nearest-neighbor divacancy configurations were considered. Not only was a markedly high binding energy (about 4eV) found, but also a strong site-dependence (cubic or hexagonal lattice sites) of the formation energies. Upon using a Madelung-type correction to deal with the electrostatic interactions between charged super-cells, the results indicated a negative-U behavior at EV+0.7eV between the charge states 1+/1– only for nearest-neighbor divacancies on different lattice sites (mixed cubic and hexagonal) in 4H-SiC, but not for all of the other cases (pure cubic or pure hexagonal) in 4H-SiC; or for the cubic divacancy in 3C-SiC.

Divacancy in 3C- and 4H-SiC - an Extremely Stable Defect. L.Torpo, T.E.M.Staab, R.M.Nieminen: Physical Review B, 2002, 65[8], 085202