Negative-U carbon vacancy in 4H-SiC: Assessment of charge correction schemes and identification of the negative carbon vacancy at the quasicubic site

The carbon vacancy (V-C) has been suggested by different studies to be involved in the Z(1)/Z(2) defect-a carrier lifetime killer in SiC. However, the correlation between the Z(1)/Z(2) deep level with V-C is not possible since only the negative carbon vacancy (V-C(-)) at the hexagonal site, V-C (h), with unclear negative-U behaviors was identified by electron paramagnetic resonance (EPR). Using freestanding n-type 4H-SiC epilayers irradiated with low energy (250 keV) electrons at room temperature to introduce mainly V-C and defects in the C sublattice, we observed the strong EPR signals of V-C(-)(h) and another S = 1/2 center. Electron paramagnetic resonance experiments show a negative-U behavior of the two centers and their similar symmetry lowering from C-3v to C-1h at low temperatures. Comparing the Si-29 and C-13 ligand hyperfine constants observed by EPR and first principles calculations, the new center is identified as V-C(-)(k). The negative-U behavior is further confirmed by large scale density functional theory supercell calculations using different charge correction schemes. The results support the identification of the lifetime limiting Z(1)/Z(2) defect to be related to acceptor states of the carbon vacancy.