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 (Vc) has been suggested by different studies to be involved in the Z1/Z2 defect-a carrier lifetime killer in SiC. However, the correlation between the Z1/Z2 deep level with Vc 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 Vc 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 C3v to C1h at low temperatures. Comparing the 29Si and 13C 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 Z1/Z2 defect to be related to acceptor states of the carbon vacancy. [ABSTRACT FROM AUTHOR]