Structure and vibrational spectra of carbon clusters in SiC

The electronic, structural, and vibrational properties of small carbon interstitial and antisite clusters are investigated by ab initio methods in 3C- and 4H-SiC. The defects possess sizable dissociation energies and may be formed via condensation of carbon interstitials, e.g., generated in the course of ion implantation. All considered defect complexes possess localized vibrational modes (LVM's) well above the SiC bulk phonon spectrum. In particular, the compact antisite clusters exhibit high-frequency LVM's up to 250 meV. The isotope shifts resulting from a C-13 enrichment are analyzed. In the light of these results, the photoluminescence centers D-II and P-U are discussed. The dicarbon antisite is identified as a plausible key ingredient of the D-II center, whereas the carbon split interstitial is a likely origin of the P-T centers. The comparison of the calculated and observed high-frequency modes suggests that the U center is also a carbon-antisite-based defect.