Carbon cluster emitters in silicon carbide

Defect qubits in 4H-SiC are outstanding candidates for numerous applications in the rapidly emerging field of quantum technology. Carbon clusters can act as emission sources that may appear after thermal oxidation of 4H-SiC or during irradiation which kicks out carbon atoms from their sites. These fluorescent carbon clusters could interfere with the already established vacancy-related qubits that generated with irradiation techniques. In this study, we systematically investigate the electronic structure, formation energy, dissociation energy, vibrational properties, and the full fluorescence spectrum of carbon clusters involving up to four carbon atoms in 4H-SiC by means of density functional theory calculations. All the possible local configurations for these carbon clusters are carefully evaluated. We find the electronic and vibronic properties of the carbon clusters depend strongly on the local configuration of the 4H-SiC lattice. By comparing the calculated and previously observed fluorescence spectra in 4H-SiC, we identify several carbon clusters as stable visible emitters in 4H-SiC. The paired carbon interstitial defects are identified as the source of the 463-nm triplet and the 456.6-nm emitters. The 471.8-nm emitter in 4H-SiC is associated with tri-carbon antisite clusters. Our findings provide plausible explanation for the origin of visible emission lines in 4H-SiC and propose the possible configurations of carbon clusters which are helpful for the quantum information processing application through qubits in 4H-SiC.