  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.
