  We report first-principles calculations that clarify the formation energies
and charge transition levels of native point defects and carbon clusters in the
4H polytype of silicon carbide (4H-SiC) under a carbon-rich condition. We
applied a hybrid functional that reproduces the experimental bandgap of SiC
well and offers reliable defect properties. For point defects, we investigated
single vacancies, antisites, and interstitials of Si and C on relevant sites.
For carbon clusters, we systematically introduced two additional C atoms into
the perfect 4H-SiC lattice with and without removing Si atoms and performed
structural optimization to identify stable defect configurations. We found that
neutral Si antisites are energetically favorable among Si-point defects in a
wide range of the Fermi level position around the intrinsic regime, whereas
negatively-charged Si vacancies and a positively-charged Si interstitial on a
site surrounded by three Si and three C atoms become favorable under n- and
p-type conditions, respectively. For C-point defects, neutral C antisites are
favorable under intrinsic and n-type conditions, whereas positively-charged C
vacancies become favorable under p-type conditions. We also found that a
di-carbon antisite is more favorable than a C-split interstitial, which is the
most stable form of single C interstitials.
