EPR and theoretical studies of positively charged carbon vacancy in 4H-SiC

The carbon vacancy is a dominant defect in 4H-SiC, and the ``EI5'' electron-paramagnetic-resonance (EPR) spectrum originates from positively charged carbon vacancies (VC⁺) at quasicubic sites. The observed state for EI5, however, has been attributed to a motional-averaged state with the C₃ᵥ symmetry, and its true atomic structure has not been revealed so far. We here report low temperature (<400.3em0exK) EPR measurements on EI5 and show that this center has a C₁ₕ-symmetric structure due to Jahn-Teller distortion. We also performed ab inito calculations of the hyperfine tensors for EI5, and obtained a good agreement between experiment and theory in not only their principal values but also their principal axis directions. A good agreement was also demonstrated for the EI6 center (hexagonal-site VC⁺) in this paper. The transition from EI5(C₁ₕ) to EI5(C₃ᵥ) was found to be thermally activated and its activation energy was measured as 0.0140.3em0exeV.