Optical transitions of the silicon vacancy in 6H-SiC studied by positron annihilation spectroscopy

Positron annihilation spectroscopy has been applied to identify Si and C vacancies as irradiation-induced defects in 6H-SiC. Si vacancies are shown to have ionization levels at E_C-0.6 eV and E_C-1.1 eV below the conduction-band edge E_C by detecting changes of positron trapping under monochromatic illumination. These levels are attributed to (2-/1-) and (1-/0) ionizations of the isolated Si vacancy. In as-grown n-type 6H-SiC, a native defect complex involving V_Si is shown to have an ionization level slightly closer to conduction band at roughly E_C-0.3 eV. These results are used further to present microscopic interpretations to effects seen in optical-absorption spectra and to electrical levels observed previously by deep-level transient spectroscopy.