Optical transitions of the silicon vacancy in6H−SiCstudied 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 $E_C-0.6\mathrm{eV}$ and $E_C-1.1\mathrm{eV}$ below the conduction-band edge $E_C$ by detecting changes of positron trapping under monochromatic illumination. These levels are attributed to $(2\mathrm{}-/1\mathrm{}-)$ and $(1\mathrm{}-/0)$ ionizations of the isolated Si vacancy. In as-grown n-type 6H-SiC, a native defect complex involving $V_{\mathrm{Si}}$ is shown to have an ionization level slightly closer to conduction band at roughly $E_C-0.3\mathrm{eV}.$ 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.