Photoionization of the silicon divacancy studied by positron-annihilation spectroscopy

The optical ionization of the silicon divacancy in 2-MeV electron-irradiated Si was studied by using positron-lifetime and positron-electron momentum distribution measurements under illumination with monochromatic light. Upon irradiation at room temperature, negative and neutral divacancies are detected in both float zone and Czochralski Si by positron-annihilation measurements in darkness. The positron-annihilation characteristics of the divacancy are determined as ${\tau }_d=300\mathrm{}\left(5\right)\mathrm{}$ $\mathrm{ps}=1.35\mathrm{}\left(2\right)\mathrm{}\times {\tau }_b,$ $S_d$ $=1.055\mathrm{}\left(3\right)\mathrm{}\times S_b,$ and $W_d=0.75\mathrm{}\left(2\right)\mathrm{}\times W_b.$ Illumination at 15 K with monochromatic 0.70–1.30 eV light has a strong effect on the positron trapping rate to the divacancies. The results can be understood in terms of optical electron and hole emission from the electron levels $V_2^{-l0\mathrm{}}$ and $V_2^{2-l-}$ of the divacancy. The changes in the positron trapping rate are due to the different sensitivities of the positron to the charge states $V_2^0,$ $V_2^{-},$ and $V_2^{2-}.$ The spectral shape of the positron trapping rate under illumination reveals an electron level at $E_v$ +0.75 eV, which is attributed to the ionization level $V_2^{2-/-}$ of the divacancy.