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 (Formula presented) and (Formula presented) 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 (Formula presented) and (Formula presented) of the divacancy. The changes in the positron trapping rate are due to the different sensitivities of the positron to the charge states (Formula presented) and (Formula presented) The spectral shape of the positron trapping rate under illumination reveals an electron level at (Formula presented) +0.75 eV, which is attributed to the ionization level (Formula presented) of the divacancy.