Optical ionization of the Si di-vacancy in samples which had been subjected to 2MeV electron irradiation was studied by performing positron-lifetime and positron-electron momentum distribution measurements under illumination with monochromatic light. After irradiation at room temperature, negative and neutral di-vacancies were detected in both float-zone and Czochralski-type material by performing positron annihilation measurements in the dark. The positron annihilation lifetime of the di-vacancy was 300ps. Illumination with monochromatic (0.70 to 1.30eV) light at 15K had a marked effect upon the positron trapping rate to di-vacancies. The results could be explained in terms of optical electron and hole emission from the electron levels, V2-l0 and V22-l-, of the di-vacancy. Changes in the positron trapping rate were attributed to differing sensitivities of the positron to the charge states, V20, V2- and V22-. The spectral shape of the positron trapping rate under illumination revealed an electron level, at Ev + 0.75eV, which was attributed to the V22-l- ionization level of the di-vacancy.

H.Kauppinen, C.Corbel, J.Nissilä, K.Saarinen, P.Hautojärvi: Physical Review B, 1998, 57[20], 12911-22