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 Czockralski Si by positron-annihilation measurements in darkness. The positron-annihilation characteristics of the divacancy are determined as tau(d)=300(5) ps = 1.35(2)x tau(b), S-d = 1.055(3) x S-b, and W-d = 0.75(2) x 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) 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-u + 0.75 eV, which is attributed to the ionization level V-2(2-/-) of the divacancy.