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 τd=300(5) ps=1.35(2)×τb, Sd =1.055(3)×Sb, and Wd=0.75(2)×Wb. 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 V₂^-l0 and V₂^2-l- of the divacancy. The changes in the positron trapping rate are due to the different sensitivities of the positron to the charge states V₂⁰, V₂^-, and V₂^2-. The spectral shape of the positron trapping rate under illumination reveals an electron level at Eᵥ +0.75 eV, which is attributed to the ionization level V₂^2-/- of the divacancy.