Positron-annihilation 2D-ACAR study of divacancy and vacancy-oxygen pairs in Si

Two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) experiments have been performed on electron-irradiated Si single crystals containing divacancies with the definite charge states, V-2(-2), V-2(-1) and V-2(0), from 15K to 295K. The 2D-ACAR spectra of unirradiated crystals have been also obtained and show characteristic anisotropies. Using these anisotropies, the 2D-ACAR spectra associated with the divacancies are separated from the partial bulk contribution. The 2D-ACAR spectra for the various charge states are practically isotropic and are very close to each other. However, using a specimen with oriented divacancies V-2(-1), we have obtained a small but definite anisotropy from the divacancies V-2(-1). The small increase in the FWHM width of the 2D-ACAR spectra from the divacancies is observed with decreasing temperature. The trapped fractions at the divacancies increase remarkably with decreasing temperature, not only for negative but also neutral divacancies. The 2D-ACAR spectra from vacancy-oxygen complexes is also obtained. These experimental 2D-ACAR spectra are expected to provide useful and potential information on electronic and atomistic structures of the divacancies in Si and need theoretical calculations, such as first-principle ones.