Experimental data on the microscopic structure of the DX center in Si-doped material were presented. Positron annihilation spectroscopy revealed the vacancy-like structure of the Si-DX center. The vacancy signal disappeared upon illumination with infra-red light, and the optical cross-section for this process was equal to the photo-ionization cross-section for the Si-DX center. The critical temperature, below which illumination made the vacancy signal disappear, was related to the persistent photo-conductivity effect. The thermal ionization energy of the Si-DX center was estimated from positron experiments, and it was demonstrated that thermal ionization of the DX center accounted for the disappearance of the vacancy at high temperatures. The temperature dependence of the positron trapping rate was typical of a negatively charged vacancy. The structural data which were obtained from positron annihilation spectroscopy were consistent with a vacancy-interstitial model that explained the metastability of the DX center in terms of 2 different lattice sites for the donor impurity.

J.Mäkinen, T.Laine, K.Saarinen, P.Hautojärvi, C.Corbel, V.M.Airaksinen, J.Nagle: Physical Review B, 1995, 52[7], 4870-83