Microscopic structure of the DX center in Si-doped AlxGa1-xAs: Observation of a vacancy by positron-annihilation spectroscopy

Experimental results on the microscopic structure of the DX center in Si-doped AlxGa1-xAs are presented. Positron-annihilation spectroscopy indicates the vacancylike structure of the Si-DX center. The vacancy signal disappears persistently by illumination with infrared light. The optical cross section for this process is equal to the photoionization cross section for the Si-DX center. The critical temperature below which illumination makes the vacancy signal disappear is correlated to the persistent photoconductivity effect. We estimate the thermal ionization energy of the Si-DX center from positron experiments and demonstrate that thermal ionization of the DX center accounts for the disappearance of the vacancy at high temperatures. Temperature dependence of the positron trapping rate is typical of a negatively charged vacancy. The structural data from positron-annihilation spectroscopy are consistent with the vacancy-interstitial model that explains the metastability of the DX center in terms of two different lattice sites of the donor impurity.