VACANCYLIKE STRUCTURE OF THE DX CENTER IN TE-DOPED ALXGA1-XAS

The positron-annihilation method was used to investigate the DX center s in Te-doped AlxGa1-xAs grown by metal-organic vapor-phase epitaxy. V acancy defects were found in all layers. The vacancy signal disappears when the DX center is ionized either optically or thermally. After th e optical ionization the vacancy signal reappears when the temperature increases over 50 K. The optical cross section of 4X10(-17) cm(2) was determined for the removal of the positron trapping at a vacancy. Bec ause the properties of the vacancy signal correlate exactly with those determined earlier for the DX(Te) center, we conclude that the vacanc y detected by positrons belongs to the atomic structure of the DX(Te) center. This result is in agreement with the vacancy-interstitial mode l, which predicts that in the case of group-VI(Te) doping the DX cente r is formed by the distortion of the Ga atom towards the interstitial position. The positron results indicate further that the open volume o f the vacancy related to the DX(Te) center is smaller compared to an i solated monovacancy in GaAs or to the vacancy in the DX(Si) center in AlxGa1-xAs. This means that the distortion in the vacancy-interstitial configuration is smaller in the DX(Te) than in the DX(Si) center.