Vacancylike structure of the DX center in Te-dopeAs

The positron-annihilation method was used to investigate the DX centers in Te-doped (Formula presented)(Formula presented)As grown by metal-organic vapor-phase epitaxy. Vacancy defects were found in all layers. The vacancy signal disappears when the DX center is ionized either optically or thermally. After the optical ionization the vacancy signal reappears when the temperature increases over 50 K. The optical cross section of 4×(Formula presented) was determined for the removal of the positron trapping at a vacancy. Because the properties of the vacancy signal correlate exactly with those determined earlier for the DX(Te) center, we conclude that the vacancy detected by positrons belongs to the atomic structure of the DX(Te) center. This result is in agreement with the vacancy-interstitial model, which predicts that in the case of group-VI(Te) doping the DX center is formed by the distortion of the Ga atom towards the interstitial position. The positron results indicate further that the open volume of the vacancy related to the DX(Te) center is smaller compared to an isolated monovacancy in GaAs or to the vacancy in the DX(Si) center in (Formula presented)(Formula presented)As. This means that the distortion in the vacancy-interstitial configuration is smaller in the DX(Te) than in the DX(Si) center.