OBSERVATION OF A VACANCY AT THE DX CENTER IN SI-DOPED AND SN-DOPED ALGAAS

A vacancy defect is observed by positron annihilation in n-type Si- an d Sn-doped AlxGa1-xAs (x greater-than-or-equal-to 0.18). The vacancy i s not observed after optical ionization of the DX center. The disappea rance of the vacancy signal is persistent below a critical temperature . Thermal ionization of the DX center removes the vacancy signal above 300 K. We conclude that the deep ground state of the DX center contai ns the vacancy. The results are in perfect agreement with the theoreti cal predictions of the large displacements of the Si and Sn atoms from the substitutional configuration when the DX state is occupied.