[en] Slow positrons were used to study impurity diffusion mechanisms in Si-doped GaAs grown on Be-doped substrates by molecular beam epitaxy. The S parameter correlates with the concentration of V_Ga acceptors increases continuously with the Si doping concentration, which supports the theoretical model that the creation of V_Ga acceptors is related to the change of the Fermi level position in Si-doped GaAs. This implies that Si diffuses as a single Si-vacancy neutral complex (Si_Ga⁺-V_Ga^-) rather than the paired Si (Si_Ga⁺-Si_As^-). When the specimens were annealed with a face-to-face contact GaAs, the S parameter value saturated at that for trapping at a monovacancy. However the formation of divacancies (V_Ga^--V_As⁺) was observed in undoped GaAs wafers annealed under the same conditions. These gives the evidence on the formation of monovacancy complexes such as (V_As⁺-Ge_Ga^-) and (V_Ga^--Si_Ga⁺) in impurity-containing GaAs because of the Coulomb interaction between the two dipoles