POSITRON STATES AT POINT-DEFECTS IN GAAS MEASURED BY 2D-ACAR

The electron-positron momentum distribution in the Ga vacancy (V-GA(3- )) and Ga antisite (Ga-As(2-)) have been obtained by performing 2D-ACA R measurements on electron-irradiated semi-insulating GaAs. These resu lts are compared to our previous results for the As vacancy. All the v acancy distributions are structureless, though their anisotropies indi cate that there is a residual bulk-like component. The magnitude of th is component varies from vacancy to vacancy and indicates how much the localized positron wave function extends in the crystal bulk around t he vacancy. Our results show that this extension of the wave function in V-Ga(3-) and V-As(-) is similar to and clearly greater than that in V-As(O). The momentum distribution sampled by the positron in Rydberg state about Ga-As(2-), though very similar to that of the bulk, can n evertheless be distinguished from the latter.