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-ACAR measurements on electron-irradiated semi-insulating GaAs. These results are compared to our previous results for the As vacancy. All the vacancy distributions are structureless, though their anisotropies indicate that there is a residual bulk-like component. The magnitude of this component varies from vacancy to vacancy and indicates how much the localized positron wave function extends in the crystal bulk around the 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_As⁰. The momentum distribution sampled by the positron in a Rydberg state about Ga _As^2-, though very similar to that of the bulk, can nevertheless be distinguished from the latter.