Using positron 2D-ACAR as a probe of point defects in GaAs: The As vacancy as a case study

Two-Dimensional Angular Correlation of positron Annihilation Radiation (2D-ACAR) experiments have been performed on n-type GaAs. By combining these results with those from positron lifetime experiments, the momentum distribution of the arsenic vacancy in its neutral (V_As⁰) and negative (V_As^-) charge states have been extracted. These distributions were all normalized to the respective positron lifetime that characterizes them. The first thing to be noticed is that the momentum distributions of the vacancies, as seen by the positron, are fairly isotropic and structureless. The distribution for V_As⁰ is more peaked than that of V_As^-, while the latter is more intense in the large momentum regions of the spectra. From this, it can be inferred that V_As^- has a smaller open volume than K_As⁰. A closer look at the momentum distribution of the vacancies reveals that they are not entirely isotropic, but, in fact, have a bulk-like component. Finally, the experimental results for bulk GaAs and KAS compare well in a qualitative manner with the momentum distributions that result from an ab-initio molecular dynamics calculation.