STUDY OF DEFECTS IN GAAS BY 2D-ACAR POSITRON-ANNIHILATION

We describe the two dimensional angular correlation of positron annihi lation radiation (2D-ACAR) method and show that it can be advantageous ly used to study the electronic structure of defects, in addition to s tandard positron lifetime and Doppler broadening measurements. Using a nnihilation fractions determined by lifetime measurements, we separate 2D-ACAR distributions for negatively charged and neutral arsenic vaca ncies in n-type GaAs. In electron-irradiated semi-insulating GaAs, we present 2D-ACAR results for the negatively charged gallium vacancy and for positron Rydberg states induced by gallium antisites. The 2D-ACAR for delocalized positrons, needed in the separation process, has been obtained from measurements on as-grown semi-insulating GaAs. Our resu lts outline the capabilities of the 2D-ACAR technique when it is appli ed to the study of defects: the electronic structure in the vacancies are found to be well described by recent molecular dynamics calculatio ns. The method also provides information about atomic relaxations arou nd the vacancy sites, they are in agreement with the calculated relaxa tions. We conclude that the 2D-ACAR method is a promising tool to inve stigate defects in semiconductors.