It was recalled that, at a vacant lattice site, positron-ion repulsion was reduced. This then led to positron trapping, which caused observable changes in the annihilation characteristics. That is, the positron lifetime increased and the positron-electron momentum distribution narrowed. Due to long-range Coulomb interactions, the charge state of a vacancy had a large effect upon positron trapping. The lattice relaxation due to the charge-state transition of a vacancy was also seen in the positron lifetime, and the ionization levels of vacancy defects could therefore be determined. The method was applied to native vacancies in GaAs, and the positrons revealed As and Ga vacancies in bulk crystals. The As vacancies had negative, neutral and positive charge states in the upper part of the band-gap, whereas the Ga vacancies were negative. The EL2 defect had a vacancy in its metastable state. A vacancy was also seen in the deep ground state of the DX center in AlGaAs. The positron results thus supported the vacancy-interstitial model for the structures of EL2 and DX centers.

P.Hautojärvi: Journal de Physique III, 1995, 5[C1], 3-14