Positron lifetime techniques were used to investigate the metastability of the point defects which were produced by the irradiation of semi-insulating material. Measurements which were performed in darkness indicated the presence of Ga vacancies and Ga antisite defects in a negative charge state. Illumination at 25K revealed another type of defect, which had a vacancy in its metastable state. The metastable vacancies could be observed most effectively after illumination with 1.1eV photons, and persisted up to annealing temperatures of between 80 and 100K. The introduction rate of the metastable defects was about 0.3/cm. This was close to values which had been reported for the As antisite. The metastable properties of the defects resembled those of the EL2 center in as-grown GaAs. These defects were suggested to be As antisites; which exhibited the metastability that was predicted by theory. In the metastable configuration, the As antisite atom relaxed away from the lattice position to leave a vacant Ga site.

K.Saarinen, S.Kuisma, J.Mäkinen, P.Hautojärvi, M.Törnqvist, C.Corbel: Physical Review B, 1995, 51[20], 14152-63