Positron annihilation and infra-red absorption techniques were used to study metastable defects in irradiated semi-insulating material. The positron results showed that an irradiation-induced metastable vacancy could be generated by illumination with 1.1eV light at 25K. The vacancy could be optically recovered by using 0.85 or 1.3eV photons at 25K. These signals were identical to those which had been detected previously, for irradiation-induced As antisite defects, by using magnetic circular dichroism techniques. It was concluded that the metastable vacancy belonged to the atomic structure of the metastable state of the As antisite. The latter absorbed infra-red light, thus suggesting that its ionization level was situated in the energy gap and not in the conduction band; as in as-grown GaAs. This difference was explained by the presence of other defects that were complexed with the As antisite in electron-irradiated GaAs.

S.Kuisma, K.Saarinen, P.Hautojärvi, C.Corbel: Physical Review B, 1996, 53[12], R7588-91