Positron lifetime and Doppler broadening experiments were performed on undoped semi-insulating material under monochromatic illumination A negative vacancy, identified as being the Ga vacancy, was observed in darkness. Under illumination with 1.42eV photons at temperatures below 150K, another type of vacancy was observed. The illumination-induced vacancy was identified as being the As vacancy, and it had a negative charge state above the ionization level; at 0.06eV below the conduction band. Under illumination, the negative charge state of the As vacancy could be populated either by the trapping of photo-electrons excited from the EL2 defect, or by the direct optical excitation of electrons from the valence band. The latter suggested a microscopic explanation for the optical near-bandedge absorption which was observed in this material. In the samples which were studied here, the concentrations of both Ga and As vacancies were between 1015 and 1016/cm3; thus indicating that they probably played a role in the electrical compensation of the material.

S.Kuisma, K.Saarinen, P.Hautojärvi, C.Corbel, C.LeBerre: Physical Review B, 1996, 53[15], 9814-30