Positron lifetime measurements of electron-irradiated semi-insulating samples indicated the presence of Ga vacancies and Ga antisites in a negative charge state. Illumination of the sample revealed another vacancy defect which was metastable at 30K and which annealed out at 100 to 120K. The introduction rate of this defect was about 0.3/cm. The metastable properties of the defect resembled those of the EL2 center in as-grown material. The defects were attributed to As antisites which, in the metastable configuration, relaxed away from the lattice site to leave a vacant Ga site.

K.Saarinen, S.Kuisma, J.Mäkinen, P.Hautojärvi, M.Törnqvist, C.Corbel: Materials Science Forum, 1995, 196-201, 1055-60