Optical processes related to arsenic vacancies in semi-insulating GaAs studied by positron spectroscopy

We have performed positron lifetime and Doppler broadening experiments under monochromatic illumination in undoped semi-insulating GaAs. A negative vacancy, identified as the Ga vacancy, is observed in darkness. Under illumination with 1.42 eV photons below 150 K another type of vacancy is observed. The illumination-induced vacancy is identified as the As vacancy and it has a negative charge state above the ionization level at 60 meV below the conduction band. Under illumination the negative charge state of the As vacancy can be populated either by the trapping of photoelectrons excited from the EL2 defect or by the direct optical excitation of electrons from the valence band. The latter process offers a microscopic explanation for the optical near-band-edge absorption observed in GaAs. in the samples studied in this work the concentrations of both Ga and As vacancies are between 10(15) and 10(16) cm(-3), indicating that they probably play a role in the electrical compensation of the material.