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Positron annihilation experiments have been performed to identify native point defects in GaN bulk crystals as well as in epitaxial layers. The results show that Ga vacancies are present at concentrations of 1017 – 1018 cm−3 in undoped GaN bulk crystals and layers, whereas the Mgdoped samples are free of Ga vacancies. The Ga vacancies are negatively charged and their concentration correlates with the intensity of the yellow luminescence. We conclude that the Ga vacancies contribute to the electrical compensation of n-type GaN and their acceptor levels are involved in the yellow luminescence transition.

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Positron-lifetime experiments have been performed in Zn-doped p-type and undoped semi-insulating GaAs in the temperature range 20 -300 K to investigate native point defects. In p-type materials with hole concentrations of 10"-10' cm, no evidence of positron trapping is observed. The temperature dependence of the positron lifetime can be explained in terms of lattice expansion associated with positron-phonon coupling. Therefore, we ascribe it to delocalized positrons. In semi-insulating GaAs, two kinds of acceptors are detected with concentrations in the range 10"-10' cm '. gallium vacancies and negative ions. The temperature dependence of the positron trapping at the Ga vacancy exhibits a slope break at about 130 K. A weakly bound Rydberg-like precursor state is invoked to explain this temperature dependence.

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 Ϫ3 , indicating that they probably play a role in the electrical compensation of the material.