We use positron annihilation spectroscopy to study 2 MeV 4 He + -irradiated InN grown by molecular-beam epitaxy and GaN grown by metal-organic chemical-vapor deposition. In GaN, the Ga vacancies act as important compensating centers in the irradiated material, introduced at a rate of 3600 cm −1 . The In vacancies are introduced at a significantly lower rate of 100 cm −1 , making them negligible in the compensation of the irradiation-induced additional n-type conductivity in InN. On the other

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... d, negative non-open volume defects are introduced at a rate higher than 2000 cm −1 . These defects are tentatively attributed to interstitial nitrogen and may ultimately limit the free-electron concentration at high irradiation fluences.