Positron annihilation is an effective method for the investigation of vacancy-type defects in semiconductors. Positrons can get trapped at vacancies, resulting in observable changes in the characteristics of the annihilation radiation. The annihilation data can be used to determine vacancy concentrations as well as to distinguish between different vacancy types. In this work, we apply positron annihilation to investigate radiation-induced defects in InN and GaN grown by molecular-beam epitaxy. InN and GaN samples were irradiated with 2-MeV ⁴He⁺ ions to fluences in the range 5×10¹⁴–2×10¹⁶ cm^-2, and subsequently subjected to rapid thermal-annealing below 500°C. This has previously been found to result in increased electron concentrations and mobilities in the InN films.

Our results show that in InN, negative In vacancies are introduced in the irradiation at a significantly low rate of 100 cm^-1, with their concentration saturating in the mid-10¹⁷ cm^-3 at an irradiation fluence of 2×10¹⁵ cm^-2. The annealing is found to result in efficient clustering of the In vacancies near the film-substrate interface, in good agreement with results on void formation obtained by transmission electron microscopy. This is completely opposite to the behaviour observed in GaN, where Ga vacancies are introduced in the irradiation at a much higher rate of 3600 cm^-1 showing no saturation, but where no clustering or recovery is seen after annealing. The cluster formation suggests that a large number of N vacancies need to be present in the InN films after irradiation, possibly being the cause of the increased n-type conductivity. Also, it is likely that the introduction rate of In vacancies is much higher than observed, which could be evidence of a similar acceptor-to-donor transition as the V_Ga ↔ (V_As + As_Ga) transition in GaAs.

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