Defects in ion-implanted GaN and their annealing properties were studied by using monoenergetic positron beams. Doppler broadening spectra of the annihilation radiation and the positron lifetimes were measured for Si+, O+, and Be+-implanted GaN grown by the metal-organic chemical vapor deposition technique. First-principles calculations were also used to identify defect species introduced by the implantation. For as-implanted samples, the major defect species was identified as Ga vacancies and/or divacancies. An agglomeration of defects starts after annealing at 400 °C, and the defect profile shifted toward the surface with the open volumes of the defects increasing. The annealing properties of defects were found to depend on the ion species, and they are discussed here in terms of defect concentrations and interactions between impurities and defects.

A relationship between intra-4f transitions of Er and vacancy-type defects in Er-doped GaN was studied by using a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured for Er-doped GaN grown by molecular beam epitaxy. A clear correlation between the defect concentration and the photoluminescence ͑PL͒ intensity was observed. The major defect species detected by positrons was identified as a Ga vacancy V Ga , and its concentration increased with increasing Er concentration ͓Er͔. For the sample with ͓Er͔ = 3.3 at. %, the maximum integrated intensity of PL was observed. The V Ga concentration was above 10 18 cm −3 and additional vacancies such as divacancies started to be introduced at this Er concentration. For the sample with higher ͓Er͔, the PL intensity decreased, and the mean size of vacancies decreased due to an introduction of precipitates and/or metastable phases.

The introduction of vacancy-type defects into InN by Mg-doping was studied using a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured for Mg-doped InN (N-polar) grown on GaN/sapphire templates using plasma-assisted molecular beam epitaxy. The concentration of In-vacancy (VIn) related defects was high near the InN/GaN interface, and the defect-rich region expanded from the interface toward the surface with increasing Mg concentration [Mg]. Using electrolyte-based capacitance-voltage analysis, we determined that the conduction type of InN with low [Mg] (≤1×1018 cm−3) was still n-type. It became p-type with increasing [Mg] (3×1018–2×1019 cm−3), but turned into n-type again above 3×1019 cm−3. The point defects introduced at the conductivity transition from p-type and n-type were found to be complexes between In-vacancy (VIn) and N-vacancy clusters such as VIn(VN)3. Below [Mg]=4×1019 cm−3, an observed behavior of positron annihilation parameters was well explained by assuming the trapping of positrons by N-vacancy clusters such as (VN)3. This fact suggests that, although isolated VN is positively charged, a VN cluster could be a positron trapping center because of the increased electron concentration in the local In-rich region.

Annihilation characteristics of positrons and positronium (Ps) in thin metal and