The effect of increasing the dosage on the electrical properties of Mg‐ion‐implanted GaN before activation annealing is investigated to obtain knowledge on the defect levels generated by ion implantation. To probe the near‐surface region, GaN metal‐oxide‐semiconductor (MOS) structures with Al2O3 are used. Two kinds of MOS diodes with Mg‐ion dosages of 1.5 × 1011 and 1.5 × 1012 cm−2 implanted at 50 keV are prepared. Although anomalous capacitance–voltage (C–V) characteristics are observed for the low‐dosage sample, they are improved by annealing at 600 °C for 3 h. However, for the high‐dosage sample, more severe and persistent frequency dispersion is observed in the C–V characteristics, which is not improved by the same annealing. On the basis of the detailed analysis of capacitance–frequency (C–f) characteristics, it is concluded that the discrete interface trap at 0.2–0.3 eV below the conduction band is responsible for the frequency dispersion observed for the high‐dosage sample. Combined with the results of deep‐level transient spectroscopy, it is highly likely that the bulk deep levels affect the C–V and C–f characteristics. The possibility that the dominant deep levels are changed by the increase in Mg‐ion dosage is discussed.
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