Effects of Pd or Pt addition to Ti/Al ohmic contact materials used for n-type Al0.2Ga0.8N grown on the GaN substrates were studied where Si with 2×1018 cm−3 were doped in the AlGaN layers. The contact metals were prepared by depositing either by the electron-beam or thermal evaporation technique, and then annealed at high temperatures for 30 s in N2 atmosphere. It was found that the addition of a small amount of Pd or Pt to the Ti/Al ohmic contact materials reduced the contact resistivities of the Ti/Al contacts. The lowest contact resistivity of 4.1×10−6 Ω cm2 was obtained in the Ti(20 nm)/Pd(5 nm)/Al(200 nm) contact after annealing at 600 °C (where a slash “/” indicates the deposition sequence). The reduction of the contact resistivities by the Pd or Pt addition was believed to be due to increase in the donor concentration in the AlGaN layers by forming Pd–Ga or Pt–Ga compounds in the contact metals after annealing at elevated temperatures.

Mg ions were implanted into Si-doped (5 × 1017 cm–3) n-GaN at a dose of 1.5 × 1011 or 1.5 × 1012 cm–2. MOS diodes were used to characterize the implanted GaN after 300 °C annealing for 3 h and after additional 500 °C annealing for 3 min. Although capacitance–voltage (C–V) characteristics varied with the dosage, the effects of acceptor-like defects induced by ion implantation were observed in the C–V characteristics independently of dosage and annealing temperature. A defect level at approximately 0.25 eV below the conduction band edge was detected electrically. By positron annihilation spectroscopy, its origin was identified as a divacancy consisting of Ga and N vacancies. It was found that its density compared with that of as-implanted GaN decreased with 300 °C annealing, and further increased with 500 °C annealing. This phenomenon was explained on the basis of the difference between the diffusion barriers of possible point defects.

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.

The encapsulant-dependent effects of long-term low-temperature annealing on defects in Mg-ion-implanted GaN have been investigated using metal-oxide-semiconductor (MOS) diodes. Annealing was carried out at 600 ℃ under nitrogen flow without or with a cap layer of Al2O3, SiO2, or SiN. For annealing at 600 ℃ for 3 h, the capacitance-voltage characteristics of the Al2O3 cap annealed samples indicated the existence of acceptor-like defects, whereas those of the capless, SiO2 cap and SiN cap annealed samples exhibited bumps, which indicated the existence of donor-like defect level at around 0.8 eV from the conduction band edge EC. A more distinct result was obtained for annealing at 600 ℃ for 30 h. Namely, annealing of samples with the Al2O3 cap layer induced an acceptor-like defect level at EC -0.9 eV, whereas that with the SiN cap layer induced a donor-like defect level at EC -0.8 eV. Secondary ion mass spectroscopy and transmission electron microscopy studies revealed that interstitial Ga (Gai) in Mg-implanted GaN diffused into the Al2O3 cap layer but not into the SiN cap layer after annealing. Most possibly, the detected EC -0.8 eV level can be assigned to interstitial Gai.

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