An interface state density ( D it) distribution near the conduction band edge ( E C) at the Al2O3/Mg-ion-implanted GaN interface was measured after ion implantation, annealing with an AlN protective cap, and cap layer removal. Mg ions were implanted into n-GaN with a Si concentration of 6 × 1017 cm−3 at a maximum Mg concentration of 2 × 1017 cm−3, resulting in the maintenance of the n-type conduction in GaN even after the activation of Mg dopants. Activation annealing was carried out at 1250 °C for 1 min using an AlN cap layer. The complete removal of the AlN cap layer was accomplished by wet etching, which was confirmed by x-ray photoelectron spectroscopy. The photoluminescence spectrum showed donor–acceptor-pair emission after annealing, indicating the activation of Mg acceptors. By applying the capacitance–voltage method to a completed metal–oxide–semiconductor diode, we derived a continuous distribution of relatively low D it below 5 × 1012 cm−2 eV−1, which increased monotonically toward E C in the range from E C − 0.15 to E C − 0.45 eV. Compared with the D it distribution of the as-implanted sample, the density of the discrete level at E C − 0.25 eV generated by divacancies markedly decreased upon 1250 °C annealing.
The gap states near the conduction band edge (E
C) in the vicinity of the interface between Mg-ion-implanted GaN and Al2O3 deposited after post-implantation annealing were investigated in the range between E
C – 0.15 eV and E
C – 0.45 eV. For this purpose, capacitance–voltage measurements were performed on MOS diodes with the n-type conduction of Mg-implanted GaN maintained by suppressing the dose. Although the gap state density D
T was reduced for the sample prepared with the dose of 1.5 × 1012 cm–2 by conventional rapid thermal annealing (RTA) at 1250 °C for 1 min using an AlN protective cap layer, further improvement was achieved by capless ultra-high-pressure annealing (UHPA) at the same temperature for the same duration. Furthermore, the D
T distributions for the samples with capless UHPA at 1400 °C for 5 min are comparable to that for the sample with conventional RTA at 1250 °C for 1 min using the cap layer.
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