We have investigated the relationship between improved electrical properties of Al 2 O 3 /AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) and electronic state densities at the Al 2 O 3 /AlGaN interface evaluated from the same structures as the MOS-HEMTs. To evaluate Al 2 O 3 /AlGaN interface state densities of the MOS-HEMTs, two types of capacitance-voltage (C-V) measurement techniques were employed: the photo-assisted C-V measurement for the near-midgap states and the frequency dependent C-V characteristics for the states near the conduction-band edge. To reduce the interface states, an N 2 O-radical treatment was applied to the AlGaN surface just prior to the deposition of the Al 2 O 3 insulator. As compared to the sample without the treatment, the N 2 O-radical treated Al 2 O 3 /AlGaN/GaN structure showed smaller frequency dispersion of the C-V curves in the positive gate bias range. The state densities at the Al 2 O 3 /AlGaN interface were estimated to be 1 Â 10 12 cm À2 eV À1 or less around the midgap and 8 Â 10 12 cm À2 eV À1 near the conduction-band edge. In addition, we observed higher maximum drain current at the positive gate bias and suppressed threshold voltage instability under the negative gate bias stress even at 150 C. Results presented in this paper indicated that the N 2 O-radical treatment is effective both in reducing the interface states and improving the electrical properties of the Al 2 O 3 /AlGaN/GaN MOS-HEMTs. V C 2013 AIP Publishing LLC. [http://dx

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Interface characterization was carried out on Al2O3/GaN structures using epitaxial n-GaN layers grown on free-standing GaN substrates with relatively low dislocation density (<3 x 10(6) cm(-2)). The Al2O3 layer was prepared by atomic layer deposition. The as-deposited metal-oxide-semiconductor (MOS) sample showed a significant frequency dispersion and a bump-like feature in capacitance-voltage (C-V) curves at reverse bias, showing high-density interface states in the range of 10(12) cm(-1) eV(-1). On the other hand, excellent C-V characteristics with negligible frequency dispersion were observed from the MOS sample after annealing under a reverse bias at 300 degrees C in air for 3 h. The reverse-bias-annealed sample showed state densities less than 1 x 10(11) cm(-1) eV(-1) and small shifts of flat-band voltage. In addition, the C-V curve measured at 200 degrees C remained essentially similar compared with the room-temperature C-V curves. These results indicate that the present process realizes a stable Al2O3/GaN interface with low interface state densities

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This paper presents a systematic characterization of electronic states at insulators/(Al)GaN interfaces, particularly focusing on insulator/AlGaN/ GaN structures. First, we review important results reported for GaN metal-insulator-semiconductor (MIS) structures. SiO 2 is an attractive material for MIS transistor applications due to its large bandgap and high chemical stability. In-situ SiN x is effective for improving the operation stability of high electron mobility transistors (HEMTs). Meanwhile, Al 2 O 3 /GaN structures have high band offsets and low interface state densities, which are also desirable for insulated gate applications. We have proposed a calculation method for describing capacitance-voltage (C-V) characteristics of HEMT MIS structures for evaluating electronic state properties at the insulator/AlGaN interfaces. To evaluate near-midgap states at insulator/ AlGaN interfaces, a photo-assisted C-V technique using photon energies less than the bandgap of GaN has been developed. Using the calculation in conjunction with the photo-assisted C-V technique, we estimate interface state density distributions at the Al 2 O 3 /AlGaN interfaces.