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In this study, we fabricated and characterized heterojunction field-effect transistors (HFETs) based on an Al0.36Ga0.64N-channel heterostructure with a dual AlN/AlGaInN barrier layer. The device fabrication was accomplished by adopting a regrown n++-GaN layer for ohmic contacts. The fabricated HFETs with a gate length of 2 μ m and a gate-to-drain distance of 6 μ m exhibited an on-state drain current density as high as approximately 270 mA mm−1 and an off-state breakdown voltage of approximately 1 kV, which corresponds to an off-state critical electric field of 166 V μ m−1. This breakdown field, as a comparison in devices without field-plate electrodes, reaches approximately four-fold higher than that for conventional GaN-channel HFETs and was considered quite reasonable as an Al0.36Ga0.64N-channel transistor. It was also confirmed that the devices adopting the dual AlN/AlGaInN barrier layer showed approximately one order of magnitude smaller gate leakage currents than those for devices without the top AlN barrier layer. [ABSTRACT FROM AUTHOR]

We investigated the relationship between the optical constants and localized states near the band-edge in high-quality crystalline Al1âˆ' x In x N alloys, with an indium content x ranging from 0.12 to 0.22, grown on a c -plane freestanding GaN substrate. Optical constants were obtained by spectroscopic ellipsometry. The tanΨ and cosÎ" spectra were fitted by the Adachi’s critical-point (AC) model. The effects of the near-band-edge localized states on the optical constants were characterized by the spectral broadening factor Îł, which was obtained by the AC model. The Îł increased with increasing x and this tendency also confirmed by the Îł obtained by the photoluminescence excitation (PLE). The bandgap energies obtained by the AC model agree well with those obtained by the PLE. It is suggested that the indium-related near-band-edge localized states cause the spectral broadening of the refractive index and extinction coefficient spectra in Al1âˆ' x In x N alloys. [ABSTRACT FROM AUTHOR]

The optical constants and bandgap energy (Eg) of semiconductors, characterized by spectroscopic ellipsometry (SE), are highly affected by the applied model dielectric function (MDF). In this study, we investigated the optical constants and Eg in low indium content (x) Al1-xInxN alloys grown on a c-plane freestanding GaN substrate by using SE, and their applied MDF dependence was investigated. The tanΨ and cosΔ spectra, in a photon energy range of 1.5–5.0 eV, of high-quality crystalline Al1-xInxN alloys were well fitted by Adachi's critical-point (ACP) model. The ACP model exhibited better spectral fitting results than did the Tauc–Lorentz model, which has often been adopted for Al1-xInxN alloys. The accuracy of the Eg values obtained by the ACP model was confirmed by optical reflectance measurements. It is suggested that the ACP model is a suitable MDF for recent high-quality crystalline Al1-xInxN alloys as well as other III-nitride semiconductors. [ABSTRACT FROM AUTHOR]