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The structure and optical characteristics of thin (∼30 nm) wurtzite AlInN films grown pseudomorphic on free-standing, c-plane GaN substrates are presented. The Al1−xInxN layers are grown by metalorganic chemical vapor deposition, resulting in films with varying In content from x = 0.142 to 0.225. They are measured using atomic force microscopy, x-ray diffraction, reciprocal space mapping, and spectroscopic ellipsometry (SE). The pseudomorphic AlInN layers provide a set where optical properties can be determined without additional variability caused by lattice relaxation, a crucial need for designing devices. They have smooth surfaces (rms < 0.29 nm) with minimum pit areas when the In content is near lattice-matched to GaN. As expected, SE shows that the refractive index increases and the bandgap energy decreases with increased In-content. Plots of bandgap energy vs In content are fitted with a single bowing parameter of 3.19 eV when using bandgap energies for AlN and InN pseudomorphic to GaN, which is lower than previous measurements and closer to theoretical predictions. 

Enhancement mode AlInN/gallium nitride (GaN) high-electron-mobility transistors (HEMTs) are fabricated by thermally oxidizing the barrier region under the gate. The oxidation is performed at 850 ∘ C in O 2 , and a SiN x mask is used to achieve selective oxidization of the AlInN layer. For comparison, a standard Schottky gate and atomic layer deposition (ALD) Al 2 O 3 metal–insulator–semiconductor (MIS) HEMTs are fabricated from the same structure and show depletion mode behavior as expected. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) mappings are performed to characterize the gate of the oxidized HEMTs, showing complete oxidation of the AlInN barrier. All the devices are tested to determine their transfer and output characteristics. The results show that the thermally oxidized gate produces a positive shift in threshold voltage at ∼ 4 V and low currents ( ∼ 2 × 10 −7 mA/mm) at zero gate voltage. The oxidized HEMTs are also subjected to postmetallization annealing (PMA) at 400 ∘ C and 500 ∘ C for 10 min flowing 1000 sccm of N 2 , retaining enhancement mode behavior and leading to a further positive shift in threshold voltage.