The valence band offsets, ⌬E V , of In 0.17 Al 0.83 N / GaN, In 0.25 Al 0.75 N / GaN, and In 0.30 Al 0.70 N / GaN heterostructures grown by metal-organic vapor phase epitaxy were evaluated by using x-ray photoelectron spectroscopy ͑XPS͒. The dependence of the energy position and the full width at half maximum of the Al 2p spectrum on the exit angle indicated that there was sharp band bending caused by the polarization-induced electric field combined with surface Fermi-level pinning in each ultrathin InAlN layer. The ⌬E V values evaluated without taking into account band bending indicated large discrepancies from the theoretical estimates for all samples. Erroneous results due to band bending were corrected by applying numerical calculations, which led to acceptable results. The evaluated ⌬E V values were 0.2Ϯ 0.2 eV for In 0.17 Al 0.83 N / GaN, 0.1Ϯ 0.2 eV for In 0.25 Al 0.75 N / GaN, and 0.0Ϯ 0.2 eV for In 0.30 Al 0.70 N / GaN. Despite the large decrease of around 1.0 eV in the band gap of InAlN layers according to the increase in the In molar fraction, the decrease in ⌬E V was as small as 0.2 eV. Therefore, the change in band-gap discontinuity was mainly distributed to that in conduction band offset.

We fabricated AlGaN/GaN high electron mobility transistors (HEMTs) on h-BN/sapphire substrates and transferred them from the host substrates to copper plates using h-BN as a release layer. In current–voltage characteristics, the saturation drain current decreased by about 30% under a high-bias condition before release by self-heating effect. In contrast, after transfer, the current decrement was as small as 8% owing to improved heat dissipation: the device temperature increased to 50 °C in the as-prepared HEMT, but only by several degrees in the transferred HEMT. An effective way to improve AlGaN/GaN HEMT performance by a suppression of self-heating effect has been demonstrated.

To systematically examine the effect of insulator deposition on the electrical properties in AlGaN/GaN heterostructures, the Si- and Al-based insulators (Si3N4, SiO2, AlN, and Al2O3) have been deposited on Al0.3Ga0.7N/GaN heterostructures. A significant increase in two-dimensional electron gas (2DEG) density (Ns) was observed for all the insulators with the order of Ns(Al2O3) > Ns(AlN) ∼ Ns(SiO2) > Ns(Si3N4) > N0 (N0: Ns without insulators). This resulted in a decrease in sheet resistance (R) with the smallest order of R(Al2O3) < R(AlN) < R(Si3N4) < R0 ∼ R(SiO2) (R0: R without insulators). This order is the same as that of Ns except for SiO2, where the 2DEG mobility largely degraded due to the diffusion of Si atoms into nitride layers. The increase in Ns was theoretically analyzed in terms of the change in the potential profile, and the following parameters were extracted: (i) the surface potential barrier (φB), and (ii) the interface charge (NInt) between an insulator and AlGaN. φB (eV) was estimated to be 1.7 (Si3N4), 2.2 (AlN), 2.7 (Al2O3), and 3.6 (SiO2), exhibiting a positive correlation between φB and the bandgap of the insulator. NInt (1013 cm-2) was estimated to be ∼0 (Si3N4), 0.1 (SiO2), 0.3 (AlN), and 0.5 (Al2O3); thus, the interface was found to be positively charged for AlN and Al2O3, whereas it was found to be almost neutral for Si3N4 and SiO2. Thus, the insulator deposition effect has been shown to be significant and to vary among insulators. The analysis shown here offers a guideline for understanding and designing the electrical properties in AlGaN/GaN heterostructures, where insulators are deposited as surface passivation and/or gate insulators.

The valence-band offset of a lattice-matched In0.17Al0.83N/GaN heterostructure grown by metal-organic vapor phase epitaxy (MOVPE) was investigated by x-ray photoelectron spectroscopy (XPS). Atomic force microscopy and angle-resolved XPS indicated that a thin In0.17Al0.83N (2.5 nm) layer was successfully grown by MOVPE on GaN. The XPS result showed that the valence band offset was 0.2±0.3 eV. This result indicates that the conduction-band offset at the In0.17Al0.83N/GaN interface is large, i.e., 0.9 to 1.0 eV, and occupies a large part of the entire band discontinuity.

We fabricated a novel heterostructure comprising InAlN/AlGaN/AlN/GaN by metal organic vapor phase epitaxy. Owing to the flat surface of the AlGaN underlayer, the obtained surface is flatter [root mean square (RMS) roughness of 0.27 nm] than that for the conventional InAlN/AlN/GaN heterostructure (RMS roughness of 0.53 nm). The electron mobility in the new structure is 1360 cm2 V-1 s-1 with NS of 1.85×1013 cm-2, which is higher that in the conventional one. The insertion of the AlGaN layer into the conventional structure is effective for improving surface morphology and electron mobility.