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Alumina incorporating nitrogen (aluminum oxynitride; AlON) for immunity against charge injection was grown on a AlGaN/GaN substrate through the repeated atomic layer deposition (ALD) of AlN layers and in situ oxidation in ozone (O 3 ) ambient under optimized conditions. The nitrogen distribution was uniform in the depth direction, the composition was controllable over a wide range (0.5-32%), and the thickness could be precisely controlled. Physical analysis based on synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) revealed that harmful intermixing at the insulator/AlGaN interface causing Ga out-diffusion in the gate stack was effectively suppressed by this method. AlON/AlGaN/GaN MOS capacitors were fabricated, and they had excellent electrical properties and immunity against electrical stressing as a result of the improved interface stability.

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Using aluminum titanium oxide (AlTiO, an alloy of Al2O3 and TiO2) as a high-k gate insulator, we fabricated and investigated AlTiO/AlGaN/GaN metal-insulator-semiconductor heterojunction field-effect transistors. From current low-frequency noise (LFN) characterization, we find Lorentzian spectra near the threshold voltage, in addition to 1/f spectra for the well-above-threshold regime. The Lorentzian spectra are attributed to electron trapping/detrapping with two specific time constants, ∼25 ms and ∼3 ms, which are independent of the gate length and the gate voltage, corresponding to two trap level depths of 0.5–0.7 eV with a 0.06 eV difference in the AlTiO insulator. In addition, gate leakage currents are analyzed and attributed to the Poole-Frenkel mechanism due to traps in the AlTiO insulator, where the extracted trap level depth is consistent with the Lorentzian LFN.

The impacts of inserting ultrathin oxides into insulator/AlGaN interfaces on their electrical properties were investigated to develop advanced AlGaN/GaN metal-oxide-semiconductor (MOS) gate stacks. For this purpose, the initial thermal oxidation of AlGaN surfaces in oxygen ambient was systematically studied by synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) and atomic force microscopy (AFM). Our physical characterizations revealed that, when compared with GaN surfaces, aluminum addition promotes the initial oxidation of AlGaN surfaces at temperatures of around 400 °C, followed by smaller grain growth above 850 °C. Electrical measurements of AlGaN/GaN MOS capacitors also showed that, although excessive oxidation treatment of AlGaN surfaces over around 700 °C has an adverse effect, interface passivation with the initial oxidation of the AlGaN surfaces at temperatures ranging from 400 to 500 °C was proven to be beneficial for fabricating high-quality AlGaN/GaN MOS gate stacks.

We have systematically investigated low-frequency noise (LFN) in AlN/AlGaN/GaN metal-insulator-semiconductor (MIS) devices, where the AlN gate insulator layer was sputtering-deposited on the AlGaN surface, in comparison with LFN in AlGaN/GaN Schottky devices. By measuring LFN in ungated two-terminal devices and heterojunction field-effect transistors (HFETs), we extracted LFN characteristics in the intrinsic gated region of the HFETs. Although there is a bias regime of the Schottky-HFETs in which LFN is dominated by the gate leakage current, LFN in the MIS-HFETs is always dominated by only the channel current. Analyzing the channel-current-dominated LFN, we obtained Hooge parameters α for the gated region as a function of the sheet electron concentration ns under the gate. In a regime of small ns, both the MIS- and Schottky-HFETs exhibit α∝ns−1. On the other hand, in a middle ns regime of the MIS-HFETs, α decreases rapidly like ns−ξ with ξ ∼ 2-3, which is not observed for the Schottky-HFETs. In addition, we observe strong increase in α∝ns3 in a large ns regime for both the MIS- and Schottky-HFETs.

We investigated BN/AlGaN/GaN metal‐insulator‐semiconductor heterojunction field‐effect transistors (MIS‐HFETs) with sputtering‐deposited BN gate dielectric. Amorphous BN thin films obtained by RF magnetron sputtering deposition were applied to MIS‐HFET device fabrication. The MIS‐HFET devices exhibit very low gate leakage current indicating good insulating properties of BN, and almost no negative drain conductance. The device characteristics and its temperature dependence were investigated, where we observe decreasing drain current and increasing gate current with increase in temperature. The relative decrease in the drain current for the low field region is larger than that for the high field region, according to the temperature dependence of low‐ and high‐field electron velocities. The temperature dependence of the gate leakage current is explained by a mechanism with thermally activated tunneling and direct tunneling. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

AlN films deposited by RF magnetron sputtering are applied to AlGaN/GaN metal–insulator–semiconductor heterostructure field-effect transistors (MIS-HFETs) as a gate dielectric. X-ray photoelectron spectroscopy (XPS) was used to characterize the AlN films, showing their chemical bonds and the bandgap by N 1s electron energy loss spectroscopy. The AlGaN/GaN MIS-HFET with a gate length of 150 nm was successfully fabricated, exhibiting low gate leakage currents for both reverse and forward biases, which are at least four orders of magnitude lower than those of reference Schottky-HFETs. Although these results support the possibility of sputtering-deposited AlN as a gate insulator, there are AlN/AlGaN interface states unfavorable for device performance, which were investigated by the frequency dispersion in the capacitance–voltage (C–V) characteristics.