We investigated the electron transport properties of n-GaN under an Ohmic-metal. Hall measurement results were compared for n-GaN (A) before Ti-based metal deposition, (B) after Ti-based metal deposition but before annealing, (C) after Ohmic annealing, and (D) after Ohmicmetal removal, where multi-probe-Hall device measurements are required for (C), while the others, (A), (B), and (D), can be characterized by conventional Hall device measurements. The multi-probe-Hall device measurements for (C) elucidated that, under the Ohmic-metal, the electron concentration is increased and the electron mobility is enhanced in comparison with those for the other cases, (A), (B), and (D). The increased electron concentration indicates that high-density doping takes place in the n-GaN by the Ohmic annealing. However, the highdensity doping is not observed after the Ohmic-metal removal. Moreover, the electron mobility enhancement under the Ohmic-metal cannot be explained by donor doping with ionized impurity scattering. These suggest that, under the Ohmic-metal, high-density donors are not formed, and high-density polarization doping owing to strain from the Ohmic-metal takes place. From theoretical calculations, we clarified that the increase in the electron density by polarization doping without donors leads to the suppression of ionized impurity scattering and consequently the electron mobility enhancement. (c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

GaN-based structures grown on SiC substrates by means of horizontal hot-wall metal-organic chemical vapor deposition (MOCVD) were systematically characterized, revealing high crystal quality. The hot-wall MOCVD grown GaN, doped by Mg and Si, respectively showed low-resistivity hole and electron transport, competitive with the state-of-the-art GaN. High concentrations of free holes (similar to 2 x 10(17) cm(-3)) were achieved for the as-grown Mg-doped GaN without thermal annealing, thanks to advantageous heating characteristics of the "hot-wall" reactor. The analysis of optical and electrical properties brought a picture, where Mg is the only impurity defining energy levels in the hot-wall MOCVD p-type doped GaN. Besides, InGaN/GaN light-emitting diodes employing such doped GaN materials in the carrier-transport layers were fabricated, resulting in high device performances. The devices exhibited bright electroluminescence with very narrow full widths at half maximum as well as negligible spectral shifts at high current levels (greater than or similar to 10 A/cm(2)). These results exemplified the rewards of the hot-wall MOCVD for development of high-quality nitrides-based structures, providing an attractive growth method to realize the demonstration of light-emitting devices with favorable properties.

The hot-wall metal-organic chemical vapor deposition (MOCVD), previously shown to enable superior III-nitride material quality and high performance devices, has been explored for Mg doping of GaN. We have investigated the Mg incorporation in a wide doping range ( 2.45 x 10( 18) cm(-3) up to 1.10 x 10(20) cm(-3)) and demonstrate GaN:Mg with low background impurity concentrations under optimized growth conditions. Dopant and impurity levels are discussed in view of Ga supersaturation, which provides a unified concept to explain the complexity of growth conditions impact on Mg acceptor incorporation and compensation. The results are analyzed in relation to the extended defects, revealed by scanning transmission electron microscopy, x-ray diffraction, and surface morphology, and in correlation with the electrical properties obtained by Hall effect and capacitance-voltage (C-V) measurements. This allows to establish a comprehensive picture of GaN:Mg growth by hot-wall MOCVD providing guidance for growth parameters optimization depending on the targeted application. We show that substantially lower H concentration as compared to Mg acceptors can be achieved in GaN:Mg without any in situ or post-growth annealing resulting in p-type conductivity in as-grown material. State-of-the-art p-GaN layers with a low resistivity and a high free-hole density (0.77 omega cm and 8.4 x 10( 17) cm(-3), respectively) are obtained after post-growth annealing demonstrating the viability of hot-wall MOCVD for growth of power electronic device structures. (C)2022 Author(s).

Electron transport properties in InAs films epitaxially grown on GaAs(001), InAs/GaAs(001) heterostructures, were systematically investigated through the dependence on crystal direction, thickness, and temperature. As a result, we found a pronounced electron mobility anisotropy, in which the mobility is highest and lowest along [1 (1) over bar0] and [110] crystal directions, respectively. The mobility anisotropy intensifies as the InAs thickness decreases, while it diminishes in thick regimes, where the InAs films are relatively immune to effects from the epitaxial heterointerface. We observed the anisotropy in a wide temperature range, 5-395 K, with an enhancement at high temperatures. Our analysis indicates that the electron mobility anisotropy can be attributed to anisotropic electron scatterings by both interface roughness and random piezoelectric polarization near the interface. Published under license by AIP Publishing.

GaN-based pyramidal quantum structures, InGaN nanostructures located on top of micro-sized GaN pyramids, were fabricated by selective-area growth on SiC substrates by means of hot-wall metal-organic chemical vapor deposition. Arrays of GaN-based pyramidal structures exhibit micro-size pyramids possessing high uniformity, precise hexagonal bases, and InGaN/GaN quantum-well layers with well-defined interfaces. Each pyramid comprises a p-i-n InGaN/GaN structure, which is separated from that of other pyramids by a dielectric layer, serving as a building block for micro-emitters. Moreover, interconnected micro-size light-emitting diodes (microLEDs) built on the GaN-based pyramidal quantum structures were demonstrated, resulting in well-determined electroluminescence in the near-ultraviolet regime with negligible spectral shifts at high current levels. The results elucidated the rewards for development of these light-emitting designs and their potential for microLED applications.

We have investigated insulator-semiconductor interface fixed charges in AlGaN/GaN metal-insulator-semiconductor (MIS) devices with Al₂O₃ or AlTiO (an alloy of Al₂O₃ and TiO₂) gate dielectrics obtained by atomic layer deposition on AlGaN. Analyzing insulator-thickness dependences of threshold voltages for the MIS devices, we evaluated positive interface fixed charges, whose density at the AlTiO/AlGaN interface is significantly lower than that at the Al₂O₃/AlGaN interface. This and a higher dielectric constant of AlTiO lead to rather shallower threshold voltages for the AlTiO gate dielectric than for Al₂O₃. The lower interface fixed charge density also leads to the fact that the two-dimensional electron concentration is a decreasing function of the insulator thickness for AlTiO, whereas being an increasing function for Al₂O₃. Moreover, we discuss the relationship between the interface fixed charges and interface states. From the conductance method, it is shown that the interface state densities are very similar at the Al₂O₃/AlGaN and AlTiO/AlGaN interfaces. Therefore, we consider that the lower AlTiO/AlGaN interface fixed charge density is not owing to electrons trapped at deep interface states compensating the positive fixed charges and can be attributed to a lower density of oxygen-related interface donors.

We fabricated and investigated an InAs/high-k/low-k structure in comparison with an InAs/low-k structure, where the former and the latter are respectively obtained by bonding of InAs/Al2O3/AlN and InAs on low-k flexible substrates (FS). The InAs/high-k/low-k (InAs/Al2O3/AlN/FS) exhibits electron mobilities immune to interface fluctuation scattering, whereas this scattering is serious for the InAs/low-k (InAs/FS). Moreover, we find that electron sheet concentrations in the InAs/high-k/low-k are significantly higher than those in the InAs/low-k. From InAs/Al2O3 interface analysis by energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy, we find that the higher electron concentrations can be attributed to natural modulation doping from Al2O3 to InAs. [ABSTRACT FROM AUTHOR]

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 ~3ms, 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. [ABSTRACT FROM AUTHOR]

We have systematically investigated low-frequency noise (LFN) in InAs films with several thicknesses (≃10-100 nm) bonded on low-k flexible substrates (InAs/FS), comparing with that in InAs films epitaxially grown on GaAs(001) substrates (InAs/GaAs). We obtain current LFN spectra exhibiting approximate 1/f characteristics and consequent effective Hooge parameters α depending on the thickness, where we find that α in the InAs/FS is larger than that in the InAs/GaAs. The behavior of α can be attributed to the fluctuation of the electron mobility dominated by surface/interface charge scattering and by thickness fluctuation scattering. [ABSTRACT FROM AUTHOR]

We have systematically investigated low-frequency noise (LFN) in AlN/AlGaN/GaN metalinsulator- semiconductor (MIS) devices, where the AlN gate insulator layer was sputteringdeposited 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 theMIS-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. [ABSTRACT FROM AUTHOR]