Search

We report a novel approach for metal organic vapor phase epitaxy of (Al)GaN heterostructures on Si substrates. An approximately 90-100 nm thick SiC buffer layer is synthesized using the reaction between Si substrate and CO gas. High-resolution transmission electron microscopy reveals sharp crystalline interfaces with epitaxial relationship between SiC/Si and AlN/SiC layers. Optimization of SiC morphology and AlN seed layer thickness facilitates the growth of GaN layers free of pits (v-defects). It is also found that Si doping eliminates these defects in the case of growth on SiC templates with non-optimized surface morphology. Thus, synthesis of thin SiC buffer layer is suggested as a solution for the interface problems at the initial stage of III-N on Si epitaxy. [ABSTRACT FROM AUTHOR]

GaN is demonstrated to be an ideal anode for Li-ion batteries (LIBs) for the first time. Amorphous GaN@Cu nanorods (a-GaN@Cu) freestanding electrode is designed via a low-temperature pulsed laser deposition method, which exhibits prominent rate capability and untralong lifespan as an anode for LIBs. With porous interconnected metal nanorods substrate to improve the structure integrity and electronic conductivity, the a-GaN@Cu electrode delivers a capacity recovery of 980 mAh g−1 after 150 cycles from 0.25 to 6.25 A g−1 and a high discharge capacity of 509 mAh g−1 after 3000 cycles at 10.0 A g−1. The lithium storage in the a-GaN is also systematically studied, which suggests a redox reaction mechanism. [ABSTRACT FROM AUTHOR]

Developing strategies that can promote charge transportation in photodevices is crucial for achieving high solar energy conversion efficiency. Herein a moisture-assisted nitridation approach is presented for the fabrication of efficient gallium-zinc oxynitride (GaN:ZnO) photoanode with compact structure to facilitate the charge transportation. With moisture-assisted nitridation, the charge separation efficiency and injection efficiency obtained on GaN:ZnO photoanode are significantly enhanced. Correspondingly, the photocurrent at 1.23 V vs reversible hydrogen electrode (RHE) has 18 folds improvement compared with that prepared without moisture assistance. Furthermore, via treating with HCl acid and modification with cobalt phosphate (CoPi) as a cocatalyst, state-of-the-art photocurrent over 2.0 mA cm−2 is achieved on independent GaN:ZnO photoanode when bias is higher than 1.4 V vs RHE. To the best of our knowledge, this is the first paradigm of moisture-assisted preparing oxynitride-based photoanode. The participation of moisture is found to improve the interconnection between adjacent GaN:ZnO nanoparticles as well as that between the GaN:ZnO film and the underlying substrate. Moreover, the volatilization of Zn can be substantially suppressed due to the modulation of reaction pathway by moisture. These two factors are confirmed to be the main reasons for the enhanced charge transportation and PEC performance obtained on GaN:ZnO photoanode. [ABSTRACT FROM AUTHOR]

Abstractauthoren We propose partially relaxed ultrathin GaN interlayer for strain control of AlN on SiC substrates. According to the degree of relaxation of the GaN interlayer, the lattice constant of the interlayer changes from that of SiC to that of bulk GaN, which leads to strain control of an AlN top layer grown on the interlayer. Growth of these layers is conducted by plasma-assisted molecular beam epitaxy. Before growing the interlayer, an AlN layer coherently grown on SiC is used as a template layer. The interlayers that have different degrees of relaxation are successfully obtained by changing the interlayer thickness. As a result, strain values of the AlN top layer grown on the interlayers are widely controlled from compressive (−0.53%) to tensile (+ 0.07%). TEM observation revealed the relaxation is induced by U-shaped half-loop dislocations originating from the GaN interlayer. [ABSTRACT FROM AUTHOR]

We present a detailed atomistic analysis of the electronic properties of m-plane InGaN/GaN quantum wells. The tight-binding model used treats realistically sized systems atomistically and accounts for compositional and structural inhomogeneities. Local variation in strain and built-in potential arising from random alloy fluctuations are explicitly included in the model. Many energy states of the supercells considered are calculated in order to determine the impact of the alloy fluctuations on the electronic structure of the system under investigation. We find that while the electrons are relatively insensitive to the local indium environment, the hole states are highly sensitive to it and are subject to very strong localization effects. These effects persist several states into the valence band. This strong localization of the hole states leads to a very broad distribution of ground state energies in different random configurations. Furthermore, we see that the localization leads to poor overlap between different hole states resulting in a reduced probability of transfer of carriers between different states. This feature should play an important role for transport properties in m-plane InGaN/GaN QWs. [ABSTRACT FROM AUTHOR]

The thermal expansion coefficient mismatch between GaN and SiC is lower than between GaN and Si. However it is usually observed that GaN on SiC/Si templates has higher tensile stress than GaN directly on Si. To provide an insight into the relationship between SiC template residual stress and GaN overlayer residual stress and crystal quality, we have analyzed the in-plane residual stress gradient present in GaN/SiC microstructure arrays of various dimensions and shapes using micro-Raman spectroscopy. It was found that the GaN overlayer residual stress is directly proportional to the SiC buffer layer residual stress. GaN films with lower residual stress also resulted in GaN films with improved crystal quality. Our study shows that the frequently encountered problems of high tensile stress in GaN films deposited on SiC templates is due to the high tensile residual stress in the SiC layers. The poorer GaN crystal quality deposited on SiC templates reported in literature is also likely due to the high tensile stress in the SiC templates. In order to grow GaN films with sufficient compressive stress and crystal quality for subsequent device processing, it is crucial to reduce the high tensile stress present in the SiC templates. [ABSTRACT FROM AUTHOR]

GaN layers have been grown on double-side-polished sapphire substrates using hydride vapor phase epitaxy. The GaN layers on each side of the substrate were alternately grown using a new susceptor with a rotating mechanism. Although a conventional GaN layer grown on one side of a sapphire substrate exhibits a small radius of curvature, the alternately double-sided-grown GaN template exhibits a large radius of curvature. The radius of curvature of the alternately double-sided-grown GaN template with a thickness of 180 μm was found to be approximately 16.4 m, and was larger than the 0.5 m radius of curvature of the conventional GaN template. [ABSTRACT FROM AUTHOR]

A high-resistance buffer layer is critical for GaN-based high electron mobility transistors (HEMTs) to suppress the drain leakage current and pre-mature device breakdown. A typical method to obtain the HR buffer layers is the acceptor impurity doping that is able to provide holes to compensate the background electrons in the buffer layers. However, the intentional doped acceptor impurities such as Mg, Fe and C will result in current collapse in GaN-based HEMTs. To address this issue, in this work, we employed a polarization doping method of holes by Al-composition grading instead of the acceptor impurity doping. The sheet resistance of the GaN-based buffer layer significantly increased due to the holes generated by the polarization field. (a) Structures of the three samples; (b) Schematic illustration of polarization-induced 3DHG in (0001)-oriented graded AlGaN layer; (c) The square resistance of the three samples calculated by using CTLM. [ABSTRACT FROM AUTHOR]

In this paper, we report an experimental observation of the two-stage turn-on characteristic in normally-OFF Al2O3/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) on Si substrate. The impact of oxide/GaN interface traps with different energy levels on the switching behavior of the device was extensively examined by simulation and verified by measurements. The interface traps at 0.4 eV below the bottom of the conduction band (Eit = EC-0.4 eV) of GaN buffer with a density of Dit = 6.5 x 1012 cm-2 was identified responsible for the ob- served two-stage turn-on characteristic. The weak Fermi-level pinning (WFLP) induced by the dynamic electron filling of interface traps may hamper the electron from accumulating in the Al2O3/GaN MOS-channel and then manifests a premature turn-on during the switching-on process. [ABSTRACT FROM AUTHOR]

In recent years, different power transistors have been developed in silicon carbide (SiC) and gallium nitride (GaN) as replacements for silicon based IGBTs. This paper presents a simulation comparison of the static and dynamic performance of silicon IGBTs with different SiC and GaN based lateral and vertical power transistors (HEMT, MOSFET and IGBT) with breakdown voltage ratings between 1.2 kV to 15 kV. The strengths and weaknesses of different technologies which make them suitable at different voltage levels have been discussed. [ABSTRACT FROM AUTHOR]

In this paper we report on the impact that the quantum well growth temperature has on the internal quantum efficiency and carrier recombination dynamics of two sets of InGaN/GaN multiple quantum well samples, designed to emit at 460 and 530 nm, in which the indium content of the quantum wells within each sample set was maintained. Measurements of the internal quantum efficiency of each sample set showed a systematic variation, with quantum wells grown at a higher temperature exhibiting higher internal quantum efficiency and this variation was preserved at all excitation power densities. By investigating the carrier dynamics at both 10 K and 300 K we were able to attribute this change in internal quantum efficiency to a decrease in the non-radiative recombination rate as the QW growth temperature was increased which we attribute to a decrease in incorporation of the point defects. [ABSTRACT FROM AUTHOR]

The mechanisms governing efficiency droop in an In0.18Ga0.82N/GaN multiple quantum well structure were investigated using a combination of ultrafast time-resolved terahertz and photoluminescence spectroscopy. From excitation fluence dependent studies, a reduction in the room temperature photoluminescence efficiency to 3% of its maximum value was observed for an excitation fluence of 0.96 mJcm-2. A correlation was found between the onset of efficiency droop and the emergence of a peak on the high-energy side of the quantum well emission with a 1/e decay time of 19.6 ps. These characteristics were attributed to the saturation of localised states and the population of higher energy delocalised states. Time-resolved studies revealed different scaling behaviours between the terahertz and photoluminescence decay dynamics, suggesting that the saturation of localised hole states may be playing a part in the onset of efficiency droop. [ABSTRACT FROM AUTHOR]

In this work, the Schottky-on-heterojunction light-emitting diodes (SoH-LED) are monolithically integrated with the AlGaN/GaN high-electron-mobility transistors (HEMT) on an AlGaN/GaN-on-Si platform commonly used for GaN lateral electronic devices. The on-chip electro-optic modulation is realized in a single device fabricated using HEMT-compatible process. Also, SoH-LED arrays are fabricated to demonstrate the on-chip addressable functionality with each SoH-LED pixel in the array individually controlled by the integrated HEMT. High-resolution micro-scale SoH-LED seven-segment displays are demonstrated to show the potential of using SoH-LEDs as a robust alternative for micro flat-panel displays. [ABSTRACT FROM AUTHOR]

In this work we employ terahertz (THz) ellipsometry to determine two-dimensional electron gas (2DEG) density, mobility and effective mass in AlGaN/GaN high electron mobility transistor structures grown on 4H-SiC substrates. The effect of the GaN interface exposure to low-flow-rate trimethylaluminum(TMA) on the 2DEG properties is studied. The 2DEGeffectivemass and sheet density are determined to be in the range of 0.30-0.32m0 and 4.3-5.5 x 1012 cm-2, respectively. The 2DEG effective mass parameters are found to be higher than the bulk effective mass of GaN, which is discussed in view of 2DEG confinement. It is shown that exposure to TMA flow improves the 2DEG mobility from 2000 cm²/Vs to values above 2200 cm²/Vs. A record mobility of 2332±61 cm²/Vs is determined for the sample with GaN interface exposed to TMA for 30 s. This improvement in mobility is suggested to be due to AlGaN/GaN interface sharpening causing the reduction of interface roughness scattering of electrons in the 2DEG. [ABSTRACT FROM AUTHOR]

In this paper, we presented AlGaN/GaN MIS-HEMT with a 6-nm ALD La2O3 as gate insulator. The gate leakage current had been greatly reduced at both positive and negative bias by using La2O3 gate dielectric. Threshold voltage for MIS-HEMT moved +1 V toward positive direction compared with the Schottky gate HEMT (HEMT) demonstrating the negatively fixed charges at La2O3/AlGaN interface. Besides, the transconductance of MIS-HEMT was not degraded. The stability of fixed and trap state introduced by La2O3 gate dielectric was discussed. [ABSTRACT FROM AUTHOR]

A new approach is proposed to the synthesis of a semipolar GaN on a Si(100) substrate at the surface of which the V‐shaped nanostructures with the characteristic size of elements as low as 100 nm are formed. It has been demonstrated that application of buffer layers of 3C‐SiC and AlN enables formation of the GaN(10–11) layer characterized by the full width at half maximum value as low as ωθ ≈45 arcmin for the X‐ray diffraction rocking curve. The model based on anisotropic nucleation of AlN on the V‐shaped nanostructure is proposed to explain the growth of the GaN layer in a single semipolar direction. In this article, Bessolov et al. focus on the growth of the GaN layer in a semipolar direction by heteroepitaxy of the GaN layer on nano‐patterned Si(100) substrate with V‐shaped nano‐grooves arranged with the period as low as 100 nm, combined with the synthesis of a nano‐sized SiC layer formed by solid‐phase substitution of silicon atoms by carbon atoms. [ABSTRACT FROM AUTHOR]

InGaN alloys have been attracting tremendous interest because of the large range of wavelengths covered when tuning the In to Ga ratio, offering extensive band gap engineering possibilities as well as the opportunity to achieve longer wavelength optoelectronic devices. Industrialization of InGaN materials with large In contents is however still limited by challenges imposed by intrinsic material properties. At typical InGaN growth temperatures the In desorption rate is lower than the In–N bond decomposition rate, making it difficult to avoid accumulation of metal In droplets on the surface. Hestroffer et al. (pp. 626–629) have established a growth diagram for InGaN grown on (0001) GaN at a constant temperature of 575 °C versus Ga and In fluxes. The authors identify different regimes, namely: N‐rich, intermediate In‐rich, droplet‐accumulation In‐rich, and Ga‐rich. Compositionally graded InGaN layers are grown by ramping the In and Ga fluxes in parallel under two distinct growth regimes. Structural and optical properties of the two types of grades are compared. The growth diagram proves to be a useful tool in controlling the metal fluxes and thereby the stoichiometry conditions on the surface during the growth of compositionally graded layers. [ABSTRACT FROM AUTHOR]