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Integrating two-dimensional (2D) layered materials with wide bandgap β-Ga2O3 has unveiled impressive opportunities for exploring novel physics and device concepts. This study presents the epitaxial growth of 2D β-In2Se3/3D β-Ga2O3 heterostructures on c-Sapphire substrates by plasma-assisted molecular beam epitaxy. Firstly, we employed a temperature-dependent two-step growth process to deposit Ga2O3 and obtained a phase-pure (2 ¯ 01) β-Ga2O3 film on c-Sapphire. Interestingly, the in-situ reflective high-energy electron diffraction (RHEED) patterns observed from this heterostructure revealed the in-plane 'b' lattice constant of β-Ga2O3 ~ 3.038Å. In the next stage, for the first time, 2D In2Se3 layers were epitaxially realized on 3D β-Ga2O3 under varying substrate temperatures (Tsub) and Se/In flux ratios (RVI/III). The deposited layers exhibited (00l) oriented β-In2Se3 on (2 ¯ 01) β-Ga2O3/c-Sapphire with the epitaxial relationship of [ 11 2 ¯ 0 ] β-In2Se3 || [010] β-Ga2O3 and [ 10 1 ¯ 0 ] β-In2Se3 || [102] β-Ga2O3 as observed from the RHEED patterns. Also, the in-plane 'a' lattice constant of β-In2Se3 was determined to be ~ 4.027Å. The single-phase β-In2Se3 layers with improved structural and surface quality were achieved at a Tsub ~ 280 °C and RVI/III ~ 18. The microstructural and detailed elemental analysis further confirmed the epitaxy of 2D layered β-In2Se3 on 3D β-Ga2O3, a consequence of the quasi-van der Waals epitaxy. Furthermore, the β-Ga2O3 with an optical bandgap (Eg) of ~ 5.04 eV (deep ultraviolet) when integrated with 2D β-In2Se3, Eg ~ 1.43eV (near infra-red) can reveal potential applications in the optoelectronic field. [ABSTRACT FROM AUTHOR]

Molecular beam epitaxy (MBE) is demonstrated as an excellent growth technique for growing a low-defect GaN channel layer, which is crucial for controlling vertical leakage current and improving breakdown voltage (BV) in GaN-based high-electron mobility transistors (HEMTs). The 3D islands to 2D growth mode transition approach was induced by modulating substrate growth temperature (Tsub), displaying an overall improvement in film quality. A comprehensive investigation was conducted into the effects of Tsub on surface morphologies, crystal quality, and the optical and electrical properties of GaN films. Optimal results were achieved with a strain-relaxed GaN film grown at 690 °C, exhibiting significantly improved surface characteristics (root-mean-square roughness, Rq = 0.3 nm) and impressively reduced edge dislocations. However, the film with the smoothest surface roughness, attributed to the effect of the Ga-rich condition, possessed a high surface pit density, negatively affecting optical and electrical properties. A reduction in defect-related yellow emission further confirmed the enhanced crystalline quality of MBE GaN films. The optimized GaN film demonstrated outstanding electrical properties with a BV of ~1450 V, surpassing that of MOCVD GaN (~1180 V). This research significantly contributes to the advancement of MBE GaN-based high electron mobility transistor (HEMT) applications by ensuring outstanding reliability. [ABSTRACT FROM AUTHOR]

The nanotribological properties of aluminum gallium nitride (AlxGa1−xN) epitaxial films grown on low-temperature-grown GaN/AlN/Si substrates were investigated using a nanoscratch system. It was confirmed that the Al compositions played an important role, which was directly influencing the strength of the bonding forces and the shear resistance. It was verified that the measured friction coefficient (μ) values of the AlxGa1−xN films from the Al compositions (where x = 0.065, 0.085, and 0.137) were in the range of 0.8, 0.5, and 0.3, respectively, for Fn = 2000 μN and 0.12, 0.9, and 0.7, respectively, for Fn = 4000 μN. The values of μ were found to decrease with the increases in the Al compositions. We concluded that the Al composition played an important role in the reconstruction of the crystallites, which induced the transition phenomenon of brittleness to ductility in the AlxGa1−xN system. [ABSTRACT FROM AUTHOR]

Abstract In this study, we used metal organic chemical vapor deposition to form gallium nitride (GaN) epilayers on c- and a-axis sapphire substrates and then used the nanoscratch technique and atomic force microscopy (AFM) to determine the nanotribological behavior and deformation characteristics of the GaN epilayers, respectively. The AFM morphological studies revealed that pile-up phenomena occurred on both sides of the scratches formed on the GaN epilayers. It is suggested that cracking dominates in the case of GaN epilayers while ploughing during the process of scratching; the appearances of the scratched surfaces were significantly different for the GaN epilayers on the c- and a-axis sapphire substrates. In addition, compared to the c-axis substrate, we obtained higher values of the coefficient of friction (μ) and deeper penetration of the scratches on the GaN a-axis sapphire sample when we set the ramped force at 4,000 μN. This discrepancy suggests that GaN epilayers grown on c-axis sapphire have higher shear resistances than those formed on a-axis sapphire. The occurrence of pile-up events indicates that the generation and motion of individual dislocation, which we measured under the sites of critical brittle transitions of the scratch track, resulted in ductile and/or brittle properties as a result of the deformed and strain-hardened lattice structure.

AbstractThe effects of H2flow rate during plasma pretreatment on synthesizing the multiwalled carbon nanotubes (MWCNTs) by using the microwave plasma chemical vapor deposition are investigated in this study. A H2and CH4gas mixture with a 9:1 ratio was used as a precursor for the synthesis of MWCNT on Ni-coated TaN/Si(100) substrates. The structure and composition of Ni catalyst nanoparticles were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The present findings showed that denser Ni catalyst nanoparticles and more vertically aligned MWCNTs could be effectively achieved at higher flow rates. From Raman results, we found that the intensity ratio of G and D bands (ID/IG) decreases with an increasing flow rate. In addition, TEM results suggest that H2plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles. As a result, the pretreatment plays a crucial role in modifying the obtained MWCNTs structures.

This work reports molecular beam epitaxy (MBE) of two-dimensional (2D) GaSe1−xTex ternary alloys that have recently attracted a lot of interest in physics for prospective electronics and optoelectronics even though they face crucial challenges in their epitaxial technology. Disregarding a distinction in crystal phase symmetry of two end-terminals, i.e., hexagonal-GaSe (h-GaSe) and monoclinic-GaTe (m-GaTe), the majority of hexagonal-GaSe1−xTex (h-GaSe1−xTex) phase on a GaN/sapphire platform is guaranteed under our specific growth conditions without a visible sign of phase transition. We have also proposed extracting the Te composition of the ternary alloy via the experimental in-plane lattice constant, which is consistent with those indicated from energy dispersion X-ray data. Fascinatingly, the experimental and predictable band emission versus Te content displays a continuous redshift from 1.78 eV (h-GaSe) to 1.25 eV (h-GaSe0.4Te0.6) then a reversible blueshift to 1.46 eV (h-GaTe). Importantly, benefiting from the presence of Te incorporated atoms, the photoresponse performance of the hexagonal ternary alloy has greatly enhanced in comparison to the h-GaSe binary in terms of photocurrent density (up to 1250 nA cm−2 for h-GaSe0.65Te0.35 at only 300 mV bias). Overall, the results pave a way for phase/physical engineering of the alloys through the MBE process and realizing self-powered wafer-scale photodetectors based on 2D Ga-based monochalcogenide epitaxial thin films. [ABSTRACT FROM AUTHOR]