β-Ga2O3 Nanostructures Research Articles

Elastic modulus of β-Ga2O3 nanowires measured by resonance and three-point bending techniques.

Due to the recent interest in ultrawide bandgap β-Ga2O3 thin films and nanostructures for various electronics and UV device applications, it is important to understand the mechanical properties of Ga2O3 nanowires (NWs). In this work, we investigated the elastic modulus of individual β-Ga2O3 NWs using two distinct techniques - in-situ scanning electron microscopy resonance and three-point bending in atomic force microscopy. The structural and morphological properties of the synthesised NWs were investigated using X-ray diffraction, transmission and scanning electron microscopies. The resonance tests yielded the mean elastic modulus of 34.5 GPa, while 75.8 GPa mean value was obtained via three-point bending. The measured elastic moduli values indicate the need for finely controllable β-Ga2O3 NW synthesis methods and detailed post-examination of their mechanical properties before considering their application in future nanoscale devices.

Growth of β-Ga2O3 nanostructures by thermal oxidation of GaN-on-sapphire for optoelectronic devices applications

Heterostructures of wide-bandgap semiconductors, like β-Ga2O3/ GaN, are being used in many high-power, high-frequency electronic and optoelectronic devices. This paper presents the growth of β-Ga2O3 thin film and nanostructures by thermally oxidizing (at 1000 °C) the planar and nano-structured GaN surfaces respectively. The nano-structures of GaN (having 300–500 nm long vertical nano-kinks) are fabricated by metal-assisted chemical etching of GaN surface for 1–2 hours. The thermally grown β-Ga2O3 samples are polycrystalline. The chemical analysis of the samples using X-ray photoelectron spectroscopy (XPS) showed enhancement of Ga-O peak intensity (at 20.21–20.3 eV) at the expense of the Ga-N peak intensity (at 19.2–19.7 eV) in the thermal oxidation process. The O1s XPS spectra of the oxidized samples exhibited prominent signature of O2- ions (530.7–530.9 eV) representing the formation of Ga2O3 with presence of adsorbed hydroxyl group at the surface (532.2 – 532.4 eV). The valance band XPS spectra disclosed the formation of an additional Ga(4 s)–O(2p) hybridization energy state, and the valence band maxima increased from 2.3 eV to 3.25 eV. The Raman spectra of the oxidized samples exhibited multiple Ag and Bg vibrational modes of β-Ga2O3 along with intense E2(high), A1(LO) modes of GaN. The time-resolved photoluminescence spectra of the samples demonstrated fast transition of charge carriers from excited state to ground state with 460–820 pico-seconds average career lifetime.

β-Ga2O3 nanostructures for photocatalytic degradation of red amaranth toxic dye

Beta gallium oxide (β-Ga2O3) microstructures composed of ∼50 nm nanoparticles were synthesized by the hydrothermal method. Using the Tauc plot method a value of ∼4.9 eV was obtained for the optical band gap of β-Ga2O3. TEM and XRD analyses revealed high crystallinity of the β-phase of gallium oxide nanostructures. Since there are few publications for the photocatalytic properties of β-Ga2O3 the obtained results contribute to better understanding of the photocatalytic effect of this material on toxic dye red amaranth. Moreover, it is shown that β-Ga2O3 is a very efficient photocatalyst leading to high percentage degradation of dyes for relatively short periods. For example, the degradation of red amaranth and rhodamine B toxic dyes under UV light irradiation reached 97% and 100% after 165 and 120 min, respectively.

Low-temperature gas sensing mechanism in β-Ga2O3 nanostructures revealed by near-ambient pressure XPS

Ga2O3 is a promising gas-sensing material that can operate over a wide temperature range. However, despite its relevance for practical applications, there is a lack of understanding regarding its sensing mechanism at temperatures below 800 °C. To reveal the sensing mechanism, GaOOH nanorods were grown on β-Ga2O3 seed layers by chemical bath deposition, then converted to β-Ga2O3 by annealing in the air at 500 °C, and the surface properties were investigated by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in the presence of oxidizing/reducing gases. Time-stable shifts in the XPS spectra were used to estimate the relative changes in conductivity in compliance with the ionosorption model. Our results indicate that the sensing mechanism at lower temperatures is governed by redox reactions, leading to an increase/decrease in the conductivity for reducing/oxidizing gases. Furthermore, we provide a detailed description of the ethanol-sensing mechanism at different temperatures.

Advanced Fabrication of 3D Micro/Nanostructures of Gallium Oxide with a Tuned Band Gap and Optical Properties.

Gallium oxide (Ga2O3) is a promising material for high-power semiconductor applications due to its wide band gap and high breakdown voltage. However, the current methods for fabricating Ga2O3 nanostructures have several disadvantages, including their complex manufacturing processes and high costs. In this study, we report a novel approach for synthesizing β-Ga2O3 nanostructures on gallium phosphide (GaP) using ultra-short laser pulses for in situ nanostructure generation (ULPING). We varied the process parameters to optimize the nanostructure formation, finding that the ULPING method produces high-quality β-Ga2O3 nanostructures with a simpler and more cost-effective process when compared with existing methods. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to characterize the samples, which indicated the presence of phosphorous. X-ray photoelectron spectroscopy (XPS) confirmed the formation of gallium oxide, along with a minor amount of phosphorus-containing compounds. Structural analysis using X-ray diffraction (XRD) revealed the formation of a monoclinic β-polymorph of Ga2O3. We also measured the band gap of the materials using reflection electron energy loss spectroscopy (REELS), and found that the band gap increased with higher nanostructure formation, reaching 6.2 eV for the optimized sample. Furthermore, we observed a change in the heterojunction alignment, which we attribute to the change in the oxidation of the samples. Our results demonstrate the potential of ULPING as a novel, simple, and cost-effective method for fabricating Ga2O3 nanostructures with tunable optical properties. The ULPING method offers a green alternative to existing fabrication methods, making it a promising technology for future research in the field of Ga2O3 nanostructure fabrication.

Probing Hyperbolic Shear Polaritons in β-Ga2O3 Nanostructures Using STEM-EELS.

Phonon polaritons, quasiparticles arising from strong coupling between electromagnetic waves and optical phonons, have potential for applications in subdiffraction imaging, sensing, thermal conduction enhancement, and spectroscopy signal enhancement. A new class of phonon polaritons in low-symmetry monoclinic crystals, hyperbolic shear polaritons (HShPs), have been verified recently in β-Ga2O3 by free electron laser (FEL) measurements. However, detailed behaviors of HShPs in β-Ga2O3 nanostructures still remain unknown. Here, by using monochromatic electron energy loss spectroscopy in conjunction with scanning transmission electron microscopy, the experimental observation of multiple HShPs in β-Ga2O3 in the mid-infrared (MIR) and far-infrared (FIR) ranges is reported. HShPs in various β-Ga2O3 nanorods and a β-Ga2O3 nanodisk are excited. The frequency-dependent rotation and shear effect of HShPs reflect on the distribution of EELS signals. The propagation and reflection of HShPs in nanostructures are clarified by simulations of electric field distribution. These findings suggest that, with its tunable broad spectral HShPs, β-Ga2O3 is an excellent candidate for nanophotonic applications.

Synthesis, phase conversion and physical characteristics of mesoporous β-Ga2O3 nanostructures for catalytic applications

Herein, α-GaOOH micro/nanorods were synthesized by facile template free hydrothermal methods. These rods were converted as mesoporous β-Ga2O3 nanostructures via thermal annealing at 900 °C for 2 h. The structural analysis (XRD) reveals the presence of orthorhombic (α-GaOOH) and monoclinic (β-Ga2O3) phases (pre- and post-annealing). The surface morphology, shape, size and porous structure, growth direction and crystalline quality of the micro/nanorods are investigated using scanning and transmission electron microscopy. Interplanar spacing of 0.37 and 0.46 nm confirms that the growth is along (2 0 1) and (−2 0 1) plane, respectively. The possible growth mechanism was proposed based on the obtained micro-graphical analyses. EDX, elemental mapping and XPS analyses revealed the existence of Ga and O and their chemical states. The characteristic Raman phonon modes confirmed the monoclinic phase of β-Ga2O3. The optical absorbance edge varies between 231 nm (GaOOH) to 290 nm (β-Ga2O3). Subsequently, the band gap varies between 5.6 and 4.4 eV. PL result showed intense ultraviolet and less intense violet-blue emissions, which can be related to the defect level emission. The BET surface area analysis confirmed the mesoporous structure with pore diameter (3.97 nm) and high specific surface area (13.87 m2/g) of Ga-3. The 98 % degradation efficiency with reaction kinetic rates of k = 0.0144/min shows a good catalytic behaviour of Ga-3 catalyst against RhB dye. The results reveal that the hydrothermally grown 1-D β-Ga2O3 mesoporous nanostructures are promising candidates for designing a catalytic and other functional nanodevices.

Fabrication of β-Ga2O3 Nanotubes via Sacrificial GaSb-Nanowire Templates.

β-Ga2O3 nanostructures are attractive wide-band-gap semiconductor materials as they exhibit promising photoelectric properties and potential applications. Despite the extensive efforts on β-Ga2O3 nanowires, investigations into β-Ga2O3 nanotubes are rare since the tubular structures are hard to synthesize. In this paper, we report a facile method for fabricating β-Ga2O3 nanotubes using pre-synthesized GaSb nanowires as sacrificial templates. Through a two-step heating-treatment strategy, the GaSb nanowires are partially oxidized to form β-Ga2O3 shells, and then, the residual inner parts are removed subsequently in vacuum conditions, yielding delicate hollow β-Ga2O3 nanotubes. The length, diameter, and thickness of the nanotubes can be customized by using different GaSb nanowires and heating parameters. In situ transmission electron microscopic heating experiments are performed to reveal the transformation dynamics of the β-Ga2O3 nanotubes, while the Kirkendall effect and the sublimation process are found to be critical. Moreover, photoelectric tests are carried out on the obtained β-Ga2O3 nanotubes. A photoresponsivity of ~25.9 A/W and a detectivity of ~5.6 × 1011 Jones have been achieved with a single-β-Ga2O3-nanotube device under an excitation wavelength of 254 nm.

Tailoring of structural and optical properties of electrosprayed β-Ga2O3 nanostructures via self-assembly

The present article demonstrates the fabrication of various β-Gallium Oxide (Ga2O3) nanostructures (NSs) by low-cost and scalable electrospraying (ES) methods via self-assembly (SA). The effect of the annealing sequences on the self-assembled β-Ga2O3 NSs has been detailed. The comparative studies of NSs and nanoflakes (NFs) growth with annealing effect at different stages of self-assembly time (SAT) have been explored further. The fabricated NSs and NFs have been investigated using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The comparative study of categorized growth samples shows better crystalline quality when moving from the category 1 (C1) sample to the category 3 (C3) sample. The annealing sequences with SAT play a significant role in crystal formation and its quality, optical properties, and morphology of the Ga2O3 NSs. The optical properties of NSs have been derived from the normal incidence of absorbance measurements. The maximum observed energy band gap is ~ 5.40 eV. Traps and impurities play a critical role in the formation and deformation of energy bands in crystals. The photoluminescence spectra further reveal the variation in the intensity of luminescence of different emission bands due to the variation in the number of defect states and impurities in the NSs.

Codoping of Al and In atoms in β-Ga2O3 semiconductors

Al and In dopants are codoped in β-Ga2O3 nanostructures via hydrothermal synthesis. Unlike single-dopant β-Ga2O3 nanostructures, less mechanical strain is induced in the β-Ga2O3 nanostructures with the codoping of Al and In atoms. This is attributed to the size compensation effect. The induced mechanical strain caused by each dopant in the β-Ga2O3 nanostructures is analyzed using X-ray diffraction. Chemical states are also compared after Al and In doping in the β-Ga2O3 nanostructures. Finally, the photocatalytic properties of various β-Ga2O3 nanostructures at different Al and In concentrations are compared via the decomposition of methylene blue from UVC irradiation with a wavelength of 254 nm. Compared to the single-dopant β-Ga2O3 nanostructures, enhanced photocatalytic activity is obtained with higher dopant concentrations when Al and In atoms are codoped in the β-Ga2O3 nanostructures. This codoping technique could be applied to various applications where higher doping concentrations are advantageous while minimizing the induced lattice deformation of the β-Ga2O3.

β-Ga2O3 Nanostructures: Chemical Vapor Deposition Growth Using Thermally Dewetted Au Nanoparticles as Catalyst and Characterization.

β-Ga2O3 nanostructures, including nanowires (NWs), nanosheets (NSHs), and nanorods (NRs), were synthesized using thermally dewetted Au nanoparticles as catalyst in a chemical vapor deposition process. The morphology of the as-grown β-Ga2O3 nanostructures depends strongly on the growth temperature and time. Successful growth of β-Ga2O3 NWs with lengths of 7–25 μm, NSHs, and NRs was achieved. It has been demonstrated that the vapor–liquid–solid mechanism governs the NW growth, and the vapor–solid mechanism occurs in the growth of NSHs and NRs. The X-ray diffraction analysis showed that the as-grown nanostructures were highly pure single-phase β-Ga2O3. The bandgap of the β-Ga2O3 nanostructures was determined to lie in the range of 4.68–4.74 eV. Characteristic Raman peaks were observed with a small blue and red shift, both of 1–3 cm−1, as compared with those from the bulk, indicating the presence of internal strain and defects in the as-grown β-Ga2O3 nanostructures. Strong photoluminescence emission in the UV-blue spectral region was obtained in the β-Ga2O3 nanostructures, regardless of their morphology. The UV (374–377 nm) emission is due to the intrinsic radiative recombination of self-trapped excitons present at the band edge. The strong blue (404–490 nm) emissions, consisting of five bands, are attributed to the presence of the complex defect states in the donor (VO) and acceptor (VGa or VGa–O). These β-Ga2O3 nanostructures are expected to have potential applications in optoelectronic devices such as tunable UV–Vis photodetectors.

Gallium Oxide Nanostructures: A Review of Synthesis, Properties and Applications.

Gallium oxide, as an emerging semiconductor, has attracted a lot of attention among researchers due to its high band gap (4.8 eV) and a high critical field with the value of 8 MV/cm. This paper presents a review on different chemical and physical techniques for synthesis of nanostructured β-gallium oxide, as well as its properties and applications. The polymorphs of Ga2O3 are highlighted and discussed along with their transformation state to β-Ga2O3. Different processes of synthesis of thin films, nanostructures and bulk gallium oxide are reviewed. The electrical and optical properties of β-gallium oxide are also highlighted, based on the synthesis methods, and the techniques for tuning its optical and electrical properties compared. Based on this information, the current, and the possible future, applications for β-Ga2O3 nanostructures are discussed.

Titanium oxide nanotube film decorated with β-Ga2O3 nanoparticles for enhanced water splitting properties

TiO2 is a wide bandgap material with inherent photogenerated charge carrier recombination problems, which reduces its photoelectrochemical (PEC) water splitting efficiency. Considerable efforts have been dedicated to reducing this charge recombination problems by decorating the film’s surface using transition metal nanoparticles (NP) to enhanced device performance. And considerable numbers of literature reports are available to this respect. However, reports are rare for the surface modification of TiO2 nanotubes (TNT) arrays with metal oxide, specially by β-Ga2O3 nanostructures and its potential applications. Herein, the PEC water splitting properties of β-Ga2O3 NP (GNP)-modified TNT arrays is described. The TNT were synthesized by anodization, whereas the GNP were fabricated by chemical vapour deposition method. X-ray diffraction (XRD) results revealed the presence of weak β-Ga2O3 peaks and reduction in crystallite size upon GNP addition. The integration of GNP on the TNT arrays resulted in a slight increase in optical bandgap from 3.4 to 3.6 eV observed by UV–vis measurements. Linear sweep voltammetry measurement for the modified TNT photocatalyst showed a 32.5% enhancement in photocurrent density compared to the bare TNT photoelectrode, measured in 1 M HCl solution. Elemental analysis confirmed the presence of β-Ga2O3, thereby supporting the XRD and UV–vis results. This study demonstrates that GNP-activated TNT arrays could be used as photoanode for enhanced photocatalytic activity in the PEC cell.

Energy levels of acceptor impurities in β-Ga2O3 nanostructures

Applying the methods of the density functional theory and ab initio pseudopotential, the characteristics of the energy levels of acceptors in β-Ga2O3 nanostructures are obtained from the first principles. Mg and Ca are most favorably included in the position of Ga1 and Ga2 in both the cluster and the film. N is most favorably included in the O1 position in the film. The spatial distribution of valence electron density, the density of states, acceptor levels, and band gap width for these nanostructures are calculated.

Cactus-like Gallium Oxide Nanostructure for Gas Sensor Applications

Pulsed laser ablation technique in distilled water (PLAL) was employed to prepare colloid of spindle-like β-Ga₂O₃ nanostructure and then deposit on the quartz substrate by the drop-casting method to produce cactus-like β-Ga₂O₃ nanostructures at 900C. The Nd-YAG Q-Switched laser is the method to formation nanostructure from a gallium metal target by using a wavelength 1064 nm, the repetition rate of about 5 Hz and the fluency of laser about 5.57 J/cm2. All the investigations of the Transmission electron microscope (TEM) and Scanning electron microscopy (SEM) showed that the formation of spindle-like β-Ga₂O3 and cactus-like β-Ga₂O₃ nanostructures, respectively. X-ray diffraction (XRD) of the cactus-like β-Ga₂O₃ nanostructures was investigated with a crystallite size of about 9.78 nm. The formation of cactus-like β-Ga₂O₃ nanostructures is the result of transformation from the prepared spindle-like β-Ga₂O₃ nanostructures with the optimum fluency of laser of about 5.57 J/cm2.

Dependence of the Structural and Optical Properties of Gallium Oxide Nanostructures on Laser Fluency

In this research, thin films of gallium oxide β-Ga₂O₃ nanostructures were prepared by pulsed laser ablation in distilled water (PLAL), then deposited on quartz substrate by the drop-casting method at 90 oC. The Nd-YAG laser was used with a wavelength of 1064 nm and a repetition rate of 5 Hz. The effect of increasing laser fluencies on the structural and optical properties was investigated by, Transmission electron microscope (TEM), Atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and spectrophotometer microscopy (UV-VIS). Crystallite size for all samples increased with the increasing the fluency of laser excepting at the fluency of laser 5.57 J/cm2 was 9.78 nm. The formation of nanoparticles, spindle-like, nanosheet and core-shell of β-Ga₂O₃ have been observed by the images of TEM at the fluencies of laser 4.77, 5.57, 5.97, and 6.36 J/cm2, respectively. The nanocluster and nanoroads of the β-Ga₂O₃ thin films can be seen by the SEM images at the all fluencies of laser 4.77, 5.97, 6.36 and 5.57 J/cm2. The‏ ‏‎ results of XRD investigations showed‏ that the diffractions patern of β-Ga₂O₃ transformed from the monoclinic phase with (-201), ‎‎(-402), and (-603) into orthorhombic phase with (002), 004), and (006) when increased ‎the fluency of laser.‎‏‎ The energy bandgap decreased with the increase of laser fluencies, and the absorption peak was located in the UV region.

Electronic and band structure studies on In and N doped β-Ga2O3 nanostructures from first-principles calculations

The electronic properties and band structures of pristine, In and N substituted β-Ga2O3 nanostructures are studied using density functional theory with GGA/PBE exchange correlation functional. The band structure gets modified with the substitution of impurities in β-Ga2O3 nanostructures. Besides, the band gap of β-Ga2O3 nanostructures increases with indium substitution. Upon substitution of impurities, the localization of charge increases. Moreover, along the oxygen sites the electron density is found to be more in β-Ga2O3 nanostructures. The findings of the present work give the perception on band structure and electronic properties of β-Ga2O3 nanostructures, which can be fine-tuned with the substitution impurities.

In situ and tunable structuring of semiconductor-in-glass transparent composite.

SummarySemiconductor-in-glass composites are an exciting class of photonic materials for various fundamental applications. The significant challenge is the scalable elaboration of composite with the desirable combination of tunable structure, high semiconductor loading ratio, and excellent transparency. Here we report that the topological engineering strategy via hybridization of the glass network former enables to surmount the aforementioned challenge. It not only facilitates the in situ precipitation of (Ga2-xAlx)O3 domains with continuously tunable composition but also allows to simultaneously refine the grain size and enhance the crystallinity. In addition, the composites exhibit excellent transparency and can host various active dopants. We demonstrate the attractive broadband optical response of the composite and achieve the pulse laser operation in mid-infrared waveband. The findings are expected to provide a fundamental principle of in situ modification in hybrid system for generation of high-performance semiconductor-in-glass composites.

Novel SnO2-coated β-Ga2O3 nanostructures for room temperature hydrogen gas sensor

Nanostructured β-Ga2O3 nanobelts are fabricated by thermal evaporation method at a temperature around of 1120 °C. Subsequently, a sol-gel processed SnO2 layer is employed to coat β-Ga2O3 nanobelts. This simple deposition technique is confirmed to be a promising route to incorporate a coating material onto nanostructured films, resulting in an attractive approach to modify the functionality of gallium oxide. In comparison with the data reported in the literature, the sensing characteristics of SnO2-functionalized β-Ga2O3 nanobelts to analyte gas H2 in the concentration range (33–1000 ppm) and within the temperature range (room temperature – 200 °C) are found to be greatly improved.

Synthesis and Photocatalytic Activity of β-Ga2O3 Nanostructures for Decomposition of Formaldehyde under Deep Ultraviolet Irradiation

An attempt to degrade volatile organic compounds (VOCs) and sterilize air simultaneously is highly desirable to improve indoor air quality. With the help of deep ultraviolet (UVC) lighting, harmful bacteria that exists in the air can be destroyed. Thus, a new photocatalytic substance that can break down VOCs under UVC irradiation is required. Here, we demonstrate the photocatalytic activity of β-Ga2O3 nanostructures, synthesized via the solvothermal method for removing formaldehyde (HCHO) under deep ultraviolet irradiation. The Raman and XRD results indicated that as-synthesized nanostructures showed β-crystalline phase with a monoclinic structure. The photoluminescence spectrum exhibited a broad and strong green emission peak at 510 nm, which was likely attributed to a considerable amount of oxygen and gallium vacancies formed during the calcinating process. The photocatalytic efficiency for decomposing HCHO at room temperature under deep ultraviolet irradiation (278 nm) of the synthesized β-Ga2O3 nanoparticles is higher than that of the β-Ga2O3 nanorods. Both nanoparticles and nanorods obeyed the pseudo-first-order Langmuir-Hinshelwood kinetic model with a degradation rate constant of 0.057 and 0.033 min−1, corresponding to the efficiency of 82% and 62% in the formaldehyde removal, respectively.