Ga2O3 Nanostructures Research Articles

Temperature modulation enables in situ growth of gallium oxide nanostructures on thin films exhibiting diverse morphologies

In this study, we successfully synthesized three gallium oxide thin films on (0001)-faceted Al2O3 substrates using hydrogen-reduced chemical vapor deposition (CVD) at different deposition temperatures, resulting in distinct morphologies and crystalline phases. These films consist of two pure-phase β-Ga2O3 films and one mixed-phase film comprising both β- and ε-Ga2O3 phases. The mixed-phase samples were subsequently annealed at high temperatures to obtain pure-phase β-Ga2O3 thin films. Furthermore, by precisely controlling the growth temperature, we achieved in-situ formation of Ga2O3 nanostructures (nanowires and nanoribbons) on the surface of the β-Ga2O3 thin films. The presence of different dominant crystal planes in the samples was confirmed through X-ray diffraction analysis (XRD) for verification purposes. In directly grown β-Ga2O3 films, the (−201) plane emerged as the primary crystallographic plane, while both the (−201) and (401) planes dominated in β-Ga2O3 films obtained via ε-Ga2O3 film annealing. Raman spectroscopy findings further supported these observations regarding the crystalline phase of the films as observed through XRD analysis. Scanning electron microscopy (SEM) images revealed distinct film morphologies resulting from variations in deposition temperatures. A flatter surface was observed when a higher proportion of ε-Ga2O3 was present in the film, however, regular cracking occurred after annealing. Atomic Force Microscope (AFM) images demonstrated that at a deposition temperature of 800 °C, regular steps were present on the film's surface. Mixed-phase films exhibited hillock morphology that disappeared after annealing, leading to a decrease in the surface roughness in root mean square (RMS) value. By elevating the growth temperature, a significantly enhanced density of gallium oxide nanoribbons can be seamlessly integrated onto the film surface. The transferred nanoribbons exhibit an ultra-thin profile with a minimum thickness of merely 12 nm, rendering them more slender and economically advantageous in comparison to conventional mechanical stripping methods. This study presents a CVD-based approach for growing gallium oxide thin films with varying morphologies by employing temperature modulation and demonstrates an innovative one-step method for growing nanostructures on gallium oxide films in situ, providing valuable insights for future device development involving gallium oxide.

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.

Enhanced Photocatalytic Performance of rGO Modulated β‐Ga2O3 for Wastewater Treatment

AbstractRapid urbanization has inevitably raised challenges by indirectly adding industry waste to the environment. This problem becomes even more severe when contamination of consumable water is considered. As a leading sector in semiconductor‐related research to face such challenges, gallium oxide and its derivatives have shown remarkable performance by exhibiting controlled, recyclable photocatalytic activities. This work demonstrates the enhancement of the photocatalytic performance of β‐Ga2O3, which is related to the structural and morphological variation induced by rGO wrapping. Firstly, the Ga2O3 nanostructures are obtained by calcinating gallium oxide hydroxide (GaOOH), which is synthesized via a simple chemical route. On the other hand, rGO wrapped β‐Ga2O3 is synthesized using in‐situ hydrothermal treatment. In addition to traditional characterizations such as X‐ray diffractometry (XRD), field emission scanning electron microscopy (FESEM), high‐resolution transmission electron microscopy (HRTEM), Raman spectroscopy, etc., the photocatalytic performance of both pristine and composite systems is studied by time‐dependent UV–Vis absorption spectrum for the degradation of different organic toxic pollutants. It is observed that the composite sample exhibited a larger effective surface area and enhanced optical absorption in the UV–Vis range as compared to the pristine gallium oxide sample. Under UV light irradiation, the rGO wrapped β‐Ga2O3 nanobars can degrade rhodamine B dye entirely within 25 min and exhibit a higher photodegradation rate of 0.1598 min−1 which became 3.5 times higher as compared to that of pure gallium oxide.

Ambient-condition strategy for rapid mass production of crystalline gallium oxide nanoarchitectures toward device application

Currently, the synthesis of nanostructures still encounters two grand challenges: one is the often-required high temperatures and/or high pressures, and the other is the scalable fabrication. Here, to break through such bottlenecks, we demonstrate an ambient-condition strategy for rapid mass production of fourth-generation semiconductor Ga2O3 nanoarchitectures assembled by single-crystalline nanosheets in a controlled manner based on sonochemistry. Their growth is fundamentally determined by the introduced ethanediamine in rationally designed source materials, which could not only protect the metal Ga against reacting with H2O into GaOOH, but facilitate the reaction of Ga with OH· radicals for target crystalline Ga2O3 nanostructures. As a proof of concept for applications, the as-fabricated Ga2O3 nanoarchitectures exhibit superb performances for electromagnetic wave absorption with a reflection loss value of 52.2 dB at 8.1 GHz, and ammonia sensing with high sensitivity and selectivity at room temperature, representing their bright future to be commercially applied in modern devices.

Morphology tuned Ga2O3 nanostructures for visible light-assisted dye-sensitized photocatalytic water remediation

Low-dimensional Ga2O3 was synthesized via a facile chemical route aiming at easy morphological tuning. Different Gallia precursors as starting materials for hydrothermal synthesis eventually led to spherical nanoparticles and fractured nanobricks as the end product. In addition to the regular characterization of phase, morphology, chemical bond, and surface-related investigations, both samples were subjected to visible-light-induced photocatalytic degradation of toxic organic pollutants. Despite its wide bandgap, the samples showed an efficient dye degradation ability under visible excitation, which was explained as originating due to the sensitization of the dyes. The nitrate-salt-originated nanobricks appeared to be more efficient in degrading rhodamine B (RhB) than the nanospheres fabricated using chloride-salt, presenting degradation rate constants of 0.0394 and 0.0057 min−1, respectively. The performances of the samples differed due to their electronic band positions. Also, morphological features like higher specific surface area, porosity, and aspect ratio enabled faster degradation of RhB for nanobricks. However, as reflected in BET studies, the lower surface area and pore density caused comparatively weaker degradation performance of the nanospheres. This remediation technology can optimize similar future catalyst systems fabricated using the hydrothermal-synthesis route for wastewater treatment.

Novel Approach to Synthesize Nanostructured Gallium Oxide for Devices Operating in Harsh Environmental Conditions

The importance of Ga2O3-based material for harsh environmental applications has attracted the interest of researchers in exploring various fabrication and growth techniques of Ga2O3-based nanomaterials using effective and low-cost processes. Herein, a demonstration to improve the wettability of liquid gallium on a rough silicon surface is presented. To control the roughness process, the silicon surface was patterned and groove-shape structures on the silicon were created using a photoelectrochemical (PEC) etching technique. Gallium oxide nanostructures were grown by thermal oxidation from liquid Ga in the presence and the absence of a silver thin film used as a catalyst. Scanning Electron Microscopy (SEM) was used to observe the morphology of the nanostructures grown on the roughed surface of the silicon substrate. The conformal deposition of Ga2O3 nanostructures inside the grooves of the PEC etched silicon surface was observed. The presence of Ag catalyst was found to completely change the morphology of Ga2O3. This method is recommended for the sustainable and low-cost synthesis of nanostructured gallium oxide for applications, including gas sensing.

Highly sensitive solar-blind deep ultraviolet photodetector based on graphene/PtSe2/β-Ga2O3 2D/3D Schottky junction with ultrafast speed

There is an emerging need for high-sensitivity solar-blind deep ultraviolet (DUV) photodetectors with an ultra-fast response speed. Although nanoscale devices based on Ga2O3 nanostructures have been developed, their practical applications are greatly limited by their slow response speed as well as low specific detectivity. Here, the successful fabrication of two-/three-dimensional (2D/3D) graphene (Gr)/PtSe2/β-Ga2O3 Schottky junction devices for high-sensitivity solar-blind DUV photodetectors is demonstrated. Benefitting from the high-quality 2D/3D Schottky junction, the vertically stacked structure, and the superior-quality transparent graphene electrode for effective carrier collection, the photodetector is highly sensitive to DUV light illumination and achieves a high responsivity of 76.2 mA/W, a large on/off current ratio of ~ 105, along with an ultra-high ultraviolet (UV)/visible rejection ratio of 1.8 × 104. More importantly, it has an ultra-fast response time of 12 µs and a remarkable specific detectivity of ~ 1013 Jones. Finally, an excellent DUV imaging capability has been identified based on the Gr/PtSe2/β-Ga2O3 Schottky junction photodetector, demonstrating its great potential application in DUV imaging systems.

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.

Comparative Study of Growth Morphologies of Ga2O3 Nanowires on Different Substrates.

Gallium oxide (Ga2O3) is a new wide bandgap semiconductor with remarkable properties that offers strong potential for applications in power electronics, optoelectronics, and devices for extreme conditions. In this work, we explore the morphology of Ga2O3 nanostructures on different substrates and temperatures. We used silver catalysts to enhance the growth of Ga2O3 nanowires on substrates such as p-Si substrate doped with boron, 250 nm SiO2 on n-Si, 250 nm Si3N4 on p-Si, quartz, and n-Si substrates by using a thermal oxidation technique at high temperatures (~1000 °C) in the presence of liquid silver paste that served as a catalyst layer. We present the results of the morphological, structural, and elemental characterization of the Ga2O3 nanostructures. This work offers in-depth explanation of the dense, thin, and long Ga2O3 nanowire growth directly on the surfaces of various types of substrates using silver catalysts.

Feasibility study on synthesis of gallium oxide nanostructures on glass substrate by chemical vapor deposition

Synthesis of metal oxide semiconductor films on amorphous glass substrate by using chemical vapor deposition (CVD) reactors operating at temperature ≥ 650 °C has seemingly been impossible over the years, due to glass substrate's tendency to undergo deformation at high temperatures. In the present work, Ga2O3 nanostructures (NSs) are being deposited on amorphous glass substrate by a CVD route. The films are fabricated at 950°C for durations of 60, 90, 120 and 150 min, respectively in a mixed atmosphere of N2 and H2. The effects of growth duration on the crystal structure, microstructure, stoichiometry and optical bandgap of Ga2O3 NSs are investigated in detail. X-ray diffraction data reveals polycrystalline Ga2O3α-β bi-phase. Crystallite size of the films is found to increase with prolong deposition time. Field-emission scanning electron microscopy unveils surface morphology comprising primarily of faceted aggregates as the underneath layer and a few nanostrips at the top. Energy dispersive X-ray spectroscopy reveals gradual rise in Ga/O atomic ratio with increased growth duration. The optical absorbance measurements of the films a show slight shift of the absorption edge to longer wavelengths and the bandgap decrease from 4.60 to 4.51 eV with an increase in reaction time, attributed to rising Ga/O ratio and an increase in film thickness. The results presented are expected to pave way for the fabrication of different nanomaterials on amorphous glass substrates by using high-temperature CVDs.

Comparison of Ga2O3 and TiO2 Nanostructures for Photocatalytic Degradation of Volatile Organic Compounds

The photocatalytic degradation of formaldehyde, acetaldehyde, toluene, and styrene are compared using monoclinic Ga2O3 and anatase TiO2 nanostructures under ultraviolet-C irradiation. These Ga2O3 and TiO2 photocatalysts are characterized using a field emission scanning electron microscope, a powder X-ray diffraction system, the Brunauer–Emmett–Teller method, and a Fourier transform infrared spectrometer. The Ga2O3 shows a higher reaction rate constant (k, min−1) than TiO2 by a factor of 7.1 for toluene, 8.1 for styrene, 3.1 for formaldehyde, and 2.0 for acetaldehyde. The results demonstrate that the photocatalytic activity ratio of the Ga2O3 over the TiO2 becomes more prominent toward the aromatic compounds compared with the nonaromatic compounds. Highly energetic photo-generated carriers on the conduction/valence band-edge of the Ga2O3, in comparison with that of the TiO2, result in superior photocatalytic activity, in particular on aromatic volatile organic compounds (VOCs) with a high bond dissociation energy.

Gallium Oxide for High‐Power Optical Applications

AbstractGallium oxide (Ga2O3) is an emerging wide‐bandgap transparent conductive oxide (TCO) with potential applications for high‐power optical systems. Herein, Ga2O3 fabricated nanostructures are described, which demonstrate high‐power laser induced damage threshold (LIDT). Furthermore, the demonstration of an electron accelerator based on Ga2O3 gratings is reported. These unique Ga2O3 nanostructures provide acceleration gradients exceeding those possible with conventional RF accelerators due to the high breakdown threshold of Ga2O3. In addition, the laser damage threshold and acceleration performance of a Ga2O3‐based dielectric laser accelerator (DLA) are compared with those of a DLA based on sapphire, a material known for its high breakdown strength. Finally, the potential of Ga2O3 thin‐film coatings as field reduction layers for Si nanostructures is shown; they potentially improve the effective LIDT and performance of Si‐based DLAs and other high‐power optical structures. These results could provide a foundation for new high‐power optical applications with Ga2O3.

Influence of Zn doping on the morphology and luminescence of Ga2O3 low-dimensional nanostructures

Zn-doped Ga2O3 nanostructures were synthesized by thermal evaporation. SEM results show that there are plenty of nanorods in the samples. The EDS analysis of the samples demonstrate that the nanostructures are mainly composed of Ga and O elements, and a small amount of Zn element is also observed. The Raman spectra indicate that the samples still keep the monoclinic structure consistent with the pure Ga2O3 crystal structure. A broad-band blue-green luminescence centered around at 2.3 eV was observed at room temperature. The luminescence intensity of the PL spectra of the Zn-doped Ga2O3 nanostructures varied with the amount of Zn doping. Besides, the PL spectra of the Zn-doped Ga2O3 nanostructures have a red-shift compared to that of the pure Ga2O3 nanostructures. The analysis of the PL spectrum of Zn-doped Ga2O3 nanostructures with a mass ratio of Ga2O3 powder to zinc powder of 10:1 indicates that the broad-band blue-green luminescence of the Zn-doped Ga2O3 nanostructures is composed of one blue luminescence (2.8 eV, 450 nm) band, one green luminescence (2.3 eV, 533 nm) band, one yellow luminescence (2.1 eV, 592 nm) band, and one red luminescence (1.9 eV, 638 nm) band, respectively. Further, their luminescence mechanisms will be discussed.

Phase junction enhanced photocatalytic activity of Ga2O3 nanorod arrays on flexible glass fiber fabric.

Ga2O3 nanostructures hold great potential applications in photocatalytic fields due to their stability, high efficiency and environmental friendliness. The construction of phase junction has been proved to be one of the most effective strategies for enhancing Ga2O3 photocatalytic activity. However, the influence of the formation process at the interface of the phase junction on the photocatalytic activity of Ga2O3 nanostructures is far less well understood. In this work, for the first time, large-area Ga2O3 nanorod arrays (NRAs) with controllable α/β phase junction were prepared in situ on a flexible glass fiber fabric by a facile and environmentally friendly three-step method. The α/β-Ga2O3 phase junction NRAs exhibit an ultra-high photocatalytic degradation rate of 97% during Ultraviolet (UV) irradiation for 60 min, which is attributed to a unique phase junction promoting efficient charge separation. However, the photocatalytic activity of α/β-Ga2O3 phase junction NRAs is not evident in the early phase transition, possibly due to the presence of defects acting as charge recombination centers.

Temperature-resolved photoluminescence, Raman and electrical properties of Li doped Ga2O3 nanostructure

In this study, Li-doped Ga2O3 nanostructures (Ga2O3: Li) were synthesized through thermal evaporation. The morphology of the Ga2O3: Li was found to be mainly composed of nanorods and nanosheets. Raman spectroscopy revealed that the Raman spectra after doping of sample had four additional peaks at 273 cm−1, 428 cm−1, 517 cm−1 and 583 cm−1, respectively. While the signal of Li ions was detected in the XPS spectra of sample. In Temperature-Resolved Photoluminescence, Ga2O3: Li showed an additional wider Red-IR emission peak centered at 696 nm, in contrast to undoped Ga2O3. Further, with decreasing temperature, the Red-IR emission peak split into several peaks. Among them, the peak at 722 nm showed the largest variations with the temperature. Further analysis of the peaks reveals that it is mainly caused by the doping of Li ions. Besides, it was found that the resistance increased significantly under illumination. Especially when the illumination light is red emission, the resistance value changes most obviously, and has a correspondence with the photoluminescence spectrum. The Red-IR characteristic of Ga2O3: Li makes it a promising candidate for application in the optoelectronic devices.

Tailored mesoporous nanocrystalline Ga2O3 for dye-selective photocatalytic degradation

Porous brick-like low dimensional Ga2O3 nanostructures were fabricated through a low-cost chemical route having different phases synthesized at different temperatures. The pore-density was varied by varying synthesis temperature and the materials exhibited a transformation from α-hexagonal to β-monoclinic phase. With traditional structural and morphological characterization, detailed contaminant removal properties were investigated for the as-prepared samples. The green cleaning efficiency was recognized to be influenced by crystal structure, surface morphology and surface charge type of the catalyst. All of the synthesized material showed promising performance in degradation of traditional organic hazardous dyes like rhodamine B (RhB), methyl orange (MO) and apparently invisible harmful water soluble chemical like phenol. An interesting feature, i.e., obtaining dye specific adsorbent out of the same materials and the mechanism for the ion selective photo-degradation process have been presented in this work. High degradation rate constants of 0.072 min−1, 0.051 min−1 and 0.18 min−1 were obtained for RhB, MO and mixed dyes respectively with almost complete removal of the dyes. The role of intermediate radicals •OH, •O2− and h+ was correlated with catalytic performances depending upon modification of band positions of the dyes. The mesoporous Ga2O3 structures are hence inferred as potential candidates for future water safety issues.

Enhanced Photocatalytic Degradation of 2-Butanone Using Hybrid Nanostructures of Gallium Oxide and Reduced Graphene Oxide Under Ultraviolet-C Irradiation

Hybrid nanostructures made of gallium oxide (Ga2O3) and reduced graphene oxide (rGO) are synthesized using a facile hydrothermal process method, where the Ga2O3 nanostructures are well dispersed on the rGO surface. The formed Ga2O3-rGO hybrids are characterized via Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), a diffuse reflectance Ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer, Brunauer–Emmett–Teller (BET), and photoluminescence (PL). Gas chromatography mass spectrometry (GC-MS) was used for analyzing volatile organic compounds (VOCs). The photocatalytic activity of the hybrid nanostructures is evaluated via the degradation of the 2-butanone, representing the VOCs under 254-nm radiation in the atmosphere. That activity is then compared to that of the Ga2O3 and commercial TiO2-P25. The Ga2O3-rGO hybrid shows enhanced photocatalytic degradation of 2-butanone compared to Ga2O3 and TiO2-P25, which is attributed to the enhanced specific surface area. The results indicate that the Ga2O3-rGO hybrid could be a promising method of enhancing photocatalytic activity and thereby effectively degrading VOCs, including the 2-butanone.

Raman and cathodoluminescence analysis of transition metal ion implanted Ga2O3 nanowires

The structural and luminescence properties of gallium oxide nanowires doped with chromium or manganese have been investigated. Undoped Ga2O3 nanostructures have been fabricated by a thermal evaporation method, while doping was subsequently achieved by ion implantation followed by thermal annealing. Scanning electron microscopy (SEM) analysis has shown that this doping process does not alter the morphology of the nanostructures. Ion implantation results in partial amorphization of the crystal lattice, as deduced from Raman spectroscopy studies. Thermal annealing at different temperatures was carried out in order to restore the crystallinity of the nanowires. Raman spectroscopy analysis demonstrates that recrystallization starts at about 700°C and a complete recrystallization is achieved at about 1000°C. Cathodoluminescence (CL) analysis has been used to study the emissions in the 300–900nm range. As-implanted nanowires virtually do not emit any light, which is related to their poor crystal quality and the implantation induced defects. Thermal annealing results in effective CL emission. In particular, a clear correlation between crystallinity of the nanowires doped with Cr and the emission from the 2E-4A2 and 4T2-4A2 intraionic transitions has been observed. On the other hand, emissions directly related to intraionic transitions of Mn have not been found in the nanowires implanted with this ion. The influence of the implantation process and annealing temperature on the observed changes in the donor-acceptor pairs (DAP) band of Ga2O3 is discussed.

Template assisted fast photocatalytic degradation of azo dye using ferric oxide–gallia nanostructures

Photocatalytic activity of Fe2O3–Ga2O3 nanostructures, synthesized via two routes (with and without glycoluril as structure directing agent), containing various proportions of Fe2O3 in presence of 15 ppm CR was investigated.

Synthesis, structure and ethanol sensing properties of Ga2O3-core/WO3-shell nanostructures

Ga2O3-core/WO3-shell nanostructures were synthesized by thermal evaporation of Ga2S3 powders followed by thermal evaporation of WO3 powders and their C2H5OH gas sensing properties were examined. Ga2O3–WO3 heterostructure formation turned out to enhance the C2H5OH gas sensing properties remarkably. Ga2O3-core/WO3-shell nanostructure sensors showed far stronger and faster response to C2H5OH gas than pristine Ga2O3 nanostructures sensors. The significant improvement in the response of the Ga2O3 nanostructures to C2H5OH gas by Ga2O3–WO3 heterostructure formation can be explained by a combination of the potential barrier-controlled carrier transport mechanism and the surface-depletion mechanism.