β-Ga2O3 Nanowires Research Articles

Catalyst-assisted heteroepitaxial strategy for highly ordered β-Ga2O3 nanoarrays and their optical property investigation

In this work, we demonstrate the growth of highly ordered β-Ga2O3 nanoarrays with (001) preferred growth plane for the first time through a facile heteroepitaxial strategy using metal Ga and c-sapphire as Ga precursor and monocrystalline substrate. The (001) preferred growth plane means that the β-Ga2O3 nanowires grow along the normal direction of the (001) plane. The β-Ga2O3 nanoarrays along (001) preferential plane exhibit inclined six equivalent directions that correspond to the six crystallographic symmetry of (0001) α-Al2O3. High-resolution transmission electron microscopy analyses confirm the good crystallinity and the existence of unusual epitaxial relationship of {310} β-Ga2O3 ǁ (0001) α-Al2O3 and <001> β-Ga2O3 or <132> β-Ga2O3 ǁ [100] α-Al2O3. UV–vis and cathodoluminescence measurements reveal the wide band gap of 4.8 eV and the strong UV-blue luminescence (300–500 nm) centered at ∼388 nm. Finally, the luminescence mechanism is further investigated with the assistance of x-ray photoelectron spectroscopy. The heteroepitaxial strategy of highly ordered β-Ga2O3 nanoarrays in this work will undoubtedly pave a solid way toward the fundamental research and the applications of Ga2O3 nanodevices in optoelectronic, gas sensor, photocatalyst and next-generation power electronics.

High-Quality Single-Crystalline β-Ga2O3 Nanowires: Synthesis to Nonvolatile Memory Applications.

One of the promising nonvolatile memories of the next generation is resistive random-access memory (ReRAM). It has vast benefits in comparison to other emerging nonvolatile memories. Among different materials, dielectric films have been extensively studied by the scientific research community as a nonvolatile switching material over several decades and have reported many advantages and downsides. However, less attention has been given to low-dimensional materials for resistive memory compared to dielectric films. Particularly, β-Ga2O3 is one of the promising materials for high-power electronics and exhibits the resistive switching phenomenon. However, low-dimensional β-Ga2O3 nanowires have not been explored in resistive memory applications, which hinders further developments. In this article, we studied the resistance switching phenomenon using controlled electron flow in the 1D nanowires and proposed possible resistive switching and electron conduction mechanisms. High-density β-Ga2O3 1D-nanowires on Si (100) substrates were produced via the VLS growth technique using Au nanoparticles as a catalyst. Structural characteristics were analyzed via SEM, TEM, and XRD. Besides, EDS, CL, and XPS binding feature analyses confirmed the composition of individual elements, the possible intermediate absorption sites in the bandgap, and the bonding characteristics, along with the presence of various oxygen species, which is crucial for the ReRAM performances. The forming-free bipolar resistance switching of a single β-Ga2O3 nanowire ReRAM device and performance are discussed in detail. The switching mechanism based on the formation and annihilation of conductive filaments through the oxygen vacancies is proposed, and the possible electron conduction mechanisms in HRS and LRS states are discussed.

Epitaxial growth of β-Ga2O3 nanowires from horizontal to obliquely upward evolution

Changing the direction of horizontal nanowires to extend them in the z direction and grow obliquely has important practical significance for achieving higher density integration and making it have more free space in multi-dimension. However, how to guide the horizontal nanowires to extend in the z direction and grow obliquely is still a challenge. In this paper, we report the regular and horizontal β-Ga2O3 nanostructure grown on c-plane Al2O3 substrates, and the evolution of β-Ga2O3 nanowires from horizontal growth to obliquely upward growth through the chemical vapor deposition (CVD) method. Field-emission scanning electron microscopy, field-emission transmission electron microscopy, X-ray diffraction, Raman spectrum and photoluminescence spectroscopy were used to investigate the morphology, structure, components, and optical properties of the as-synthesized β-Ga2O3 nanowires. Due to the change of the growth interface at the Au catalysts, there are four different growth modes for the growth of β-Ga2O3 nanowires. The preparation route reported in this study is more simple, general and low-cost compared to other common growth methods for Ga2O3 nanomaterials, and these studies about the evolution of nanowires growth can provide a reference for the change of growth direction of nanowires and simplify planar device processing technology.

Microstructural and optical properties of Ag assisted β-Ga2O3 nanowires on silicon substrate

Abstract In this paper, Silver nanoparticle assisted vertically aligned β-Ga2O3 nanowires (Ag NP-β-Ga2O3 NW) were fabricated on Silicon (Si) -substrate by Glancing angle deposition (GLAD) technique. With the integration of Ag NP, the XRD pattern exhibits a reduction in the crystallite size of β-Ga2O3 NW. The EDS analysis confirms the presence of Ag NP, thereby further supporting the XRD result. The SEM image analysis shows a consistency in the size (40–50 nm) of NW growth for Ag NP-β-Ga2O3 NW. The photoluminescence spectra show an enhancement in the Ag NP-β-Ga2O3 NW structure. Lastly, the optical absorption spectra of β-Ga2O3 NW and Ag NP-β-Ga2O3 NW reveals that the presence of Ag NP in β-Ga2O3 NW enhance the photon absorption in the UV and the visible region as compared to β-Ga2O3 NW due to the synergistic action of Ag NP and β-Ga2O3 NW. This study demonstrates that high-quality Ag NP-β-Ga2O3 NW nanostructure can be grown in large quantities for a variety of applications, including photodetector, gas sensor, and power devices applications.

Controlled lateral epitaxial growth in vertical β-Ga2O3 nanowires on sapphire by MOCVD

Lateral epitaxial growth in vertical β-Ga2O3 nanowires was studied on sapphire substrates via metal organic chemical vapor deposition by controlling growth temperature and O2 flow rate. The lateral epitaxial growth rate of β-Ga2O3 rise up with increasing growth temperature and decreasing O2 flow rate, which are related to reaction kinetics and parasitic reaction, respectively. In addition, a growth model is proposed to explain the morphology of nanowires when growth condition changes. These results can provide important guidance on addressing slow coalescence rate of β-Ga2O3 nuclear islands on heterogeneous substrates and open new doors to β-Ga2O3 optoelectronic and electronic devices on cost-effective foreign substrates.

Ultraviolet photodetector based on vertical β-Ga2O3 nanowire array on GaN substrate

Driven by the requirement to ultraviolet detection, β-Ga2O3 UV photodetectors have attracted great attention. Using a metal organic chemical vapor deposition (MOCVD) reactor, we grew β-Ga2O3 nanowires array on a GaN substrate using Ga as catalyst. The density of the nanowires was optimized employing the substrate patterning technology. A UV detector based on the graphene/β-Ga2O3-nanowire-array was realized by micro-fabrication techniques. The device has a wide range of UV response covering UVC-UVA band and the peak response reaches 30.82 mA W−1 at 258 nm corresponding to the band gap of β-Ga2O3. The rapid response speed (<1 s) is comparable to that of most reported Ga2O3 nanowire ultraviolet photodetectors.

Morphology of Ga2O3 Nanowires and Their Sensitivity to Volatile Organic Compounds

Gas sensitive structures made of nanowires exhibit extremally large specific surface area, and a great number of chemically active centres that can react with the ambient atmosphere. This makes the use of nanomaterials promising for super sensitive gas sensor applications. Monoclinic β-Ga2O3 nanowires (NWs) were synthesized from metallic gallium at atmospheric pressure in the presence of nitrogen and water vapor. The nanowires were grown directly on interdigitated gold electrodes screen printed on Al2O3 substrates, which constituted the gas sensor structure. The observations made with transmission electron microscope (TEM) have shown that the nanowires are monocrystalline and their diameters vary from 80 to 300 nm with the average value of approximately 170 nm. Au droplets were found to be anchored at the tips of the nanowires which may indicate that the nanowires followed the Vapor–Liquid–Solid (VLS) mechanism of growth. The conductivity of β-Ga2O3 NWs increases in the presence of volatile organic compounds (VOC) even in the temperature below 600 °C. The gas sensor based on the synthesized β-Ga2O3 NWs shows peak sensitivity to 100 ppm of ethanol of 75.1 at 760 °C, while peak sensitivity to 100 ppm of acetone is 27.5 at 690 °C.

Influence of Silver as a Catalyst on the Growth of β-Ga2O3 Nanowires on GaAs.

A simple and inexpensive thermal oxidation process was performed to synthesize gallium oxide (Ga2O3) nanowires using Ag thin film as a catalyst at 800 °C and 1000 °C to understand the effect of the silver catalyst on the nanowire growth. The effect of doping and orientation of the substrates on the growth of Ga2O3 nanowires on single-crystal gallium arsenide (GaAs) wafers in atmosphere were investigated. A comprehensive study of the oxide film and nanowire growth was performed using various characterization techniques including XRD, SEM, EDS, focused ion beam (FIB), XPS and STEM. Based on the characterization results, we believe that Ag thin film produces Ag nanoparticles at high temperatures and enhances the reaction between oxygen and gallium, contributing to denser and longer Ga2O3 nanowires compared to those grown without silver catalyst. This process can be optimized for large-scale production of high-quality, dense, and long nanowires.

High performance solar-blind UV detector based on β-Ga2O3/GaN nanowires heterojunction

Gallium nitride nanowires and β-Ga2O3 nanowires were prepared by chemical vapor deposition. A solar-blind ultraviolet detector was fabricated based on β-Ga2O3/GaN nanowires heterojunction. The UV detector exhibits excellent response properties to deep ultraviolet with wavelength of 254 nm. The device exhibits a photo-to-dark current ratio of 1.375 × 103 and a high detectivity of 1.2 × 1011 Jones at 10 V bias Therefore, the responsivity of the UV detector is 27.5 mA/W under 254 nm ultraviolet light (8 mW/cm2) at 10 V bias voltage. The detector also has the advantages of easy preparation, low dark current, high rejection ratio, fast response, good stability and repeatability. The design method of β-Ga2O3/GaN nanowires heterojunction has the advantages of novelty and simplicity, which provides a new approach to fabricate a heterojunction detector.

Post deposition annealing effect on the electrical properties of β-Ga2O3 Nanowire

This study reports the fabrication of vertically aligned β-Ga2O3 nanowires over the silicon substrate by the glancing angle deposition technique. Thermal annealing was done to the fabricated sample in ambient atmospheric conditions at 900 °C for 1 h. The annealed sample shows a higher average crystallite size (20.19 nm) when equated to the as-deposited one (13.72 nm). The annealed sample also shows a lower turn-on voltage of + 2.5 V. A maximum responsivity of 152.34 A/W and an internal gain of 675.02 were obtained for the annealed device at 260 nm. Low noise equivalent power (8.4 × 10–14 W) and high sensitivity towards light were observed for the annealed device which makes it a promising contender for optoelectronics device application.

Solar-blind ultraviolet photodetector based on vertically aligned single-crystalline β-Ga2O3 nanowire arrays

Abstract Vertically aligned nanowire arrays, with high surface-to-volume ratio and efficient light-trapping absorption, have attracted much attention for photoelectric devices. In this paper, vertical β-Ga2O3 nanowire arrays with an average diameter/height of 110/450 nm have been fabricated by the inductively coupled plasma etching technique. Then a metal-semiconductor-metal structured solar-blind photodetector (PD) has been fabricated by depositing interdigital Ti/Au electrodes on the nanowire arrays. The fabricated β-Ga2O3 nanowire PD exhibits ∼10 times higher photocurrent and responsivity than the corresponding film PD. Moreover, it also possesses a high photocurrent to dark current ratio (I light/I dark) of ∼104 and a ultraviolet/visible rejection ratio (R 260 nm/R 400 nm) of 3.5 × 103 along with millisecond-level photoresponse times.

VLS growth of pure and Au decorated β-Ga2O3 nanowires for room temperature CO gas sensor and resistive memory applications

High-density single crystalline β-Ga2O3 nanowires on silicon (1 0 0) substrates were grown by vapour-liquid-solid growth method. We have characterized the pure β-Ga2O3 nanowires along with the Au-decorated β-Ga2O3 nanowires. The CO gas sensors at room temperature (RT) have been studied for pure and Au decorated nanowires with multiple-networked array and single nanowire devices. The diameter of the 1D nanostructure ranged from 127 ± 5 nm. The synthesized nanowires were studied using Field Emission Scanning Electron Microscope (FESEM), Transmission Electron Microscope (TEM), Energy Dispersive X-ray Spectroscopy (EDS), Gracing Incidence X-ray Diffraction (GI-XRD), Photoluminescence (PL), Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Using the Focussed Ion Beam (FIB) technique, single nanowire gas sensor devices were fabricated. Single nanowire RT CO gas sensors using the proposed Au decorated β-Ga2O3 nanowire achieved remarkable sensitivity for 100 ppm CO gas at room temperature. Besides, we have compared the RT CO gas sensing properties of multiple-networked Au decorated β-Ga2O3 nanowires with single Au-decorated β-Ga2O3 nanowire and single pure β-Ga2O3 nanowire. In addition, bipolar resistive switching property is inspected for the Au/pure β-Ga2O3 nanowires/p-Si and Au/Au decorated β-Ga2O3 nanowires/p-Si structures.

1D semiconductor nanowires for energy conversion, harvesting and storage applications

Abstract The accomplishment of 1D semiconductor nanowires (SN) in the field of energy has attracted intense interest in recent years due to their advantageous properties (e.g., large surface area, unique surface chemistry, and tunable transport properties). Considerable efforts were devoted to explore 1D-SN building blocks as the harvesting channel/unit (e.g., in thermal, chemical, mechanical, and solar energy applications) and as the storage media (for electrochemical energy). A wide bandgap tuning of SN in the range of 0.39 eV (in case of InAs nanowires) to 4.66 eV (in case of β-Ga2O3 nanowires) due to quantum size effect makes them a suitable candidate for optoelectronic applications. This review focuses on 1D-SN wherein the travel of electron and photon is confined in two directions but in one dimension. The SN emerged as promising nanostructures for developing electronic devices of high carrier-mobilities (e.g., >12000 cm2V−1s−1 for holes and 3000 cm2V−1s−1 for electrons in case of Ge nanowires). A list of efficient fabrication strategies (e.g., vapor-liquid-solid [VLS], hard-template approaches, and solution-phase) are discussed along with ultrafast electron transport dynamics of SN and piezoelectric nanowires. The control on electrons, photons, and phonons transport makes 1D-SN ideal for solid-state energy conversion, harvesting, and storage applications. State-of-the-art 1D-SN energy nano-systems have been demonstrated to yield diverse outcomes of high significance including single-nanowire and array-based photovoltaic cells (InP nanowires with a maximum power conversion efficiency up to 17.8%), nanogenerators (SiGe nanowires with a maximum power output of 7.1 μW/cm2), supercapacitors (core-shell hierarchical CoS@MoS2 nanowire array with an energy density of 95.7 Wh kg−1 at power density of 711.2 W kg−1), and lithium-air batteries (3D freestanding hierarchical CuCo2O4 nanowires@Ni foam with an excellent specific capacity of 13654 mAh g−1 at 0.1 mA cm−2). This review will serve as a key platform to understand 1D-SN to fabricate the next-generation novel nano-systems for developing efficient energy devices of high performance.

Ge incorporation in gallium oxide nanostructures grown by thermal treatment

Effective doping of semiconductor oxide nanostructures is needed to control n-type conductivity; however, out-diffusion of impurities in high quality crystals is still a bottleneck in nanotechnology. Here, germanium-doped gallium oxide (β-Ga2O3) nanowires have been grown by a thermal evaporation method. Two temperatures, close to Ge melting point, were chosen for the dynamic annealing of metallic gallium and germanium powders. At 840 °C, Ga2O3 nanowires and nanobelts grow along the b or c direction, leaving gallium germanate particles at the tip in both cases. Wider structures were produced at 940 °C, decorated with a Ge nanocrystal at the end. Transmission electron microscopy has been used to establish the vapor–liquid–solid-like growth process for the formation of these structures. The influence of Ge incorporation in the nanostructures has been assessed by cathodoluminescence and micro-Raman spectroscopy. The presence of Ge in β-Ga2O3 nanostructures is essential for electronic and optoelectronic applications since Ge is potentially the most effective n-type dopant for β-Ga2O3.

Catalyst-Assisted Large-Area Growth of Single-Crystal β-Ga2O3 Nanowires on Sapphire Substrates by Metal-Organic Chemical Vapor Deposition.

In this work, we have achieved synthesizing large-area high-density β-Ga2O3 nanowires on c-plane sapphire substrate by metal–organic chemical vapor deposition assisted with Au nanocrystal seeds as catalysts. These nanowires exhibit one-dimensional structures with Au nanoparticles on the top of the nanowires with lengths exceeding 6 μm and diameters ranging from ~50 to ~200 nm. The β-Ga2O3 nanowires consist of a single-crystal monoclinic structure, which exhibits strong (01) orientation, confirmed by transmission electronic microscopy and X-ray diffraction analysis. The PL spectrum obtained from these β-Ga2O3 nanowires exhibits strong emissions centered at ~360 and ~410 nm, respectively. The energy band gap of the β-Ga2O3 nanowires is estimated to be ~4.7 eV based on an optical transmission test. A possible mechanism for the growth of β-Ga2O3 nanowires is also presented.

Controllable Ga catalyst deposition on GaN template and fabrication of ordered vertical β-Ga2O3 nanowire array

Controllable deposition of Ga droplets on a GaN template is discussed in detail in the present work. It is found that the Ga droplet density shows apparent nonlinear dependence on the flux of triethylgallium under certain annealing conditions. A simplified model based on the weakening of cluster mobility is proposed to explain the phenomenon. In addition, based on the different mobility of Ga on different substrates, a SiO2 mask with ordered nanopore array is designed and optimized for controllable catalyst deposition. By further optimizing the deposition and annealing process, a higher filling rate of nanopores is obtained, and ordered β-Ga2O3 nanowire arrays perpendicular to substrates are then achieved. The controllable uniform vertical nanowire array would further promote optimization of nanowire growth and device fabrication.

Characterization and photoluminescence of Sn-doped β-Ga2O3 nanowires formed by thermal evaporation

Sn-doped β-Ga2O3 nanowires were fabricated by V-L-S mechanism using a thermal process at 850 °C. XRD patterns revealed that the Sn4+ ions substituted at Ga3+ lattice sites. HRTEM confirmed that both the Sn-doped and Sn-free β-Ga2O3 nanowires grew in the [0 0 2] direction. Photoluminescence results showed the enhanced intensities in 2.52 and 2.80 eV by 4.36 and 4.01 times upon to 10 wt% Sn, due to the intrinsic defects VO2- and Schottky defects VO2-andVGa3+; and extrinsic defects of Sn4+Ga3+ and Sn4+i. Our results show that the Sn-doped Ga2O3 nanowires have potential applications in nanophotonics and light-emitting devices in long-wavelengths.

Gallium Oxide Nanowire with Twinning Structure and Its Photoluminescence Property.

In this paper, β-Ga₂O₃ nanowires were synthesized by vapor transport method at different temperatures. The as-prepared samples were analyzed for crystal structure by X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED), and for morphology using scanning electron microscopy (SEM). The results show that the Ga₂O₃ nanowires present a monoclinic structure, the length and diameter of the Ga₂O₃ nanowires increased with the growth temperature. A majority of the Ga₂O₃ nanowires present longitudinal twinning structures. A broad photoluminescence emission band was observed from the Ga₂O₃ nanowires at room temperature, which is caused by different kinds of vacancy defects. Our study shows an unusual twinning structure of β-Ga₂O₃ nanowires, which may be helpful to understand the growth mechanism of nanowires.

Synthesis and optoelectronic properties of ultrathin Ga2O3 nanowires

Synthesis using a gallium sulfide precursor and Au or Ag catalysts produces ultrathin single-crystalline β-Ga2O3 nanowires at low substrate temperatures.

Synthesis and Characterization of Monoclinic Gallium Oxide Nanomaterials for High-Concentration Ethanol Vapor Detection

This study was conducted to synthesize Gallium Oxide nanomaterials from its bulk form using a low-cost and repeatable method and investigate on its potential as sensing element for Ethanol (EtOH). Horizontal Vapor Phase Growth (HVPG) Technique was utilized to synthesize monoclinic Gallium Oxide (β-Ga2O3) nanowires. Images obtained from the Scanning Electron Microscope showed high density nanowires specifically in the area of highest temperature gradient. SEM also showed that the grown structures using HVPG Technique were at the nanoscale with diameters ranging from 51.60 to 908.38 nm. Higher surface-to-volume ratio was also noted in the area of highest temperature gradient which was subjected to an applied magnetics field. The Gallium to Oxygen ratio was verified via EDX to be approximately 2:3 which agrees with the atomic ratio of Ga2O3. The monoclinic structure of the grown nanomaterials was investigated using Raman Spectroscopy. Raman peaks of the samples were at 199 and 486 cm−1 which was accounted to the presence of two Raman-active modes of the b-polymorph of Ga2O3. The Raman spectrum of the grown nanowires confirmed that the material is monoclinic in structure and belongs to C2/m space group. The I-V Curve of the grown nanowires were also determined using two-point probe which illustrates a non-linear curve similar to that of a semiconductor material. Furthermore, additional fundamental properties such as resistivity and specific conductance of the materials were also determined via van der Pauw Technique. Results showed that the material has high specific conductance and low resistivity. The synthesized nanomaterial was responsive to Ethanol vapor. Exposure to the said compound increased its resistance. Graphs showed that there is a significant increase in the resistance after exposure to ethanol vapor.