β-Ga2O3 Nanowires Research Articles

Study of photoconduction properties of CVD grown β-Ga2O3 nanowires

We have investigated photoconductive properties and photoconduction mechanism in single crystalline β-Ga2O3 nanowires. To investigate photoconduction, metal-semiconductor-metal (MSM) based photodetectors were fabricated using single crystalline β-Ga2O3 nanowires grown by CVD technique. The current-voltage characteristics of these photodetectors were measured with varying incident laser powers. It was observed that the photocurrent was almost linear with the incident power. Under illumination, the photocurrent increased by three orders of magnitude. The photoconduction mechanism in β-Ga2O3 nanowires has also been discussed. The photoconduction properties of nanowires demonstrate the possibility of future applications of these nanowires in sensors and photodetectors.

Urchin-like artificial gallium oxide nanowires grown by a novel MOCVD/CVD-based route for random laser application

A novel procedure based on a two-step method was developed to obtain β-Ga2O3 nanowires by the chemical vapor deposition (CVD) method. The first step consists in the gallium micro-spheres growth inside a metal-organic chemical vapor deposition environment, using an organometallic precursor. Nanoscale spheres covering the microspheres were obtained. The second step involves the CVD oxidization of the gallium micro-spheres, which allow the formation of β-Ga2O3 nanowires on the micro-sphere surface, with the final result being a nanostructure mimicking nature's sea urchin morphology. The grown nanomaterial is characterized by several techniques, including X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray, transmission electron microscopy, and photoluminescence. A discussion about the growth mechanism and the optical properties of the β-Ga2O3 material is presented considering its unknown true bandgap value (extending from 4.4 to 5.68 eV). As an application, the scattering properties of the nanomaterial are exploited to demonstrate random laser emission (around 570 nm) when it is permeated with a laser dye liquid solution.

Growth and characterization of β-Ga2O3 nanowires obtained on not-catalyzed and Au/Pt catalyzed substrates

In recent years, a large interest has been reported on low-dimensional β-Ga2O3 structures, like nanowires, nanobelts, nanorods or nanosheets, because of their peculiar and sometimes superior properties. These properties, however, can be strongly affected by the growth procedure, especially if metal growth catalysts are used. In this work we report the successful synthesis of β-Ga2O3 nanowires/nanobelts using a simple combination of thermal evaporation of a metallic Ga source and controlled oxidation. The same growth procedure has been used to grow nanostructures on different kind of substrates (silicon and alumina), without catalyst as well as with Au or Pt deposited on the substrates, in order to promote the nucleation of nanowires. The morphological, structural and optical properties of the obtained nanostructures have been characterized and compared. Different growth distributions on the substrates and possible growth mechanisms have been highlighted, while a strong increase in luminescence intensity has been observed on samples grown with Au and Pt catalysts.

Sn doped β-Ga2O3 and β-Ga2S3 nanowires with red emission for solar energy spectral shifting

Sn doped β-Ga2O3 nanowires have been grown on Si(001) via the vapor–liquid–solid mechanism at 800 °C over a broad range of compositions. These have a monoclinic β-Ga2O3 crystal structure and minimum resistances for 1–2 at. % Sn but we observe the emergence of tetragonal rutile SnO2 which dominates with increasing content of Sn. All of the nanowires exhibited photoluminescence at 2.7 eV but a red shift of the emission occurred from 2.7 eV to 1.8 eV after post growth processing under H2S above 500 °C. The red emission is related to deep donor to acceptor transitions and the formation of monoclinic β-Ga2S3 and has been exploited for spectral shifting in a Si solar cell resulting into an increase of the power conversion efficiency from 7.2% to 8.3%.

Ultrafast Spectroscopy and Red Emission from β-Ga2O 3/β-Ga 2S 3 Nanowires.

Ultrafast pump-probe and transient photoluminescence spectroscopy were used to investigate carrier dynamics in β-Ga2O3 nanowires converted to β-Ga2O3/Ga2S3 under H2S between 400 to 600 °C. The β-Ga2O3 nanowires exhibited broad blue emission with a lifetime of 2.4 ns which was strongly suppressed after processing at 500–600 °C giving rise to red emission centered at 680 nm with a lifetime of 19 μs. Differential absorption spectroscopy reveals that state filling occurs in states located below the conduction band edge before sulfurization, but free carrier absorption is dominant in the β-Ga2O3/Ga2S3 nanowires processed at 500 to 600 °C for probing wavelengths >500 nm related to secondary excitation of the photo-generated carriers from the mid-gap states into the conduction band of Ga2S3.

Molecular Origin of Valence Band Anisotropy in Single β-Ga2O3 Nanowires Investigated by Polarized X-ray Absorption Imaging

The origin of anisotropy at the molecular and electronic structure levels in individual β-Ga2O3 nanowires grown along the crystallographic b direction was studied using linearly polarized X-ray absorption imaging at both gallium L3 and oxygen K edges. The O K-edge spectrum shows significant linear dichroism for the electric field vector of the X-rays oriented parallel and perpendicular to the nanowire long axis. The contributions from the three nonequivalent oxygen sites to the observed spectral anisotropy were elucidated by using FDMNES calculations in the framework of the multiple scattering theory. The role of relevant O and Ga orbitals in the linearly polarized O K-edge absorption was determined based on the point group symmetry arguments. The results of this work suggest mixed covalent and ionic character of the Ga–O bond in individual nanowires, with the dominant contribution of O 2pz orbitals to the absorption spectrum for the electric field vector oriented perpendicular, and O 2px,y orbitals for t...

Study of Growth and Characterization of Fe-Catalyzed β-Ga2O3 Nanowires

ABSTRACTIn the present study, Fe as a catalyst was used to grow single crystalline monoclinic gallium oxide (β-Ga2O3) nanowires using chemical-vapor-deposition method. The morphology, structure and luminescence properties of the as-grown β-Ga2O3 nanowires were investigated using various characterization techniques. The diameter of the as-grown nanowires was in the range of 50 to 100 nm, and the lengths up to tens of micrometers. The structural investigation of the nanowires was carried out using X-ray diffraction that showed monoclinic phase of Ga2O3. Further, the transmission electron microscope (TEM) investigations along with selected area diffraction pattern revealing single crystalline nature of the nanowires. The as-grown β-Ga2O3 nanowires had preferred orientation along [1-1-1] direction. The high resolution TEM image showed regular arrangement of atoms and the lattice spacing between (1-1-1) planes was around 0.266 nm. The luminescence properties of the as-grown nanowires were measured using cathodoluminescence (CL) spectroscopy. The CL measurements of β-Ga2O3 nanowires revealed a strong broad UV-blue emission band and a weak red emission band.

Effects of humidity and ultraviolet characteristics on β-Ga2O3 nanowire sensor

Monoclinic gallium oxide (β-Ga2O3) nanowires (NWs) were synthesized via a vapor–liquid–solid mechanism by heating a GaN/sapphire template.

Synthesis and characterization of β-Ga2O3 nanowires on amorphous substrates using radio-frequency powder sputtering

We investigated the growth mechanism and microstructure of β-Ga2O3 nanowires (NWs) deposited on amorphous SiN/Si(001), SiOx/Si(001), and glass substrates using radio-frequency powder sputtering. During growth, the β-Ga2O3 changed from an initial amorphous thin film into NWs. Since the deposition was performed in an Ar gas environment, the initial amorphous thin films were non-stoichiometric Ga oxide (Ga2O3−x). Oxygen-deficient deposition led to the formation of intermediate thin films by the phase separation of Ga2O3−x into metallic Ga clusters and stoichiometric β-Ga2O3. The Ga clusters acted as catalyst seeds in the growth of the β-Ga2O3 NWs through the self-catalytic vapor–liquid–solid mechanism. We found that the growth of β-Ga2O3 NWs is possible at temperatures greater than 450°C, above which phase separation and Ga cluster formation occur. We also observed that NWs inherit the planar defects, such as twin boundaries, of the host Ga seed.

Study of iron-catalysed growth of β-Ga2O3 nanowires and their detailed characterization using TEM, Raman and cathodoluminescence techniques

In this paper, we demonstrate a new catalyst (Fe) to grow single crystalline beta-gallium oxide (β-Ga2O3) nanowires (NWs) via the vapour–liquid–solid mechanism using the chemical vapour deposition technique. The structural studies of these NWs showed the highly crystalline monoclinic phase of Ga2O3. This was confirmed by detailed scanning transmission electron microscope investigations demonstrating the NW to be single crystalline β-Ga2O3, growing along the normal of the plane. We also compared Raman and cathodoluminescence (CL) properties of the as-grown β-Ga2O3 NWs with a bulk Ga2O3 single crystal grown by the Czochralski method. It was observed that Raman peak positions of a single β-Ga2O3 NW had a red frequency shift of about 0.3–1.4 cm−1 as compared to a bulk Ga2O3 single crystal, which was in fact quite small. In addition, the CL measurements of β-Ga2O3 NWs and the bulk Ga2O3 single crystal exhibited similar spectra, having a strong broad UV–blue emission band and a weak red emission band. Moreover, the structural, morphological and optical properties of Fe-catalysed β-Ga2O3 NWs were comparable to those of Au-catalysed β-Ga2O3 NWs.

β-Ga2O3 nanowires for an ultraviolet light selective frequency photodetector

The behaviour of β-Ga2O3 nanowires as photoconductive material in deep ultraviolet photodetectors to operate in the energy range 3.0–6.2 eV has been investigated. The nanowires were grown by a catalyst-free thermal evaporation method on gallium oxide substrates. Photocurrent measurements have been carried out on both undoped and Sn-doped Ga2O3 nanowires to evidence the influence of the dopant on the photodetector performances. The responsivity spectrum of single nanowires show maxima in the energy range 4.8–5.4 eV and a strong dependence on the pulse frequency of the excitation light has been observed for undoped nanowires. Our results show that the responsivity of β-Ga2O3 nanowires can be controlled by tuning the chopper frequency of the excitation light and/or by doping of the nanowires. Non-linear behaviour in characteristic current–voltage curves has been observed for Ga2O3 : Sn nanowires. The mechanism leading to this behaviour has been discussed and related to space-charged-limited current effects. In addition, the responsivity achieved by doped nanowires at lower bias is higher than for undoped ones.

Low-temperature growth of highly crystalline β-Ga2O3 nanowires by solid-source chemical vapor deposition.

Growing Ga2O3 dielectric materials at a moderately low temperature is important for the further development of high-mobility III-V semiconductor-based nanoelectronics. Here, β-Ga2O3 nanowires are successfully synthesized at a relatively low temperature of 610°C by solid-source chemical vapor deposition employing GaAs powders as the source material, which is in a distinct contrast to the typical synthesis temperature of above 1,000°C as reported by other methods. In this work, the prepared β-Ga2O3 nanowires are mainly composed of Ga and O elements with an atomic ratio of approximately 2:3. Importantly, they are highly crystalline in the monoclinic structure with varied growth orientations in low-index planes. The bandgap of the β-Ga2O3 nanowires is determined to be 251 nm (approximately 4.94 eV), in good accordance with the literature. Also, electrical characterization reveals that the individual nanowire has a resistivity of up to 8.5 × 107 Ω cm, when fabricated in the configuration of parallel arrays, further indicating the promise of growing these highly insulating Ga2O3 materials in this III-V nanowire-compatible growth condition.PACS77.55.D; 61.46.Km; 78.40.Fy

Highly Sensitive $\beta{-}{\rm Ga}_{2}{\rm O}_{3}$ Nanowire Nanowires Isopropyl Alcohol Sensor

We report the growth of β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanowires using a vapor phase transport method and the fabrication of β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanowire gas sensor. The β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanowire sensor shows reversible response to isopropyl alcohol (IPA) gas at temperature as low as 100 °C. With 300 ppm IPA gas introduced, it was found that measured responses were ~ 7.5%, 13.5%, and 23% when the sensor was operated at 100 °C, 200 °C, and 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, respectively. It was also found that the responses measured at 300 °C were ~ 9%, 15.5%, 24%, and 33% when the injected IPA gas concentrations were 100, 200, 300, and 400 ppm, respectively.

The effect of deposition time on the structural and optical properties of β-Ga2O3 nanowires grown using CVD technique

In the present work, we have investigated the effect of deposition time on the morphological, structural, and photoluminescence (PL) properties of β-Ga2O3 NWs grown by CVD technique. The diameter and length of the as-grown NWs varied for the deposition time of 1–4 h, from 50 to 100 nm and 5–15 μm, respectively. The crystalline quality of the NWs improved with increasing the deposition time. The detailed transmission electron microscopy (TEM) and fast Fourier Transformation (FFT) measurements revealed that the as-grown β-Ga2O3 NWs were single crystalline. Furthermore, we have studied the variation of PL spectra of the NWs with deposition time and provided an energy band diagram to give a plausible explanation of the origin of different emissions in the PL spectra. The PL spectra showed a broad strong UV-blue emission band and a weak red emission for 1 h deposited sample. We suggested that the UV and red emission from β-Ga2O3 NWs are related to oxygen vacancies and impurities such as nitrogen, respectively. It was observed that the intensity of UV emission decreased with deposition time and this reduction was attributed primarily to the reduction of oxygen vacancies in the NWs.

$\beta\hbox{-}{\rm Ga}_{2}{\rm O}_{3}$ Nanowires-Based Humidity Sensors Prepared on GaN/Sapphire Substrate

We report the growth of β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanowires (NWs) on GaN substrate and the fabrication of β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> NW humidity sensors. It was found that average length and diameter were ~10 μm and 100 nm, respectively. It was also found that the current of the NWs increased as we increased the relative humidity because of the n-type nature of β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . Through measuring current-voltage characteristics of the fabricated device at 25 °C, it was found that the measured currents were 5.99×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , 6.42×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , 6.76×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , 7.33×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , and 7.93×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> A when measured with 35%, 50%, 65%, 85%, and 95%-relative humidity, respectively.

High-detectivity nanowire photodetectors governed by bulk photocurrent dynamics with thermally stable carbide contacts

Photodetectors fabricated from one-dimensional semiconductors are always dominated by the surface states due to their large surface-to-volume ratio. Therefore, the basic 5S requirements (high sensitivity, high signal-to-noise ratio, high spectral selectivity, high speed, and high stability) for practical photodetectors are difficult to satisfy. We report on high-temperature and high-detectivity solar-blind deep-ultraviolet (DUV) photodetectors based on β-Ga2O3 nanowires, in which the photoresponse behavior is dominated by the bulk instead of the surface states. Ohmic contact to the β-Ga2O3 nanowires was achieved by using a thermally stable tungsten carbide electrode. As a result, the DUV responsivity at 250 nm shows the highest values—4492 A W−1 at room temperature (RT) and 3000 A W−1 at 553 K (280 ° C)—among the DUV photodetectors. The detectivity is as high as 1.26×1016 cm Hz1/2 W−1 at RT, and still remains 4.1×1014 cm Hz1/2 W−1 at as high a temperature as 553 K. The photocurrent dynamics from the β-Ga2O3 nanowire is discussed in terms of the bulk dominated photoresponse behavior. Other wide bandgap DUV detectors based on nanostructures could also be developed for high-temperature applications based on this work.

A comparative study of β-Ga2O3 nanowires grown on different substrates using CVD technique

Abstract A comparative study of beta gallium oxide (β-Ga2O3) nanowires (NWs) grown on different substrates such as silicon (Si), sapphire, and GaN/sapphire has been reported. Field emission scanning electron microscopy (FESEM) results revealed that the β-Ga2O3 NWs grown on GaN/sapphire substrate were comparatively more aligned than grown on other substrates. The diameter of the NWs varied from 150 to 400 nm, and their length ranging up to tens of micrometers. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) showed the single crystalline monoclinic nature of NWs. The Raman spectra acquired from three samples revealed similar features consisting of two active modes such as mid and high frequency. However, low frequency mode was absent in our results. The cathodoluminescence (CL) spectra of β-Ga2O3 NWs on different substrates showed a strong broad UV-blue emission band and a weak red emission band in all the samples. Hence, the morphologies and structural properties of the β-Ga2O3 NWs grown on three substrates showed some observable changes, while their optical properties were quite similar.

Electron-Field-Emission Properties of Gallium Compound by Ammonification of Ga 2O$_{3}$ Nanowires

The authors report the growth of β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanowires and the conversion of β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanowires to gallium nitride (GaN) nanowires through ammonification, and the fabrication of nanowires-based field emitters. The threshold field of Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanowires was 5.65 V/μm. After ammonification under 750, 800 and 900°C, the threshold fields became smaller which were 3.82, 3.03 and 2.12 V/μm, respectively. While the ammonification temperature was increased to 950°C, the threshold field drastically increased to 13.13 V/μm.

Fabrication and UV-sensing properties of one-dimensional β-Ga2 O3 nanomaterials

β-Ga2O3 nanowires and nanobelts with a length range of tens of micrometers to a few millimeters were successfully fabricated by a simple in situ chemical vapor deposition (CVD) method under normal pressure without any catalysts. The morphology and microstructure of the samples were studied by SEM, XRD, and EDS. The results indicated that the morphology of Ga2O3 depends on the effective heating time. Ga2O3 nanostructures were mainly collected at the gallium source position instead of in the downstream area. So, we propose that the growth process can be explained by a vapor–solid (VS) model. The ultraviolet (UV) sensing properties of a single nanowire were tested. From the nonlinear I–V curves, we find that a single nanowire behaves as a rectifier and has asymmetric characteristics. The photoresponse characterization presents that response time and recovery time were about 0.32 and 0.08 s, respectively. Our result indicated that a single β-Ga2O3 nanowire possesses excellent UV sensing properties and it should be a good candidate for UV-sensor development.

UV Enhanced Field Emission for β-Ga2O3 Nanowires

This letter presents vapor-liquid-solid growth of β-Ga2O3 nanowires (NWs) on cost-effective SiO2/Si template and the fabrication of β-Ga2O3 NW field emitters. It is found that the β-Ga2O3 NWs grown at 950°C are structurally uniform, defect free, and well-oriented with pure monoclinic structure. It is also found that turnon fields are 2.0, 3.9, and 5.8 V/μm whereas field-enhancement factors β are 1890, 2760, and 4489, for the samples grown at 850°C, 900 °C, and 950°C, respectively. For the sample prepared at 950°C, it is found that we could further reduce the turnon field from 2 to 1.2 V/μm whereas enhance the field-enhancement factor β from 4489 to 6926 by ultraviolet irradiation.