GaN Powders Research Articles

Gas sensing properties of multiple networked GaN/WO3 core-shell nanowire sensors

GaN nanowires and GaN-core/WO3-shell nanowires were synthesized by the thermal evaporation of GaN powders followed by the sputter-deposition of WO3 and their gas sensing properties were examined. The multiple networked pristine GaN nanowire sensors showed responses of approximately 125%, 140%, 146%, 159%, and 183% to 1, 2, 3, 4, and 5ppm NO2 gases, respectively. These responses are comparable to those obtained previously using metal oxide semiconductor one-dimensional nanostructure sensors. The responses of the nanowires to 1, 2, 3, 4, and 5ppm NO2 gases were improved 1.3, 1.4, 1.6, 1.7 and 1.8 fold, respectively, further through the encapsulation of GaN nanowires with a WO3 thin film. The improvement in the response of GaN nanowires to NO2 gas by encapsulation is attributed to the modulation of electron transport at GaN–WO3 heterojunction. The electron transport in the core-shell nanowires is modulated by the heterojunction with an adjustable energy barrier height, resulting in an enhanced sensing property of the core-shell nanostructures.

Fabrication and Photocatalytic Performance of GaN/ZnO Composites

GaN powder was firstly synthesized by the Sol-Gel method using polyvinylpyrrolidone (PVP) and gal- lium nitrate (Ga(NO3)3) as surfactant and gallium source, respectively. Then, GaN/ZnO composites were prepared by solid phase method through mechanical mixing GaN powder and ZnO powder with different proportions. The composites were characterized by using X-Ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), high-resolution X-ray electron microscope (HRTEM) and photoluminescence (PL) spectra. The photocatalytic performance of GaN/ZnO composites as catalyst was tested by degradation of methyl- ene blue in aqueous solution under white light irradiation. The results show that the photocatalytic property of GaN/ZnO composites is obviously improved comparing with that of pure GaN and ZnO powders. The photocata- lytic activity is analyzed by pseudo-first-order kinetics equation, showing that the best photodegradation rate con- stant k is 0.11 min -1 for the GaN/ZnO composites with GaN and ZnO mixing ratio of 1: 2.

Structure, Optical Properties, and Photocatalytic Activity towards H2Generation and CO2Reduction of GaN Nanowires via Vapor-Liquid-Solid Process

High quality single crystalline GaN nanowires with large aspect ratio (>100) are synthesized on n-type Si (111) substrate via Au-catalyzed vapor-liquid-solid process. Morphology, crystal structure, and optical property of the as-synthesized GaN nanowires are characterized by means of X-ray diffraction, scanning/transmission electron microscopy, UV-vis diffuse reflection spectroscopy, and room temperature photoluminescence. The results indicate that the as-prepared GaN nanowires with a large aspect ratio are well crystallized in the hexagonal wurtzite structure, and a slight blue shift appears in both the absorption edge and emission peak probably due to the quantization effect. Photocatalytic H2evolution over the as-prepared GaN nanowires is performed with the incorporation of Pt or Rh as the cocatalyst, exhibiting greatly enhanced capability compared to the GaN powder tested under the same conditions. Moreover, photocatalytic CO2reduction over the GaN nanowires is also successfully realized using Pt or Rh as the cocatalyst, depending on which the products show a strong selectivity inherently related to the reductive electrons transferred by cocatalyst.

Synthesis and influence of annealing atmosphere on the luminescence properties of ZnGa2O4 nanowires

ZnGa2O4 nanowires were synthesized on Si substrates by using the thermal evaporation of a mixture of Zn and GaN powders. Scanning electron microscopy showed that the diameters and the lengths of the nanowires ranged from a few tens to a few hundreds of nanometers and up to a few hundreds of micrometers, respectively. The ZnGa2O4 nanowires were found to have a face-centered cubic-structured monocrystalline phase. The photoluminescence properties of the ZnGa2O4 nanowires appeared to depend strongly on the annealing atmosphere. The ZnGa2O4 nanowires annealed in a hydrogen atmosphere showed a relatively weak broad visible emission band, ranging from 500 to 700 nm. In contrast, the ZnGa2O4 nanowires annealed in an oxygen atmosphere showed a relatively strong near-ultraviolet emission band centered at approximately 380 nm. On the other hand, the ZnGa2O4 nanowires annealed in an argon atmosphere showed a sharp, taller ultraviolet emission peak centered at approximately 380 nm, as well as a broad green emission band centered at approximately 510 nm. The origins of the enhanced luminescence in ZnGa2O4 nanowires due to annealing in different atmospheres are discussed.

Structural characterizations of GaN nanowires grown on Si (111) substrates by thermal evaporation

In this work wurtzite GaN nanowires (NWs) on silicon (111) substrates were grown, using commercial GaN powder by thermal evaporation in an atmosphere of argon (Ar) gas. Structural and optical characterizations were performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution X-ray diffraction (HR-XRD) and photoluminescence (PL) spectroscopy. The obtained results of GaN NWs indicated that the NWs on Si (111) substrates are of good quality single-crystal hexagonal GaN due to the small lattice mismatch between the NWs and substrates compared to the growth of GaN NWs on other substrates.

Structure and surface effect of field emission from gallium nitride nanowires

Gallium nitride nanowires (GaN NWs) were synthesized by plasma-enhanced hot filament chemical vapor deposition under different ratios of nitrogen to hydrogen, which the GaN powder and nitrogen gas were used as the Ga and N sources. The characterization results indicate that the GaN NWs are grown in wurtzite crystalline structure with different length, diameters and surface adsorption. The field emission of GaN NWs was measured in the high vacuum condition of ∼10−6Pa, which the results show that the turn-on field of GaN NWs changes from 0.86 to 2.8V/μm depending on their structures and the current density can reach up to 830μA/cm2 at the field of 6V/μm. Combined the characterization results with the work function theory related to field emission, the origin of the field emission enhancement was analyzed, which associates with their surface potential and geometric structure. These results can enrich our knowledge on the field emission of GaN NWs and are highly related to the development of the next-generation of GaN nano-electronic devices.

Self-catalyst growth of novel GaN nanowire flowers on Si (111) using thermal evaporation technique

We investigated the effect of substrate temperature on nanowire (NW) flower GaN epitaxial layers grown on catalyst-free Si (111) through physical vapor deposition via the thermal evaporation of GaN powder at 1150 °C in the absence of NH3 gas. The NW flowers were grown at various substrate temperatures from 1000 °C to 1100 °C for 60 min in N2 ambient. The surface morphology as well as the structural and optical properties of GaN NW flowers were examined by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, X-ray diffraction, and photoluminescence (PL). The results showed that the increase in substrate temperature resulted in a variation in crystal quality and surface morphology. SEM showed that the substrate temperature has a stronger effect on NW density and growth rate with respect to time. The average length of GaN flowers is estimated to be longer than 300 μm after 1 h at 1100 °C, which corresponds to a fast growth rate of more than 200 μm h−1 at all substrate temperatures. The PL measurements showed strong near-band-edge (NBE) emission with a weak deep level emission. The green-yellow emission (GYE) can be attributed to N vacancies or to the VGa–ON-complexes. The NBE peak exhibited a redshift with increasing substrate temperature, which results from the increase in strain level. The growth mechanism of the polycrystalline GaN NWs was also discussed.

Effect of B2O3 Additives on GaN Powder Synthesis from GaOOH

In this study, GaN powders were synthesized from gallium oxide-hydroxide (GaOOH) through an ammonification process in an <TEX>$NH_3$</TEX> flow with the variation of <TEX>$B_2O_3$</TEX> additives within a temperature range of <TEX>$300-1050^{\circ}C$</TEX>. The additive effect of <TEX>$B_2O_3$</TEX> on the hexagonal phase GaN powder synthesis route was examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transformation infrared transmission (FTIR) spectroscopy. With increasing the mol% of <TEX>$B_2O_3$</TEX> additive in the GaOOH precursor powder, the transition temperature and the activation energy for GaN powder formation increased while the GaN synthesis limit-time (<TEX>$t_c$</TEX>) shortened. The XPS results showed that Boron compounds of <TEX>$B_2O_3$</TEX> and BN coexisted in the synthesized GaN powders. From the FTIR spectra, we were able to confirm that the GaN powder consisted of an amorphous or cubic phase <TEX>$B_2O_3$</TEX> due to bond formation between B and O and the amorphous phase BN due to B-N bonds. The GaN powder synthesized from GaOOH and <TEX>$B_2O_3$</TEX> mixed powder by an ammonification route through <TEX>${\beta}-Ga_2O_3$</TEX> intermediate state. During the ammonification process, boron compounds of <TEX>$B_2O_3$</TEX> and BN coated <TEX>${\beta}-Ga_2O_3$</TEX> and GaN particles limited further nitridation processes.

Sublimation Growth of AlN and GaN Bulk Crystals on SiC Seeds

Growth techniques of high quality AlN and GaN bulk crystals on SiC seeds by sublimation sandwich method are presented. GaN crystals were grown in the temperature range of 1100-1250 °C and with addition of ammonia (NH3) to prevent GaN decomposition. GaN powder or metallic Ga was used as the source. AlN crystals up to 2 inch diameter have been grown on SiC seeds in the temperature range of 1950 -2050 0С. Kinetic mechanisms and transport features in the sandwich cell are discussed. The achieved high crystal quality has allowed producing semiconductor devices on their basis, in particular, ultraviolet LEDs

Synthesis of one-dimensional Ga 2 O 3 nanostructures via high-energy ball milling and annealing of GaN

β-Gallium oxide (β-Ga2O3) is a promising material for sensors that can withstand high-temperature, high-pressure, and corrosive environment in advanced fossil fuel power plants. Synthesis of one-dimensional nanostructures of β-Ga2O3, which are of particular interest, by ball milling of gallium nitride followed by annealing in nitrogen flow is attractive due to relatively inexpensive equipment and simple procedures, but the long milling time is an obstacle for widespread use of this method. In the present work, high-energy mechanical milling of GaN in a planetary ball mill is used for shortening the milling time in the fabrication of one-dimensional Ga2O3 nanostructures. Effects of milling parameters on the morphology of GaN powders as well as on the morphology and chemical composition of the obtained Ga2O3 structures are studied. When annealing was conducted in ultra-high purity (O2<1 ppb) nitrogen flow, a variety of Ga2O3 one-dimensional nanostructures such as rods, belts, sheets, and leaf-like shapes were obtained.

Comparison of morphology, structure and optical properties of GaN powders prepared by Ga2O3 nitridation and gallium nitridation

Two kinds of GaN powders were grown using the chemical vapor transport process, which involved different Ga sources, i.e., Ga2O3 and Ga metal. Although both GaN powders have a wurtzite crystal structure, which was confirmed by powder X-ray diffraction, their structural and optical properties were different. The powders prepared using the gallium nitridation method revealed a hexagonal morphology. The particles were approximately 2μm in size, and we determined the large face (0002) by selective area electron diffraction, indicating that they had good crystallinity. In addition, the oxygen content was relatively low, similar to the GaN bulk grown by hydride vapor phase epitaxy. A strong band-edge luminescence was observed in the photoluminescent and cathodoluminescent spectra. However, the powders prepared using the Ga2O3 nitridation method had an irregular morphology, and several tens of nanometer in size. The grain boundaries and the amorphous regions can be seen using high-resolution transmission electron microscopy imaging. Moreover, the oxygen concentration was relatively high. In this paper, we describe the different properties of these two kinds of powders using growth kinetics.

Close space vapor transport of gallium nitride in vacuum

GaN synthesized by close space vapor transport in vacuum using GaN powder as the source material is reported. According to X-ray diffraction measurements, samples grown onto sapphire substrates are polycrystalline with the wurzite structure, while those grown onto fused silica substrates are amorphous. Scanning electron microscope and energy dispersive spectroscopy results show that the surface of the samples is composed of rounded droplets of GaN. Room temperature photoluminescence shows near band-edge emission and a broad defect band. Possible mechanisms allowing the deposition of GaN from the vapors of GaN powder are discussed.

Broad visible emission from GaN nanowires grown on n-Si (1 1 1) substrate by PVD for solar cell application

Nanostructured gallium nitrides (GaNs) were grown on a catalyst–free Si (111) substrates using physical vapor deposition via thermal evaporation of GaN powder at 1150°C in the absence of NH3 gas for different deposition time. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDX) results indicated that the growth of GaN nanostructure varies with deposition time. Both X-ray diffraction (XRD) patterns and Raman spectra reveals a hexagonal GaN with wurtzite structure. Photoluminescence (PL) showed that the UV emission was suppressed, and the visible band emission was enhanced with increasing deposition time. Enhancement of visible band emission from the GaN NWs is due to the increasement of deep level states, which was resulted from growth process. Current–voltage (IV) characteristics of GaN/Si heterostructure were measured and good rectifying behavior was observed for this photodiode (PD). The forward current under illumination was almost three times than that in the dark current at +5V. Responsivity of the photodetector was 10.5A/W at range from 350nm to 500nm, which rapidly increased to 13.6A/W at 700nm. We found that the fabricated photodiode PD has an infra-red (IR) photoresponse behavior. The analysis of optical and electrical properties indications that the grown GaN in the absent of NH3 is a promising optical material and has potential applications in photo voltage solar cell.

One-step synthesis of SnO2-core/Ga2O3-shell nanowires by thermal evaporation

SnO2-core/Ga2O3-shell nanowires were synthesized by one-step thermal evaporation of a mixture of SnO2, GaN and graphite powders at 1000°C. Transmission electron microscopy and energy-dispersive X-ray spectroscopy concentration profiles of Sn, Ga, and O across the diameter of the nanowire indicated clearly that the nanowire consisted of a single crystal SnO2 core and a single crystal Ga2O3 shell with a shell layer thickness of ∼20nm. A possible growth mechanism of SnO2-core/Ga2O3-shell nanowires in the one-step synthesis process is proposed.

From powder to nanowire: a simple and environmentally friendly strategy for optical and electrical GaN nanowire films

Gallium nitride nanowires (GaN NWs) were successfully grown on silicon substrates deposited with a thin gold film by plasma-assisted hot filament chemical vapor deposition (CVD), where GaN powder was used as the Ga source and nitrogen and hydrogen gases were used for the reactive gases. The diameters of synthesized GaN NWs are in a range of 40–150 nm and the lengths of some of the NWs are up to over 10 μm. The GaN NWs crystallize in a wurtzite structure and they are grown in alignment or bending modes depending on the growth conditions. The photoluminescence spectra exhibit five emission bands at about 379, 396, 467, 481 and 491 nm, which are related to the band edge emission, defects and impurities of GaN NWs, respectively. Furthermore, the excellent field emission (FE) properties of GaN NWs were observed, and the FE results reveal that the GaN NWs have a low turn-on field of 1.66 V μm−1. It supplies a simple strategy to prepare GaN NW films for optical and electrical applications.

Influence of In2O3 capping and annealing on the luminescence properties of Ga2O3 nanorods

Ga2O3 nanorods were synthesized by thermal evaporation of GaN powders, and the influence of In2O3 capping and subsequent annealing on their luminescence properties was examined. The results of transmission electron microscopy and x-ray diffraction analyses indicated that the cores and shells of the annealed coaxial nanorods are monoclinic-structured single-crystal Ga2O3 and body-centered cubic-structured single-crystal In2O3, respectively. Photoluminescence (PL) measurements revealed that the blue emission band of Ga2O3 nanorods centered at approximately 460 nm was increased in intensity slightly by In2O3 coating and was increased in intensity further by subsequent thermal annealing. The PL peak was red-shifted from ∼460 to ∼530 nm by oxygen annealing. In contrast, the PL emission intensity of the nanorods was increased significantly and the PL peak was red-shifted from ∼460 to ∼590 nm by annealing in a reducing atmosphere. In addition, the origin of the PL intensity enhancement and of the PL peak shift by annealing is discussed.

Gan Nanowire Growth by Thermal Evaporation Method

In this research we introduce an inexpensive method to produce highly crystalline GaN Nanowires (NWs) grown on porous zinc oxide (PZnO) using commercial GaN powder, either in argon gas or combination of nitrogen and Ar gas atmosphere, by thermal evaporation. Morphological structural studies using transmission electron microscope (TEM) and scanning electron microscopy (SEM) measurements showed the role of porosity and different gas flowing, in the alignment and nucleation of these NWs. The NWs grown under flow of mix gases have very different diameters of between 50 and 200 nm, but those which were grown in Ar gas atmosphere, have rather uniform diameter of around 50 nm. The length of the GaN NWs was uniform, (around 10 µm). Optical and structural characterizations were performed by energy-dispersive X-ray spectroscopy (EDX) and high resolution X-ray diffraction (HR-XRD). Results revealed that these NWs are of single-crystal hexagonal GaN with [oooı] and [ıoīı] growth directions for the NWs grown under Ar and mixed gas flow.

Growth and analysis of GaN nanowire on PZnO by different-gas flow

In this research we have used an inexpensive method to fabricate highly crystalline GaN nanowires (NWs) on porous zinc oxide (PZnO) on Si (111) wafer by thermal evaporation using commercial GaN powder, either in argon (Ar) gas atmosphere or a combination of Ar and nitrogen (N2) gas atmosphere without any catalyst. Micro structural studies by scanning electron microscopy (SEM) and transmission electron microscope (TEM) measurements reveal the role of different gas flowing, in the nucleation and alignment of the GaN NWs. The GaN NWs different diameters ranging between 50 and 200nm for the NWs grown under flow of mix gases, but the NWs which were grown under Ar gas only, have uniform diameter of around 50–60nm, also their lengths are almost the same (around 10μm). Further structural and optical characterizations were performed using high resolution X-ray diffraction (HR-XRD), energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. Results indicated that the NWs are of single-crystal hexagonal GaN with [0001] and [1 0 1¯ 1] growth directions for NWs grown under Ar and mixed gas flow.

Controllable synthesis of SnO2 nanowires and nanobelts by Ga catalysts

We report the morphology control of one-dimensional (1D) SnO2 nanostructures by Ga catalysts using thermal evaporation method. Gallium (Ga), either from decomposition of GaN powder or from Ga metal, is adopted as a catalyst for the growth of long SnO2 nanowires and nanobelts. At similar experimental conditions, quantities of nanobelts are formed instead of nanowires when the temperature and reaction time are increased. Such approach enables us to synthesize various morphologies of SnO2 nanobelts with different side facets. Novel nanobelts with [001] growth direction with high energy side facets are obtained for the first time, which is attributed to the large amount of oxygen vacancies introduced in the nanobelts by the Ga catalysts.

Synthesis of Au in Ga2O3 peapodded nanorods and their photoluminescence properties

Gold (Au) in gallium oxide (Ga2O3) peapodded one-dimensional (1D) nanostructures were synthesized by thermal evaporation of GaN powders on Si (100) substrates coated with Au thin films followed by annealing. At the initial stages of annealing, Au formed a continuous linear core located along the long axis of each Ga2O3 nanorod. The morphology of the Au core changed from a continuous line to a discrete line, and then to a droplet-like chain, finally evolving into a peapod in which crystalline Au nanoparticles were encapsulated in crystalline Ga2O3 with increasing annealing temperature. The Ga2O3 nanorods with the Au core showed an emission band at ∼410nm in the violet region. The violet emission intensity increased rapidly with increasing annealing temperature. The intensity of emission from the Au in Ga2O3 pepodded nanorods (annealed at 800°C) was approximately six times higher than that of the emission from the Au-core/Ga2O3-shell nanorods with a continuous linear shaped-Au core (annealed at 600°C). This intense violet emission from the core-shell nanorods annealed at 800°C might have originated mainly from the surface plasmon resonance (SPR) of Au-peapods whereas the less intense violet emission from the core-shell nanorods annealed at 600 or 700°C might have originated mainly from the band-to-band transition of the Ga2O3 shells.