Unconventional gallium oxide nanowires

Zigzag and helical beta-Ga(2)O(3) one-dimensional nanostructures were produced by thermal evaporation of gallium oxide in the presence of gallium nitride. High-resolution TEM analysis indicates that each individual zigzag nanostructure has a periodic arrangement of three distinct blocks: two structurally perfect blocks mirrored with respect to each other on the (002) plane, and one stacking-fault-rich block sandwiched between them. In a zigzag nanostructure, the growth orientation of a beta-Ga(2)O(3) crystal changes alternately in three blocks. The zigzag nanostructure as a whole has the [001] axial direction. In addition to zigzag nanostructures, single-crystalline helical nanowires were also obtained.     

Growth mechanism and elemental distribution of beta-Ga2O3 crystalline nanowires synthesized by cobalt-assisted chemical vapor deposition

Long beta-Ga2O3 crystalline nanowires are synthesized on patterned silicon substrates using chemical vapor deposition technique. Advanced electron microscopy indicates that the as-grown beta-Ga2O3 nanowires are consisted of poly-crystalline (Co, Ga)O tips and straight crystalline beta-Ga2O3 stems. The catalytic cobalt not only locates at the nanowire tips but diffuses into beta-Ga2O3 nanowire stems several ten nanometers. A solid diffusion growth mechanism is proposed based on the spatial elemental distribution along the beta-Ga2O3 nanowires at nanoscale.     

Synthesis of single-crystal GaN nanowires on Si(111) substrate by ammonification technology

GaN nanowires have been synthesized on Si(111) substrate through ammoniating Ga₂O₃/BN films under flowing ammonia atmosphere at the temperature of 900 °C. The as-synthesized GaN nanowires were characterized by X-ray diffraction (XRD), selected-area electron diffraction (SAED), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM). The results demonstrated that the nanowires are hexagonal wurtzite GaN and possess a smooth surface with diameters ranging from 40 to 100 nm and lengths up to several tens of micrometers. The growth mechanism of crystalline GaN nanowires is discussed briefly.     

Growth process from amorphous GaN to polycrystalline GaN on Si (111) substrates

Amorphous GaN (a-GaN) films on Si (111) substrates have been deposited by RF magnetron sputtering with GaN powder target. The growth process from amorphous GaN to polycrystalline GaN is studied by XRD, SEM, PL and Raman. XRD data mean that annealing under flowing ammonia at 850-950 °C for 10 min converts a-GaN into polycrystalline GaN (p-GaN). The growth mechanism can be mostly reaction process through N³⁻ in amorphous GaN replaced by N³⁻ of NH₃. Annealing at 1000 °C, the appearance of GaN nanowires can be understood based on the vapor-liquid-solid (VLS) mechanism. In addition, XRD, PL and Raman measurement results indicate that the quality of GaN films increases with increasing temperature. The tensile stress in the films obtained at 1000 °C is attributable to the expansion mismatch between GaN and Si, with the gallium in the film playing a negligible role.     

Fabrication of needle-shaped GaN nanowires by ammoniating Ga2O3 films on MgO layers deposited on Si (111) substrates

Needle-shaped GaN nanowires have been synthesized on Si (111) substrate through ammoniating Ga₂O₃/MgO films under flowing ammonia atmosphere at the temperature of 950 °C. The as-synthesized GaN nanowires were characterized by X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM). The results demonstrate that these nanowires are hexagonal GaN and possess a smooth surface with an average diameter about 200 nm and a length ranging from 5 μm to 15 μm. In addition, the diameters of these nanowires diminish gradually. The growth mechanism of crystalline GaN nanowires is discussed briefly.     

Coaxial metal-oxide-semiconductor (MOS) Au/Ga2O3/GaN nanowires

Coaxial metal-oxide-semiconductor (MOS) Au-Ga2O3-GaN heterostructure nanowires were successfully fabricated by an in situ two-step process. The Au-Ga2O3 core-shell nanowires were first synthesized by the reaction of Ga powder, a mediated Au thin layer, and a SiO2 substrate at 800 degrees C. Subsequently, these core-shell nanowires were nitridized in ambient ammonia to form a GaN coating layer at 600 degrees C. The GaN shell is a single crystal, an atomic flat interface between the oxide and semiconductor that ensures that the high quality of the MOS device is achieved. These novel 1D nitride-based MOS nanowires may have promise as building blocks to the future nitride-based vertical nanodevices.     

Synthesis of gallium oxide nanomaterials on source material supply and their growth behavior

High purity and single crystalline beta-Ga2O3 nanomaterials with various morphologies were obtained through the simple thermal evaporation of metal gallium powder on a gold-coated silicon substrate in argon. In this report, the growth behavior of the beta-Ga2O3 nanomaterials as a function of synthesis time and source material supply was delicately surveyed via FESEM and HRTEM. The synthesis time and source material supply affected morphology, growth rate and growth mechanism of the grown nanomaterials. It was confirmed that the growth mechanism of the beta-Ga2O3 nanomaterials was varied in the order of VLS, combination of VLS and VS, and VS, by increasing the synthesis time without regard to the supply of the source material. When the source materials supply was increased, many beta-Ga2O3 nanomaterials with various morphologies, such as sheet, triangle, and belt-like were appeared. It was confirmed that the oxidation reaction of gallium and oxygen for the formation of gallium oxide nanomaterials carried out the precipitation of gallium at the same time due to the supersaturation of the gallium atoms in gold catalyst. The growth and formation mechanism of the beta-Ga2O3 nanomaterials are discussed herein.     

Gallium nitride nanowires doped with magnesium

GaN nanowires doped with Mg have been synthesized on Si (111) substrate through ammoniating Ga₂O₃ films doped with Mg under flowing ammonia atmosphere. The Mg-doped GaN nanowires were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL). The results demonstrate that the nanowires were single crystalline with hexagonal wurzite structure. The diameters of the nanowires ranged 20-30 nm and the lengths were about hundreds of micrometers. The intense PL peak at 359 nm showed a blueshift from the bulk band gap emission, attributed to Burstein-Moss effect. The growth mechanism of the crystalline GaN nanowires is discussed briefly.     

采用Al缓冲层在蓝宝石衬底上合成GaN多晶薄膜

采用CVD法以金属镓(Ga)和氨气(NH3)为原料,在镀有Al膜的蓝宝石衬底上成功地制备了GaN多晶薄膜。采用高分辨X射线衍射仪(HRXRD)、场发射电子扫描电镜(FESEM)、原子力显微镜(AFM)和光致发光能谱(PL)对样品进行了成分、形貌、表面粗糙度和发光性能分析。结果表明,制备的GaN薄膜为结晶性较好的六方纤锌矿GaN多晶薄膜,用266nm的激光作为激发光源时,光致发光谱中除出现354nm的近带边发射峰外,同时还观察到中心波长位于530nm附近的黄光发光峰及中心波长位于约637nm的红光发光峰。     

Investigation on the role of indium in the removal of metallic gallium from soft and hard sputtered GaN (0001) surfaces

Cleaning of GaN by argon sputtering and subsequent annealing introduces metallic gallium on the GaN surface. Once formed, this metallic gallium can be difficult to remove. It has a strong influence on the Fermi level position in the band gap and poses a problem for subsequent epitaxial growth on the surface. We present a method of removing metallic gallium from moderately damaged GaN surfaces by deposition of indium and formation of an In-Ga alloy that can be desorbed by annealing at ~ 550 °C. After the In-Ga alloy has desorbed, photoemission spectra show that the Ga3d bulk component becomes narrower indicating a smoother and more homogeneous surface. This is also reflected in a sharper low energy electron diffraction pattern. On heavily damaged GaN surfaces, caused by hard sputtering, larger amount of metallic gallium forms after annealing at 600 °C. This gallium readily alloys with deposited indium, but the alloy does not desorb until a temperature of 840 °C is reached and even then, traces of both indium and metallic gallium could be found on the surface.     

Synthesis and cathodoluminescence of beta-Ga2O3 nanowires with holes

Gallium oxide nanowires were synthesized on Si (001) substrate by chemical vapor deposition, using a Ga/Ga2O3 mixture as a precursor and Au as a catalyst. The structure of the as-synthesized products was examined by X-ray powder diffraction and high-resolution transmission electron microscopy, and found to be monoclinic beta-Ga2O3. The morphologies of the beta-Ga2O3 nanowires were characterized by scanning electron microscopy. The majority of the nanowires contain holes along their length, but a few were also found without holes. The holes are believed to be formed by the reaction of adsorbed Ga droplets on reactive terminating surfaces of the nanowires. For nanowires where these reactive surfaces are not exposed, the reaction of Ga is retarded. Cathodoluminescence (CL) of the nanowires was measured. Three emission bands centered at 376, 454, and 666 nm, respectively, were observed.     

Study of using aqueous NH3 to synthesize GaN nanowires on Si(1 1 1) by thermal chemical vapor deposition

High-quality GaN nanowires (NWs) and zigzag-shaped NWs were grown on catalyst-free Si(1 1 1) substrate by thermal chemical vapor deposition (TCVD). Gallium (Ga) metal and aqueous NH₃ solution are used as a source of materials. Ga vapor was directly reacts with gaseous NH₃ under controlled nitrogen flow at 1050 °C. Scanning electron microscopy (SEM) images showed that the morphology of GaN displayed various densities of NWs and zigzag NWs depending on the gas flow rate, and increased nitrogen flow rate caused density reduction. The GaN NWs exhibited clear X-ray diffraction analysis (XRD) peaks that corresponded to GaN with hexagonal wurtzite structures. The photoluminescence spectra showed that the ultraviolet band emission of GaN NWs had a strong near band-edge emission (NBE) at 361–367 nm. Yellow band emissions were observed at low and high flow rates due to nitrogen and Ga vacancies, respectively. Moderate N₂ flow resulted in a strong NBE emission and a high optical quality of the NWs. This study shows the possibility of low-cost synthesis of GaN nanostructures on Si wafers using aqueous NH₃ solution.     

Fabrication of single-crystalline gallium nitride nanowires by using alginate as template

Hexagonal gallium nitride nanowires were synthesized successfully by solvothermal method with alginate as template. The microstructure, morphologies and compositions of the as-prepared product were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), high resolution transmission electron microscopy (HRTEM), and energy dispersive X-ray (EDX). Results suggested that the rod-like nanowires were hexagonal single-crystalline GaN growing along [001] direction. The photoluminescence spectra (PL) of the GaN revealed that the as-synthesized sample possesses excellent optical properties.     

Synthesis and characterization of LnPO4 x nH2O (Ln = La, Ce, Gd, Tb, Dy) nanorods and nanowires

Aqueous precipitation method has been used to synthesize lanthanide orthophosphates LnPO4 x nH2O (Ln = La, Ce, Gd, Tb, Dy) with high purity and yield. It has been shown by XRD, TGA, and FTIR characterization that the as-synthesized samples are the LnPO4 hydrates (LnPO4 x nH2O) with hexagonal rhabdophane-type structure. The X-ray diffraction peaks and absorption of PO4(3-) groups show a systematic shift from LaPO4 x nH2O to DyPO4 x nH2O due to the effect of lanthanide ionic contraction. The value of n in LnPO4 x nH2O depends on the lanthanide element and synthetic condition. Field-emission scanning electron microscopy observations show the morphology of as-synthesized samples, which consist entirely of nanorods/nanowires with diameters of 30-100 nm and lengths ranging from several hundreds of nanometers to several micrometers. The anisotropic growth of crystals should be responsible for the formation of nanorods/nanowires, which is related to the hexagonal crystal structure.     

One-step growth of curled GaN nanowires using chemical vapor deposition method

The studies of curled GaN nanowires grown on sapphire and silicon substrate using chemical vapor deposition method have been reported in this article. The mean diameters of the nanowires grown on sapphire and silicon were 108.1 nm and 98 nm respectively. A growth model was proposed to describe the growth of nanowires. X-ray diffraction pattern and Raman spectroscopy revealed that the nanowires were hexagonal wurtzite in structure. Gaussian fitting was done on photoluminescence spectra, which revealed two sub-bands that could be attributed to band emission and surface disorder caused by impurities. The absence of yellow luminescence signified undoped case and minimal shallow level defects.     

General synthesis of manganese-doped II-VI and III-V semiconductor nanowires

A general approach for the synthesis of manganese-doped II-VI and III-V nanowires based on metal nanocluster-catalyzed chemical vapor deposition has been developed. High-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy studies of Mn-doped CdS, ZnS, and GaN nanowires demonstrate that the nanowires are single-crystal structures and homogeneously doped with controllable concentrations of manganese ions. Photoluminescence measurements of individual Mn-doped CdS and ZnS nanowires show characteristic pseudo-tetrahedral Mn2+ ((4)T1-->(6)A1) transitions that match the corresponding transitions in bulk single-crystal materials well. Photoluminescence studies of Mn-doped GaN nanowires suggest that manganese is incorporated as a neutral (Mn3+) dopant that partially quenches the GaN band-edge emission. The general and controlled synthesis of nanowires doped with magnetic metal ions opens up opportunities for fundamental physical studies and could lead to the development of nanoscale spintronic devices.     

The growth of ultralong and highly blue luminescent gallium oxide nanowires and nanobelts, and direct horizontal nanowire growth on substrates

We report the growth of ultralong β-Ga(2)O(3) nanowires and nanobelts on silicon substrates using a vapor phase transport method. The growth was carried out in a tube furnace, with gallium metal serving as the gallium source. The nanowires and nanobelts can grow to lengths of hundreds of nanometers and even millimeters. Their full lengths have been captured by both scanning electron microscope (SEM) and optical images. X-ray diffraction (XRD) patterns and transmission electron microscope (TEM) images have been used to study the crystal structures of these nanowires and nanobelts. Strong blue emission from these ultralong nanostructures can be readily observed by irradiation with an ultraviolet (UV) lamp. Diffuse reflectance spectroscopy measurements gave a band gap of 4.56?eV for these nanostructures. The blue emission shows a band maximum at 470?nm. Interestingly, by annealing the silicon substrates in an oxygen atmosphere to form a thick SiO(2) film, and growing Ga(2)O(3) nanowires over the sputtered gold patterned regions, horizontal Ga(2)O(3) nanowire growth in the non-gold-coated regions can be observed. These horizontal nanowires can grow to as long as over 10?μm in length. Their composition has been confirmed by TEM characterization. This represents one of the first examples of direct horizontal growth of oxide nanowires on substrates.     

Synthesis of nanocrystalline GaN from Ga2O3 nanoparticles derived from salt-assisted spray pyrolysis

Gallium nitride (GaN) nanoparticles were successfully produced from nano-sized gallium oxide (Ga₂O₃) particles under a flow of ammonia gas. The gallium oxide nanoparticles were prepared by salt-assisted spray pyrolysis (SASP). Highly crystalline Ga₂O₃ nanoparticles with an average diameter of approximately 10 nm were obtained at various temperatures when a flux salt (LiCl, 5 mol/l) was added to the precursor solution. The effects of the crystallinity of the Ga₂O₃ particles and nitridation time on transformation to GaN were characterized using X-ray diffraction and scanning/transmission electron microscopy. Highly crystalline GaN nanoparticles with a mean size of 23.4 nm and a geometric standard deviation of 1.68 nm were obtained when Ga₂O₃ nanoparticles with relatively low crystallinity were used as the starting material. The resulting GaN nanoparticles showed a photoluminescence peak at 364 nm under UV excitation at 254 nm.     

Cover Picture: Direction‐Dependent Homoepitaxial Growth of GaN Nanowires (Adv. Mater. 2/2006)

GaN nanowires with vastly different morphologies depending upon their growth direction can be produced by direct nitridation and vapor transport of Ga in disassociated ammonia, report Sunkara and co‐workers on p. 216. Nanowires grown along the c‐direction develop hexagonal‐prism island morphologies, while wires grown along the a‐direction form uniform, belt‐shaped morphologies. A “ballistic” phenomenon involving the 1D transport of adatoms on the non‐polar surfaces of <0001> GaN nanowires is proposed to explain the prismatic island morphologies.     

Growth of Ga-doped ZnS nanowires constructed by self-assembled hexagonal platelets with excellent photocatalytic properties

A new process for making single crystalline undoped and Ga-doped ZnS nanowires with simple evaporation and condensation procedures on Si and GaN is introduced. The process does not need additional catalysts or precursors. The growth mechanism is studied using transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. TEM images show that the undoped ZnS nanowires exhibit an ordinary straight morphology, whereas the Ga-doped nanowires are composed of aligned hexagonal platelets, connected in the center into nanowires to maximize surface area. The Ga 2p3 and S 2p peaks in the XPS results confirm the presence of Ga doping in the form of Ga-S bonding. Raman spectra show that the ZnS LO peak is red-shifted from 349 to 347 cm(-1), indicative of a tensile stress caused by the Ga dopants. The growth mechanism and photocatalytic activity of the Ga-doped ZnS nanowires are discussed. We also demonstrate the excellent photocatalytic activity of Ga-doped ZnS nanowires as compared to those of undoped ZnS nanowires and Ga-doped ZnS nanosheets.