Hydrothermal growth of ZnO nanorods on a-plane GaN/sapphire template

ZnO nanorod arrays are grown on a-plane GaN template/r-plane sapphire substrates by hydrothermal technique. Aqueous solutions of zinc nitrate hexahydrate and hexamethylenetetramine were employed as growth precursors. Electron microscopy and X-ray diffraction measurements were carried out for morphology, phase and growth orientation analysis. Single crystalline nanorods were found to have off-normal growth and showed well-defined in-plane epitaxial relationship with the GaN template. The 〈0 0 0 1〉 axis of the ZnO nanorods were observed to be parallel to the 〈1 0 1¯ 0〉 of the a-plane GaN layer. Optical property of the as-grown ZnO nanorods was analyzed by room temperature photoluminescence measurements.     

In2O3 nanorod arrays grown at grain-boundary triple junctions of Cu–Sn alloy substrate

In this article, an alternative method for site-specific growth of In₂O₃ nanorod arrays, which relies on the vapor–liquid–solid growth mechanism, is demonstrated using Cu–Sn (5 at% Sn) alloy as substrate. By annealing Cu–Sn alloy slightly below the solidus line, grain-boundary triple junctions can be wetted preferentially. As a result, the catalyzing Cu droplets will be present at the sites of grain-boundary triple junctions, which will control the growth of In₂O₃ nanorods at defined locations. This growth technique provides a cost-effective and simple approach to fabricate ordered nanorod arrays with the sites controlled, which may benefit nanorod device applications.     

Growth of epitaxial ZnO films on Si (1 1 1) substrates with Cr compound buffer layer by plasma-assisted molecular beam epitaxy

Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1)_ZnO‖(1 1 1)_Si and [1 1 2¯ 0]_ZnO‖[0 1 1]_Si. Full-width at half-maximums (FWHMs) of (0 0 0 2) and (1 0 1¯ 1) X-ray rocking curves (XRCs) were 1.379° and 3.634°, respectively, which were significantly smaller than the FWHMs (4.532° and 32.8°, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be ∼5×10⁹ cm⁻². Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.     

A simple method to synthesize α-Si3N4, β-SiC and SiO2 nanowires by carbothermal route

α-Si₃N₄ nanowires, β-SiC nanowires and SiO₂ amorphous nanowires are synthesized via the direct current arc discharge method with a mixture of silicon, activated carbon and silicon dioxide as the precursor. The α-Si₃N₄ nanowires, β-SiC nanowires and SiO₂ amorphous nanowires are about 50–200 nm in stem diameter and 10–100 μm in length. α-Si₃N₄ nanowires and β-SiC nanowires consist of a solid single-crystalline core along the [0 0 1] and [1 1 1] directions, respectively, wrapped within an amorphous SiO_x layer. The direct current arc plasma-assisted self-catalytic vapor–solid and/or vapor–liquid–solid (VLS) growth processes are proposed as the growth mechanism of the nanowires.     

Shape evolution of zinc oxide from twinned disks to single spindles through solvothermal synthesis in binary solvents

Shape evolution of ZnO crystals from twinned disks to single spindles was studied through solvothermal synthesis in binary solvents N,N-diethylformamide (DEF) and methanol (MeOH). The MeOH content in DEF had large influence on the morphology of the obtained ZnO crystals. In MeOH-free DEF, well-shaped ZnO twinned disks with perfect mirror symmetry could be formed through the assembly of ZnO₄⁶⁻–julolidinium–ZnO₄⁶⁻ growth units on the (0 0 0 1) growth interfaces. For small amounts of MeOH (MeOH/DEF=0.04), elongated twinned disks were formed since the growth along the polar c-axis was enhanced. With increasing MeOH content (MeOH/DEF=0.1), twinned rods with reduced mirror symmetry were formed. When a large amount of MeOH was added to DEF (MeOH/DEF=0.5), single spindles rather than twinned disks or twinned rods were obtained. A similar shape evolution of zinc oxide was observed in binary solvents DEF and N,N-dimethylformamide (DMF), suggesting that the growth of ZnO crystals with tuneable shape and size can be controlled by the composition of the binary solvent mixture.     

Growth of hexagonal and cubic InN nanowires using MOCVD with different growth temperatures

We have performed a detailed investigation of the metal-organic chemical vapor deposition (MOCVD) growth and characterization of InN nanowires formed on Si(1 1 1) substrates under nitrogen rich conditions. The growth of InN nanowires has been demonstrated by using an ion beam sputtered (∼10 nm) Au seeding layer prior to the initiation of growth. We tried to vary the growth temperature and pressure in order to obtain an optimum growth condition for InN nanowires. The InN nanowires were grown on the Au+In solid solution droplets caused by annealing in a nitrogen ambient at 700 °C. By applying this technique, we have achieved the formation of InN nanowires that are relatively free of dislocations and stacking faults. Scanning electron microscopy (SEM) showed wires with diameters of 90–200 nm and lengths varying between 3 and 5 μm. Hexagonal and cubic structure is verified by high resolution X-ray diffraction (HR-XRD) spectrum. Raman measurements show that these wurtzite InN nanowires have sharp peaks E₂ (high) at 491 cm⁻¹ and A₁ (LO) at 591 cm⁻¹.     

Growth of low defect density GaP layers on Si substrates within the critical thickness by optimized shutter sequence and post-growth annealing

We have developed a growth procedure for realizing a low defect density GaP layer on an Si substrate. The growth procedure consists of two parts. One is the post-growth annealing for the annihilation of stacking faults (SFs). We have investigated an annihilation mechanism with molecular beam epitaxy grown GaP layers. 1-monolayer-thick SFs typically generate from the GaP/Si interface in a non-annealed GaP layer. In a 700 °C annealed GaP layer, generation points of these SFs tend to shift toward the GaP surface. In a 730 °C annealed GaP layer, SFs density is effectively decreased. These results suggest that SFs are annihilated through the climb motion of two partial dislocations during the post-growth annealing. Another one is the optimized shutter sequence for migration enhanced epitaxy. We have revealed that it is effective for the suppression of both three-dimensional growth and melt-back etching to increase in a stepwise manner the number of supplied Ga atoms per cycle. As a result, the generation of threading dislocations and pits is remarkably suppressed. A root mean square surface roughness of 0.13 nm is obtained within the critical thickness. We have estimated etch pit density (EPD) to be ∼7×10⁵ cm⁻² with a GaPN/GaP/Si structure. To the best of our knowledge, this value is same as that of commercially available GaP substrates and is the lowest one in the EPD of GaP/Si heteroepitaxy.     

Dense vertical nanoplates arrays and nanobelts of indium doped ZnO grown by thermal treatment of ZnS–In2O3 powders

Dense vertical arrays of indium doped ZnO nanoplates have been grown by thermal treatment of compacted ZnS–In₂O₃ powders with 0.35 at% of In. The distribution of nanoplates is related to the grain structure of the substrate. Only a small content of In has been detected in the plates by energy dispersive X-ray spectroscopy, but comparison with previous works shows that its presence in the precursor determines the growth of the nanoplates. Increase in the amount of In in the precursor leads to the growth of long indium doped ZnO nanobelts. Cathodoluminescence spectra of the nanobelts show a 23 meV blue shift of the band edge emission.     

Crystal growth and characterization of GaCrN nanorods on Si substrate

GaCrN nanorods were grown on GaN nanorods by RF-plasma-assisted molecular beam epitaxy. GaN nanorods were grown on Si (0 0 1) substrates with native SiO₂. Cr doping into GaCrN nanorods was conducted at substrate temperatures of 800 and 550 °C. Cross-sectional transmission electron microscopy images revealed that the diameter of GaCrN nanorod gradually increases with growth proceeding at 550 °C, while the growth at 800 °C does not change the nanorod diameter. Low-temperature growth enhances the growth perpendicular to the c-axis and decreases the growth along the c-axis. It was found that the solubility limit of Cr atoms in GaCrN is much higher for the low-temperature growth than for the high-temperature growth. It was also found that the highest saturation magnetization is obtained at some optimum Cr cell temperature.     

Improving the property of ZnO nanorods using hydrogen peroxide solution

Zinc oxide (ZnO) nanorod arrays made by the hydrothermal method were treated with hydrogen peroxide (H₂O₂) solution through two different approaches. One is to immerse ZnO nanorod sample into H₂O₂ solution. The other is a pre-treatment of spin-coating H₂O₂ solution on the seed layer before the growth of the ZnO nanorods. In the first approach, we found that the ultraviolet (UV) emission peak of the ZnO nanorod photoluminescence (PL) spectra was strongly dependent on the immersion time. In the second approach, the H₂O₂ solution influences not only the quality of the seed layer, but also the amount of the oxygen interstitial defects in the ZnO nanorods grown thereon. As a result, the UV emission intensity from the ZnO nanorods is enhanced almost five times. The ZnO nanorod arrays with few oxygen interstitial defects are prepared by the hydrogen peroxide treatment and expected to enable the fabrication of optoelectronic device with excellent performance.     

A study on structural formation and optical property of wide band-gap Be0.2Zn0.8O layers grown by RF magnetron co-sputtering system

Wide band-gap BeZnO layers were grown on Al₂O₃ (0 0 0 1) substrate using radio-frequency magnetron co-sputtering. The rate of Be_xZn_1−xO crystallized as a hexagonal structure was x=0.2. From the X-ray photoelectron spectroscopy measurement, the O–Zn bonds relating the crystal structure and the Be–O bonds related to the deviation of the stoichiometry in the BeZnO layer were caught at 530.4 and 531.7 eV in the O 1s spectrum, respectively. Thus, the observance on the Be 1s peak of 113.2 eV associated with the bonding Be–O indicates that the sputtered Be atoms are substituted for the host-lattice site in ZnO. This Be–O bonding shows a relatively low intense and broadening spectrum caused by large fluctuation of Be content in the BeZnO layer. From the photoluminescence and transmittance measurement, the free exciton and the neutral donor-bound exciton (D⁰, X) emissions were observed at 3.7692 and 3.7313 eV, respectively, and an average transmittance rate over 95% was achieved in a wide ultraviolet (UV)–visible region. Also, the binding energy for the (D⁰, X) emission was extracted to be 37.9 meV. Through the wide band-gap material BeZnO, we may open some possibilities for fabricating a ZnO-based UV light-emitting diode to be utilized as a barrier layer comprised of the ZnO/BeZnO quantum well structure and/or an UV light emitting material itself.     

Characterization of AlN single crystal fabricated by a novel growth technique, “pyrolytic transportation method”

Aluminum nitride single crystal fabricated by a novel growth technique “pyrolytic transportation method”, which is advantageous for industrial process because of using α-Al₂O₃ as a source material instead of aluminum nitride, was characterized. This growth technique shows high growth rate up to 1.6 mm/h. High crystalline quality was indicated by X-ray topogragh and X-ray rocking curve. Full width at half maximum of (0 0 0 2) and (1 0 −1 0) were excellent values of 90 and 148 arcsec, respectively. Homoepitaxial overgrowth by hydride vapor phase epitaxy was successfully conducted. No dislocation was observed at the interface between the substrate and overgrowth layer by transmission electron microscopy.     

In-situ visualization of a super-accelerated synthesis of zinc oxide nanostructures through CO2 laser heating

A simple growth technique capable of growing a variety of zinc oxide (ZnO) nanostructures with record growth rates of 25 μm/s is demonstrated. Visible lengths of ZnO nanowires, nanotubes, comb-like and pencil-like nanostructures could be grown by employing a focused CO₂ laser-assisted heating of a sintered ZnO rod in ambient air, in few seconds. For the first time, the growth process of nanowires was videographed, in-situ, on an optical microscope. It showed that ZnO was evaporated and presumably decomposed into Zn and oxygen by laser heating, reforming ZnO nanostructures at places with suitable growth temperatures. Analysis on the representative nanowires shows a rectangular cross-section, with a [0 0 0 1] growth direction. With CO₂ laser heating replacing furnace heating used conventionally, and using different reactants and forming gases, this method could be easily adopted for other semiconducting inorganic nanostructures in addition to ZnO.     

Growth and microstructural features of laser ablated LiCoO2 thin films

Thin films of LiCoO₂ were prepared by pulsed laser deposition technique and the properties were studied in relation to the deposition parameters. The films deposited from a sintered composite target (LiCoO₂+Li₂O) in an oxygen partial pressure of 100 mTorr and at a substrate temperature of 300 °C exhibited preferred c-axis (0 0 3) orientation perpendicular to the substrate surface. The AFM data demonstrated that the films are composed of uniform distribution of fine grains with an average grain size of 80 nm. The grain size increased with an increase in substrate temperature. The (0 0 3) orientation decreased with increase in (1 0 4) orientation for the films deposited at higher substrate temperatures (>500 °C) indicating that the films’ growth is parallel to the substrate surface. The composition of the experimental films was analyzed using X-ray photoelectron spectroscopy (XPS). The binding energy peaks of Co(2p_3/2) and Co(2p_1/2) are, respectively, observed at 779.3 and 794.4 eV, which can be attributed to the Co³⁺ bonding state of LiCoO₂. The electrochemical measurements were carried out on Li//LiCoO₂ cells with a lithium metal foil as anode and LiCoO₂ film as cathode of 1.5 cm² active area using a Teflon home-made cell hardware. The Li//LiCoO₂ cells were tested in the potential range 2.6-4.2 V. Specific capacity as high as 205 mC/cm² μm was measured for the film grown at 700 °C. The growth of LiCoO₂ films were studied in relation to the deposition parameters for their effective utilization as cathode materials in solid-state microbattery application.     

Fabrication and photocatalytic property of ZnO/SrTiO3 core/shell nanorod arrays

ZnO/SrTiO₃ core/shell nanorod arrays were fabricated by a facile two‐step method. ZnO nanorod arrays were first hydrothermally grown on Si substrate. Then, using liquid phase deposition method, SrTiO₃ were deposited onto the ZnO nanorods to form core/shell nanorod structures. The morphologies and structures of the products were characterized by scanning electron microscopy, transmission electron microscopy, and X‐ray diffraction. The photocatalytic behavior of the nanorod arrays was also examined through the photodegradation of methylene blue solution under UV irradiation. It was found that the core/shell nanorod arrays with deposition time of 10 min showed higher photocatalytic activity than bare ZnO nanorod arrays. This enhancement was attributed to the efficient charge separation at the ZnO/SrTiO₃ interface.     

In-situ and ex-situ ZnO nanorod growth on ZnO homo-buffer layers

Vertically well-aligned ZnO nanorods were fabricated in-situ and ex-situ on ZnO homo-buffer layers using catalyst-free metal-organic chemical vapor deposition. Field-emission electron microscopy measurements demonstrated that the nanorods were well aligned and had a uniform diameter of 70–100 nm depending on the growth temperature, irrespective of growth conditions, in-situ and ex-situ. X-ray diffraction measurements demonstrated that the ZnO nanorods and the ZnO buffer layers had a wurtzite structure, and that the crystal quality of the nanorods grown on a smooth surface was better than that of the nanorods grown on a rough surface. Field-emission transmission electron microscopy measurements revealed the presence of a disordered layer at the interface of the nanorod and the buffer layer.     

Hydride-assisted growth of GaN nanowires on Au/Si(0 0 1) via the reaction of Ga with NH3 and H2

High quality, straight GaN nanowires (NWs) with diameters of 50 nm and lengths up to 3 μm have been grown on Si(0 0 1) using Au as a catalyst and the direct reaction of Ga with NH₃ and N₂:H₂ at 900 °C. These exhibited intense, near band edge photoluminescence at 3.42 eV in comparison to GaN NWs with non-uniform diameters obtained under a flow of Ar:NH₃, which showed much weaker band edge emission due to strong non-radiative recombination. A significantly higher yield of β-Ga₂O₃ NWs with diameters of ≤50 nm and lengths up to 10 μm were obtained, however, via the reaction of Ga with residual O₂ under a flow of Ar alone. The growth of GaN NWs depends critically on the temperature, pressure and flows in decreasing order of importance but also the availability of reactive species of Ga and N. A growth mechanism is proposed whereby H₂ dissociates on the Au nanoparticles and reacts with Ga giving Ga_xH_y thereby promoting one-dimensional (1D) growth via its reaction with dissociated NH₃ near or at the top of the GaN NWs while suppressing at the same time the formation of an underlying amorphous layer. The higher yield and longer β-Ga₂O₃ NWs grow by the vapor liquid solid mechanism that occurs much more efficiently than nitridation.     

Structure evolution of Zn cluster on Si (0 0 1) substrate for ZnO nanostructure growth

In this paper, Monte Carlo simulations are carried out for Zn cluster supported on a static Si (0 0 1) substrate to estimate the morphological evolution of self-catalysis growth of ZnO nanostructures. The tight-binding many-body potential and the Lennard–Jones potential are used to describe Zn–Zn and Zn–Si interactions, respectively. The dynamic processes of Zn cluster in the temperature field decomposing and wetting effects are visualized through the simulation. The Zn atomic aggregates that randomly disperse on the Si (0 0 1) substrate with different shapes, such as a dimer, trimer, multimer and atomic chain, would act as catalytic nucleation sites for the following growth of the ZnO nanostructure. This phenomenon provides a sound explanation for the formation of randomly orientated and diversified ZnO nanostructures on the Si (0 0 1) substrate.     

Fabrication of (1 1 1)-oriented Si layers on SiO2 substrates by an aluminum-induced crystallization method and subsequent growth of semiconducting BaSi2 layers for photovoltaic application

We have prepared (1 1 1)-oriented Si layers on SiO₂ (fused silica) substrates from amorphous-Si(a-Si)/Al or Al/a-Si stacked layers using an aluminum-induced crystallization (AIC) method. The X-ray diffraction (XRD) intensity from the (1 1 1) planes of Si was found to depend significantly on growth conditions such as the thicknesses of Si and Al, deposition order (a-Si/Al or Al/a-Si on SiO₂), deposition technique (sputtering or vacuum evaporation) and exposure time of the Al layer to air before the deposition of Si. The crystal orientation of the Si layers was confirmed by θ−2θ, 2θ XRD and electron backscatter diffraction (EBSD). The photoresponse properties of semiconducting BaSi₂ films formed on the (1 1 1)-oriented Si layers by the AIC method were measured at room temperature. Photocurrents were clearly observed for photon energies greater than 1.25 eV. The external quantum efficiencies of the BaSi₂ were also evaluated.     

Microstructure and dielectric properties of La2O3 films prepared by ion beam assistant electron-beam evaporation

Ultrathin La₂O₃ gate dielectric films were prepared on Si substrate by ion assistant electron-beam evaporation. The growth processing, interfacial structure and electrical properties were investigated by various techniques. From XRD results, we found that the La₂O₃ films maintained the amorphous state up to a high annealing temperature of 900 °C for 5 min. From XPS results, we also discovered that the La atoms of the La₂O₃ films did not react with silicon substrate to form any La-compound at the interfacial layer. However, a SiO₂ interfacial layer was formed by the diffusion of O atoms of the La₂O₃ films to the silicon substrate. From the atomic force microscopy image, we disclosed that the surface of the amorphous La₂O₃ film was very flat. Moreover, the La₂O₃ film showed a dielectric constant of 15.5 at 1 MHz, and the leakage current density of the La₂O₃ film was 7.56 × 10⁻⁶ A/cm² at a gate bias voltage of 1 V.