Tip-growth mode and base-growth mode of Au-catalyzed zinc oxide nanowires using chemical vapor deposition technique

Tip-growth and base-growth modes of Au-catalyzed zinc oxide nanowires (ZnO NWs) were synthesized on Au-film pre-deposited silicon substrates using Chemical Vapor Deposition (CVD) technique. The diameter of tip-growth Au-catalyzed ZnO NWs was proportional to the Au film thickness, whereas the areal density of these NWs was inversely proportional to the Au film thickness. It would be more appropriate to explain the growth of Au-catalyzed ZnO NWs by a combination of Vapor–Liquid–Solid and Vapor–Solid (VLS–VS) mechanisms instead of the conventional VLS mechanism, regardless of tip-growth or base-growth mode of Au-catalyzed ZnO NWs. The competition between the VLS and VS mechanism in the effectiveness of capturing the adsorbed Zn and O atoms would determine the final morphology of ZnO NWs. In addition, Au catalyst promoted the growth rate of NWs as compared to the self-catalyzed ZnO NWs.     

Growth and photoluminescence properties of vertically aligned SnO2 nanowires

Vertically aligned SnO₂ nanowires (NWs) were grown for the first time by a vapor–liquid–solid method on c-sapphire with gold as a catalyst under Ar gas flow. Electron backscatter diffraction analysis indicated the NWs are single crystalline having the rutile structure, grow vertically along the [1 0 0] direction, and exhibit a consistent epitaxial relationship where lattice mismatch is estimated to be 0.3% along the SnO₂ [0 1 0] direction. The growth of these NWs is sensitive to many parameters, including growth duration, substrate type, source vapor concentration, and the thickness of the catalyst layer. Photoluminescence measurements at room temperature showed that the vertically aligned NWs exhibit an intense transition at 3.64 eV, a near band-edge transition which is rarely observed in SnO₂.     

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 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.     

Preparation of single-crystalline ZnO films on ZnO-buffered a-plane sapphire by chemical bath deposition

High-quality zinc oxide (ZnO) films were successfully grown on ZnO-buffered a-plane sapphire (Al₂O₃ (1 1 2¯ 0)) substrates by controlling temperature for lateral growth using chemical bath deposition (CBD) at a low temperature of 60 °C. X-ray diffraction analysis and transmission electron microscopy micrographs showed that the ZnO films had a single-crystalline wurtzite structure with c-axis orientation. Rocking curves (ω-scans) of the (0 0 0 2) reflections showed a narrow peak with full width at half maximum value of 0.50° for the ZnO film. A reciprocal space map indicated that the lattice parameters of the ZnO film (a=0.3250 nm and c=0.5207 nm) were very close to those of the wurtzite-type ZnO. The ZnO film on the ZnO-buffered Al₂O₃ (1 1 2¯ 0) substrate exhibited n-type conduction, with a carrier concentration of 1.9×10¹⁹ cm⁻³ and high carrier mobility of 22.6 cm² V⁻¹ s⁻¹.     

Investigation of nonpolar (1 1 2¯ 0) a-plane ZnO films grown under various Zn/O ratios by plasma-assisted molecular beam epitaxy

Structural and optical properties of nonpolar a-plane ZnO films grown with different II/VI ratios on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy were investigated. Even by increasing the II/VI ratio across the stoichiometric flux condition a consistent surface morphology of striated stripes along the ZnO 〈0 0 0 1〉 direction without any pit formation was observed, which is contrary to polar c-plane ZnO films. Root mean square surface roughness, full width at half maximum values of X-ray rocking curves, defect densities, and photoluminescence were changed with the II/VI ratio. The sample grown with stoichiometric flux condition showed the lowest value of rms roughness, the smallest threading dislocation and stacking fault densities of ∼4.7×10⁸ cm⁻² and ∼9.5×10⁴ cm⁻¹, respectively, and the highest intensity of D^oX peak. These results imply that the stoichiometric flux growth condition is suitable to obtain superior structural and optical properties compared to other flux conditions.     

Chemically assisted vapour transport for bulk ZnO crystal growth

A chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10⁻³ to 1 atm) and temperatures (800–1200 °C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46 mm in diameter and 8 mm in thickness, with growth rates up to 400 μm/h. These values are compatible with an industrial production rate. N type ZnO crystals (μ=182 cm²/(V s) and n=7 10¹⁵ cm⁻³) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10–30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] <10⁺¹⁵ cm⁻³). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented.     

MBE growth of PbSe thin film with a 9×10 cm etch-pits density on patterned (1 1 1)-oriented Si substrate

PbSe thin film was grown on a patterned Si substrate with (1 1 1)-orientation by molecular-beam epitaxy (MBE). On the mesa, a low dislocation density of 9×10⁵ cm⁻² was confirmed by the etch-pits density (EPD) wet-etching technique. The photoluminescence (PL) intensity at room temperature from the low dislocation PbSe film was much higher than that from the PbSe film grown on the planar area, which further indicated the high-quality of PbSe thin film grown on patterned Si substrate.     

SIMS and Raman characterizations of ZnO:N thin films grown by MOCVD

Nitrogen was incorporated into ZnO films grown by metalorganic chemical vapour deposition (MOCVD) on ZnO substrates using DMZn-TEN, tert-butanol and diallylamine, respectively, as zinc, oxygen and doping sources. The carrier gas was either hydrogen or nitrogen and the partial pressure ratio (R_VI/II) was varied in order to favor the nitrogen incorporation and/or reduce carbon related defects. The ZnO films have been characterized by Micro-Raman scattering and SIMS measurements. SIMS measurements confirm the nitrogen incorporation with concentrations extending from ∼10¹⁹ cm⁻³ to ∼4×10²⁰ cm⁻³. Raman spectra show nitrogen local vibration modes in films grown at low R_VI/II ratio and using H₂ as carrier gas. However, a vibration band attributed to carbon clusters dominates the Raman spectra for films grown with N₂ carrier. The contribution of N complexes was discussed. The strain was calculated for the as-grown and annealed films and it changes from tensile to compressive after annealing.     

Fabrication of UV detectors based on ZnO nanowires using silicon microchannel

Aligned ZnO nanowires were grown by metal organic chemical vapor deposition on patterned silicon substrate. The shape of nanostructures was greatly influenced by the micropatterned surface. The aspect ratio, packing fraction and the number density of nanowires on top surface are around 10, 0.8 and 10⁷ per mm², respectively, whereas the values are 20, 0.3 and 5×10⁷ per mm², respectively, towards the bottom of the cavity. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy confirmed the single-crystalline growth of the ZnO nanowires along the [0 0 0 1] direction. Photosensitivity of the nanowires, grown on both top and bottom surface of the microchannel, was observed. However, the nanowires grown on bottom surface have shown better UV response with base line recovery at dark condition.     

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.     

A route to single-crystalline ZnO films with low residual electron concentration

Single-crystalline ZnO films have been grown on a-plane sapphire in plasma assisted molecular beam epitaxy by introducing a high-temperature ZnO buffer layer. The residual electron concentration of the films can be lowered to 1.5×10¹⁶ cm⁻³, comparable with the best value ever reported for ZnO films grown on a rare and costly substrate of ScAlMgO₄. A 3×3 reconstruction has been observed on the films grown in this route, which reveals that the films have very smooth surface. X-ray phi-scan spectrum of the films shows six peaks with 60° intervals, and two-dimensional X-ray diffraction datum indicates the single-crystalline nature of the films. Low temperature photoluminescence spectrum of the films shows a dominant free exciton emission and five phonon replicas, confirming the high quality of the films.     

A facile route to arsenic-doped p-type ZnO films

Undoped zinc oxide (ZnO) films have been prepared on sapphire substrates in a molecular beam epitaxy technique, and the films were annealed in air ambient along with a GaAs wafer. Arsenic in the GaAs wafer will evaporate, and enter into the ZnO films. In this facile way, arsenic-doped p-ZnO has been obtained. Hall measurements reveal that the hole concentration and Hall mobility of the ZnO:As films obtained in this way can reach 3.7×10¹⁷ cm⁻³ and 2.8 cm² V⁻¹ S⁻¹, respectively. X-ray photoelectron spectroscopy confirms the incorporation of arsenic into the ZnO films. The activation energy of the acceptors derived from temperature-dependent Hall measurement is about 164 meV.     

Synchrotron X-ray diffraction studies of heteroepitaxial ZnO films grown by pulsed laser deposition

Heteroepitaxial ZnO films were grown by pulsed laser deposition on various substrates such as GaN-buffered C-Al₂O₃, C-Al₂O₃, A-Al₂O₃, and R-Al₂O₃. The epitaxy nature of the films was investigated mainly by synchrotron X-ray diffraction. The results showed that the GaN interlayer plays a positive role in growing an unstrained, well-aligned epitaxial ZnO film on the basal plane of Al₂O₃. Importantly, the ZnO film grown on R-Al₂O₃ has two differently aligned domains. The dominant (1 1 0) oriented domain has much better alignment in the in-plane direction than the minor portion of (0 0 1) oriented domain, while in the out-of-plane direction the two domains have almost the same mosaic distribution.     

Development of m-plane GaN anisotropic film properties during MOVPE growth on LiAlO2 substrates

The anisotropic film properties of m-plane GaN deposited by metal organic vapour phase epitaxy (MOVPE) on LiAlO₂ substrates are investigated. To study the development of layer properties during epitaxy, the total film thickness is varied between 0.2 and 1.7 μm. A surface roughening is observed caused by the increased size of hillock-like features. Additionally, small steps which are perfectly aligned in (1 1 −2 0) planes appear for samples with a thickness of ∼0.5 μm and above. Simultaneously, the X-ray rocking curve (XRC) full width at half maximum (FWHM) values become strongly dependent on incident X-ray beam direction beyond this critical thickness. Anisotropic in-plane compressive strain is initially present and gradually relaxes mainly in the [1 1 −2 0] direction when growing thicker films. Low-temperature photoluminescence (PL) spectra are dominated by the GaN near-band-edge peak and show only weak signal related to basal plane stacking faults (BSF). The measured background electron concentration is reduced from ∼10²⁰ to ∼10¹⁹ cm⁻³ for film thicknesses of 0.2 μm and ∼1 μm while the electron mobilities rise from ∼20 to ∼130 cm²/V s. The mobilities are significantly higher in [0 0 0 1] direction which we explain by the presence of extended planar defects in the prismatic plane. Such defects are assumed to be also the cause for the observed surface steps and anisotropic XRC broadening.     

The impact of an intermediate temperature buffer on the growth of GaN on an AlN template by hydride vapor phase epitaxy

The present study focused on the effect of an intermediate-temperature (IT; ∼900 °C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29×10⁷ cm⁻²) and a higher edge-dislocation density (8.157×10⁹ cm⁻²) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.     

Ordered arrays of high-quality single-crystalline α-Si3N4 nanowires: Synthesis,properties and applications

Ordered arrays of high-quality single-crystalline α-Si₃N₄ nanowires (NWs) have been synthesized via thermal evaporation and detailed characteristics of the NWs have been analyzed by employing scanning electron microscope (SEM) along with energy dispersive spectroscopy (EDS), high-resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), X-ray photospectroscopy (XPS), infrared (IR), photoluminescence (PL) and in situ I–V measurements by STM/TEM holder. The microscopic results revealed that the NWs having diameter in the range of ~30–100 nm and length in microns. Furthermore, the NWs are found to be single crystalline grown along [0 0 1] direction. The elemental composition and valence states of elements are analyzed by EDS and XPS. The room temperature PL spectra exhibit a broad range visible emission band. The electron transport property of a single NW illustrates the symmetric I–V curve of a semiconductor. The possible growth mechanism is also briefly discussed.     

Defect reduction in nonpolar and semipolar GaN using scandium nitride interlayers

Nonpolar (1 1 2¯ 0) and semipolar (1 1 2¯ 2) GaN films were grown on sapphire by metalorganic vapour phase epitaxy using ScN interlayers of varying thicknesses. A 5 nm interlayer reduced basal plane stacking fault (BSF) densities in nonpolar films by a factor of 2 and threading dislocation (TD) densities by a factor of 100 to (1.8±0.2)×10⁹ cm⁻². An 8.5 nm interlayer reduced BSF densities in semipolar films by a factor of 5 and reduced TD densities by a factor of 200 to (1.5±0.3)×10⁸ cm⁻². Nonpolar film surface roughnesses were reduced by a factor of 20.     

Czochralski-growth of germanium crystals containing high concentrations of oxygen impurities

Oxygen-containing germanium (Ge) single crystals with low density of grown-in dislocations were grown by the Czochralski (CZ) technique from a Ge melt, both with and without a covering by boron oxide (B₂O₃) liquid. Interstitially dissolved oxygen concentrations in the crystals were determined by the absorption peak at 855 cm⁻¹ in the infrared absorption spectra at room temperature. It was found that oxygen concentration in a Ge crystal grown from melt partially or fully covered with B₂O₃ liquid was about 10¹⁶ cm⁻³ and was almost the same as that in a Ge crystal grown without B₂O₃. Oxygen concentration in a Ge crystal was enhanced to be greater than 10¹⁷ cm⁻³ by growing a crystal from a melt fully covered with B₂O₃; with the addition of germanium oxide powder, the maximum oxygen concentration achieved was 5.5×10¹⁷ cm⁻³. The effective segregation coefficients of oxygen in the present Ge crystal growth were roughly estimated to be between 1.0 and 1.4.     

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.