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.     

Low temperature Si homo-epitaxy by reduced pressure chemical vapor deposition using dichlorosilane,silane and trisilane

Dichlorosilane (DCS), silane and trisilane have been investigated as Si precursors for low temperature (<700 °C) Si reduced pressure chemical vapor deposition. DCS and silane are limited to growth temperatures higher than 600–650 and 500 °C, respectively. At lower temperatures, absence of either Cl or H desorption from the surface impedes Si growth with acceptable growth rate (>5 Å/min). Trisilane permits the growth of Si at lower temperatures below 350 °C due to a specific growth mechanism enhancing H desorption. Layers grown at temperatures lower than 500 °C are defective, irrespective of the carrier gas, pressure and precursor flow used.     

Cracking assisted nucleation in chemical vapor deposition of silicon nanoparticles on silicon dioxide

Deposition of sub-monolayer silicon on SiO₂/Si(1 0 0) greatly facilitates nucleation in subsequent thermal chemical vapor deposition (CVD) of silicon nanoparticles. Sub-monolayer seeding is accomplished using silicon atoms generated via disilane decomposition over a hot tungsten filament. The hot-wire process is nonselective towards deposition on silicon and SiO₂, is insensitive to surface temperature below 825 K, and gives controlled coverages well below 1 ML. Thermal CVD of nanoparticles at 1×10⁻⁴ Torr disilane and temperatures ranging from 825 to 925 K was studied over SiO₂/Si(1 0 0) surfaces that had been subjected to predeposition of Si or were bare. Seeding of the SiO₂ surface with as little as 0.01 ML is shown to double the nanoparticle density at 825 K, and densities are increased twenty fold at 875 K after seeding the surface with 30% of a monolayer.     

Rate-determining process in chemical vapor deposition of SiC on off-axis -SiC

Homoepitaxial growth on off-axis α-SiC at reduced pressures in a horizontal cold-wall chemical vapor deposition (CVD) system operating at has been investigated. The growth rate was found inversely proportional to the square root of total pressure or the partial pressure of H₂, a carrier gas. A model to explain the experimental results is proposed, where the rate-determining process in CVD is competition between Si species and hydrogen atoms for C (carbon) dangling bonds at SiC step edges.     

Growth of SiC nanowires within stacked SiC fiber fabrics by a noncatalytic chemical vapor infiltration technique

An atmospheric pressure chemical vapor infiltration (CVI) process without metallic catalysts was applied for the growth of SiC nanowires within stacked SiC fiber fabrics. We investigated the effect of the concentration of a reactant gas (CH₃SiCl₃, MTS) on the growth behavior and microstructure of the SiC nanowires. At high concentration of MTS in a H₂+MTS mixture gas, one-dimensional (1D) SiC deposits with diameters of several hundreds of nanometers were formed. Microstructures of the 1D SiC deposits exhibited a strong positional dependency throughout the thickness direction of the stacked fabric due to a depletion of the MTS gas. On the other hand, single-crystalline SiC nanowires with average diameters of 50–60 nm could be obtained at a low concentration of MTS. The SiC nanowires also exhibited a homogeneous growth both in the plane of each fabric layer and throughout the thickness of the sample.     

Polar dependence of impurity incorporation and yellow luminescence in GaN films grown by metal-organic chemical vapor deposition

We have investigated the unintentional impurities, oxygen and carbon, in GaN films grown on c-plane, r-plane as well as m-plane sapphire by metal-organic chemical vapor deposition. The GaN layer was analyzed by secondary ion mass spectroscopy. The different trend of the incorporation of oxygen and carbon has been explained in the polar (0 0 0 1), nonpolar (1 1 2¯ 0) and semipolar (1 1 2¯ 2) GaN by a combination of the atom bonding structure and the origin direction of the impurities. Furthermore, it has been found that there is a stronger yellow luminescence (YL) in GaN with higher concentration of carbon, suggesting that C-involved defects are originally responsible for the YL.     

Structural and infrared properties of zinc oxide film and nanowires

The structural and infrared properties of the highly (00.2) oriented ZnO film, randomly grown Au-catalyzed ZnO nanowires (NWs) and vertically aligned self-catalyzed ZnO NWs were compared. In the XRD analysis, (0 0 2) diffraction intensity of self-catalyzed ZnO NWs was enhanced mainly attributed to the preferential growth of NWs in [0 0 0 1] as compared to the ZnO film and the randomly grown Au-catalyzed ZnO NWs. The high UV-to-green emission ratio of self-catalyzed ZnO NWs in room temperature PL measurement indicates that they had a better crystal quality as compared to Au-catalyzed ZnO NWs and ZnO film. Infrared spectroscopy has been used to characterize these films and nanowires too. The phonon peak 407 cm⁻¹ which related to the transverse optical (TO) vibrations perpendicular to the optical axis was observed in the IR reflectivity measurements on the highly c-oriented ZnO film. The IR peaks that appeared in the 550–580 cm⁻¹ region of the spectra of the specimens could be assigned to the ZnO NWs as it was not observed in the ZnO film. These peaks were observed in the 550–580 cm⁻¹ region in both s- and p-polarized light for the randomly grown Au-catalyzed ZnO NWs. In contrast, the IR peak at 580 cm⁻¹ was clearly shown in p-polarized light but not in the s-polarized light for vertically aligned ZnO NWs. This indicated that the vibration was polarized along the vertically aligned ZnO NWs. The (00.2) orientation of the ZnO specimens could be identified by comparing the p- and s-polarized IR spectra.     

Morphological,structural and electrical investigations on non-polar a-plane ZnO epilayers

We report on the growth of non-polar a-plane ZnO by CVD on r-plane-sapphire-wafers, a-plane GaN-templates and a-plane ZnO single-crystal substrates. Only the homoepitaxial growth approach leads to a Frank–van-der–Merwe growth mode, as shown by atomic force microscopy. The X-ray-diffraction spectra of the homoepitaxial thin films mirror the excellent crystalline quality of the ZnO substrate. The morphological and the structural quality of the homoepitaxial films is comparable to the best results for the growth on c-plane ZnO-substrates. The impurity incorporation, especially of group III elements, seems to be reduced when growing on the non-polar a-plane surface compared to the c-plane films as demonstrated by secondary ion mass spectrometry (SIMS). Optical properties have been investigated using low temperature photoluminescence measurements. We employed capacitance–voltage measurements (C–V) to measure the background carrier density and its profile from substrate/film interface throughout the film to the surface. In thermal admittance spectroscopy (TAS) specific traps could be distinguished, and their thermal activation energies and capture cross sections could be determined.     

Electrical properties and deep levels spectra of bulk SiC crystals grown by hybrid physical–chemical vapor transport method

Concentrations of nitrogen shallow donors, boron shallow acceptors, charge carriers, and electron traps were measured as a function of position along the growth axis in a series of undoped 6H–SiC boules grown by sublimation method with and without addition of hydrogen to the growth atmosphere. Elemental analysis by secondary ion mass spectrometry and measurements of electrical properties indicate that the addition of hydrogen suppresses nitrogen incorporation and formation of all electron traps. Concentration of boron is not affected by hydrogen presence. The addition of hydrogen to the growth ambient improves the uniformity of nitrogen incorporation and deep trap distribution along the growth axis. The results are interpreted as due to increased carbon transport and corresponding shift of crystal stoichiometry toward carbon-rich side of the SiC existence range.     

Fluid-dynamics during vapor epitaxy and modeling

Epitaxial deposition of thin or thick solid films is one of the most important growth processes in opto- and micro-electronic device production. The performance of growth apparatuses depend strongly on the physical and chemical aspects involved in the deposition process, such as the fluid dynamic features and the deposition chemistry. These phenomena can be well described through a macroscale modeling approach based on fundamental conservation equations. These models can be successfully adopted to optimize existing processes and to design new reactors where the “flat area” matching industrial needs is always increasing in time. Here, a macroscale model for deposition reactors has been derived highlighting the hypotheses necessary to fit the general conservation equations for these systems. Moreover, attention has been placed on the estimation of the necessary physical and chemical parameters. Macroscale aspects have been addressed with particular emphasis on the role of fluid flow within the reactor to reveal desired or undesired flow paths and their effect on process performance parameters. In particular, horizontal, vertical and barrel reactor types have been examined.     

Epitaxial growth of graphitic carbon on C-face SiC and Sapphire by chemical vapor deposition (CVD)

Epitaxial, graphitic carbon thin films were directly grown on C-face/(0 0 0 1¯) SiC and (0 0 0 1) sapphire by chemical vapor deposition (CVD), using propane as a carbon source and without any catalytic metal on the substrate surface. Raman spectroscopy shows the signature of multilayer graphene/graphite growth on both the SiC and sapphire. Raman 2D-peaks have Lorentzian lineshapes with FWHM of ∼60 cm⁻¹ and the ratio of the D-peak to G-peak intensity (I_D/I_G) linearly decreases (down to 0.06) as growth temperature is increased. The epitaxial relationship between film and substrates were determined by X-ray diffraction. On both substrates, graphitic layers are oriented parallel to the substrate, but exhibit significant rotational disorder about the surface normal, and predominantly rhombohedral stacking. Film thicknesses were determined to be a function of growth time, growth temperature, and propane flow rate.     

Ordered arrays of epitaxial silicon nanowires produced by nanosphere lithography and chemical vapor deposition

Gold dot arrays on (1 1 1) Si substrates obtained through nanosphere lithography (NSL) combined with sputtering and annealing in Ar at 1000 °C are used to catalyze vapor liquid solid (VLS) epitaxial growth of silicon nanowires (Si NWs) using chemical vapor deposition (CVD) with SiH₄ in Ar. The NWs grow primarily epitaxially on the underlying (1 1 1) Si wafer following the four independent 〈1 1 1〉 directions. The diameter distribution of the wires reflects the diameter distribution of the catalyst gold dot arrays and is therefore predictable. The wire length depends on the size of the gold catalyst for the same CVD parameters. The wire position is foreseeable within the limits of the pattern geometrical quality, but one-to-one growth of NWs to gold dots is not always observed, probably due to (very locally) the remaining presence of silicon oxide. Overall, this inexpensive patterning method for obtaining high-quality crystalline VLS Si NWs by CVD fulfills the requirements of many device applications, where patterning control, quality and reproducibility of the nanostructures are crucial.     

Accelerated surface flattening by alternating Ga flow in hydride vapor phase epitaxy

GaN films were grown on c$c$-plane sapphire substrates by using hydride vapor phase epitaxy (HVPE) with a pulsed flow of HCl over Ga metal. NH₃${\mathrm{NH}}_3$ gas supply was controlled to flow in a constant rate or in a modulated way. The surface morphology dependence of these films on the various flow modulation schemes was investigated. Depending on the duty cycle of NH₃${\mathrm{NH}}_3$ flow, the surface morphology of GaN films was sensitively modified. This sensitive response of surface morphology of GaN films to the flow modulation was attributed to diffusion efficiency variation of Ga species under different gas environment. Under proper modulation conditions, flattened top-surface morphology of nucleated domains was found to be obtained.     

Predicted nitrogen doping concentrations in silicon carbide epitaxial layers grown by hot-wall chemical vapor deposition

A simple quantitative model for the surface adsorption of nitrogen has been developed to simulate the doping incorporation in intentionally doped 4H–SiC samples during epitaxial growth. Different reaction schemes are necessary for the two faces of SiC. The differences are discussed, and implications to the necessary model adjustments are stressed. The simulations are validated by experimental values for a large number of different process parameters with good agreement.     

Coexistence properties of phase separation and CuPt-ordering in InGaAsP grown on GaAs substrates by organometallic vapor phase epitaxy

CuPt-ordering and phase separation were directly investigated in In_1-xGa_xAs_yP_1-y with a low arsenic content grown by organometallic vapor phase epitaxy on GaAs substrates. CuPt-ordering and phase separation in samples grown at the substrate temperatures of 630 and 690 °C were characterized by transmission electron diffraction and transmission electron microscopy. Although the immiscibility of InGaAsP was enhanced at the lower substrate temperature, the sample grown at 630 °C showed less phase separation than the 690 °C-grown sample. The degree of CuPt-ordering was significantly enhanced in the sample grown at 630 °C. The results demonstrated that the CuPt-ordering originating from surface reconstruction of P(2×4) suppressed the phase separation even in the miscibility gap. The detailed characterization of the phase separation clearly revealed a vertical composition modulation (VCM) in InGaAsP for the first time. The mechanism of the VCM formation is discussed based on the modulated-strain field on the surface.     

Chemical vapor transport of zinc sulfide

Crystals of cubic zinc sulfide with different isotopic compositions have been grown by iodine vapor transport for basic research purposes (vibrational, electronic, and thermodynamic properties). The synthesis reaction in sulfur vapor was found to be controlled by solid-state diffusion of zinc atoms through a ZnS passivation layer. Crystals up to 5 mm in length were grown from small amounts of source material. The presence of argon reduced the nucleation density and favored the formation of facets.     

Heteroepitaxial growth of Cu2O thin film on ZnO by metal organic chemical vapor deposition

Cuprous oxide (Cu₂O) thin films were grown epitaxially on c-axis-oriented polycrystalline zinc oxide (ZnO) thin films by low-pressure metal organic chemical vapor deposition (MOCVD) from Copper(II) hexafluoroacetylacetonate [Cu(C₅HF₆O₂)₂] at various substrate temperatures, between 250 and 400 °C, and pressures, between 0.6 and 2.1 Torr. Polycrystalline thin films of Cu₂O grow as single phase with [1 1 0] axis aligned perpendicular to the ZnO surface and with in-plane rotational alignment due to (2 2 0)_Cu2O(0 0 0 2)_ZnO; [0 0 1]_Cu2O[1 2¯ 1 0]_ZnO epitaxy. The resulting interface is rectifying and may be suitable for oxide-based p–n junction solar cells or diodes.     

Synthesis and optical properties of purified translucent,orthorhombic boron nitride films

Large-area (>1 cm²) freestanding translucent orthorhombic boron nitride (oBN) films have been synthesized by magnetron sputtering at a low radio-frequency power of 120 W. The structural characterizations were performed by means of X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. It is demonstrated that oBN is a direct band gap semiconductor (E_g∼3.43 eV). Excited by ultraviolet laser (wavelength at 325 nm), the oBN films emit strong white light, which can be seen by the naked eyes in the dark. In the photoluminescence spectrum, besides the ultraviolet near-band-edge radiative recombination emission, there are three visible emission bands (centered at 400, 538, and 700 nm) arising from the defect-related deep-level centers of oBN, which are mixed to form the white light emission. The hardness and elastic modulus of oBN films are 11.5 and 94 GPa, respectively, examined by nanoindentation measurements.     

InGaN growth with various InN mole fractions on m-plane ZnO substrate by metalorganic vapor phase epitaxy

We have demonstrated In_xGa_1−xN epitaxial growth with InN mole fractions of x=0.07 to 0.17 on an m-plane ZnO substrate by metalorganic vapor phase epitaxy for the first time. The crystalline quality of the epilayers was found to be much higher than that of epilayers grown on a GaN template on an m-plane SiC substrate.