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.     

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.     

Growth and characterization of CdSe single crystals by modified vertical vapor phase method

Good quality, large single crystals of CdSe were grown by the modified growth method (i.e., vertical unseeded vapor phase growth with multi-step purification of the starting material in the same quartz ampoule without any manual transfer between the steps). Lower temperature gradients (8–9°C/cm) at the growth interface were used for the crystal growth. As-grown CdSe crystals was characterized by X-ray diffraction, scanning electron microscopy, energy dispersive analyzer of X-rays, high-resistance instrument measurement, and etch-pit observation. It is found that there are two cleavage faces of (1 0 0) and (1 1 0) orientations on the crystal, the resistivity is about 10⁸ Ω cm, and the density of etch pits is about 10^3–4/cm². The crystal was cut into wafers and was fabricated into detectors. The detectors were tested using an ²⁴¹Am radiation source. γ-ray spectra at 59.5 keV were obtained. The results demonstrated that the quality of the as-grown crystals was good. The crystals were useful for fabrication of room-temperature-operating nuclear radiation detectors. Therefore, the modified growth technique is a promising, convenient, new method for the growth of high-quality CdSe single crystals.     

Homogeneous SiGe crystals grown by using the traveling liquidus-zone method

It is indispensable to estimate a diffusion coefficient in a solution zone in order to grow a homogeneous crystal by using the traveling liquidus-zone (TLZ) method. To estimate the diffusion coefficient of Ge in the SiGe solution zone, result of a two-dimensional numerical simulation is compared with an experimental result. From the comparison, the diffusion coefficient is estimated to be 9.5×10⁻⁵ cm²/s. By using this coefficient, a sample translation rate for obtaining a homogeneous SiGe crystal is determined. By translating samples with appropriate rates, homogeneous Si_0.5Ge_0.5 crystals are successfully grown. The typical Ge composition is 0.496±0.006 for more than 13 mm long. The experimental result shows the homogeneity of ±1.2% in the mole fraction. This deviation corresponds to the variation of less than ±0.03% in the lattice constant. Since this variation is negligibly small, the homogeneity is excellent. Thus it is found that the TLZ method is the universal growth technique, which is applicable to the crystal growth of not only the III–V compounds but also the IV–IV compounds.     

Catalytic growth of In2O3 nanobelts by vapor transport

Indium oxide (In₂O₃) nanobelts have been fabricated by thermal evaporation of metallic indium powders with the assistance of Au catalysts. The as-synthesized nanobelts are single-crystalline In₂O₃ with cubic structure, and usually tens of nanometers in thickness, tens to hundreds of nanometers in width, and several hundreds of micrometers in length. The room temperature photoluminescence spectrum of In₂O₃ nanobelts features a broad emission band at 620 nm, which could be attributed to oxygen deficiencies in the as-synthesized belts. The formation of In₂O₃ nanobelts follows a catalyst-assistant vapor—liquid–-solid growth mechanism, which enables the controlled growth of individual belts on predetermined sites.     

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.     

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.     

Oriented polycrystalline α-HgI2 thick films grown by physical vapor deposition

This work investigates the growth of polycrystalline α-HgI₂ thick films from physical vapor deposition. By varying the growth conditions, the as-deposited thick films are characterized by scanning electron microscopy, X-ray diffraction, current–voltage and photoconductivity measurements. The growth mechanism and its effects to the properties of these polycrystalline α-HgI₂ thick films are then discussed. Finally, the best deposition conditions for polycrystalline α-HgI₂ thick films compactly formed by separated columnar monocrystallines with uniform orientation along c-direction and with good crystallinity are reported.     

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.     

Extended defects in bulk GaN and III-nitrides grown on this substrate

A short review of the structural perfection of high-pressure grown bulk crystals is given. As-grown undoped and Mg-doped crystals are described. The dependence of defect arrangement and quality of the surface on growth polarity is described. A high perfection of homoepitaxial layers grown on these substrates is shown. However, growth of thick layers by HVPE may lead to the formation of differently arranged dislocations and the formation of low angle grain boundaries associated with cracks. It is shown that the introduction of dopant or growth of mismatched layers on undoped high-pressure substrates may lead to the formation of additional defects.     

Low-temperature Si epitaxy on large-grained polycrystalline seed layers by electron–cyclotron resonance chemical vapor deposition

The epitaxial thickening of polycrystalline Si films on glass substrates is of great interest for the realization of crystalline Si thin film solar cells and other large-area thin film devices. In this paper we report on the epitaxial growth of Si at temperatures below on polycrystalline seed layers using electron–cyclotron resonance chemical vapor deposition. The Si seed layers were prepared by aluminum-induced crystallization. The quality of the ECRCVD-grown films strongly depends on the orientation of the underlying seed layer grains. Due to a mainly favorable orientation of the seed layers more than 73% of the substrate area were epitaxially thickened. It turned out that a (1 0 0) preferential orientation is favorable for epitaxial thickening. This, however, is not the only requirement for successful low-temperature epitaxial growth of Si.     

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.     

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.     

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.     

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.     

Study on the properties of CuInSe2 ingots grown from the melt using stoichiometric and non-stoichiometric charges

CuInSe₂ (CIS) ingots have been prepared by direct reaction of stoichiometric and non-stoichiometric proportions of high-purity Cu, In and Se. Two approaches, namely the one-ampoule process (quartz crucible) and two-ampoule process (graphite crucible) were investigated to grow the crystals, using starting charges with excess copper, and (nearly stoichiometric and with excess indium), respectively. The effect of deviation from stoichiometry in the charge on the physical properties of the resulting polycrystals is presented. Compositional analysis of the best part of the ingots with starting metals ratio (Cu/In) greater than or equal to 1 showed that the matrix preserved the original character of the charge and evidenced that the CIS chalcopyrite structure, -CIS, tolerates well a large In excess. In contrast, the composition of the crystal prepared with a 10% Cu excess was nearly-stoichiometric, with chemical images revealing the formation of heterogeneous phases besides -CIS. The inclusions precipitation was found to increase toward the ingot base. Interestingly, powder X-ray diffraction measurements revealed the presence of secondary phases rather in all the samples. The corresponding diffraction peaks were however few and very weak, with intensities of less than 3% the maximum value recorded for the CIS (1 1 2) plane.     

Properties of Ge-doped,high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy

We investigated the properties of Ge-doped, high-quality bulk GaN crystals with Ge concentrations up to 2.4×10¹⁹ cm⁻³. The Ge-doped crystals were fabricated by hydride vapor phase epitaxy with GeCl₄ as the dopant source. Cathodoluminescence imaging revealed no increase in the dislocation density at even the highest Ge concentration, with values as low as 3.4×10⁶ cm⁻². The carrier concentration, as determined by Hall measurement, was almost identical to the combined concentration of Ge and unintentionally incorporated Si. The electron mobilities were 260 and 146 cm² V⁻¹ s⁻¹ for n=3.3×10¹⁸ and 3.35×10¹⁹ cm⁻³, respectively; these values are markedly larger than those reported in the past for Ge-doped GaN thin films. The optical absorption coefficient was quite small below the band gap energy; it slightly increased with increase in Ge concentration. Thermal conductivity, estimated by the laser-flash method, was virtually independent of Ge concentration, maintaining an excellent value around 2.0 W cm⁻¹ K⁻¹. Thermal expansion coefficients along the a- and m-axes were approximately constant at 5.0×10⁻⁶ K⁻¹ in the measured doping concentration range.     

The photoluminescence of ZnO thin films grown on Si (1 0 0) substrate by plasma-enhanced chemical vapor deposition

High-quality ZnO thin films have been grown on a Si(1 0 0) substrate by plasma-enhanced chemical vapor deposition (PECVD) using a zinc organic source (Zn(C₂H₅)₂) and carbon dioxide (CO₂) gas mixtures at a temperature of 180°C. A strong free exciton emission with a weak defect-band emission in the visible region is observed. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption peak in the absorption spectra, are closely related to the gas flow rate ratio of Zn(C₂H₅)₂ to CO₂. Full-widths at half-maximum of the free exciton emission as narrow as 93.4 meV have been achieved. Based on the temperature dependence of the PL spectra from 83 to 383 K, the exciton binding energy and the transition energy of free excitons at 0 K were estimated to be 59.4 meV and 3.36 eV, respectively.     

Correlations between photoluminescence and Hall mobility in GaN/sapphire grown by metalorganic chemical vapor deposition

We have investigated photoluminescence (PL) and electron Hall mobility for unintentionally doped GaN epitaxial layers grown by low-pressure metalorganic chemical vapor deposition on c-plane Al₂O₃ substrates. Four GaN films having identical dislocation density but remarkably different electron Hall mobility were exploited. At low temperature (12 K), a PL line associated with a bound exciton was observed and strong correlations were found between the Hall mobility and the PL intensity of the exciton transition. That is, relative PL intensity of the bound exciton to a donor-bound exciton monotonously increased with decreasing the electron mobility of the GaN films. This correlation was interpreted in terms of electrical compensation. Efforts to find the chemical origin of the PL line led to the conclusion that the BE line originated neither from threading dislocations nor from extrinsic point defects. Intrinsic acceptors such as Ga vacancy and GaN anti-site were suspected as plausible origin.     

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.