Preparation of single-helix carbon microcoils by catalytic CVD process

Three-dimensional (3D) single-helix spring-like carbon microcoils (SH-CMCs) were obtained by the catalytic pyrolysis of acetylene at 800-820 °C over the Fe-Ni alloy catalysts; their growth morphologies and microstructure were examined. The diameter of carbon fiber, from which the carbon nanocoils was formed, was about 0.5 μm, the coil diameter was about 1-2 μm, and the coil pitch was about the same with the coil diameter. The SH-CMCs were generally grown by a double-directional growth mode.     

Effect of hydrogen dilution in preparation of carbon nanowall by hot-wire CVD

Carbon nanowall films prepared on the stainless steel substrates by hot-wire chemical vapor deposition using CH₄ with different hydrogen dilution ratios and structure variation in the CNWs against hydrogen dilution have been studied. In the scanning electron microscope images, the wall height and width in the samples prepared with the hydrogen dilution ratio, H₂/(CH₄ + H₂), between 10% and 25% were larger than that prepared without hydrogen dilution. In the Raman spectra for the samples prepared with the H₂/(CH₄ + H₂) below 25%, the intensity ratio of the G peak to the D peak, I_G/I_D, increased with increasing the H₂/(CH₄ + H₂). In the samples prepared with the H₂/(CH₄ + H₂) over 25%, the wall size and the I_G/I_D decreased.     

Formation of isolated carbon nanofibers with hot-wire CVD using nanosphere lithography as catalyst patterning technique

Recently the site-density control of carbon nanotubes (CNTs) has attracted much attention as this has become critical for its many applications. To obtain an ordered array of catalyst nanoparticles with good monodispersity nanosphere lithography (NSL) is used. These nanoparticles are tested as catalyst sites in hot-wire chemical vapor deposition (HWCVD) of carbon nanostructures. Aside from using NSL also nickel (Ni) nano-islands are made by thermal annealing of a thin Ni film and tested as catalyst sites. Multiwall CNTs, isolated carbon nanofibres, and other nanostructures have been deposited using HWCVD. Tungsten filaments held at ~ 2000 °C are used to decompose a mixture of ammonia, methane and hydrogen. The structures have been characterized with Scanning Electron Microscopy, High Resolution Transmission Electron Microscopy, Raman spectroscopy and Rutherford Backscattering Spectroscopy.     

Synthesis of carbon nanowalls by hot-wire chemical vapor deposition

Carbon nanowall (CNW) is a carbon nano-material which has a wall structure that stood on substrates. CNWs can be synthesized by hot-wire chemical vapor deposition (HWCVD) using methane without hydrogen dilution. The synthesis of CNWs by HWCVD is discussed along with reviewing the experimental results. The growth of CNWs is affected by hydrogen dilution ratio and substrate surface temperature. Based on these results, it is suggested that hydrogen radical density and substrate surface temperature are the important parameters for the synthesis of CNWs. The growth process of CNWs is also discussed.     

Synthesis of nanometal oxides and nanometals using hot-wire and thermal CVD

We report the synthesis of nano-oxides of molybdenum, tungsten, and zinc. Molybdenum oxide (MoO₃) and tungsten oxide (WO_x) were produced by hot-wire CVD with molybdenum and tungsten filaments, respectively while zinc oxide (ZnO) was produced by thermal CVD. When high purity molybdenum wire was oxidized at ambient system atmosphere, nanorods and nanostraws of MoO₃ with length ranging from ∼ 20-80 nm and diameters ranging from ∼ 5-15 nm were produced. Also, the oxidation of the tungsten filament led to the deposition of tungsten oxide nanorods (10-25 nm diameter and 75-90 nm long) and nanospheres with diameters of ∼ 60 nm. Each oxide was reduced to its metallic form by annealing in a hydrogen environment to produce metallic nanoparticles. Nanorods and nanoribbons of ZnO with diameters ranging from 20-65 nm and lengths up to 2 μm were also produced.     

Modeling and measurement of film deposition in a one-dimensional hot-wire CVD system

A novel hot-wire chemical vapor deposition (HW-CVD) geometry was employed to study the deposition of Teflon-like films from hexafluoropropylene oxide (HFPO). In this configuration hot wires were replaced by thin ribbons, and under proper operating conditions the complex HW-CVD geometry is simplified to a one-dimensional system. The kinetics of both HFPO decomposition and Teflon deposition were measured as a function of operating conditions. A hybrid 2-D CFD/1-D stagnation flow model was used to interpret the results. At relatively low ribbon temperatures good agreement between model and experiment was observed. Deviations observed at higher ribbon temperatures were attributed to gas-phase polymerization of CF₂ moieties, and participation of these oligomers in the deposition process.     

Microcrystalline silicon for solar cells deposited at high rates by hot-wire CVD

Using two tungsten (W) filaments and a filament–substrate spacing of 3.2 cm, we have explored the deposition of microcrystalline silicon (μc-Si) solar cells, with the i-layer deposited at high deposition rates (R_d), by the hot-wire CVD (HWCVD) technique. These cells were deposited in the n–i–p configuration on textured stainless steel (SS) substrates, and all layers were deposited by HWCVD. Thin, highly crystalline seed layers were used to facilitate crystallite formation at the n–i interface. Companion devices were also fabricated on flat SS substrates, enabling structural measurements (by XRD) to be performed on i-layers used in actual device structures. Using a filament temperature of 1750 °C, device performance was explored as a function of i-layer deposition conditions, including variations in i-layer substrate temperature (T_sub) using constant H₂ dilution, and also variations in H₂ dilution during i-layer deposition. The intent of the latter is to affect crystallinity at the top surface of the i-layer (i–p interface). We report device performance resulting from these studies, with all i-layers deposited at R_d>5 Å/s, and correlate them with i-layer structural studies. The highest device efficiency reported is 6.57%, which is a record efficiency for an all-hot-wire solar cell.     

Hydrogen passivation and junction formation on APIVT-deposited thin-layer silicon by hot-wire CVD

The hot-wire chemical vapor deposition (HWCVD) technique was employed to deposit μc-Si emitters and a-SiN_x:H passivation/antireflection films, and to hydrogenate silicon thin layers grown by atmospheric-pressure iodine vapor transport (APIVT). Photovoltaic devices with HWCVD μc-Si emitters on APIVT epitaxial silicon exhibit greater than 8% efficiency, similar to those made with diffused junctions. On polycrystalline APIVT-Si layers, a HWCVD-deposited μc-Si emitter reduces open-circuit voltage loss caused by grain boundaries. Hot-wire hydrogenation improves Hall mobility by approximately 50%. HWCVD a-SiN_x:H films improve minority-carrier lifetime significantly after thermal annealing at temperatures up to 500 °C.     

Monte Carlo simulations on large-area deposition of amorphous silicon by hot-wire CVD

Scale-up of hot-wire CVD reactors for commercial production of a-Si:H based solar cells requires understanding of the large-area deposition process. Therefore, the process was simulated using the Direct Simulation Monte Carlo-method (G.A. Bird, Clarendon Press, Oxford (1994)), considering reactions at the filaments, in the gas phase and at the substrate, and in particular large-area deposition by modeling the gas shower and the filament grid, which were found to determine the uniformity and quality of the a-Si:H films (Thin Solid Films 395 (2001) 61; Solar Energy Mater. Solar Cells 73 (2002) 321). The distance between the filament grid and the substrate (d_fil–S) and the distance between the filaments (d_fil) were systematically varied, and the simulation results were compared to experimental results obtained in our large-area deposition system (Thin Solid Films 395 (2001) 61; Solar Energy Mater. Solar Cells 73 (2002) 321). The experimentally obtained optimum filament-to-substrate distance was supported by an optima in the simulated Si₂H₄-concentration. For other species, the existence was confirmed but a definite value for optimum d_fil–S could not be concluded. The simulations also confirmed the influence of the filament geometry on the uniformity as obtained in the experiments.     

Photo-carrier transport in microcrystalline silicon films prepared by hot-wire CVD

Electronic transport properties of hydrogenated microcrystalline silicon films prepared by hot-wire CVD have been discussed. Near room temperature, the photo-transport occurs by thermionic emission of electrons over potential barriers between grain boundaries. The potential barrier height, which dominates the carrier mobility, decreases with increasing illumination intensity. The mobility-lifetime product (10^− 7-10^− 8 cm² V^− 1) at 300 K obtained experimentally is comparable with those obtained in films prepared by plasma enhanced CVD.     

Preparation of SnO2 thin films at low temperatures with H2 gas by the hot-wire CVD method

H₂ additional effect for crystallization of SnO₂ films prepared by the hot-wire CVD method was investigated. The crystallization of SnO₂ films starts at 170 °C. The selectivity enhancement of the solar cell substrate will contribute to reduce the cost of silicon thin film solar cells. The atomic hydrogen assisted nano-crystallization exists for the depositions of SnO₂ films by the hot-wire CVD method. Furthermore, the addition of H₂ gas improved the electrical conductivity up to 5.3 × 10⁰ S/cm. However, these effects are limited in the deposition condition of a small amount of hydrogen. Addition of much higher hydrogen concentration starts an etching effect of oxygen atoms.     

Properties of hetero-structured SiCX films deposited by hot-wire CVD using SiH3CH3 as carbon source

The hetero-structured SiC_X films have been deposited by hot-wire CVD using SiH₃CH₃ as the carbon source gas. Although the carbon source gas ratio and filament temperature in the deposition using SiH₃CH₃ were smaller than those using C₂H₆, the carbon content in the sample deposited using SiH₃CH₃ was similar to that deposited using C₂H₆. The optical energy gap in the sample deposited using SiH₃CH₃ was larger than that deposited using C₂H₆. The sample deposited using SiH₃CH₃ under optimized condition showed a wide optical energy gap of 1.99 eV and a large dark conductivity of 15.1 S/cm. The p-type sample deposited using SiH₃CH₃ under the optimized condition has been used as a window layer material in p-i-n a-Si:H based solar cells.     

Deposition of silicon nitride thin films by hot-wire CVD at 100 °C and 250 °C

Silicon nitride thin films for use as passivation layers in solar cells and organic electronics or as gate dielectrics in thin-film transistors were deposited by the Hot-wire chemical vapor deposition technique at a high deposition rate (1-3 ?/s) and at low substrate temperature. Films were deposited using NH₃/SiH₄ flow rate ratios between 1 and 70 and substrate temperatures of 100 °C and 250 °C. For NH₃/SiH₄ ratios between 40 and 70, highly transparent (T ~ 90%), dense films (2.56-2.74 g/cm³) with good dielectric properties and refractive index between 1.93 and 2.08 were deposited on glass substrates. Etch rates in BHF of 2.7 ?/s and < 0.5 ?/s were obtained for films deposited at 100 °C and 250 °C, respectively. Films deposited at both substrate temperatures showed electrical conductivity ~ 10^− 14 Ω^− 1 cm^− 1 and breakdown fields > 10 MV cm^− 1.     

Mass spectrometric study of gas-phase chemistry in the hot-wire CVD processes of SiH4/NH3 mixtures

The gas-phase reaction products of SiH₄, NH₃ and their mixtures from a hot-wire CVD chamber were investigated using laser ionization time-of-flight mass spectrometry. Both vacuum ultraviolet laser single photon ionization and laser-induced electron impact ionization were used. The main products observed from a 50% NH₃/He sample were H₂ and N₂. The study of an NH₃/SiH₄ mixture (P_NH3:P_SiH4 = 100:1) has shown that the NH₃ dissociation on the filament was suppressed by the presence of SiH₄ in the system. Signals from Si(NH₂)₄ and Si(NH₂)₃ species were identified as products from the 100:1 NH₃/SiH₄ mixture. The spectrum for a 1:1 NH₃/SiH₄ mixture was dominated by mass peaks characteristic of SiH₄ chemistry in the reactor, i.e. H₂, Si₂H₆, and Si₃H₈, at low temperatures. The extent to which the decomposition of NH₃ is suppressed is enhanced with more SiH₄ molecules in the system.     

Improvement of the efficiency of triple junction n-i-p solar cells with hot-wire CVD proto- and microcrystalline silicon absorber layers

Hot-wire chemical vapour deposition (HWCVD) was applied for the deposition of intrinsic protocrystalline (proto-Si:H) and microcrystalline silicon (μc-Si:H) absorber layers in thin film solar cells. For a single junction μc-Si:H n-i-p cell on a Ag/ZnO textured back reflector (TBR) with a 2.0 μm i-layer, an 8.5% efficiency was obtained, which showed to be stable after 750 h of light-soaking. The short-circuit current density (J_sc) of this cell was 23.4 mA/cm², with a high open-circuit voltage (V_oc) and fill factor (FF) of 0.545 V and 0.67.Triple junction n-i-p cells were deposited using proto-Si:H, plasma-deposited proto-SiGe:H and μc-Si:H as top, middle and bottom cell absorber layers. With Ag/ZnO TBR's from our lab and United Solar Ovonic LLC, respective initial efficiencies of 10.45% (2.030 V, 7.8 mA/cm², 0.66) and 10.50% (2.113 V, 7.4 mA/cm², 0.67) were achieved.     

Influence of co-catalyst on growth of carbon nanotubes using alcohol catalytic CVD method

This paper describes the influence of a co-catalyst on growth of carbon nanotubes (CNTs) by alcohol catalytic chemical vapour deposition (ACCVD) method. Silicon wafers covered with thermal oxide or polycrystalline diamond thin film were used as substrates. Ni thin film supported with Al, Cu or Ti was used as a catalyst. The films were deposited by pulsed laser deposition technique. Comparison of the various types of the co-catalyst (Al, Cu, Ti) leads to the conclusion that Cu co-catalyst is suitable for producing very thin single wall carbon nanotubes (SWCNTs) and combination of Al and Ni provide a good condition to the catalytic growth of CNTs. In addition, we observed also the influence of the various diffusion barriers (thermal oxide and polycrystalline diamond) on growth of CNTs. Prepared samples were analysed by Raman spectroscopy (RS) and scanning electron microscopy (SEM).     

Quantitative analysis of tungsten, oxygen and carbon concentrations in the microcrystalline silicon films deposited by hot-wire CVD

In order for hot-wire chemical vapor deposition to compete with the conventional plasma-enhanced chemical vapor deposition technique for the deposition of microcrystalline silicon, a number of key scientific problems should be cleared up. Among these points, the concentration of tungsten (nature of the filament), as well as the concentration of oxygen and carbon (elements issued when vacuum is broken between two runs), should not exceed threshold values, beyond which electronic properties of the films could be degraded, as in the case of monocrystalline silicon. Quantitative chemical analysis of these elements has been carried out using the secondary ion mass spectrometry technique through depth profiles. It has been shown that for a high effective filament surface area (S_f=27 cm²), the W content increases steadily from 5×10¹⁴ to 2×10¹⁸ atoms cm⁻³ when the filament temperature T_f increases from 1500 to 1800 °C. For a fixed T_f, the W content increases with the effective surface area S_f. Thus, considering our reactor geometry, the W content does not exceed the detection limit (5×10¹⁴ atoms cm⁻³) when T_f and S_f are limited to 1600 °C and 4 cm², respectively. For O and C elements, under deposition conditions of high dilution of silane in hydrogen (96%), O and C concentrations approaching 10²⁰ atoms cm⁻³ have been obtained. The introduction of an inner vessel inside the reactor, the addition of a load-lock chamber and a decrease in substrate temperature to 300 °C have led to a drastic decrease in these contents down to 3×10¹⁸ atoms cm⁻³, compatible with the realization of 6% efficiency HWCVD μc-Si:H solar cells.     

Small-angle neutron scattering studies of hot-wire CVD a-Si:H

Several a-Si:H and a-Si:D films prepared by hot-wire chemical vapor deposition have been examined by small-angle neutron scattering (SANS) to search for H non-uniformity in this material. The SANS measurements were supplemented by small-angle X-ray scattering measurements. The differences in H/D detection sensitivity of these two techniques allow distinction of the scattering mechanisms. Two- or three-phase models are used to interpret the results quantitatively. Significant H non-uniformity, as well as a small fraction of microvoids, was found in the best-quality material. Samples grown with higher deposition rates or lower substrate temperatures have much larger void fractions. The size scale of the heterogeneity spans a range from 2 nm to more than 50 nm, with the largest features assigned to surface roughness.     

Preparation of n-type nanocrystalline 3C-SiC films by hot-wire CVD using N2 as doping gas

N-type nanocrystalline 3C-SiC films were prepared by hot-wire chemical vapor deposition from SiH₄/CH₄/H₂ and N₂ as a doping gas and the structural and electrical properties were investigated. The gas flow rates of SiH₄, CH₄ and H₂ were 1, 1 and 200 sccm, respectively. As the N₂ gas flow rate was increased from 0 to 10 sccm, the conductivity and the activation energy improved from 0.05 to 0.3 S/cm and from 45 to 28 meV, respectively. The Hall Effect measurement proved that the improvement of the electrical properties was caused by the increase in the carrier concentration. On the other hand, in the N₂ gas flow rate between 10 and 50 sccm, the conductivity and the activation energy remained unchanged. The crystallinity deteriorated with increasing N₂ gas flow rate. This gave rise to the unchanged electronic properties in spite of the increase in the intake of N atoms.     

Deposition of device quality silicon nitride with ultra high deposition rate (> 7 nm/s) using hot-wire CVD

The application of hot-wire (HW) CVD deposited silicon nitride (SiN_x) as passivating anti-reflection coating on multicrystalline silicon (mc-Si) solar cells is investigated. The highest efficiency reached is 15.7% for SiN_x layers with an N/Si ratio of 1.20 and a high mass density of 2.9 g/cm³. These cell efficiencies are comparable to the reference cells with optimized plasma enhanced (PE) CVD SiN_x even though a very high deposition rate of 3 nm/s is used. Layer characterization showed that the N/Si ratio in the layers determines the structure of the deposited films. And since the volume concentration of Si-atoms in the deposited films is found to be independent of the N/Si ratio the structure of the films is determined by the quantity of incorporated nitrogen. It is found that the process pressure greatly enhances the efficiency of the ammonia decomposition, presumably caused by the higher partial pressure of atomic hydrogen. With this knowledge we increased the deposition rate to a very high 7 nm/s for device quality SiN_x films, much faster than commercial deposition techniques offer [S. von Aichberger, Photon Int. 3 (2004) 40].