Hot-wire Chemical Vapor Deposition Research Articles

Crystalline tantalum carbide and ditungsten carbide formation via hot wire chemical vapor deposition using the precursor of 1-methylsilacyclobutane

An efficient method for the preparation of crystalline tantalum carbide (TaC) was reported using a new methyl-substituted precursor, 1-methylsilacyclobutane (MSCB), in the hot-wire chemical vapor deposition process with a tantalum filament. The ability of MSCB to produce methyl radicals on heated Ta filament allows for the formation of crystalline TaC (cubic Fm-3m) in a wide temperature range of 1600–2200°C. This corroborates the key role of the radical species formed via the precursor's decomposition on the hot metal wire in the type of metal alloys formed on the wire surface. The growth rate of TaC achieved at 0.12μm/min and 0.35μm/min, respectively, for 1600°C and 2200°C is much higher than the value obtained from using CH4/H2 mixtures as the C-containing precursor. Crystalline ditungsten carbide (W2C) was formed by treating the tungsten filament with MSCB at the relatively high temperatures of 2200–2400°C, with a deposition rate of 2.1μm/min at 2400°C. The method reported in this work is a useful method to produce crystalline TaC or W2C without the use of the corrosive metal halide precursors.

Synthesis of orthorhombic-molybdenum trioxide (α-MoO3) thin films by hot wire-CVD and investigations of its humidity sensing properties

In present work, we report synthesis of orthorhombic-molybdenum trioxide (α-MoO 3 ) thin films using home-build hot wire-CVD (HW-CVD) method simply by heating the Mo filament in a controlled O 2 atmosphere. The formation of α-MoO 3 was confirmed by low angle-XRD and Raman spectroscopy. Low angle-XRD analysis revealed that α-MoO 3 crystallites have orientations along (110), (101) and (111) directions while Raman spectroscopy analysis shows two prominent vibrational modes ~819 and ~994 cm -1 associated with Mo 2 -O and Mo=O respectively. SEM and TEM analysis show the formation of nano-sheets like morphology of α-MoO 3 thin films. The SAED pattern shows highly crystalline nature of α-MoO 3 . The humidity-sensing properties were investigated at room temperature by fabricating the two probe device. The humidity sensing results showed n-type behavior of α-MoO 3 . The maximum humidity sensitivity of ~6957% along with response time of ~66 s and recovery time of ~5 s were observed for α-MoO 3 thin film humidity sensor device. Our results have opened up a new avenue to grow α-MoO 3 for humidity sensor applications.

Atomic hydrogen induced defect kinetics in amorphous silicon

Near-infrared evanescent-wave cavity ring-down spectroscopy (CRDS) has been applied to study the defect evolution in an amorphous silicon (a-Si:H) thin film subjected to a directed beam of atomic H with a flux of (0.4–2) × 1014 cm−2 s−1. To this end, a 42 ± 2 nm a-Si:H film was grown on the total internal reflection surface of a folded miniature optical resonator by hot-wire chemical vapor deposition. A fully reversible defect creation process is observed, with a nonlinear dependence on H flux, with a time resolution of 33 ms and a relative sensitivity of 10−7. Using polarizing optics, the CRDS signal was split into s- and p-polarized components, which, combined with E-field calculations, provides depth sensitivity. Extensive kinetic modeling of the observed process is used to determine rate constants for the hydrogen–material interactions and defect formation in a-Si:H, as well as revealing a high diffusion coefficient for atomic H on the order of 10−11 cm2 s−1. A novel reaction pathway is proposed, whereby H inserted into weak Si–Si bonds recombines with mobile H, resulting in a limited penetration depth for atomic H from the gas-phase on the order of 10–15 nm.

Growth of silicon nitride by nitridation of amorphous silicon at low temperature in hot-wire CVD

Silicon nitride (SiNx) layers of varying thickness were fabricated in a a-Si/SiNx/a-Si heterostructure via nitridation of the bottom a-Si layer, using ammonia (NH3) gas, in the hot-wire chemical vapor deposition (HWCVD) chamber at 250°C. The thickness of the SiNx in the film was varied by changing the nitridation time (tN) in range of 1–30min. Raman analysis showed that the a-Si and SiNx layers formed in the films were amorphous. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results indicated Si3N4 was formed in the nitrided a-Si. Presence of strong SiH2 (2086cm−1) peaks in the FTIR spectrum suggested the hydrogen in a-Si effused at 250°C. XPS Si2p spectra and high resolution transmission electron microscopy images showed thin SiNx layer of thickness 1.8–7.6nm was formed. Formation of SiNx thicker than 7.6nm even at a low temperature (250°C) is suggestive of the H effusion promoting both the reaction and the diffusion of the nitridants in a-Si. The kinetics of the growth of SiNx during nitridation of the a-Si layer closely matches the growth process described by diffusion model, wherein the reaction of nitridants and a-Si is controlled by diffusion in the SiNx layer formed.

A new tubular hot-wire CVD for diamond coating

A new tubular hot-wire chemical vapor deposition (HWCVD) system using a tubular quartz vacuum chamber has been fabricated. The filaments in this system can heat the substrate and act as a gas activator and thermally activator for gas species at the same time. The nano- and microcrystalline diamond coatings on the surface of steel AISI 316 substrates have been grown. To assess the results, SEM and FESEM images and Raman spectroscopy investigations have been applied. The results reveal that micro- and nanocrystalline diamond structures have been formed in the coatings, but the disordered diamond and some non-diamond phases, such as graphitic carbons, are also present in the coating layers. The analytical measurements show the growth of diamond films with well-faceted crystals in (111) direction. However, intrinsic stress, secondary nucleation, and poor adhesion are the main issues of future research for this new designed HWCVD.

Novel chemical cleaning of textured crystalline silicon for realizing surface recombination velocity

In this study, the development of a novel chemical cleaning method suitable for textured surfaces of crystalline silicon (c-Si) used for solar cells is demonstrated. To remove contaminants from such textured structures, chemicals have to penetrate into their complicated fine structures. Thus, the viscosity, reaction activity, and surface tension of the chemicals are adjusted by increasing the reaction temperature or introducing a surfactant. Actually, the use of concentrated (conc.) sulfuric acid (H2SO4) of 140 °C and the introduction of methanol (CH3OH) to other chemicals contribute to the improvement of the cleaning ability in textured structures. The present cleaning method in conjunction with plasma-damage-less catalytic chemical vapor deposition (Cat-CVD), often called hot-wire CVD passivation with silicon–nitride (SiNx)/amorphous silicon (a-Si) stacked layers, also contributes to the decrease in the surface recombination velocity (SRV) of c-Si. The maximum estimated SRV (SRVmax), evaluated under the assumed absence of recombination in bulk c-Si, is less than 1.1 cm/s for textured surfaces, and the real SRV, evaluated by changing the c-Si substrate thickness, is less than 0.2 cm/s.

Textured and micropillar silicon heterojunction solar cells with hot-wire deposited passivation layers

The passivating quality of hydrogenated amorphous silicon thin films is essential for achieving high open-circuit voltages in silicon heterojunction solar cells. This work reports our progress towards the use of hot wire chemical vapor deposition for fabricating these passivation layers. We have achieved 19.4% energy conversion efficiency for a solar cell with a conventional alkali-etched pyramidal texture. Next, we tested radial heterojunction devices that consist of up to 20-μm high micropillar arrays. Proper passivation of these devices is challenging, since the structures feature an enlarged surface area and vertical walls. We have made use of the ability of hot wire chemical vapor deposition to deposit conformal thin films at high deposition rate. The micropillar solar cells show only limited reduction of open-circuit voltage in comparison to a flat reference solar cell and have an improved overall performance. We created additional stacked multijunction silicon-based junctions on top of the micropillars to increase the open-circuit voltage of the solar cells for future applications in which a higher potential is required such as water splitting devices.

Interface states study of intrinsic amorphous silicon for crystalline silicon surface passivation in HIT solar cell**Project supported by the National Natural Science Foundation of China (Grant Nos. 51361022 and 61574072) and the Postdoctoral Science Foundation of Jiangxi Province, China (Grant No. 2015KY12).

Intrinsic hydrogenated amorphous silicon (a-Si:H) film is deposited on n-type crystalline silicon (c-Si) wafer by hot-wire chemical vapor deposition (HWCVD) to analyze the amorphous/crystalline heterointerface passivation properties. The minority carrier lifetime of symmetric heterostructure is measured by using Sinton Consulting WCT-120 lifetime tester system, and a simple method of determining the interface state density from lifetime measurement is proposed. The interface state density measurement is also performed by using deep-level transient spectroscopy (DLTS) to prove the validity of the simple method. The microstructures and hydrogen bonding configurations of a-Si:H films with different hydrogen dilutions are investigated by using spectroscopic ellipsometry (SE) and Fourier transform infrared spectroscopy (FTIR) respectively. Lower values of interface state density are obtained by using a-Si:H film with more uniform, compact microstructures and fewer bulk defects on crystalline silicon deposited by HWCVD.

Controlling the c-Si/a-Si:H interface in silicon heterojunction solar cells fabricated by HWCVD

This paper deals with the engineering of the hetero-interface between intrinsic amorphous silicon (i-a-Si:H) layer and the n-type crystalline silicon (c-Si) wafer during the fabrication of the Silicon Heterojunction (SHJ) solar cell by the Hot Wire Chemical Vapor Deposition technique. It is known that this interface and the associated surface passivation of the c-Si is key to obtaining high efficiency heterojunction solar cells. The monitoring of this interface was carried out using high-resolution transmission electron microscopy (HRTEM). The HRTEM data of the c-Si/a-Si:H interface reveals a drastic dependence on the filament temperature (Tf) used during the deposition of the i-a-Si:H layer. Detailed analysis of the solar cell characteristics indicates that the cells where one has an abrupt crystalline/amorphous interface shows higher conversion efficiency compared to those where we have a rough and a defective interface or where there are indications of local epitaxy in the a-Si:H layer. The second parameter which was engineered is the bulk defect density of the intrinsic a-Si:H layer. Though the thickness of i-a-Si:H layer in case of SHJ solar cells is only around 5nm and serves the purpose of passivating the dangling bonds on the c-Si wafer, the bulk defect density of this layer cannot be ignored. We have achieved a-Si:H films with acceptable bulk defect density without dilution of the silane gas with hydrogen. The bulk defect density of the i-a-Si:H layer has been determined by the constant photocurrent method (CPM) and is correlated to the performance of SHJ solar cells. A direct consequence of these control parameters was observed in the improvement of the external quantum efficiency near 600nm wavelength region.

Wide gap microcrystalline silicon carbide emitter for amorphous silicon oxide passivated heterojunction solar cells

Wide gap n-type microcrystalline silicon carbide [µc-SiC:H(n)] is highly suitable as window layer material for silicon heterojunction (SHJ) solar cells due to its high optical transparency combined with high electrical conductivity. However, the hot wire chemical vapor deposition (HWCVD) of highly crystalline µc-SiC:H(n) requires a high hydrogen radical density in the gas phase that gives rise to strong deterioration of the intrinsic amorphous silicon oxide [a-SiOx:H(i)] surface passivation. Introducing an n-type microcrystalline silicon oxide [µc-SiOx:H(n)] protection layer between the µc-SiC:H(n) and the a-SiOx:H(i) prevents the deterioration of the passivation by providing an etch resistance and by blocking the diffusion of hydrogen radicals. We fabricated solar cells with µc-SiC:H(n)/µc-SiOx:H(n)/a-SiOx:H(i) stack for the front side and varied the µc-SiOx:H(n) material properties by changing the microstructure of the µc-SiOx:H(n) to evaluate the potential of such stack implemented in SHJ solar cells and to identify the limiting parameters of the protection layer in the device. With this approach we achieved a maximum open circuit voltage of 677 mV and a maximum energy conversion efficiency of 18.9% for a planar solar cell.

Density of states measurements in a-Si:H and a-Si:H/nc-Si:H multilayer structures prepared by hot wire chemical vapor deposition

In this study, the density of states (DOS) were determined in single layer hydrogenated amorphous silicon (a-Si: H) and a-Si:H/nc-Si:H (hydrogenated nanocrystalline silicon) multilayered structures, prepared by hot wire chemical vapor deposition, by measuring space-charge limited currents in sandwich geometry using ITO(Indium Tin Oxide)/a-Si:H/Ag and ITO{a‐Si:H/nc‐Si:H}n/Ag structures. Comparisons showed that the DOS of a-Si:H layer in a-Si:H/nc-Si:H multilayer structures were higher than that of single layer a-Si:H film for the same shift in ΔEfn. The higher DOS in multilayer structures may be due to the existence of interface states between the a-Si:H and nc-Si:H layers, which could also change the local electric field at the junction under the no-bias condition. These interface states were also shown to be responsible for the observed persistent photoconductivity in these a-Si:H/nc-Si:H multilayer structures.

A-SiOx:H passivation layers for Cz-Si wafer deposited by hot wire chemical vapor deposition

In order to get the high photoelectric conversion efficiency a-Si:H/c-Si solar cells, high quality intrinsic hydrogenated passivation layer between the a-Si:H emitter layer and the c-Si wafer is necessary. In this work, hot wire chemical vapor deposition (HWCVD) is used to deposite intrinsic oxygen-doped hydrogenated amorphous silicon (a-SiOx:H) and hydrogenated amorphous silicon (a-Si:H) films as the intrinsic passivation layer for a-Si:H/c-Si solar cells. The passivation effect of the films on the c-Si surface is shown by the effective lifetime of the samples that bifacial covered by the films with same deposition parameters, tested by QSSPC method. The imaginary part of dielectric constant (ε2) and bonds structure of the layers are analyzed by Spectroscopic Ellipsometry(SE) and Fourier Transfom Infrared Spectroscopy(FTIR). It is concluded that: (1) HWCVD method can be used to make a-SiOx:H films as the passivation layer for a-Si:H/c-Si cells and the oxidation of the filament can be overcome by optimizing the deposition parameters. In our experiments, the lowest surface recombination velocity of the c-Si wafer is 3.0cm/s after a-SiOx:H films passivation. (2) Oxygen-doping in the amorphous silicon layers can increase H content and the band-gap of films, similar as the phenomenon of the films deposited by PECVD.

The influence of precursor gas pressure on structure and properties of fluoropolymer coatings by hot wire CVD

In this paper we investigated the deposition process of the fluoropolymer coatings by the method of Hot Wire Chemical Vapor Deposition. The dependence of precursor gas pressure on deposition rate, structure and properties of the formed film was investigated. The study has shown that the increasing of precursor gas pressure leads to change of the film structure from porous to continuous with globules and further to solid that have different wettability. Depending on the mode of deposition was obtained the fluoropolymer coating with different structure: the wetting angle changed from 60 to 170°. A mechanism of the formation of fluoropolymer coating was discussed.

Role of free-radical chain reactions and silylene chemistry in using methyl-substituted silane molecules in hot-wire chemical vapor deposition

The reaction chemistry of four methyl-substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS), and tetramethylsilane (TMS), has been studied in the hot-wire (catalytic) chemical vapor deposition (HWCVD) process, both on heated tungsten or tantalum filaments and in the gas phase. The four molecules dissociate catalytically on hot W or Ta surfaces to form ·CH3, ·H and H2. In a HWCVD reactor setup where the pressure is relatively high at 8–533Pa, two reaction mechanisms involving silylene and free radical intermediate, respectively, operate with the four precursor gases. For MMS, its reaction chemistry is characterized by the exclusive involvement of methylsilylene intermediate. Reaction products when using DMS as a precursor gas come from silylene chemistry and free-radical chain reactions. Filament temperature and pressure affect strongly the competition between the two mechanisms. Specifically, silylene chemistry dominates at low temperatures of 1200–1300°C and a low pressure of 16Pa, and free-radical chain reactions take over at high temperatures and pressures. With the increasing methyl substitutions in TriMS and TMS, free-radical chain reactions become predominant. A clear transition in the operating reaction mechanism from silylene chemistry to free-radical chain reactions has been shown when the number of SiCH3 bonds increases in the methyl-substituted silane molecules.

Role of oxygen and nitrogen in n-type microcrystalline silicon carbide grown by hot wire chemical vapor deposition

N-type microcrystalline silicon carbide (μc-SiC:H(n)) deposited by hot wire chemical vapor deposition provides advantageous opto-electronic properties for window layer material in silicon-based thin-film solar cells and silicon heterojunction solar cells. So far, it is known that the dark conductivity (σd) increases with the increase in the crystallinity of μc-SiC:H(n)films. However, due to the fact that no active doping source is used, the mechanism of electrical transport in these films is still under debate. It is suggested that unintentional doping by atmospheric oxygen (O) or nitrogen (N) contamination plays an important role in the electrical transport. To investigate the impact of O and N, we incorporated O and N in μc-SiC:H(n) films and compared the influence on the microstructural, electronic, and optical properties. We discovered that, in addition to increasing the crystallinity, it is also possible to increase the σd by several orders of magnitude by increasing the O-concentration or the N-concentration in the films. Combining a high concentration of O and N, along with a high crystallinity in the film, we optimized the σd to a maximum of 5 S/cm.

Junction Formation With HWCVD and TCAD Model of an Epitaxial Back-Contact Solar Cell

In this paper, we present morphological and electrical characteristics of a junction formed of Si p-type films deposited on an n-type silicon wafer using a hot wire chemical vapor deposition (HWCVD) tool. We describe the fabrication process and study the influence of diborane flow and postprocess annealing in improving junction characteristics. Our morphological studies undertaken using atomic force microscopy show that the initial deposition suffered from voids rather than being a uniform film; however, this improves significantly under our annealing treatment. The improvement in morphology was observed in the electrical characteristics, with estimated $V_{\text{oc}}$ doubling and rectification of the junction improving by several orders of magnitude. Fitting of the current–voltage curves to a two-diode model showed that increasing the diborane flow in the process helps reduce the saturation current and ideality factors, while increasing the shunt resistance. Electrochemical capacitance–voltage (ECV) and quasi-steady-state photoconductance measurements are used to characterize the deposited films further. A solar cell device with a silicon epitaxy emitter is modeled using industry-standard 3-D modeling tools and input parameters from experimental data, and the impact of defects is studied. A potential efficiency approaching 25% is shown to be feasible for an optimized device.

Non-classical crystallization of silicon thin films during hot wire chemical vapor deposition

The deposition behavior of silicon films by hot wire chemical vapor deposition (HWCVD) was approached by non-classical crystallization, where the building block of deposition is a nanoparticle generated in the gas phase of the reactor. The puzzling phenomenon of the formation of an amorphous incubation layer on glass could be explained by the liquid-like property of small charged nanoparticles (CNPs), which are generated in the initial stage of the HWCVD process. Using the liquid-like property of small CNPs, homo-epitaxial growth as thick as ~150nm could be successfully grown on a silicon wafer at 600°C under the processing condition where CNPs as small as possible could be supplied steadily by a cyclic process which periodically resets the process. The size of CNPs turned out to be an important parameter in the microstructure evolution of thin films.

Deposition of plasmon gold–fluoropolymer nanocomposites

Degradation-resistant two-dimensional metal–fluoropolymer composites consisting of gold nanoparticles coated with a thin fluoropolymer film were deposited on a substrate by hot wire chemical vapour deposition (HWCVD) and ion sputtering. The morphology and optical properties of the obtained coatings were determined. The thickness of the thin fluoropolymer film was found to influence the position of the surface plasmon resonance peak. Numerical calculations of the optical properties of the deposited materials were performed using Mie theory and the finite-difference time-domain (FDTD) method. The calculation results are consistent with the experimental data. The study shows that the position of the resonance peak can be controlled by changing the surface concentration of particles and the thickness of the fluoropolymer coating. The protective coating was found to prevent the plasmonic properties of the nanoparticles from changing for several months.

Electrochemical Performance of Carbon-Coated Cauliflower-like WO3 for Li-Ion Batteries

Metal oxides have been considered and investigated as alternative anodes for use in lithium ion batteries. The research focus is on two different mechanism groups: intercalation–deintercalation reaction and conversion (redox) reaction. The intercalation–deintercalation mechanism is that transition metal oxides and other compounds with a multidimensional layer structure can reversibly intercalate lithium ions into the lattice without destroying their crystal structure. While conversion mechanism usually produces metallic nanoparticles embedded in an insulating Li2O matrix and the shows generally much higher theoretical capacity than that of already-commercialized graphite. Among the metal oxides, tungsten oxides (WO2, WO3, and WO3-x H2O) as one of the important metal oxides with respect to physicochemical properties, have been extensively investigated for such applications as gas sensors, photocatalysis, electrochromic devices, electronic devices, dye-sensitized solar cells, supercapacitors, and lithium ion batteries. Interestingly, the tungsten oxides have better electronic conductivity (10–10-6 S cm-1) than some of the other oxides; in addition, they also rely on designing a nanostructured particle with a small radius and right crystalline phase for fast lithium-ion insertion and superior electrochemical performance. From this point of view, a wide variety of approaches have been pursued over the years for the synthesis of tungsten oxides, such as nanoparticles, nanowires, nanorods, and nanoflowers using various approaches which include a hydrothermal reaction, combustion process, sol–gel process, solution-phase reaction, template directed method, mechanochemical activation, and hot-wire chemical vapor deposition (HWCVD) method. However, most of the previous approaches led to the formation of monoclinic W18O49, hexagonal WO3, and WO3-xH2O nanostructures. There have been very few studies on nanostructured monoclinic WO3 (m-WO3). In this study, we will present a facile hydrothermal process that obtains cauliflower-like carbon-coated WO3 oxide with a high rate-capability and good cycling performance for lithium-ion batteries. The motivation in this study is to explore the possibility of using m-WO3 as an anode material for lithium-ion batteries and to elucidate its reaction mechanism with lithium during the charge–discharge process.

A 3-D ZnO-Nanowire Smart Photo Sensor Prepared With Through Silicon via Technology

The authors report the fabrication of a 3-D ZnO-nanowire/MOSFET smart photo sensor using through silicon via technology. It was found that the MOSFET, prepared by hot-wire chemical vapor deposition, exhibits standard saturation and pinch-off characteristics. The dynamic response of the 3-D ZnO-nanowire/MOSFET smart photo sensor was stable and reproducible with an ON–OFF current contrast ratio larger than one order of magnitude. As we turned OFF the ultraviolet illumination, it was found that the decay time was smaller than 1 s for the 3-D ZnO-nanowire/MOSFET smart photo sensor, which was significantly shorter than that observed from the conventional ZnO nanowire photo sensor.