Hot-wire Chemical Vapor Deposition Technique Research Articles

Role of chamber pressure on crystallinity and composition of silicon films using silane and methane as precursors in hot-wire chemical vapour deposition technique

Hot-wire chemical vapour deposition is a versatile technique to deposit a-Si:H and nc-Si films at higher deposition rate (~5–10 Å/s) as compared to Plasma enhanced chemical vapour deposition (1–2 Å/s). We report the deposition of highly crystalline Si films at very high deposition rate (≥40 Å/s) by adding methane to silane during thermal/catalytic decomposition. A series of films were deposited by varying the chamber pressure between 10 and 100 Pa at a substrate temperature of 300 °C and filament temperature 2000 °C. The hydrogen diluted silane (10% silane in hydrogen) and pure methane were used as precursors and gas flow rate ratio was kept constant at 10. Films prepared at lower pressure (≤20 Pa) were more crystalline and do not contain any trace of carbon atoms. Bandgap was found to increase from 1.24–1.63 eV when pressure was increased. It was observed that chamber pressure plays a key role in determining the crystallinity, disorder and composition of these films. Addition of methane to hydrogen diluted silane increased deposition rate and crystallinity of Si films at low pressure (≤20 Pa). Above 20 Pa pressure, carbon atoms signature was obtained. SiC films were obtained when pressure was >100 Pa.

Synthesis of vertically aligned carbon nanoflakes by hot-wire chemical vapor deposition: Influence of process pressure and different substrates

The vertically aligned carbon nanoflakes thin films were synthesized on Si (100) and alkali free borosilicate glass substrates at relatively low substrate temperature of 400 °C, without any catalyst or surface pre-treatment, using hot-wire chemical vapor deposition (HWCVD) technique. Raman Spectroscopy studies reveal that the crystallinity of the films increases with increase in process pressure. Our studies show that HWCVD is a versatile technique for the synthesis of these films at low temperature compared to plasma enhanced CVD on both semiconducting Si as well as insulating glass substrates.

Effect of the presence of Si-oxide/sub-oxide surface layer(s) on ‘micron-sized’ Si wires towards the electrochemical behavior as anode material for Li-ion battery

We report here a systematic study concerning the effects of different Si oxides and sub-oxides (viz., SiO2, Si2O3, SiO, Si2O), in varying contents, as present on the surface of amorphous Si shell of ‘micron-sized’ Si wires (SiMWs), towards the electrochemical behavior as anode material for Li-ion batteries. This has been carried out by exposing the SiMWs, as stand-alone electrodes sans any conducting additive/binder/interlayer, to different surface treatments and air exposure/non-exposure prior to cell assembly. The 5–6 μm long SiMWs, having crystalline core (∼25 nm in diameter) and amorphous shell (∼0.22 μm thick), were grown on stainless steel substrate via hot-wire chemical vapour deposition technique. The optimal combination of ‘native’ SiO2, SiO and Si2O, as present on the surface of the ‘as-deposited’ SiMWs, yields the best possible 1st cycle coulombic efficiency (>98%), near-theoretical reversible Li-storage capacity (∼3780 mAh g−1) and also superior cyclic stability, as compared to the SiMWs subjected to either HF treatment/etching or additional annealing or extra care to totally prevent any air exposure (post deposition and prior to characterization/cycling). Electrochemical impedance measurements also supported the above results by showing nearly negligible build-ups of the resistances due to SEI layer and towards charge transfer for the ‘as-deposited’ SiMWs (i.e., the ones showing the best electrochemical performances) upon electrochemical cycling, unlike for the other investigated SiMW-types (having greater/lesser O-containing surface layers). Li-ion ‘full cell’ developed with the as-deposited SiMW as anode and home-made LiFePO4 as cathode showed appreciable capacity retention over 100 cycles, despite the anode being made of micron-sized Si, sans any conducting additive or buffer interlayer.

Growth of high density NiSi/SiC core-shell nanowires by hot-wire chemical vapour deposition for electrochemical applications

In this work, NiSi/SiC core-shell nanowires were grown on crystal silicon and Ni foil substrates by hot-wire chemical vapour deposition technique. The growth of these core-shell nanowires were facilitated by NiSi nanoparticles. These core-shell nanowires were found to be grown at substrate temperatures above 350°C. The nanowires consisted of single crystalline NiSi and polycrystalline SiC nanocolumn as core and shell nanowires, respectively. Increase of the substrate temperature enhances the growth of high density vertically aligned nanowires. The electrochemical properties of NiSi/SiC core-shell nanowires also have been investigated. The specific capacitance of the NiSi/SiC core-shell nanowires obtained from the cyclic voltammetry curve is about 75mF/cm2, which is nearly three times higher than the intrinsic NiSi nanowires (21mF/cm2) and four times higher than the blank Ni foil (17mF/cm2). The superior electrochemical performances of the nanowire electrode were attributed to the synergetic effects of the core-shell structure and the large specific surface area of the high density vertically aligned nanowires. Moreover, the nanowires grown directly on the Ni foil as a supporting electrode showed to improve their charge transport efficiencies during the electrochemical processes.

Influences of hydrogen dilution on the growth of Si-based core–shell nanowires by HWCVD, and their structure and optical properties

Si-based core–shell nanowires were grown on Ni-coated crystal silicon substrates using a hot-wire chemical vapor deposition technique. The NiSi nanoparticles acted as catalysts that facilitated the growth of the core–shell nanowires without any hydrogen dilution as well as that ranging from 20 to 99 %. These nanowires were structured by single-crystalline NiSi cores and amorphous shells with consisting of nanocrystallites embedded within an amorphous matrix. Raman results reveal crystallization of amorphous Si to crystalline Si up to the crystalline volume fraction of 92.3 % for the nanowires grown with hydrogen dilution. An increase in hydrogen dilution enhanced the decomposition rate and the gas-phase reactions for SiC shell formation, while further increases up to 99 % suppressed the growth of the nanowires. Moreover, a phased transition from Si to SiC occurred with increases in hydrogen dilution above 20 %. The nanowires demonstrated superior optical absorption in the visible region, revealing their significant light-trapping ability. This paper discusses the influences of hydrogen dilution on the structure and optical properties of these core–shell nanowires.

Transformation from amorphous to nano-crystalline SiC thin films prepared by HWCVD technique without hydrogen dilution

Silicon carbide (SiC) thin films were deposited on Si(111) by the hot wire chemical vapour deposition (HWCVD) technique using silane (SiH4) and methane (CH4) gases without hydrogen dilution. The effects of SiH4 to CH4 gas flow ratio (R) on the structural properties, chemical composition and photoluminescence (PL) properties of the films deposited at the different gas flow ratios were investigated and compared. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra revealed a structural transition from amorphous SiC to cubic nano-crystalline SiC films with the increase in the gas flow ratio. Raman scattering confirmed the multi-phased nature of the films. Auger electron spectroscopy showed that the carbon incorporation in the film structure was strongly dependent on the gas flow ratio. A similar broad visible room-temperature PL with two peaks was observed for all SiC films. The main PL emission was correlated to the band to band transition in uniform a-SiC phase and the other lower energy emission was related to the confined a-Si : H clusters in a-SiC matrix. SiC nano-crystallites exhibit no significant contribution to the radiative recombination

Growth And Characterization Of Large Grained Poly-Si Films Grown On Biaxially Textured Ni-W Substrate By Hot-wire CVD

Polycrystalline Si (Poly-Si) film with highly crystalline nature, and having most of the grains in the range of 50-100 µm has been grown over biaxially textured Ni-W substrate by Hot-wire chemical vapor deposition technique, using a single buffer layer of CeO2 thin film. This result has been achieved for the SiH4 source gas diluted to 95% with added H2 gas, and for the substrate temperature of 840±10oC and a deposition pressure of 40 mTorr. XRD analysis shows that the Poly-Si films have grown with (111) and (200) orientations. Raman studies reveal that a crystalline volume fraction of 95.3% has been achieved. The imaginary part of pseudo dielectric function, <ε2>, as extracted from ellipsometric data, shows two prominent shoulders at energy positions 3.4 eV and 4.2 eV corresponding to the optical absorption of crystalline Si, indicating a high crystallinity of the Poly-Si film. SEM micrograph shows that the grown Poly-Si film is following the morphology and grain size as that of biaxially textured Ni-W substrate. SIMS analysis of the total multilayer structure shows a formation of very sharp interfaces, with no diffusion between Si and Ni, indicating that a single buffer layer of CeO2 is sufficient to avoid the formation of nickel silicide while growing Si over Ni substrate. Thus, these results are very encouraging for the fabrication of Poly-Si film based solar cells with increased efficiency by minimizing the undesired recombination of charge carriers at grain boundaries.

Low cost vacuum web coating system

A low cost solution for a mini roll to roll web coating system is presented. The design is very simple and involves only three active rolls, two winding/unwinding rolls and a cooling drum. No extra load cells are used to control the web winding mechanism operation. To reach such result it has been necessary to develop an adequate control solution which acts on the two winding roll torques to make the web moving properly. The effect of the control mechanism is to increase electronically the total mechanical inertia of the roll to roll system. In such manner the stick-slip motion of the web, induced by the dry friction affecting the rotation of the rolls, is avoided. The effectiveness of this strategy has been corroborated: a first test showed that the web moves continuously while it is kept tense; in a second experiment a-Si material has been deposited by hot-wire chemical vapor deposition technique. For that material the optical transmission measurements at several points over the deposited area indicate a satisfactory uniformity. The presented tests validate the goodness of the new control method.

Growth of Wide-Bandgap Nanocrystalline Silicon Carbide Films by HWCVD: Influence of Filament Temperature on Structural and Optoelectronic Properties

Silicon carbide (SiC) thin films have been deposited using a hot-wire chemical vapor deposition technique on quartz substrates with a mixture of silane, methane, and hydrogen gases as precursors at a reasonably high deposition rate of approximately 15 nm/min to 50 nm/min. The influence of the filament temperature (TF) on the structural, optical, and electrical properties of the SiC film has been investigated using x-ray diffraction, Raman scattering, Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, ultraviolet–visible–near infrared transmission spectroscopy, and dark conductivity (σd) studies. Films deposited at low TF (1800°C to 1900°C) are amorphous in nature with high density of Si–Si bonds, whereas high-TF (≥2000°C) films are nanocrystalline embedded in an amorphous SiC matrix with higher concentration of Si–C bonds and negligible concentration of Si–Si bonds. The bandgap (Eg) varies from 2.5 eV to 3.1 eV and σd (50°C) from ∼10−9 Ω−1 cm−1 to 10−1 Ω−1 cm−1 as TF is increased from 1900°C to 2200°C. This increase in Eg and σd is due to microstructural changes and unintentional oxygen doping of the films.

Post-nitriding on μc-InXGa1 − XN films using hot-wire chemical vapor deposition technique

Indium gallium nitride (InXGa1−XN) is focused as a photo-absorption material for solar cells because of the variable band gap in the range from 0.6 to 3.4eV. Microcrystalline-InXGa1−XN (μc-InXGa1−XN) films prepared by radio frequency (rf)-sputtering do not show good photosensitivity due to a formation of nitrogen vacancies in the films. In this paper, a post-nitriding at the surface of μc-InXGa1−XN films was examined by using a hot-wire chemical vapor deposition technique with various gases, N2, NH3, H2 and Ar. Atomic ratio of nitrogen at the surface of μc-InXGa1−XN films increased about 10% by an exposure of mixture gas of N2 and NH3.

Dual hot-wire arrangement for the deposition of silicon and silicon carbide thin films

A dual hot-wire arrangement has been designed and investigated for the deposition of various thin film materials by the hot-wire chemical vapor deposition (HWCVD) technique. Tantalum and rhenium wires were used for silicon and silicon carbide depositions with hydrogen diluted silane and monomethylsilane, respectively. It is shown that the both types of filaments are mechanically stable after alternate depositions of silicon and silicon carbide with a total deposition time of about 80h. Good material quality of the deposited films is demonstrated. By taking advantage of this dual hot-wire arrangement, it is possible to deposit both kinds of thin film materials with the individual optimum deposition conditions in a single HWCVD chamber.

Permeation barrier performance of Hot Wire-CVD grown silicon-nitride films treated by argon plasma

In this work SiNx thin films have been deposited by Hot-Wire Chemical Vapor Deposition (HW-CVD) technique to be used as encapsulation barriers for flexible organic electronic devices fabricated on polyethylene terephthalate (PET) substrates. First results of SiNx multilayers stacked and stacks of SiNx single-layers (50nm each) separated by an Ar-plasma surface treatment are reported. The encapsulation barrier properties of these different multilayers are assessed using the electrical calcium degradation test by monitoring changes in the electrical conductance of encapsulated Ca sensors with time. The water vapor transmission rate is found to be slightly minimized (7×10−3g/m2day) for stacked SiNx single-layers exposed to argon plasma treatment during a short time (2min) as compared to that for stacked SiNx single-layers without Ar plasma treatment.

Influence of filament-to-substrate distance on the spectroscopic, structural and optical properties of silicon carbide thin films deposited by HWCVD technique

Silicon carbide (SiC) thin films were deposited using hot wire chemical vapor deposition (HWCVD) technique from pure silane and methane gas mixture. The effect of filament distance to the substrate on the structural and optical properties of the films was investigated. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Raman scattering spectroscopy and UV–Vis–NIR spectroscopy were carried out to characterize SiC films. XRD patterns of the films indicated that the film deposited under highest filament-to-substrate distance were amorphous in structure, while the decrease in distance led to formation and subsequent enhancement of crystallinity. The Si–C bond density in the film structure obtained from FTIR data, showed significant increment with transition from amorphous to nano-crystalline structure. However, it remained almost unchanged with further improvement in crystalline volume fraction. From Raman data it was observed that the presence of amorphous silicon phase and sp 2 bonded carbon clusters increased with the decrease in distance. This reflected in deterioration of structural order and narrowing the optical band gap of SiC films. It was found that filament-to-substrate distance is a key parameter in HWCVD system which influences on the reactions kinetics as well as structural and optical properties of the deposited films.

Real-time monitoring of the silicidation process of tungsten filaments at high temperature used as catalysers for silane decomposition

The scope of this work is the systematic study of the silicidation process affecting tungsten filaments at high temperature (1900 °C) used for silane decomposition in the hot-wire chemical vapour deposition technique (HWCVD). The correlation between the electrical resistance evolution of the filaments, Rfil(t), and the different stages of the their silicidation process is exposed. Said stages correspond to: the rapid formation of two WSi2 fronts at the cold ends of the filaments and their further propagation towards the middle of the filaments; and, regarding the hot central portion of the filaments: an initial stage of silicon dissolution into the tungsten bulk, with a random duration for as-manufactured filaments, followed by the inhomogeneous nucleation of W5Si3 (which is later replaced by WSi2) and its further growth towards the filaments core. An electrical model is used to obtain real-time information about the current status of the filaments silicidation process by simply monitoring their Rfil(t) evolution during the HWCVD process. It is shown that implementing an annealing pre-treatment to the filaments leads to a clearly repetitive trend in the monitored Rfil(t) signatures. The influence of hydrogen dilution of silane on the filaments silicidation process is also discussed.

Effect of annealing on structural properties of SiOx films obtained by HWCVD technique

SiOx films were deposited in the temperature range of 800–1000 °C by hot wire chemical vapor deposition (HWCVD) technique. Annealing treatment for as-deposited samples was performed in nitrogen (N2) atmosphere at 1100 °C. The effect of annealing atmosphere on the structural properties of the deposited films was investigated by means of micro Raman, X-ray diffraction (XRD), Photoluminescence (PL) and high resolution electron microscopy (HRTEM) measurements. Micro-Raman studies revealed an increase in the crystalline volume fraction (Xc) in the deposited films after annealing. XRD patterns of the annealed films showed the formation of silicon crystals. HRTEM images confirmed the presence of silicon nanocrystals (Si-ncs) in both as-deposited and annealed films. Furthermore, the annealing treatment on the as-deposited samples at 900 and 1000 °C resulted in the observation a large number of Si-ncs with sizes between 3–15 nm. PL measurement shows a broad emission from 400 to 1100 nm. This emission was related with both Si-ncs and interfacial defects present in SiOx films.

Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires

Silicon/zinc oxide (Si/ZnO) core-shell nanowires (NWs) were prepared on a p-type Si(111) substrate using a two-step growth process. First, indium seed-coated Si NWs (In/Si NWs) were synthesized using a plasma-assisted hot-wire chemical vapor deposition technique. This was then followed by the growth of a ZnO nanostructure shell layer using a vapor transport and condensation method. By varying the ZnO growth time from 0.5 to 2 h, different morphologies of ZnO nanostructures, such as ZnO nanoparticles, ZnO shell layer, and ZnO nanorods were grown on the In/Si NWs. The In seeds were believed to act as centers to attract the ZnO molecule vapors, further inducing the lateral growth of ZnO nanorods from the Si/ZnO core-shell NWs via a vapor-liquid-solid mechanism. The ZnO nanorods had a tendency to grow in the direction of [0001] as indicated by X-ray diffraction and high resolution transmission electron microscopy analyses. We showed that the Si/ZnO core-shell NWs exhibit a broad visible emission ranging from 400 to 750 nm due to the combination of emissions from oxygen vacancies in ZnO and In2O3 structures and nanocrystallite Si on the Si NWs. The hierarchical growth of straight ZnO nanorods on the core-shell NWs eventually reduced the defect (green) emission and enhanced the near band edge (ultraviolet) emission of the ZnO.

Effect of substrate to filament distance on formation and photoluminescence properties of indium catalyzed silicon nanowires using hot-wire chemical vapor deposition

Si nanowires have been synthesized by hot-wire chemical vapor deposition technique, with Indium nanocones employed as catalysts with different substrate to filament distances ranging from 6 to 3cm. Reducing the substrate to filament distance resulted in the retention of more atomic H radicals on the growth sites. The atomic H radicals acted to induce the catalytic growth and enhance the crystallinity of the Si nanowires. The Si nanowires showed tapering structures due to the radial growth of columnar Si nanocrystallites on the middle and base walls of the nanowires. The oxide-related defects on the outer layer of the Indium nanocones and Si nanowires, as well as the Si nanocrystallites on walls of the Si nanowires, contributed to the visible orange and red photoluminescence.

Study on the role of filament temperature on growth of indium-catalyzed silicon nanowires by the hot-wire chemical vapor deposition technique

Silicon nanowires (SiNWs) were synthesized from indium catalysts on the Si(111) substrate using the hot-wire chemical vapor deposition technique. A tungsten filament with purity of 99.95% was employed for both the evaporation of an indium wire as catalyst and the decomposition of the precursor gas silane diluted in hydrogen. In this study, we investigated the role of the filament temperature (Tf) on the growth and structural properties of the SiNWs. A threshold Tf for the successive growth of the SiNWs via a vapor-liquid-solid process was observed at Tf between 1400 and 1500 °C. For Tf of 1400 °C and below, only a layer of Si shell cladding was formed on the indium core. An increase in Tf above the threshold resulted in a significant increase in the number density and the aspect ratio of the SiNWs. X-ray diffraction and micro-Raman measurements indicated an enhancement in crystallinity of the SiNWs with the increase in Tf. Fourier transform infrared analysis showed an enhancement in the presence of Si–O and Si–H related bonds with the increase in Tf. The Si–O bond is mostly originated from the native oxide layer of SiNWs, while Si–H bond suggests that Si–Hx species were responsible for the growth.

Microcrystalline silicon carbide window layers in thin film silicon solar cells

Crystalline silicon carbide alloys have a very high potential to be used as transparent conductive window layers in thin-film solar cells provided they can be prepared in thin-film form and at compatible deposition temperatures. Using Hot-Wire Chemical Vapor Deposition (HWCVD) technique, silicon carbide in microcrystalline form (μc-SiC:H) has been prepared at low substrate temperature. Monomethylsilane and hydrogen are used as the precursor gases. The material is highly conductive, unintentionally n-type in nature, and possess wide optical gap. These properties have been utilized by employing the μc-SiC:H layer as the window layer in n-side illuminated single junction microcrystalline silicon (μc-Si:H) solar cells. Some typical issues related to this device research and development are described here. In particular the nucleation of the intrinsic μc-Si:H absorber layer on the n-type μc-SiC:H layer needs attention. With proper device design high short-circuit current densities of 29.6mA/cm2 and quantum efficiencies up to 90% are obtained resulting in a maximum conversion efficiency of 9.6% within a 2-μm-thick μc-Si:H solar cell.

Synthesis of indium-catalyzed Si nanowires by hot-wire chemical vapor deposition

Indium (In) catalyzed silicon nanowires (SiNWs) were synthesized by using hot-wire chemical vapor deposition (HWCVD) technique. Indium droplets were deposited on Si substrates by hot-wire evaporation of In wire, which was immediately followed by the growth of SiNWs from the droplets. Three sets of samples were prepared by varying the length of In wires, l, as 3, 1 and 0.5 mm. The sizes of In catalyst droplets decreased from 271.4 ± 66.8 to 67.4 ± 16.6 nm when the l was reduced from 3 to 0.5 mm. Larger size of In droplets (271.4 ± 66.8 nm) was found to induce the growth of worm-like NWs. The decrease in size of In catalyst droplets induced the formation of aligned and tapered NWs with smaller tips. The smallest value of tapering parameter, T p of 40.5 nm/μm is correlated to the SiNWs induced by the smallest size of In droplets (67.4 ± 16.6 nm). The as-grown SiNWs showed high purity and good crystalline structure.