Horizontal Chemical Vapor Deposition Research Articles

Numerical investigation on the effect of gas-phase dynamics on graphene growth in chemical vapor deposition

Chemical vapor deposition (CVD) is a crucial technique to prepare high-quality graphene because of its controllability. In the research, we perform a systematic computational fluid dynamics numerical investigation on the effect of gas-phase reaction dynamics on the graphene growth in a horizontal tube CVD reactor. The research results indicate that the gas-phase chemical reactions in the CVD reactor are in a nonequilibrium state, as evidenced by the comparison of species mole fraction distributions during the CVD process and under chemical equilibrium conditions. The effect of gas-phase reaction dynamics on the deposition rate of graphene under different conditions is studied, and our research shows that the main causes of change in graphene growth rates under different conditions are gas-phase reaction dynamics and active species transport. The results of numerical simulation agree well with the experimental phenomena. The research results also indicate that, for methane, the main limiting factor of graphene growth is the surface kinetic reaction rate. Conversely, for active species, the main limiting factor of graphene growth is species transport. Our research suggests that the growth rate of graphene can be regulated from the perspective of the gas reaction mechanism. This method has theoretical guiding significance and can be extended to the preparation of large-area graphene.

Comparing Ultralong Carbon Nanotube Growth from Methane over Mono- and Bi-Metallic Iron Chloride Catalysts.

This research endeavours to study the growth of ultralong carbon nanotubes (UL-CNTs) from methane using diverse catalysts, namely FeCl3, bi-metallic Fe-Cu, Fe-Ni, and Fe-Co chlorides. Aqueous catalyst solutions were evenly dispersed on silica substrates and grown at 950 °C in the presence of hydrogen via a horizontal chemical vapour deposition (CVD) furnace. The samples underwent characterisation by Raman spectroscopy, scanning electron microscopy (SEM), and optical microscopy to identify the quality of CNTs and enumerate individual UL-CNTs. Our findings revealed that FeCl3, as a mono-metallic catalyst, generated the longest UL-CNTs, which measured 1.32 cm, followed by Fe-Cu (0.85 cm), Fe-Co (0.7 cm), and Fe-Ni (0.6 cm), respectively. The G/D ratio (graphene to defects) from the Raman spectroscopy was the highest with the FeCl3 catalyst (3.09), followed by Fe-Cu (2.79), Fe-Co catalyst (2.13), and Fe-Ni (2.52). It indicates that the mono-iron-based catalyst also produces the highest purity CNTs. Moreover, this study scrutinises the vapour-liquid-solid (VLS) model for CNT growth and the impact of carbide formation as a precursor to CNT growth. Our research findings indicate that forming iron carbide (Fe3C) is a crucial transition phase for amorphous carbon transformation to CNTs. Notably, the iron catalyst generated the longest and densest CNTs relative to other iron-based bi-metallic catalysts, which is consistent with the temperature of carbide formation in the mono-metallic system. From correlations made using the phase diagram with carbon, we conclude that CNT growth is favoured because of increased carbon solubility within the mono-metallic catalyst compared to the bi-metallic catalysts.

Study on the Surface Structure of N-Doped 4H-SiC Homoepitaxial Layer Dependence on the Growth Temperature and C/Si Ratio Deposited by CVD

The quality of the N-doped 4H-SiC homoepitaxial layers grown via hot-wall horizontal chemical vapor deposition (CVD) was evaluated at various C/Si ratios (1.0–1.2) and growth temperatures (1570–1630 °C). The microstructure and morphology of the epilayers were studied through a comparative analysis of the AFM patterns under different growth conditions. X-ray photoelectron spectroscopy and Raman spectroscopy revealed the quality of the 4H-SiC epilayers and the amount of N-doping. It was found that an increase in the C/Si ratio enabled obtaining a quite smooth epitaxial layer surface. Moreover, only the 4H-SiC crystal type was distinguished in the epilayers. In addition, the epitaxial quality was gradually improved, and the amount of defect-related C-C bonds significantly dropped from 38.7% to 17.4% as the N doping content decreased from 35.3% to 28.0%. An increase in the growth temperature made the epitaxial layer surface smoother (the corresponding RMS value was ~0.186 nm). According to the Raman spectroscopy data, the 4H-SiC forbidden mode E1(TO) in the epilayers was curbed at a higher C/Si ratio and growth temperature, obtaining a significant enhancement in epitaxial quality. At the same time, more N dopants were inserted into the epilayers with increasing temperature, which was opposite to increasing the C/Si ratio. This work definitively shows that the increase in the C/Si ratio and growth temperature can directly enhance the quality of the 4H-SiC epilayers and pave the way for their large-scale fabrication in high-power semiconductor devices.

Single-Walled Carbon Nanotube Synthesis Yield Variation in a Horizontal Chemical Vapor Deposition Reactor.

The controlled synthesis of single-walled carbon nanotubes (SWNTs) is essential for their industrial application. This study investigates the synthesis yield of SWNTs, which depends on the positions of the samples on a horizontal chemical vapor deposition (CVD) system. Methane and Fe thin films were used as the feedstock and catalyst for SWNTs synthesis, respectively. A high-resolution scanning electron microscope was used to examine the synthesis yield variation of the SWNTs along the axial distance of the reactor. The morphology and crystallinity of the fabricated SWNTs were evaluated by atomic force microscopy and Raman spectroscopy, respectively. We observed that the highest synthesis yield of the SWNTs was obtained in the rear region of the horizontal reactor, and not the central region. These results can be applied to the synthesis of various low-dimensional nanomaterials, such as semiconducting nanowires and transition metal dichalcogenides, especially when a horizontal CVD chamber is used.

Hydrogen free direct growth carbon nanorod as a promising electrode in symmetric supercapacitor applications

Carbon nanorods (CNRs) are significant one-dimensional carbon-based nanostructured materials. A new method to prepare direct growth fast-standing carbon nanorod as a promising electrode in symmetric supercapacitor applications has been explored. In this study, the CNRs are directly grown on SS plate in the three-zone horizontal chemical vapor deposition unit. The hydrogen free environment is used to grow the CNRs. The substrate position was studied by controlling the substrate temperature, time and precursor gas flow rate. Raman, Fourier transform infrared spectra (FTIR), high-resolution transmission electron microscopy (HR-TEM) explored CNRs formation. The CNRs are well assembled in the substrate and showed excellent electrical contact with it. The biological SP-150 instrument measured electrochemical activity of CNR-developed electrodes. The symmetric device was fabricated with two same electrodes and the device energy and power density was studied. Symmetric supercapacitor devices expressed 22 W h/kg energy density at low 1 A/g. The 96.65% capacity is maintained for 10,000 charge discharge cycles in symmetrical supercapacitor device.

Research on rapid growth of monolayer graphene by vertical cold-wall CVD method

Chemical vapor deposition (CVD) is one of the most important methods for the preparation of graphene. Graphene is usually prepared using horizontal tube CVD (HT-CVD) method. However, the preparation of this method is too long, generally more than 2 h. In this article, a method called vertical cold-wall CVD (VCW-CVD) method was introduced for graphene preparation. Due to the vertical angle between direction of gas flow and sample surface, the reaction process of this method for graphene preparation is only 1 min. Accordingly, in this article, the modified parameter of graphene preparation by this method was discussed. Besides, the VCW-CVD graphene had an equal quality with the HT-CVD graphene, which was proved by measurement of conductivity, transmittance and heat stability.

Kinetic studies of few-layer graphene grown by flame deposition from the perspective of gas composition and temperature.

Studies on depositions of chemical vapour deposition (CVD) diamond films have shown that flame combustion has the highest deposition rates without involving microwave plasma and direct current arc. Thus, here we report on our study of few-layer graphene grown by flame deposition. A horizontal CVD reactor was modified for the synthesis of flame deposition of few-layer graphene on a Cu substrate. It was found that graphene obtained has comparable quality to that obtained with other flame deposition setups reported in the literature as determined from Raman spectroscopy, sheet resistance, and transmission electron microscopy. Calculation of the chemical kinetics reveals a gas phase species that has a close correlation to the growth rate of graphene. This was further correlated with van't Hoff analysis of the reaction, which shows that the growth reaction has a single dominating mechanism for temperatures in the range of 400 °C to 1000 °C. Arrhenius analysis also was found to be in good agreement with this result. This study shows few-layer graphene growth proceeds through different pathways from a CVD grown graphene and also highlights flame deposition as a viable method for graphene growth.

Application of γ-alumina as catalyst support for the synthesis of CNTs in a CVD reactor

The continuous search for effective and efficient engineered materials towards solving human challenges is in continuous needs. This study reports the preparation of alumina from kaolin as catalyst support using methanol as precipitant for the synthesis of multiwalled carbon nanotubes (MWCNTs) in a horizontal chemical vapour deposition (CVD) reactor. The process involved thermal treatment of kaolin in a static air furnace, leaching of the treated kaolin in tetraoxosulphate(VI) acid , precipitation in methanol medium, washing in distilled water and calcination at . The alumina in the kaolin was successfully leached and a cubic structure alumina of high thermal stability was synthesized. The alumina was then characterised and utilised as support for the bimetallic catalyst (Fe-Ni) using wet impregnation method. The produced catalyst was used to produce CNTs in a CVD reactor. The thermal properties, surface morphologies, surface functional groups, crystallinity, elemental analyses, and surface areas of the starting kaolin, alumina, prepared bimetallic catalyst (Fe-Ni/alumina) and the synthesized MWCNTs were determined using TGA/DTA, HRSEM, FTIR, XRD, EDX, and BET respectively. The XRD analysis of the bimetallic catalyst was employed to determine its average crystalline particle size to be 26.23 nm using Scherer equation. Tip growth mechanism of MWCNTs was observed during the MWCNTs nucleation process with 41.2 nm external diameters, length and 340 aspect ratio using dynamic light scattering (DLS) technique. The purified MWCNTs showed high antibacterial activities at long exposure time. The results of the various analysis conducted show that the synthesized alumina has potential properties as catalyst support for MWCNTs growth via CVD technique.

Self‐Induced Nucleation Growth of GaN Columns by Chemical Vapor Deposition

GaN columns are grown by (1 1 1) GaAs nitridation at 1000 °C. GaAs wafers are used as a substrate and Ga source. The nitridation is performed using a hydrogen and ammonia flow mix into a horizontal chemical vapor deposition (CVD) system at atmospheric pressure. XRD in correlation with pole figure shows that the structure is wurtzite with a (0 0 0 2) preferential plane. The SEM images show the growth of GaN columns with an agglomerate formation on top. EDS results show that the agglomerate is Ga rich, suggesting a self‐induced nucleation growth. The growth mechanism is discussed briefly. The average diameter and length of the columns are around 1.2 and 10 µm, respectively. The method reported here do not use a template or foreign metallic particles catalyst.

(Invited) SiC Growth Parameter Evolution Utilizing Infrared Thermal Imaging Towards Realization of Extremely Low Dislocation Bulk SiC

The NASA Glenn Research Center (GRC) has employed infrared thermal imaging technology in a hot wall chemical vapor deposition (CVD) system for thermal measurement of silicon carbide (SiC) crystals mounted vertically in the gas stream. The use of infrared thermal imaging technology revealed deficiencies in growth parameters that had previously not been observable utilizing traditional noncontact monitoring techniques. GRC has been investigating the lateral expansion (⊥ to (0001)) of pseudo SiC fiber seed crystals using a custom designed horizontal flow hot wall CVD system in proof of concept research, for a radically new bulk growth process. This bulk growth process would, ideally produce SiC boules with just one single centrally located screw dislocation (SD). Dislocations in general have a negative impact on SiC devices and device fabrication. Current commercially available SiC substrates have a dislocation density of ~1000 cm2. A key requirement of this process is the lateral homoepitaxial growth expansion of a SiC fiber without the introduction of additional defects. This requires maintaining growth temperatures along the length of the SiC fiber. In the absence of purpose grown fiber like seed crystals, pseudo seed crystals were created by mechanically dicing thin slivers from a-plane and m-plane oriented boule slices. The slivers (~17000 x 2000 x 400) µm’s were subsequently subjected to gaseous etching in hydrogen to produce needle like seed crystals with fiber like dimensions with a width of ~100 µm’s. The seed crystals were oriented vertically in a gas stream of the horizontal CVD system. Traditional optical pyrometry coupled with a small CCTV aligned along the horizontal axis was used to be provide real time thermal and optical data of the CVD. Due to the extremely small cross-sectional area of the seed crystals, the thermal radiation was insufficient to be measured with optical pyrometry and was ineffective in resolving the temperature of the vertical slivers. Only the temperature of the carrier holding the seed crystals could be resolved. A commercially available infrared thermal imaging system equipped with an extra-long working distance (530mm) lens replaced the pyrometer and CCTV. The thermal imager requires that the target be only 3 x 3 pixels to obtain accurate temperatures. When coupled with long working distance optics the temperature of the seed crystals could be resolved. A thermal gradient that was previously unobserved was now viewable. Growth parameters were adjusted to compensate for the gradient preventing nucleation of 3C-SiC at the tip of the seed crystal. A growth rate (⊥ to (0001)) of ~80 μm/hr was achieved. Total lateral growth was > 3.5mm. Initial analysis by synchrotron white beam x-ray topography (SWBXT) confirms, that the fiber growth was homoepitaxial, matching the polytype of the respective underlying region of the seed sliver. Although the as grown SiC contained a high density of basal plane dislocations (BPD’s) it appears that the BPD’s originated in the pseudo seed crystal without additional BPD’s introduced during the growth.

CVD growth and properties of boron phosphide on 3C-SiC

Improving the crystalline quality of boron phosphide (BP) is essential for realizing its full potential in semiconductor device applications. In this study, 3C-SiC was tested as a substrate for BP epitaxy. BP films were grown on 3C-SiC(100)/Si, 3C-SiC(111)/Si, and 3C-SiC(111)/4H-SiC(0001) substrates in a horizontal chemical vapor deposition (CVD) system. Films were produced with good crystalline orientation and morphological features in the temperature range of 1000–1200°C using a PH3+B2H6+H2 mixture. Rotational twinning was absent in the BP due to the crystal symmetry-matching with 3C-SiC. Confocal 3D Raman imaging of BP films revealed primarily uniform peak shift and peak widths across the scanned area, except at defects on the surface. Synchrotron white beam X-ray topography showed the epitaxial relationship between BP and 3C-SiC was (100)〈011〉BP||(100)〈011〉3C-SiC and (111)〈112̅〉BP||(111)〈112̅〉3C-SiC. Scanning electron microscopy, Raman spectroscopy and X-ray diffraction analysis indicated residual tensile strain in the films and improved crystalline quality at temperatures below 1200°C. These results indicated that BP properties could be further enhanced by employing high quality bulk 3C-SiC or 3C-SiC epilayers on 4H-SiC substrates.

Study of surface defects and crystallinity of MWCNTs growth in FCCVD

Abstract This research investigated the structural growth of multiwalled carbon nanotubes (MWCNTs) in a double stage horizontal chemical vapor deposition (CVD) reactor. Ethylene was used as a carbon source for nucleation of nanotubes. Ferrocene catalyst weight was varied from 0.1 to 0.2 g to demonstrate the growth of MWCNTs on Si/SiO2/Al2O3 substrate. The obtained data revealed that the weight of the catalyst significantly affects the diameter, crystallinity, alignment and yield of the nanotubes. Lower inner-shell spacing and the ratio of D-Raman peak intensity and G-Raman peak intensity (ID/IG ratio) were obtained with 0.15 g of ferrocene, which was an indication of relatively pure carbon nanotubes (CNTs) growth. Raman spectra also confirmed the highly crystalline and relatively pure CNTs structures with ID/IG ratio of 0.700. TGA data revealed the formation of 97% pure nanotubes with oxidation temperature of 620°C. However, above and below the optimum (0.15 g of ferrocene), some of the grown CNTs were found defective and few black spots were also seen in TEM micrographs.

The electrical transport properties of nitrogen doped carbon microspheres

Four samples of nitrogen-doped carbon microspheres were synthesised using a horizontal chemical vapour deposition process. Characterization of the samples using scanning electron microscopy, Raman spectroscopy and electron paramagnetic resonance spectroscopy showed spherical graphitic carbon microparticles with substitutional nitrogen in the lattice structure. The average diameter of the microspheres was 1.7 μm. Electron paramagnetic resonance spectroscopy was also used as a technique to measure the percentage of substitutional nitrogen in each sample. This was determined to be 3.4% in two of the samples and 1.7% in the other two. Temperature dependent electrical transport measurements were performed on the samples and resistance measurements showed clear semiconducting behaviour in two of the samples and a transition from metallic to semiconducting behaviour in the other two samples. IV characteristics measurements display curves with increasing non-linearity as temperature decreases in two samples and saturation behaviour is seen at higher temperatures in the other two samples. An anomaly is present in the IV characteristics at 300 K in all samples. A combination of fluctuation induced tunnelling and electronphonon scattering is used to model the data. These models provide a satisfactory description of the data for both the IV characteristics and the resistance measurements.

Harmonic mechanical excitations of steady convective instabilities: A means to get more uniform heat transfers in mixed convection flows?

In laminar mixed convection flows, steady thermoconvective patterns generate non uniform heat and/or mass transfers at walls that can be detrimental in some industrial processes. For instance the longitudinal thermoconvective patterns of Poiseuille–Rayleigh–Bénard (PRB) flows generate non uniform thin films or coatings when they are present in cold wall horizontal Chemical Vapor Deposition (CVD) reactors. The aim of this paper is to show that, when the basic steady flow is convectively unstable against an unsteady flow regime, introducing small harmonic mechanical excitations in the basic flow may enable to obtain more uniform time averaged heat transfers. More specifically, three-dimensional direct numerical simulations are used to characterize the temperature field and wall heat transfer associated with unsteady wavy convective instabilities of PRB flows that result from harmonic excitations of the longitudinal thermoconvective rolls at channel inlet. A design of experiments is used to build cubic response surfaces of the different quantities analyzed (growth length of the wavy rolls, magnitude of their spanwise oscillations, wall Nusselt number …) on a wide range of the flow parameters. Air PRB flows (Pr=0.71) in channels of aspect ratios equal to Width/Height=10 and 150⩽Length/Height⩽300, for Reynolds numbers 100⩽Re⩽300 and Rayleigh numbers 5000⩽Ra⩽16,000 are considered. Comparisons with experiments are presented and a good agreement is obtained. The optimal conditions to have uniform heat transfers on the horizontal walls of PRB flows correspond to the minimal growth length of the wavy rolls until saturation and the maximum magnitude of their spanwise oscillations. They are approximately obtained for moderate Reynolds number (Re≈150), high Rayleigh numbers (Ra≈15,000), low excitation frequency and rather high excitation magnitude. A discussion of these results for the applications to CVD in horizontal rectangular reactors at atmospheric pressure is finally proposed.

Template-free synthesis of vertically aligned crystalline indium oxide nanotube arrays by pulsed argon flow in a tube-in-tube chemical vapor deposition system

Vertically aligned crystalline indium oxide (IO) nanotube arrays without using any special templates or expensive epitaxial substrates were synthesized on Si (100) substrates by using a tube-in-tube horizontal chemical vapor deposition (CVD) system.

Multiwall carbon nanotubes continuously filled with micrometre-length ferromagnetic α-Fe nanowires

Carbon nanotubes filled with continuous crystalline nanowires of nanometre-scale diameter and micrometre-scale length of the ferromagnetic phase α-Fe were produced with a new chemical vapour deposition method. We report a new two-stage approach, a perturbed-vapour method of synthesis followed by a post-synthesis heat treatment that produces multiwall carbon nanotubes filled with at least 19 micrometre-length nanowires of α-Fe. Previously reported synthesis routes use steady-state conditions to guarantee nanowire continuity but result only in small (less than one-micrometre length) nanowires comprising isolated or mixed phases of either α-Fe, Fe3C, or γ-Fe. Here flower-like clusters of carbon nanotubes continuously filled with α-Fe were produced by perturbation of a laminar ferrocene (Fe(C5H5)2) vapour flow in a conventional horizontal chemical vapour deposition reactor. Single-phase filling was achieved by a post-synthesis annealing at 500°C for 15h in Ar flow. Electron microscopy studies revealed the high quality of the structural integrity of both nanotubes and encapsulated nanowires. These nanostructures possess a high coercivity of 580Oe and a very high saturation magnetization of 189.5emu/g comparable with bulk α-Fe.

Investigation of surface morphology and ion activation of aluminium implanted 4H-SiC

Multiple-energy aluminium (Al+) implantation into 4H-SiC (0001) epilayer and activation anneal with a graphite encapsulation layer were investigated in this paper. Measurements showed that the implanted Al+ box doping profile was formed and a high ion activation ratio of 78% was achieved by 40 min annealing at 1600°C using a horizontal chemical vapor deposition (CVD) reactor. The step bunching effect associated with the high temperature post implantation activation annealing (PIA) process was dramatically suppressed by using the graphite encapsulation layer. And a flat and smooth surface with a small average surface roughness (RMS) value of around 1.16 nm was achieved for the implanted 4H-SiC after the PIA process. It was demonstrated that this surface protection technique is a quite effective process for 4H-SiC power devices fabrication.

Detailed Experimental Study of Mean and Gradient Stresses in Thin 3C-SiC Films Performed Using Micromachined Cantilevers

A detailed experimental study of the mean and gradient stress, existing in the as-grown cubic silicon carbide epilayers, is presented in this paper. (100) and (111) oriented epiwafers with considerable film thickness variation have been elaborated using a horizontal low pressure chemical vapor deposition reactor. The mean and gradient stress within the 3C-SiC film were estimated from the static mechanical deformation of micromachined clamped-free beams. For both studied orientations, we observe a stress gradient inversion phenomenon that can be explained in terms of creep occurring in 3C-SiC film.

Experimental and numerical investigation of the position-dependent growth of carbon nanotube–alumina microparticle hybrid structures in a horizontal CVD reactor

The hybrid structures of carbon nanotubes (CNTs) and alumina microparticles were produced in a horizontal chemical vapor deposition (CVD) reactor using ferrocene/xylene/acetylene mixture as the catalyst–carbon source. At a given temperature and hydrogen ratio, the CNT diameter, number density, growth rate and their hybrid structures varied greatly along the axial direction of the reactor. This non-uniform growth is attributed to the position-dependent chemical reaction kinetics inside the reactor, mainly along the gas flow direction. Mass spectrometry was used to identify and quantify the chemical species of the exhaust gas. A numerical simulation of the reacting gas flow in the reactor was conducted in parallel by taking into account the space-dependent pyrolysis kinetics of the catalyst and carbon sources, the reactor temperature gradient and the fluid dynamics. A good agreement existed between the modeling and the experimental results. A double-end-injection method was proposed based on the above results, and the uniform hybrid structures were synthesized in a larger zone inside the CVD reactor. A new kind of CNT structure containing iron crystal particles at their two extremities was obtained under certain conditions, which can be very useful for various CNT junction applications.

Built-In Electric Field in High Quality GaN/AlGaN Quantum Wells

We report studies on electric field built in GaN/Al0.09Ga0.91N structure of nominally 6 nm wide quantum well. The sample was grown in horizontal metal-organic chemical vapor deposition reactor using innovative technology that decreases the density of screw dislocations. Firstly, using visible and mid infra-red interference pattern along the sample, the layer thickness and consequently the quantum well width was determined to vary linearly with the position. Secondly, photoluminescence spectra was taken at different positions. Correlation of those two measurements allows us to determine the built-in electric field to be 0.66 MV/cm, which is considerably larger than previously reported for similar structures.