Assisted Chemical Vapor Deposition Process Research Articles

Controllable growth of high-density tapered N-doped SiC nanowires arrays

Tapered nitrogen-doped SiC nanowires (N-doped SiCNWs) arrays were vertically grown on the carbon cloth utilizing Si, SiO2, C, and dicyandiamide powders as raw materials via a simple Fe(NO3)3 assisted chemical vapor deposition process. High-density tapered N-doped SiCNWs possessed homogeneous morphology. The diameter of the synthesized nanowire increased from ∼120 nm at the tail to ∼1.30 μm at the tip, while the length could be up to tens of micrometers. Moreover, the as-prepared N-doped SiCNWs were examined to be 2H/3C–SiC mixed crystals. The appearance of 2H crystal might be responsible for the high density defects in 3C–SiC. And since the interesting tapered shape was significantly affected by the catalyst droplet, the growth of N-doped SiCNWs may be governed by Fe-assisted VLS mechanism. In addition, the main emission peak of PL spectrum of N-doped SiCNWs was located at 474 nm, exhibiting a certain blue shift, which was mainly attributed to the special tapered shape, defects, 2H/3C–SiC heterostructure and the quantum confinement effect of the N-doped SiCNWs.

Influence of different hydrocarbons on the height of MWCNT carpets: Role of catalyst and hybridization state of the carbon precursor

The length of multi-walled carbon nanotubes (MWCNT) has a particular relevance for a wide range of application. To that end, in an aerosol assisted chemical vapour deposition process MWCNT carpets with different heights were synthesized on an oxidized silicon substrate by using ferrocene as Fe-catalyst precursor and some hydrocarbons with different hybridization state. By variation of the synthesis temperature from 600 °C up to 900 °C, for every used hydrocarbon different carpet heights were measured depending on the temperature. Cyclohexane generated the highest MWCNT carpets and for all carbon precursor the maximum carpet height was measured at 850 °C. To elucidate this, the thermal decomposition of the used hydrocarbons was discussed and the corresponding main decomposition products methane, ethene and benzene were identified. Subsequently, these substances were directly used in a second chemical vapour deposition equipment to simulate the growth mechanism by the intermediate carbon precursors. It was found, that with comparable content of carbon in the gas phase, ethene is much higher efficient for growing high MWCNT carpets than methane. Moreover, benzene was figured out to be also an adequate carbon precursor under specific gas phase conditions, explaining the carpet heights when using toluene and xylene.

Mechanical stability measurements of surface modified nanoparticle agglomerates

A low pressure impactor is used to measure the mechanical stability of nanoparticle agglomerates. The stability at different impaction velocities is quantified as changes in the projection area of agglomerate fragments deposited on TEM grids mounted on the impaction plate. Platinum particles produced in a spark discharge generator are used as model particles. The mechanical stability of unmodified Pt-agglomerates is compared with the ones of thermally pretreated and coated agglomerates. The coatings are produced in a plasma assisted chemical vapor deposition process using the post-discharge environment of a dielectric barrier discharge. The coating process allows the coating of particles with silica at ambient temperature and so minimizes structural changes of the agglomerates before impaction. The precursors utilized are tetraethyl orthosilicate for silica and hexamethyldisiloxane for silica-organic coatings. Both coatings improve the stability of the agglomerates as indicated by reduced structural changes during impaction. The silica-organic coating, which is applied at 200 °C, seems to prevent sinter-related changes to the particles, which is observed for the uncoated particles at this temperature. This is further corroborated by the deposition behavior of these coated particles, which resembles more that of the original particles at 24 °C. Furthermore, the rebound behavior of coated agglomerates impacting on the TEM grid is shown. It is found that a minority of the particles rebound back from the TEM grid into the gas, which occurs in parallel to the structural changes.

AACVD and gas sensing properties of nickel oxide nanoparticle decorated tungsten oxide nanowires

Here, we show that the aerosol assisted chemical vapor deposition process is suitable for growing single crystalline tungsten oxide nanowires loaded with nickel oxide nanoparticles.

(Invited) Controllable Growth of High-Performance Gasb Thin Nanowires By a Surfactant Assisted CVD Process

Although various device structures based on GaSb nanowires have been realized, further performance enhancement suffers from uncontrolled radial growth during the nanowire synthesis, resulting in non-uniform and tapered nanowires with diameters larger than few tens of nanometers. However, high-quality and uniform GaSb NWs with diameters down to 20 nm have been prepared by using a sulfur surfactant technique in our previous works, which efficiently stabilizes the high-energy sidewalls for homogenous NW formation during the chemical vapor deposition (CVD) process.1 At the same time, the growth direction and corresponding carrier mobilities are still found to be heavily depended on the NW diameters, in which the <111>-orientated GaSb NWs exhibit the highest hole mobility because of the optimized surface passivation and roughness in preference to the minimized surface traps, indicating the potential applications of these phase pure NWs for high-performance electronics.2 Using CMOS-compatible Pd catalysts and a complementary experimental and theoretical approach, we demonstrate successfully the formation of high-performance, <111>-oriented GaSb nanowires (NWs) via vapor-solid-solid (VSS) growth mode in surfactant-assisted chemical vapor deposition. When back-gated field-effect transistors (FETs) are fabricated using these NWs, the Pd-catalyzed GaSb NW exhibits a superior peak hole mobility of ~330 cm2V-1s-1, and this value is close to the mobility limit for a NW channel diameter of ~30 nm with a free carrier concentration of ~1018 cm-3, indicating excellent homogeneity of the NW phase purity, the growth orientation, the surface morphology and the electrical characteristics. Contact printing process is also used to fabricate large-scale assembly of Pd-catalyzed GaSb NW parallel arrays, and a calculated peak hole mobility of ∼65cm2V-1s-1 is achieved, confirming the potential construction and application of these high-performance electronic devices. 3 Acknowledgment We acknowledge “Qilu young scholar” program of Shandong University.

InN Nanowires: Growth and Optoelectronic Properties

An overview on InN nanowires, fabricated using either a catalyst-free molecular beam epitaxy method or a catalyst assisted chemical vapor deposition process, is provided. Differences and similarities of the nanowires prepared using the two techniques are presented. The present understanding of the growth and of the basic optical and transport properties is discussed.

Surface nanostructuring via combined all atmospheric pressure processing, coating, patterning and etching

Manipulation of surface nanostructure is reported by a sequential multi stage novel combination of all atmospheric pressure based processing. Firstly a thermal chemical vapour deposition (CVD) process is used to deposit a thin zinc oxide film. A flame assisted CVD process is then used to produce the patterning layer. Subsequently an atmospheric pressure dielectric barrier discharge plasma is used to etch the surface to further nanostructure it. We report that this approach can be used to generate a low surface energy organic surface treatment using atmospheric pressure plasma enhanced chemical vapour deposition. The films were characterised by means of atomic force microscopy, transmission spectroscopy, scanning electron microscopy and contact angle.

On the carbo-thermal reduction of silica for carbon nano-fibre formation via CVD

ABSTRACTThe formation of carbon nanostructures using silica nanoparticles from quartz substrates as a catalyst in an aerosol assisted chemical vapor deposition process was examined. The silica particles are reduced to silicon carbide via a carbothermal reduction process. The recyclability of the explored quartz substrates is also presented. The addition of triethyl borate improves the efficiency of the carbothermal reduction process and carbon nanotubes formation. Moreover, the addition of hydrogen during the chemical vapor deposition leads to the helical carbon nanostructures formation.

Synthesis and mechanical properties of carbon nanotubes produced by the water assisted CVD process

AbstractCatalyst activity during the carbon nanotubes (CNTs) growth by chemical vapor deposition (CVD) is enhanced when water, a weak oxidizer, is introduced together with the carbon source. The height as well as the CNTs density can be controlled by fine‐tuning the water content. The characterization of the mechanical properties of CNTs produced by the water assisted CVD process clearly indicates that high quality materials are produced. CNTs with diameter smaller than 12 nm exhibit Young's modulus higher than 400 GPa.

The running-in of amorphous hydrocarbon tribocoatings: a comparison between experiment and molecular dynamics simulations

Abstract Amorphous hydrocarbon (a-C: H) films have enormous potential as low friction, wear resistant coatings. Here, we present a plasma assisted chemical vapour deposition process for a-C: H that exhibits growth rates of 100 nm min– 1 and higher. The tribological performance of the resulting a-C: H films has been studied experimentally by reciprocating sliding of an a-C: H-coated Si3N4 ball on an a-C: H-coated 100Cr6 steel substrate and by subsequent micro Raman spectroscopy of the wear track. Running-in of the coatings is observed and characterised by a rapid decrease in the friction coefficient accompanied by a significant increase in sp2 hybridisation in the wear track. In order to gain a deeper understanding of the underlying running-in mechanisms, the sliding of two a-C: H films under a load of 5 GPa has been studied by classical molecular dynamics employing a range-corrected Brenner bond-order potential. The simulations reproduce the experimental trends and explain the running-in by a combination of smoothing and chemical passivation of both tribosurfaces. Consequently, both mechanisms should be controlled in order to produce tribological coatings for applications with optimum energy-efficiency.

Hydrogen in Chemical Vapour Deposited Carbon Nanotubes: An Active Site for Functionalization

The presence of hydrogen in as-grown carbon nanotubes (CNTs) synthesized by microwave plasma (MP) chemical vapour deposition (CVD) technique is demonstrated. Our results showed that the MPCVD, as-grown CNTs were hydrogenated consisting of C-H bonds; whereas, the tubes synthesized by arc discharge consisted of non-hydrogenated multi-walled CNTs. Fourier transform infrared spectroscopy (FTIR) and micro-Raman spectroscopy techniques were used to detect C-H bonding in the as-grown CNTs. The effective functionalization of as-grown hydrogenated CNTs grown using a microwave CVD process is first time demonstrated by laser assisted CVD process. It was found that the laser-assisted CVD process resulted in the termination of hydrogen and the oxidation of as-grown CNT structure leading to the carboxylic group attachment. The FTIR results show the presence of -OH and C=O bonds in the functionalized samples. However, the non-hydrogenated CNTs could not be effectively functionalized by the same process, probably due to the fact that it did not contain active sites pre-requisite for functionalization, as did the CVD grown samples. The functionalization of CVD grown tubes is believed to take place at the 'active' hydrogen-terminated sites on the CNT surfaces.

Preparation of a multilayered composite catalyst for Fischer–Tropsch synthesis in a micro-chamber reactor

A 20 wt% Co/SiO2 catalyst grafted on a stainless steel substrate was used for Fischer–Tropsch synthesis (FTS) in a micro-chamber reactor. The fabrication of the catalyst was a complex process involving several steps. The first one, the pre-treatment of the AISI316L substrate through a plasma assisted chemical vapour deposition process (PACVD), allowed the formation of a silicon based polymer. This thin polymeric film was thermally degraded into a silicon oxide bonding layer. The coating of this bonding layer with sol–gel precursor of the Co/SiO2 catalysts was done with a spray coating process. The sequential combination of these steps led to a multilayered catalyst with a complex morphology. This material demonstrated high activity under FTS reaction. Activity and selectivity were higher under micro-chamber conditions than in a classical fixed-bed catalytic test. This was attributed to the increase of mass and heat transfers within the micro-device. Moreover distribution of metallic masses within the catalyst showed significant evolutions post FTS, including steel compound migrations and possible cobalt segregation.

Effect of gravity on the growth of vertical single-walled carbon nanotubes in a chemical vapor deposition process

Vertically aligned single-walled carbon nanotubes (SWCNTs) can be fabricated on the Co∕Si(100) substrate in a gravity assisted chemical vapor deposition process. The Co∕Si(100) substrate is tilted such that its surface normal points downwards in the chemical vapor deposition process. All the SWCNTs are lined up with the direction of gravity. The length of SWCNTs increases with growth time. A modified vapor-liquid-solid mechanism is proposed to explain the role of gravity in the precipitation of SWCNTs and in the lineup behavior of SWCNTs.

Aerosol and plasma assisted chemical vapor deposition process for multi-component oxide La0.8Sr0.2MnO3 thin film

It has been of increasing interest to develop a chemical vapor deposition (CVD) process to prepare thin films of multi-component oxides, such as fluorite and perovskite type materials, which exhibit unique functional properties and potential applications. The lack of the proper volatile precursors and the difficulty in composition control are the major barriers to sluggish the achievement of this purpose. Recently, we developed an aerosol assisted CVD method to deposit perovskite-type oxide La1−xSrxMnO3, a mixed oxygen ion/electron conductor. Furthermore, microwave plasma activation to the aerosol phase is applied to promote the deposition process. This paper reports briefly the preliminary results from this novel technique.

Scanning tunnelling microscopy of diamond deposition at the nanometre scale holes on highly orientated pyrolytic graphite

Recently, (1 0 0) highly textured diamond thin films were successfully synthesized on Si(1 0 0) [1], but complete heteroepitaxial diamond films have not yet been synthesized. This is because the mechanisms of nucleation and growth of diamond by chemical vapour deposition (CVD) are not fully understood. To clarify the mechanism of the nucleation process at the various nucleation sites, continuous observation of the nucleation and growth process in the same area, ideally in situ observation, is required. Scanning tunnelling microscopy (STM) is one of the candidates for this purpose, since STM uses a microscope with atomic resolution which can be operated in various environments, even in CVD conditions. Moreover, STM can also be used for atomic scale spectroscopy (scanning tunnelling spectroscopy) [2] and for the fabrication of nanometre scale structures [3]. For these reasons, STM is expected to be applied not only to the observation of the nucleation process but also to the fabrication of artificial nucleation sites. STM observations of nucleation in diamond CVD have been reported [4, 5]; however, roughened Si wafers scratched by diamond powder have been used exclusively as the substrates. In addition, different areas were observed before and after the depositions. Therefore, it has been impossible to correlate the nucleation process and nucleation sites so far, although information about growth on the nanometre scale have been reported. In this letter, we report the observation of diamond nucleation in a hot-filament assisted CVD process at the nanometre scale holes fabricated artificially on highly orientated pyrolytic graphite (HOPG) by STM. Fig. 1 shows a schematic diagram of the apparatus used in this study. A commercial STM (NanoScope II, Digital Instruments Inc.) was in the STM part, and the deposition was performed by a hot-filament assisted method in the CVD part. To transfer a substrate from the STM part to the CVD part, and vice versa, an Invar rod fixed on the end of a micrometre head was used with a guiding rail and a stopper. The deviation of the substrate position before and after transferring the substrate was less than 3 im, which was enough to access the same area of the substrate surface by the STM. To minimize the deviation of the position, the distance between the STM part and the CVD part should be as short as possible. However, the piezoelectric elements of STM do not function well in a high temperature environment, so they were separated by 10 mm with a thermal shield mounted between them. Deposition time was controlled by a shutter inserted between the filament and the substrate. All of the STM studies were carried out in the constant-current mode at room temperature after exposure to air with etched Pt–Ir tips (Materials Analytical Services). The tunnelling current (It) and the bias voltage (Vb) were 0.5 nA and 100 mV, respectively. HOPG was used as a substrate, since large atomically flat and inert surfaces can be obtained easily by cleaving. However, nucleation of diamond hardly occurs on the as-cleaved (0 0 0 1) graphite surface [6]. Prior to deposition in this study, therefore, 3 3 3 holes at intervals of 200 nm were fabricated on an HOPG substrate by STM in air by raising Vb to 4 V with It ˆ 0.5 nA for 1 s, with the aim of forming preferential nucleation sites [7]. Fig. 2a shows an STM image of the nine holes, of about 40 nm diameter. After transferring the substrate to the CVD part, the chamber was evacuated by a rotary pump, and then CVD was performed on the substrate without auxiliary heating. The experimental conditions were as follows: gases: 2% CH4 in H2 (CH4 1 sccm and H2 49 sccm); total pressure: 6.7 3 103 Pa (50 torr); Ta filament temperature: 2100 8C (measured by an optical pyrometer); fila-

The synthesis of large-area diamond thin films using straight tungsten wires mounted under tension as the filaments

Diamond thin films with an area larger than 40 × 40 mm 2 were synthesized using straight tungsten wires mounted under tension as the filaments in a hot-filament assisted chemical vapour deposition process. Hydrogen plus methane and acetone was used as feed gases. The nucleation of diamond on silicon substrate was examined by scanning electron microscopy (SEM). The deposited diamond films at different carbon concentrations (1.0–2.5%) were examined by X-ray diffraction, SEM and Raman spectroscopy.

Effect of Mechanical Stress on Polycrystalline Diamond Schottky Diode I-V Characteristics

ABSTRACTSchottky diodes to be used for mechanical stress effect studies were fabricated using aluminum contacts to polycrystalline diamond thin films grown by a hot-filament assisted chemical vapor deposition process. Compressive stress was found to have a large effect on the forward biased current-voltage characteristics of the diode. At selected values of constant forward biased current, a linear relationship between voltage and stress, for stress less than 10 N was observed. The stress sensitivity of the diode was as high as 0.74 V/N at 1 mA forward bias. This study shows that polycrystalline diamond Schottky diodes are stress sensitive devices and have potential as mechanical sensors.

A variable temperature x-ray photoelectron spectroscopic study of the surface conversion of diethylaluminum azide to AlN

Variable temperature x-ray photoelectron spectroscopy was used to study the thermal reaction of condensed thin films of diethylaluminum azide, yielding AlN. This precursor has been shown to be a useful chemical vapor deposition (CVD) reagent for the preparation of AlN thin films by a thermally assisted CVD process. Reaction progress was monitored easily with x-ray photoelectron spectroscopy (XPS) due to the chemical uniqueness and thus the spectral uniqueness of the various functionalities in the molecule. The results are consistent with a mechanism proposed in other studies. They indicate commencement of the surface reaction at about 300 °C and formation of AlN at temperatures above 500 °C. Activation of the diethylaluminum azide by various means causes the elimination of molecular nitrogen concomitant with the formation of an intermediate which is suggested to be a nitrene radical. This intermediate is found to have a high desorption energy, and, its subsequent reaction to yield AlN is confined to the surface.

Structural order in Si–N and Si–N–O films prepared by plasma assisted chemical vapor deposition process

Local bonding modes and the nearest-neighbor environment of the constituent atoms in glow discharge silicon nitride and oxynitride films have been probed with Fourier-transform infrared spectroscopy. The silicon nitride films prepared under a wide range of (NH3+N2)/SiH4 ratio and rf power levels all show characteristic absorption bands of N–H, Si–H, and Si–N bonds. The Si–H stretch mode absorption peak shifts to higher wave numbers with the increasing number of N–H bonds in the material. This behavior is observed for all power levels and gas flow combinations. The observed universal dependence of the Si–H stretch frequency on the concentration of the N–H bonds, and the sign of the shift indicate that the change in the frequency is caused by the increasing number of nitrogen atoms at the Si site. Arguments advanced on the basis of the electronegativity of the constituents suggest that the formation of N–H and Si–H bonds at the Si–N site leads to opposite shifts in the Si–N frequency. Thermal annealing studies show that the N–H and Si–H bonds dissociate at and above 600 °C. The Si–N stretch mode absorption in the oxynitride films shifts towards the higher wave numbers with the increasing number of Si–O bonds in the material. The refractive index correspondingly approaches to that of a-SiO2. The observed one-to-one correspondence between the Si–N peak position and the refractive index decisively indicates that the oxynitride films are comprised of a complete network of bonds between the Si, N, and O without any phase separation.

Deposition of TiN and Ti(O,C,N) hard coatings by a plasma assisted chemical vapor deposition process

Using conventional plasma nitriding equipment with a dc power supply, TiN coatings have been deposited from TiCl4, H2, N2, and Ar onto tool steel substrates at temperatures about 500 °C. Coatings of the type Ti(O,C,N) have also been deposited using an organic titanium compound instead of TiCl4. The advantage of this compound is the absence of any chlorine products in the deposited layers.