Laser-induced Chemical Vapor Deposition Research Articles

Position-controlled synthesis of single-walled carbon nanotubes on a transparent substrate by laser-induced chemical vapor deposition

The synthesis of single-walled carbon nanotubes (SWCNTs) on a transparent substrate with multiple-catalyst layer (Fe/Al/Cr: 0.5/15/500nm) using laser-induced chemical vapor deposition is reported. Ethylene (C2H4) mixed with hydrogen (H2) and a continuous wave Nd:YVO4 laser (532nm) were used as the precursor gas and the irradiation source, respectively. It was found that the density and quality of the SWCNT dots varied sensitively to laser irradiance and chamber pressure. From subsequent micro-Raman analyses at different excitation sources (488, 514, 633, and 785nm), the diameters of the SWCNTs were estimated to be within the range of 0.8–2nm and that the SWCNT dots were composed of both semiconducting and metallic SWCNTs. It is demonstrated that an array of SWCNT dots can be fabricated at precisely controlled positions of a transparent substrate at room temperature with no need of catalysis patterning.

The rapid growth of vertically aligned carbon nanotubes using laser heating

Growth of densely packed vertically aligned carbon nanotubes (VA-CNTs)using laser-induced chemical vapor deposition with visible laser (λ = 532 nm)irradiation at room temperature is reported. Using a multiple-catalyst layer (Fe/Al/Cr) on quartzas the substrate and an acetylene–hydrogen mixture as the precursor gas, VA-CNT pillars with 60 µm heightand 4 µm diameter were grown at a high rate of around 1 µm s−1 with good reproducibility. It is demonstrated that the fabrication of uniformpillar arrays of VA-CNTs can be achieved with a single irradiation for each pillarusing LCVD with no annealing or preprocessing of the substrate. Here, laser fastheating is considered the primary mechanism facilitating the growth of VA-CNTpillars. Field emission characteristics of an array of VA-CNT pillars were thenexamined to investigate their potential application in vacuum electronic devices.

Direct writing of carbon nanotube patterns by laser-induced chemical vapor deposition on a transparent substrate

Dot array and line patterns of multi-walled carbon nanotubes (MWCNTs) were successfully grown by laser-induced chemical vapor deposition (LCVD) on a transparent substrate at room temperature. In the proposed technique, a Nd:YVO 4 laser with a wavelength of 532 nm irradiates the backside of multiple catalyst layers (Ni/Al/Cr) through a transparent substrate to induce a local temperature rise, thereby allowing the direct writing of dense dot and line patterns of MWCNTs below 10 μm in size to be produced with uniform density on the controlled positions. In this LCVD method, a multiple-catalyst-layer with a Cr thermal layer is the central component for enabling the growth of dense MWCNTs with good spatial resolution.

A study of the Soret effect in laser-induced chemical vapor deposition

This paper presents an investigation of the modeling of the process of pyrolytic laser-induced chemical vapor deposition (LCVD) applied to study the Soret effect. LCVD is a thermally activated process characterized by strongly coupled mass and energy transport phenomena, together with chemical reactions, which are difficult to investigate experimentally. A physical and numerical model based on a commercial computational fluid dynamics package is developed and used to simulate a reactor operating at conditions of room temperature and pressure. The proposed numerical methodology will allow us to assess and analyze the effect of various factors controlling the process, and in particular the Soret effect. This numerical model is validated by comparison with the measured growth rate of the fiber. While several studies have proposed simulations of the LCVD process, this is among the first attempts at including the Soret effect in the numerical modeling at the micro-scale level. It is expected that the fundamental insights thus obtained will guide experimental investigations which can be applied to establish reactor design and process control guidelines.

Fabrication of micro carbon pillar by laser-induced chemical vapor deposition

Argon ion laser was used as the induced light source and ethane (C2H4) was selected as the precursor gas, in the variety ranges of laser power from 0.5 W to 4.5 W and the pressure of the precursor gas from 225×133.3 Pa to 680×133.3 Pa, the experiments of laser induced chemical vapor deposition were proceeded for fabrication of micro carbon pillar. In the experiments, the influences of power of laser and pressure of work gas on the diameter and length of micro carbon pillar were investigated, the variety on averaged growth rate of carbon pillar with the laser irradiation time and moving speed of focus was discussed. Based on experiment data, the micro carbon pillar with an aspect ratio of over 500 was built through the method of moving the focus.

Highly sensitive TGA diagnosis of thermal behaviour of laser-deposited materials

Thermal gravimetric analysis (TGA) combining the use of digital microbalances, gas-chromatography and mass spectroscopy is demonstrated as a highly sensitive technique for the determination of thermal behaviour of polymeric materials obtained in very low quantities in laser-induced chemical processes. The article is a brief review on some authors’ TGA studies of materials prepared by laser ablation of polymers, laser-induced chemical vapour deposition and laser back-side etching in the gas phase. The TGA-diagnosed materials are (i) chemical vapour-deposited N-containing polyoxocarbosilane and Fe/polyoxocarbosilane composites, (ii) crosslinked polymers ablatively deposited from pure and metal particles-loaded poly(vinyl acetate), poly(vinyl chloride-co-vinyl acetate) (PVACVC) and poly[oxy(tetramethyldisilane-1,2-diyl)] (POTMD), and (iii) carbon/polyoxocarbosilane composites obtained through gas-phase etching process.

Growth and modification of thin a-Si:H/a-Ge:H bi-layers to sacrificial c-SiGe alloys through ArF-Excimer laser assisted processing

A combination of ArF-Excimer laser assisted techniques has been used for depositing and modifying ultra thin amorphous Si/Ge bi-layer structures. The first step consisted in producing, at low substrate temperatures, thin bi-layer coatings through Laser induced Chemical Vapour Deposition (LCVD) in both, large areas as well as in small regions of Si(1 0 0) wafers. In the second step, these bi-layer structures have been modified through Pulsed Laser Induced Epitaxy (PLIE) for obtaining heteroepitaxial SiGe alloys with a thin buried Ge rich layer, while keeping a shallow upper Si rich surface with good crystalline quality. Threshold for epitaxial alloy formation has been determined by Raman spectroscopy and estimated to be above 200 mJ/cm 2. Optical profilometry has been used for evaluating the thickness of the structures and the lateral dimensions of patterned features. SEM, TOF-SIMS and XPS have been used to corroborate the results. For testing IC compatibility, some samples have been overgrown with epitaxial Si and etched through conventional IC processing techniques, revealing that the laser processed layers are suitable to be used as sacrificial layers for producing Micro-Electro-Mechanical Systems (MEMSs) or Silicon-on-Nothing (SON) devices.

Self-assembly of tunable and highly uniform tungsten nanogratings induced by afemtosecond laser with nanojoule energy

Tunable and highly uniform tungsten nanogratings on sapphires are demonstrated byscanning a 400 nm femtosecond-pulsed laser with pulse energy less than half of a nanojouleduring laser-induced chemical vapor deposition. These gratings possess very gooduniformity in both the grating period (standard deviation less than 1.5%) and the toothlength (less than 4%), and can be tuned by controlling the laser power and scanning speed.We show that nanogratings can be easily fabricated on non-planar substrates such as glassoptical fibers, and can be turned into larger-area one-dimensional gratings by anappropriate offset during multiple scans. A model based on local field enhancementand interference between the incident light and plasmon radiation is proposed toexplain the anisotropic shape and the periodic appearance of the observed tungstennanogratings.

Aligned carbon nanotubes catalytically grown on iron-based nanoparticles obtained by laser-induced CVD

Iron-based nanoparticles are prepared by a laser-induced chemical vapor deposition (CVD) process. They are characterized as body-centered Fe and Fe 2O 3 (maghemite/magnetite) particles with sizes ≤5 and 10 nm, respectively. The Fe particles are embedded in a protective carbon matrix. Both kind of particles are dispersed by spin-coating on SiO 2/Si(1 0 0) flat substrates. They are used as catalyst to grow carbon nanotubes by a plasma- and filaments-assisted catalytic CVD process (PE-HF-CCVD). Vertically oriented and thin carbon nanotubes (CNTs) were grown with few differences between the two samples, except the diameter in relation to the initial size of the iron particles, and the density. The electron field emission of these samples exhibit quite interesting behavior with a low turn-on voltage at around 1 V/μm.

Carbon nanotubes grown by catalytic CO 2 laser-induced chemical vapor deposition on core-shell Fe/C composite nanoparticles

The synthesis of carbon nanotubes (CNTs) by catalytic laser-induced chemical vapor deposition (C-LCVD) was investigated. C-LCVD uses both ex situ synthesized catalyst nanoparticles and the controlled decomposition of gas-phase hydrocarbon mixtures. As catalysts, Fe/C composites of the core-shell type were used. A continuous-wave CO 2 laser was employed to irradiate the ethylene/acetylene hydrocarbon precursors and to simultaneously heat a silicon substrate on which the carbon nanotubes were grown. The effects on carbon nanotube growth of both the iron-based nanocomposite particles and of the ethylene concentration were studied. The analysis suggests the feasibility of the C-LCVD process, in which the core-shell Fe/C catalysts comply with the prerequisite conditions of the CNT growth namely dispersion and supersaturation.

Demonstration of Microcoil Heaters for Microthrusters

The resilience and performance of carbon microcoil heaters made by laser-induced chemical vapor deposition (LCVD) to be used as an efficient means for increasing the specific impulse of cold/hot gas microthrusters were investigated. Two naked carbon coils and two tungsten coated carbon coils coated through LCVD were used in this experiment. Each pair of coated and uncoated carbon coils were heated resistively in a thermal cycling between 300-1173K and 973-1173K for 2 h in 0.000003 mbar and 2 bar N. The results show that at these temperatures the carbon microcoils and nitrogen propellant were compatible while the tungsten coated microcoils started degrading. It was observed that LVCD-deposited carbon and tungsten-coated carbon microcoils can withstand low to medium-high temperatures for extended periods of time during thermal cycling without showing signs of degradation.

Deposition of tungsten nanogratings induced by a single femtosecond laser beam

Tungsten nanogratings with sub-100nm linewidths and subwavelength periods are fabricated by laser-induced chemical vapor deposition using a single 400 nm femtosecond pulsed laser beam without any beam shaping. Combining advantages of parallel and direct-write processing, this method can produce various nanograting structures on a wide range of substrates in a single step.

Electrothermal characterization of tungsten-coated carbon microcoils for micropropulsion systems

Laser-induced chemical vapor deposition is used to deposit tungsten-coated carbon microcoils from the gas phase. Because the microcoils are used as heating elements in cold/hot gas microthrusters in nanosatellites, it is important to characterize their electrothermal behavior so that the performance of the thruster may be predicted. The coils are deposited using an argon-ion laser (wavelength 514.5nm) at 360mW and 930mbar of C2H4. Bent arms are then deposited at the ends of the 1mm long coils using 600mW of laser power and 700mbar of C2H4. The arms serve as electrical contacts and as mechanical supports to hold the coils in the thruster by a locking mechanism. A layer of tungsten is then applied to the carbon coils by the hydrogen-reduction of WF6 using a 20:1 (H2:WF6) pressure ratio at a total pressure of 105mbar and 400mW of laser power. High-resolution scanning electron microscopy analysis showed the tungsten coating to be 1.5–3.5μm thick on the body of the coil and less than 2μm on the contact arms. The tungsten coating reduced the resistance of the carbon coils by an order of magnitude and reached higher temperatures at lower voltages. In vacuum, the tungsten coating is the dominant current carrier below 1300–1700°C; above this range carbon dominates. Peak temperatures for the tungsten-coated coils are 2050°C in vacuum and 1940°C in N2 – several hundred degrees higher than the non-coated coils.

Open-air laser-induced chemical vapor deposition of silicon carbide coatings

This study demonstrates the open-air deposition of amorphous hydrogenated silicon carbide (a-SiC:H) by laser-induced chemical vapor deposition (LCVD) using an enclosureless (open-air) reactor system. Films are deposited on fused quartz substrates using the precursor gas trimethylsilane (TrMS). Based on Auger electron spectroscopy (AES), Fourier transform infrared absorption spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), the film's chemical composition and microstructure is determined to be composed of a form of silicon carbide (SiC:H) with organic moiety. To understand the temperature-dependence of film growth during deposition, varying deposition conditions were employed to correlate the SiC film deposition rate and deposition temperature.

The electrothermal feasibility of carbon microcoil heaters for cold/hot gas microthrusters

With the miniaturization of spacecraft the need for efficient, accurate and low-weight attitude control systems is becoming evident. To this end, the cold/hot gas microthruster system of this paper incorporates carbon microcoils—deposited via laser-induced chemical vapor deposition—for heating the propellant gas (nitrogen) before the nozzle inlet. By increasing the temperature of the propellant gas for such a system, the specific impulse (Isp) of the microthruster will increase. The benefits of a higher Isp are lower propellant mass, higher thrust and shorter burning times. Therefore, the feasibility of achieving this increase with the carbon microcoils is investigated. The carbon microcoils have been characterized experimentally with respect to their electrothermal performance, i.e. resistance, temperature, parasitic heat losses and degradation in ambient. The resulting heat losses from the heater and the heated gas have been estimated through a combination of experiments, numerical simulation and approximate analytical expressions. At high powers, degradation of the carbon material leads to coil failure in ambient where trace oxygen was present. Thus, the next generation of carbon microcoils to be tested will have a protective coating to extend their lifetime. Theoretical modeling showed that an increase in the propellant gas temperature from 300 to 1200 K and a corresponding two-fold increase in the Isp can be achieved if 1.0 W of power is supplied to each coil in a three-coil thruster. These simulation results show that if the coils are capable of dissipating 1 W of heat at 1700 K coil temperature, the doubling of the Isp may be achieved. Comparing to the electrothermal characterization results we find that the carbon coils can survive at 1700 K if protected, and that they can be expected to reach 1700 K at power below 1 W.

Growth kinetics and microstructure of carbon nanotubes using open air laser chemical vapor deposition

Open air laser-induced chemical vapor deposition (LCVD) was used to rapidly deposit a carbon nanotube forest on a moving substrate prepared with gold–palladium nanoparticles. Butane and propane were used as the carbon source, which was combined with hydrogen gas. Nanotube growth kinetics and alignment characteristics were strongly dependent on laser power density, substrate velocity, catalyst particle size, and the type of precursor mixture used. An increase in the hydrogen concentration improved the purity of the nanotubes by reducing the formation rate of amorphous carbon on the catalytic surface. A multi-stage growth model was proposed.

UV-laser-assisted processing of thin silicon–germanium–carbon films

The aim of enhancing the performance of solar cells and microelectronic devices through band-gap engineering as well as the demand of using cheap organic substrates or advanced materials like “high- k dielectrics” for nanoelectronic based technologies has caused an increasing attention in alternative processes for growing thin silicon–germanium–carbon alloys. Laser-induced chemical vapour deposition (LCVD), excimer laser-assisted crystallisation (ELC) and pulsed laser-induced epitaxy (PLIE) are such alternative techniques that are relatively cheap and capable to provide films in a wide range of composition and crystalline structure. This contribution shows some typical results achieved combining these laser-assisted techniques, thus demonstrating the potential of integrated laser-assisted “single-chamber” processes. The growth of amorphous hydrogenated SiGeC coatings at 250 °C with, on nanometer-scale, well-tailored thickness on large area as well as on small selected areas using LCVD is presented and discussed. The subsequent irradiation of these films in a single-chamber process for the dehydrogenation of the amorphous material, the modification of these coatings to polycrystalline or nanocrystalline films via ELC or to heteroepitaxial alloys on Si(100) wafers through PLIE, is also shown and evaluated. The selected samples that will be shown have been extensively studied through Fourier transform infrared and X-ray photoelectron spectroscopy, time of flight secondary ion mass spectrometry, Rutherford backscattering, X-ray diffraction, atomic force microscopy and scanning electron as well as conventional and high-resolution transmission electron microscopy.

Growth kinetics and microstructure of carbon deposited on quartz plates and optical fibers by open-air laser-induced chemical vapor deposition

Pyrolytic carbon has been successfully deposited on fused silica optical fibers and fused quartz plates by open-air laser-induced chemical vapor deposition. Raman spectroscopy, scanning electron microscopy, ion-beam sputtering and scanning white-light interferometry were used to investigate the microstructure, composition and uniformity of the carbon film. The pyrolytic carbon was derived from methane, acetylene, propane and butane precursors. Both the axial and the lateral growth rate and threshold deposition (pyrolysis) temperature of the carbon film deposited on a quartz plate were measured. Knowledge gained from deposition on fused quartz plates was used to determine the deposition conditions on optical fibers. The experimental results indicate that among the different precursors used in this study, only acetylene and propane can be used to deposit pyrolytic carbon on moving optical fibers because their threshold deposition temperature is sufficiently low to prevent fiber softening. The effect of laser power, moving fiber speed and precursor gas on the thickness, deposition rate and microstructure of the carbon layer were studied.

Continuous deposition of carbon nanotubes on a moving substrate by open-air laser-induced chemical vapor deposition

Continuous deposition of carbon nanotubes under open-air conditions on a moving fused quartz substrate is achieved by pyrolytic laser-induced chemical vapor deposition. A CO 2 laser is used to heat a traversing fused quartz rod covered with bimetallic nanoparticles. Pyrolysis of hydrocarbon precursor gas occurs and subsequently gives rise to rapid growth of a multi-wall carbon nanotube forest on the substrate surface. A “mushroom-like” nanotube pillar is observed, where a random orientation of carbon nanotubes is located at the top of the pillars while the growth is more aligned near the base. The typical carbon nanotube deposition rate achieved in this study is approximately 50 μm/s. At high power laser irradiation, various carbon microstructures are formed as a result of excessive formation of amorphous carbon on the substrate. High-resolution transmission and scanning electron microscopy, and X-ray energy-dispersive spectrometry are used to investigate the deposition rate, microstructure, and chemical composition of the deposited carbon nanotubes.

Investigation of open-air laser-induced chemical vapor deposition of carbon on moving optical fibers

Using a CO2 laser as a heat source, carbon coatings have been successfully deposited on moving optical fibers in an open-air laser-induced chemical vapor deposition (LCVD) reactor. Applications include fusion splice recoat and in-line coating of optical fibers. The relationship between operating parameters and the carbon deposition temperature and rate was investigated. Results indicate that they are strongly dependent on the laser power density and the optical fiber's traverse velocity. In order to provide a deeper understanding of the fundamental principles that govern laser heating and the carbon LCVD processes, a heat transport model was developed to predict the fiber surface temperature during deposition. The surface temperatures obtained from experiments compared well with the predicted temperature. Based on the temperature calculations, various kinetic parameters, including the activation energy EA and pre-exponential factor k0 for the carbon deposition process, are determined. The optical fiber signal loss at 1550 nm induced by the LCVD process is observed to be less than 10–3 dB at low laser power density.