Microwave Plasma-assisted Chemical Vapor Deposition Research Articles

Growth and characterisation of carbon nanostructures obtained by MPACVD system using CH 4/CO 2 gas mixture

In this study, CH 4/CO 2 gas mixture in Microwave Plasma Assisted Chemical Vapour Deposition (MPACVD) system is used for the growth of different Carbon Nanostructures (CNSs). Fe–SiO deposited on Si(100) in which Fe particles are embedded in a 50 nm SiO film were used as the catalyst. By varying the amount of CH 4 to CO 2, we have observed that it is possible to obtain a variety of CNSs including Carbon Nanotips (CNTPs) at 45% of CH 4, bundle of Multi-wall Carbon Nanotubes (MWCNTs) at 48–50% of CH 4, and Carbon Nanowalls (CNWs) at 53% of CH 4 at a temperature around 900 °C. The diagnostic of the used plasma discharge is carried out by optical emission spectroscopy (OES). It shows the presence of atomic hydrogen H, CH, C 2, CO and OH radicals. Scanning electron microscopy observations show that the obtained CNTPs are not perpendicular to the surface, due to the no applied polarisation to the substrate holder during the growth. The top of the CNTPs size and length are respectively around 10 nm and 3 μm. Raman spectroscopy measurements exhibit a variation in the D and G lines intensity when varying the CH 4 amount in the gas mixture. Correlations between OES characterisations and microstructural analyses are established.

Tribological testing of self-mated nanocrystalline diamond coatings on Si 3N 4 ceramics

Due to their much lower surface roughness compared to that of microcrystalline diamond, nanocrystalline diamond (NCD) films are promising candidates for tribological applications, in particular when deposited on hard ceramic materials such as silicon nitride (Si 3N 4). In the present work, microwave plasma-assisted chemical vapour deposition of NCD is achieved using Ar/H 2/CH 4 gas mixtures on plates and ball-shaped Si 3N 4 specimens either by a conventional continuous mode or by a recently developed pulsed regime. The microstructure, morphology, topography and purity of the deposited films show typical NCD features for the two kinds of substrate shapes. Besides, tribological characterization of the NCD/Si 3N 4 samples is carried out using self-mated pairs without lubrication in order to assess their friction and wear response. Worn surfaces were studied by SEM and AFM topography measurements in order to identify the prevalent wear mechanisms. Friction values reached a steady-state minimum of approximately 0.02 following a short running-in period where the main feature is a sharp peak which attained a maximum around 0.44. Up to the critical load of 35 N, corresponding to film delamination, the equilibrium friction values are similar, irrespective of the applied load. The calculated wear coefficient values denoted a very mild regime ( K ∼ 1 × 10 − 8 mm 3 N − 1 m − 1 ) for the self-mated NCD coatings. The predominant wear mechanism was identified as self-polishing by micro-abrasion.

Carbon Nanotubes as Potential Cold Cathodes for Vacuum Microelectronic Applications

ABSTRACTMaterials science is playing a dramatic role in discovering new materials with tailored physical properties. Cold cathodes/field emitters are one of the examples. Electron field emitting materials are of vital importance for a variety of vacuum microelectronic devices including field emission displays for flat panel displays, electron microscopes, X-ray generators, and vacuum lamps. This is the driving force to investigate the advanced nanostructured carbons as cold cathodes as one of the potential candidates. Recently, they are also being proposed for thermionic power generators. The rationale is that reducing one or more dimensions of a system below some critical length changes the systems' physical properties, where carbon nanotubes (CNTs) in the class of carbon nanostructures serve as a model example. In this paper, synthesis and characterization of vertically aligned multiwall and single-/double-wall carbon nanotube films using a microwave plasma-assisted chemical vapor deposition technique for vacuum microelectronics is presented. Recent advances in their synthesis, processing, and characterization indicate that the above mentioned potential is slowly being realized. Experiments showed that by continuous reduction in the thickness of the catalyst film produces hollow concentric tubes in contrast to bamboo-like multiwalled tubes with larger thickness. To assess the electron field emission properties, besides the traditional field emission (I-V) properties, temperature dependent field electron emission microscopy (T-FEEM) enabling real-time imaging of electron emission providing information on emission site density, temporal variation of the emission intensity, and insight into the role of adsorbates from nanotube films will be discussed. Physics based models (such as negative or low electron affinity, geometric enhancement, surface dipole, tunneling due to adsorbates, structure modification due to doping etc.) will be described to support the experimental observations in addition to weak thermionic field emission contribution. These findings provided a great insight into the field emission mechanism and a contrasting comparison between small and large diameter carbon nanotubes.

Irradiation-induced structural modifications in multifunctional nanocarbons

ABSTRACTSevere environmental tolerability is the prime factor in the development of novel space materials exhibiting excellent physical properties accompanied by lightweight, reusability, and multifunctional capabilities. Diamond is known for its reputation being radiation hard besides a range of outstanding properties (electronic, optical, mechanical, and chemical) and hence it is preferable in harsh environments. Carbon nanotubes are also of great interest because of several unsurpassable physical properties and it needs to be shown that they are physically stable and structurally unaltered when subjected to irradiation. Therefore, a family of novel nanocarbons (nanodiamond and nanotubes) films deposited by microwave plasma-assisted chemical vapor deposition (MWCVD) technique was subjected to gamma radiation (1, 5, and 20 Mrads) and to medium energy electron-beam irradiation to study their effects on the microscopic structure and corresponding physical properties to establish property-structure correlation. Microstructural and physical properties characterizations prior to and post-irradiation include scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy (RS), X-ray diffraction (XRD), field emission (FE), and high-resolution transmission electron microscopy (HRTEM). Dramatic improvement in the field emission properties for microcrystalline diamond and relatively small but systematic behavior for nanocrystalline diamond with increasing radiation dose is discussed in terms of the critical role of defects which tends to form clusters. The results also indicate that nanocrystalline carbon tends to reach a state of damage saturation they are discussed in terms of sp3, sp2+δ ←→ sp2 C inter-conversion. The effects of electron beam irradiation on the carbon nanotube show that multi¡Vwalled nanotubes tend to be relatively more robust than those of single–walled. This is because increased exposure on an individual bundle of single-walled nanotubes promoted graphitization, pinching, and cross-linking similar to polymers forming intra-molecular junction (IMJ) within the area of electron beam focus, possibly through aggregates of amorphous carbon. Formation of novel nanostructures (nano– ring and helix– V like) due to irradiation is also observed. These studies gleam on the dynamics of nano-manufacturing and a regime of possible relevance to these materials for a) short-term space missions; b) radiation hard programmable logic circuits; and c) radiation pressure sensors.

Soft X-ray angle-resolved photoemission spectroscopy of heavily boron-doped superconducting diamond films

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of microwave plasma-assisted chemical vapor deposition diamond films with different B concentrations in order to study the origin of the metallic behaviorof superconducting diamond. SXARPES results clearly show valence band dispersions with a bandwidth of ~ 23 eV and with a top of the valence band at gamma point in the Brillouin zone, which are consistent with the calculated valenceband dispersions of pure diamond. Boron concentration-dependent band dispersions near the Fermi level (Ef) exhibit asystematic shift of Ef, indicating depopulation of electrons due to hole doping. These SXARPES results indicate that diamond bands retain for heavy boron doping and holes in the diamond band are responsible for the metallic states leading to superconductivity at low temperature. A high-resolution photoemission spectroscopy spectrum near Ef of a heavily boron-doped diamond superconductor is also presented.

Composition, mechanical properties and friction behavior of nickel/hydrogenated amorphous carbon composite films

Nickel/hydrogenated amorphous carbon composite films have been deposited on silicon and stainless steel substrates by combining sputter-deposition of metal and microwave plasma-assisted chemical vapor deposition of carbon from argon–methane mixtures of various concentrations. The composition and crystallographic structure of films were investigated as functions of the CH 4 concentration by Rutherford backscattering spectroscopy and X-ray diffraction techniques, respectively. The carbide phase, Ni 3C, was detected in Ni/C films deposited from a gas phase containing more than 8 vol.% of CH 4. The grain size of Ni and Ni 3C was determined as a function of the carbon content. The maximum magnitude of the compressive residual stresses was 0.6 GPa for films containing 25 at.% of carbon (Ni 3C). At carbon concentrations above 50 at.%, the stress magnitude diminished to 0.2 GPa. The hardness and elastic modulus of films deduced from nanoindentation measurements were studied as functions of the carbon content. The ball-on-disk tribological tests were conducted in room air at the temperature of 20 °C under a load of 1 N with a sliding speed of 50 mm/s. The friction coefficient was determined as a function of the carbon content in the films. The minimum value of 0.25 was obtained from films containing about 75 at.% of carbon.

Low temperature growth of single-walled carbon nanotubes: Small diameters with narrow distribution

Low temperature (500–700 °C) synthesis of high-quality single-walled carbon nanotubes (SWNTs) via microwave plasma assisted chemical vapor deposition (MPCVD) was demonstrated by pyrolysis of methane over the Fe–Mo bimetallic catalyst nanoparticles supported on porous MgO powders. In comparison with higher temperature (800 °C) growth by the conventional thermal CVD using the same batch of catalyst, the SWNTs grown by the low temperature MPCVD show small diameters with narrow distribution. Moreover, it was found that the lower the growth temperature, the smaller and more homogenous the tube diameter, showing the feasibility of controlling the SWNTs diameters by tuning the growth temperature.

Electrical properties of thin nanocrystalline diamond based structures

Electrical structures based on thin films of nanocrystalline diamond are described as well as their electrical properties. Diamond films with thickness between 0.25 and 1.0 μm and with average grain sizes of 300 nm and less are formed by microwave plasma-assisted chemical vapor deposition. Electrical properties of free-standing diamond films and of substrate mounted films are measured over a wide range of voltages with two-terminal electrical contacts. The films exhibited field-activated conductivity consistent with a Poole–Frenkel reduction in ionization energy associated with overlapping Coulombic potentials, well-modeled by Poole's Law. Metallization was performed on diamond films supported on the original silicon substrate and also on free-standing diamond. For the latter, the silicon substrate was removed by back-etching and the diamond film was transferred to a different structure. Both symmetrical and asymmetrical current–voltage characteristics have been obtained. Depending on the type of contacts, the rectification ratios of two-terminal structures range from 1 to 11,000 at high applied voltages.

Synthesis of thick, uniform, smooth ultrananocrystalline diamond films by microwave plasma-assisted chemical vapor deposition

The deposition of uniform, low-stress, thick and thin films of ultrananocrystalline diamond (UNCD) is investigated. The process methods and apparatus that enable the uniform and smooth deposition of both thin and thick (> 50 μm) UNCD across 3 in. diameters are described. UNCD films are synthesized by microwave plasma-assisted CVD using Ar/H 2/CH 4 input gas mixtures over a wide pressure range (60–240 Torr) and temperature range (400–850 °C). Films were grown on 3 in. diameter silicon substrates with thicknesses ranging from 58 nm to greater than 50 μm. Film surface roughness as low as 10 nm (AFM) was obtained. Film uniformities of 70 to over 95% were achieved on 3 in. diameter silicon substrates. The growth rate increased as pressure, percent hydrogen and percent methane in the gas mixture, and microwave power increased. The highest growth rate 1.12 μm/h was achieved at 180 Torr, H 2/Ar/CH 4 = (4:100:2) sccm and 1.5 kW absorbed power. Overall, a robust, repeatable process has been demonstrated for the deposition of UNCD films.

Large-sized tubular graphite cones with nanotube tips

Tubular graphite cones (TGCs) have been grown on planar steel substrates by microwave plasma-assisted chemical vapor deposition with a high concentration of methane and at a high substrate temperature. The largest TGCs can reach 110μm in length and 10μm in diameter at the root. Unique TGCs terminated in long extruding carbon nanotube tips are realized. Scanning micro-Raman spectroscopy of individual TGCs shows a high crystallinity of the tips and more disordered structure of the roots. A possible growth mechanism of TGCs is presented.

Superconductivity in polycrystalline diamond thin films

Superconductivity was discovered in heavily boron-doped diamond thin films deposited by the microwave plasma assisted chemical vapor deposition (MPCVD) method. Advantages of the MPCVD deposited diamond are the controllability of boron concentration in a wide range, and a high boron concentration, especially in (111) oriented films, compared to that of the high-pressure high-temperature method. The superconducting transition temperatures are determined to be 8.7 K for T c onset and 5.0 K for zero resistance by transport measurements. And the upper critical field is estimated to be around 7 T.

Experimental study of a pulsed microwave plasma assisted chemical vapour deposition of carbon nanotubes

AbstractIn this study we present the first results achieved on the growth of carbon nanotubes (CNT) in both continuous mode (CM) and pulsed mode (PM) microwave plasma assisted chemical vapour deposition (MPACVD). The interest of this work is to determine the influence of the discharge parameters on the CNT properties such as diameter, length, density … etc. The experiments were carried out at a sustrate temperature around 900 °C. The catalyst used was nanostructured Fe–SiO deposited on silicide. It was found that the formation of multiwalls carbone nanotubes (MWCNTs) occurs when varying the discharge parameter from the CM to the PM and a selective growth of bundles of MWCNTs is obtained in the optimal PM plasma mode conditions. The measured diameter of isolated MWCNTs in the CM is around 20 nm. We measured a decrease of the amorphous carbon structures by a factor 7 and an increase of the CNT density by a factor 3 from the CM to the PM, confirming that the pulsed mode is more suitable for the MWCNT growth. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of Free-Standing Diamond Films on Stainless Steel

Free-standing diamond films have been successfully deposited on stainless steel substrates using microwave plasma-assisted chemical vapor deposition. Although iron, which is the main element of stainless steel, is known to inhibit the nucleation of diamond and enhance the formation of graphite, we were able to grow relatively thick films (- 1.2 mm). The films were easily detachable from the substrates. The poor adhesion made it possible to obtain free-standing diamond films without chemical etching. Raman spectroscopy showed the 1332 cm-1 characteristic Raman peak of diamond and the 1580 cm-1, 1350 cm-1 bands of graphite on the growth surface and backside of the films, respectively. By energy dispersive x-ray spectroscopy it was only possible to detect iron on the back of the films, but not on the surface. The role of iron in the film growth is discussed.

Preparation and Characterization of Boron-Doped Diamond Powder: A Possible Dimensionally Stable Electrocatalyst Support Material

Electrically conducting diamond powder was prepared by coating insulating diamond powder ( diam, ) with a thin boron-doped layer using microwave plasma-assisted chemical vapor deposition. Deposition times from 1 to 6 h were evaluated. Scanning electron microscopy (SEM) revealed that the diamond powder particles become more faceted and more secondary growths form with increasing deposition time. Fusion of neighboring particles was also observed with increasing growth time. The first-order diamond phonon line appeared in the Raman spectrum at ca. for deposition times up to 4 h, and was downshifted to as low as for some particles after the 6-h growth. Electrical resistance measurements of the bulk powder (no binder) confirmed that a conductive diamond overlayer formed, as the conductivity increased from near zero (insulating, ) for the uncoated powder to after the 6-h growth. Ohmic behavior was seen in current-voltage curves recorded for the 4-h powder between . Cyclic voltammetric i-E curves for and were recorded to evaluate the electrochemical properties of the conductive powder when mixed with a polytetrafluoroethylene binder. At scan rates between 10 and , for both redox systems was high, ranging from 140 to 350 mV, consistent with significant ohmic resistance within the powder/binder electrode. Our results at this point suggest that the resistance is mainly due to poor particle-particle connectivity. Anodic polarization at 1.6 V vs for 1 h (25°C) was performed to evaluate the morphological and microstructural stability of the conductive diamond in comparison with graphite and glassy carbon (GC) powders. The total charge passed during polarization was largest for the GC powder and smallest for conductive diamond powder . SEM images taken of conductive diamond powder after polarization showed no evidence of microstructural degradation, while significant morphological and microstructural changes were seen for the GC powder.

Infrared absorption study of hydrogen incorporation in thick nanocrystalline diamond films

We present an infrared (IR) optical absorbance study of hydrogen incorporation in nanocrystalline diamond films. The thick nanocrystalline diamond films were synthesized by microwave plasma-assisted chemical vapor deposition and a high growth rate about 3.0μm∕h was achieved. The morphology, phase quality, and hydrogen incorporation were assessed by means of scanning electron microscopy, Raman spectroscopy, and Fourier-transform infrared spectroscopy (FTIR). Large amount of hydrogen bonded to nanocrystalline diamond is clearly evidenced by the huge CH stretching band in the FTIR spectrum. The mechanism of hydrogen incorporation is discussed in light of the growth mechanism of nanocrystalline diamond. This suggests the potential of nanocrystalline diamond for IR electro-optical device applications.

Increased field-emission site density from regrown carbon nanotube films

Electron field-emission properties of as-grown, etched, and regrown carbon nanotube thin films were investigated. The aligned carbon nanotube films were deposited by the microwave plasma-assisted chemical vapor deposition technique. The surface of the as-grown film contained a carbon nanotube mat of amorphous carbon and entangled nanotubes with some tubes protruding from the surface. Hydrogen plasma etching resulted in the removal of the surface layer, and regrowth on the etched surface displayed the formation of a new carbon nanotube mat. The emission site density and the current–voltage dependence of the field emission from all of the samples were analyzed. The results showed that the as-grown sample had a few strong emission spots and a relatively high emission current density (∼20μA∕cm2 at 1V∕μm), while the regrown sample exhibited a significantly increased emission site density.

Correlation between the characteristics of a pulsed microwave plasma used for diamond growth and the properties of the produced films

Previous studies have pointed out the advantages of using a pulsed discharge for diamond growth by microwave plasma assisted chemical vapour deposition (MPACVD): increase of the growth rate and/or enhancement of the diamond quality. It has been shown that the duration of the pulse and that of the pause between two pulses have a particular influence on the deposition process. Optimal values can be determined that depends mainly on the geometry and size of the reactor and on the working conditions.In the present work, new results are obtained in a MPACVD reactor working in a pulsed plasma mode, H2/CH4 gas mixture and 6 kW peak power. These conditions are similar to those of industrial reactors. The most important species in such process are CHx, C2Hy and H, which are considered, respectively, as the key species for diamond deposition, graphite deposition and graphite etching. A particular interest was devoted to the atomic hydrogen evolution during the discharge phase and the afterglow. Quantitative measurements of H-atom ground state concentration are carried out by two-photon Laser Induced Fluorescence (LIF). The study of the kinetic processes which are responsible of atomic hydrogen creation and loss permits us to determine the production rate of H-atoms in pulsed mode as compared with the continuous one (CW), and leads to a better understanding of its influence on the diamond growth rate and its chemical quality as estimated by micro-Raman spectroscopy.The correlation established between the results of the diamond film analysis and the plasma characteristics points out the benefit of this study for the optimisation and the control of the diamond growth process by pulsed MPACVD.

Excitonic emission and N‐ and B‐incorporation in homoepitaxial CVD‐grown diamond investigated by cathodoluminescence

Diamond is a very large bandgap material arising high expectations either for optoelectronic applications or for active semiconducting layers in specific electronic devices to be used under extreme conditions of pressure, temperature, wear or radiation, as well as in chemically aggressive environments. Unintentionally boron-doped diamond layers were grown by microwave plasma-assisted chemical vapour deposition (CVD) on {001}-oriented undoped Ib substrates with the addition of oxygen gas during growth. The relative quantities of nitrogen and boron incorporated in the diamond lattice are evaluated by cathodoluminescence (CL) spectra recorded at 5 K. Two different effects are shown to limit nitrogen incorporation: the substrate crystalline quality and the addition of oxygen into the precursor during the growth. First, the CL spectra are shown to change strongly near the edges of the substrate in the regions corresponding to different bulk crystal growth modes. Some regions show a luminescence governed by UV emission while in other regions, where the H3 defect-related luminescence of the substrate is much stronger, the film UV emission is reduced. Second, the relative importance of the free exciton emission with respect to those from the nitrogen-related H3 centre and from the boron-bound exciton is shown to increase with the addition of oxygen during growth. Such observations are of first importance to improve the spectral emission and absorption threshold of the diamond material in the deep UV range. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrochemical characteristics of boron-doped, undoped and nitrogen-doped diamond films

Boron-doped, undoped and nitrogen-doped diamond films were synthesized by microwave plasma assisted chemical vapor deposition (MP-CVD). Raman spectroscopy, XPS, EPMA and UV–Vis were used to characterize the properties of the synthesized films. Electrochemical characteristics for several redox systems on the three kinds of diamond films were examined. For Li +/Li ( E 0=−3.05 V) and H +/H 2 ( E 0=0.00 V) redox couples, the marked differences in cyclic voltammetric (CV) behaviors were observed on the nitrogen-doped diamond films, whereas for Fe(CN) 3−/ 4− ( E 0=0.36), Au/AuCl 4 − ( E 0=1.00) and O 2/H 2O ( E 0=1.23 V) couples, the CV behaviors on the nitrogen-doped films were similar to those on the boron-doped or undoped diamond films. The significant differences of CV behaviors could be explained by hypothesizing that the electron transfers of the redox species in the solution on diamond electrodes happened at the top of valence band together with the surface doping model suggested by F. Maier and colleagues [F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472].

Nanocrystalline diamond coating of silicon nitride ceramics by microwave plasma-assisted CVD

The thermal and chemical compatibility of silicon nitride ceramics (Si 3N 4) to diamond ensures an adequate adhesion for tribological applications, such as mechanical seals, biomaterials or cutting tools for hard machining operations. Most of the recent research efforts on these components focused on nanocrystalline diamond (NCD) due to its small grain size and surface smoothness. The present study reports for the first time NCD coating of Si 3N 4 substrates by microwave plasma-assisted chemical vapour deposition (MPACVD) using Ar/H 2/CH 4 gas mixture and varying the substrate temperature (650–1050 °C) and deposition time. Optimal deposition conditions result in growth rates up to 3.0 μm h −1 that allowed the production of continuous NCD films with low roughness (Ra≈22 nm) and grain size of about 10 nm at the lower temperatures and longer deposition times.