Microwave Plasma-enhanced Chemical Vapor Deposition Process Research Articles

Diamond nucleation in carbon films on Si wafer during microwave plasma enhanced chemical vapor deposition for quantum applications

Nucleation is important in processing of good quality diamond crystals and textured thin films by microwave plasma enhanced chemical vapor deposition (MPECVD) for applications in quantum devices and systems. Bias-enhanced nucleation (BEN) is one approach for diamond nucleation in situ during MPECVD. However, the mechanism of diamond nucleation by BEN is not well understood. This paper describes results on the nucleation of diamond within a carbon film upon application of electric field during the BEN-facilitated MPECVD process. The nature of the diamond film and nuclei formed is characterized by SEM (scanning electron microscopy), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM). The HRTEM images and associated diffraction patterns of the nucleation layer show that the diamond nuclei are formed within the carbon film close to the Si (100) substrate surface under the influence of microwaves and electric fields that lead to formation of the textured diamond film and crystal upon further growth. These results are expected to develop diamond films of optimum quality containing a nitrogen-vacancy center for application in quantum systems.

Catalyst-Less and Transfer-Less Synthesis of Graphene on Si(100) Using Direct Microwave Plasma Enhanced Chemical Vapor Deposition and Protective Enclosures.

In this study, graphene was synthesized on the Si(100) substrates via the use of direct microwave plasma-enhanced chemical vapor deposition (PECVD). Protective enclosures were applied to prevent excessive plasma etching of the growing graphene. The properties of synthesized graphene were investigated using Raman scattering spectroscopy and atomic force microscopy. Synthesis time, methane and hydrogen gas flow ratio, temperature, and plasma power effects were considered. The synthesized graphene exhibited n-type self-doping due to the charge transfer from Si(100). The presence of compressive stress was revealed in the synthesized graphene. It was presumed that induction of thermal stress took place during the synthesis process due to the large lattice mismatch between the growing graphene and the substrate. Importantly, it was demonstrated that continuous horizontal graphene layers can be directly grown on the Si(100) substrates if appropriate configuration of the protective enclosure is used in the microwave PECVD process.

Synthesis of SiV-diamond particulates via the microwave plasma chemical deposition of ultrananocrystalline diamond on soda-lime glass fibers

We report the synthesis of silicon-vacancy (SiV) incorporated spherical shaped ultrananocrystalline diamond (SiV-UNCD) particulates (size ∼1 μm) with bright luminescence at 738 nm. For this purpose, different granular structured polycrystalline diamond films and particulates were synthesized by using three different kinds of growth plasma conditions on the three types of substrate materials in the microwave plasma enhanced CVD process. The grain size dependent photoluminescence properties of nitrogen vacancy (NV) and SiV color centers have been investigated for different granular structured diamond samples. The luminescence of NV center and the associated phonon sidebands, which are usually observed in microcrystalline diamond and nanocrystalline diamond films, were effectively suppressed in UNCD films and UNCD particulates. Micron sized SiV-UNCD particulates with bright SiV emission has been attained by transfer of SiV-UNCD clusters on soda-lime glass fibers to inverted pyramidal cavities fabricated on Si substrates by the simple crushing of UNCD/soda-lime glass fibers in deionized water and ultrasonication. Such a plasma enhanced CVD process for synthesizing SiV-UNCD particulates with suppressed NV emission is simple and robust to attain the bright SiV-UNCD particulates to employ in practical applications.

Effect of Experimental Factors in the Growth of Carbon Nanotubes from CO2 by MPECVD Process

The effects of experimental factors such as type of catalyst (nickel and cobalt) and substrate (iron and silicon wafer) in the growth of carbon nanotubes (CNT) from CO2 by microwave plasma-enhanced chemical vapor deposition (MPECVD) was systematically studied. Catalyst size and CNT grown were examined using scanning electron microscope (SEM). Furthermore, gas chromatography (GC) was used to analyze the effluent gas. Moreover, suitable type of catalyst and substrate were determined in terms on the amount of CNT grown, purity, and carbon conversion.Keywords : carbon nanotubes, chemical vapor deposition, nanotechnology

Transformation of polymer composite nanofibers to diamond fibers and films by microwave plasma-enhanced CVD process

In this work, polyvinyl alcohol (PVA) fibers were used as a polymer matrix containing ultra-dispersed diamond (UDD) nanoparticles. Growth of diamond fiber-like structures and films by microwave plasma-enhanced chemical vapor deposition was studied as a function of UDD concentration in the PVA matrix. The influence of surface tension (fibers radii) for nucleation/seeding is discussed. Using a high UDD concentration in the polymer matrix lead to the formation of fiber-like structures. The composite PVA polymer nanofibers with the highest concentration of UDD nanoparticles resulted in the growth of nearly continuous diamond film at low thickness of 250 nm.

Carbon nanowalls grown by microwave plasma enhanced chemical vapor deposition during the carbonization of polyacrylonitrile fibers

We used microwave plasma enhanced chemical vapor deposition (MPECVD) to carbonize an electrospun polyacrylonitrile (PAN) precursor to form carbon fibers. Scanning electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy were used to characterize the fibers at different evolution stages. It was found that MPECVD-carbonized PAN fibers do not exhibit any significant change in the fiber diameter, whilst conventionally carbonized PAN fibers show a 33% reduction in the fiber diameter. An additional coating of carbon nanowalls (CNWs) was formed on the surface of the carbonized PAN fibers during the MPECVD process without the assistance of any metallic catalysts. The result presented here may have a potential to develop a novel, economical, and straightforward approach towards the mass production of carbon fibrous materials containing CNWs.

A Comparative Study of Three Different Chemical Vapor Deposition Techniques of Carbon Nanotube Growth on Diamond Films

This paper compares between the methods of growing carbon nanotubes (CNTs) on diamond substrates and evaluates the quality of the CNTs and the interfacial strength. One potential application for these materials is a heat sink/spreader for high‐power electronic devices. The CNTs and diamond substrates have a significantly higher specific thermal conductivity than traditional heat sink/spreader materials making them good replacement candidates. Only limited research has been performed on these CNT/diamond structures and their suitability of different growth methods. This study investigates three potential chemical vapor deposition (CVD) techniques for growing CNTs on diamond: thermal CVD (T‐CVD), microwave plasma‐enhanced CVD (MPE‐CVD), and floating catalyst thermal CVD (FCT‐CVD). Scanning electron microscopy (SEM) and high‐resolution transmission electron microscopy (TEM) were used to analyze the morphology and topology of the CNTs. Raman spectroscopy was used to assess the quality of the CNTs by determining the ID/IG peak intensity ratios. Additionally, the CNT/diamond samples were sonicated for qualitative comparisons of the durability of the CNT forests. T‐CVD provided the largest diameter tubes, with catalysts residing mainly at the CNT/diamond interface. The MPE‐CVD process yielded non uniform defective CNTs, and FCT‐CVD resulted in the smallest diameter CNTs with catalyst particles imbedded throughout the length of the nanotubes.

Effect of Oxygen for Diamond Film Synthesis with C-Hexane in Microwave Plasma Enhanced CVD Process

The purpose of this paper is to decide the optimum synthesis conditions of polycrystalline diamond films according to the ratio of gas mixture. Diamond films were deposited with cyclo-hexane as a carbon precursor by the microwave plasma enhanced chemical vapor deposition process. The optimum oxygen ratio to cyclo-hexane was reached about 125 % under the fixed 0.3% c-hexane in hydrogen. Oxygen plays a role in etching the graphitic components of carbon sp2 bond effectively. By OES measurement, the best synthesis conditions found out about 12.5 % and 15.75 %, which is the emission intensity ratios of CH(B-X) and Hβ on Hα, respectively. Also, the electron temperature was similar about 5,000 to 5,200 K in this work.

N-ion implantation of micro‐nanocrystalline duplex structured diamond films for enhancing their electron field emission properties

The improvement on the electron field emission (EFE) properties of duplex-structured diamond films by N-ion implantation/post-annealing processes was investigated. The duplex-structured diamond films were synthesized by a two-step microwave plasma enhanced CVD process. Transmission electron microscopy (TEM) examinations reveal that all the as-prepared, N-ion implanted and post-annealed diamond films contained large microcrystalline-diamond (MCD) aggregates sparsely distributed among the matrix of ultra-small diamond grains. While the granular structure of the MCD aggregates was insignificantly modified due to the N-ion implantation/post-annealing processes, that of the UNCD regions was markedly altered. The EFE process for the MCD/UNCD films can be turned on at (E0)MCD/UNCD=8.21V/μm, which is even smaller than the E0-field for the UNCD films ((E0)UNCD=13.34V/μm). These N-ion implanted/post-annealed diamond films attained an EFE current density of (Je)MCD/UNCD=0.4mA/cm2 at an applied field of 20.0V/μm that is even larger than the Je-value for the UNCD films ((Je)UNCD<0.05mA/cm2 at the same applied field). Presumably, the enhanced EFE properties are resulted from the presence of nano-graphites in the small-grain region of MCD/UNCD films that form an interconnected path, facilitating the transport of electrons.

The role of nano-graphite phase on the enhancement of electron field emission properties of ultrananocrystalline diamond films

Here, we demonstrated with examples that the presence of nano-graphite phase markedly enhanced the electron field emission (EFE) properties of the ultrananocrystaline diamond films. The nano-graphite phase was induced by different mechanism. In the two-step microwave plasma enhanced CVD (MPECVD) process, the nano-graphite phase was induced at the interfaces of large-grain and ultra-small-grains regions, which was clearly illustrated by transmission electron microscopy (TEM). The EFE process was thus markedly enhanced, viz. low turn-on field of 9.2V/μm with large emission current density of 800μA/cm2 at 27.5V/μm was achieved. In the Au-ion irradiation of UNCD films, the presence of nano-graphite phase was not clearly resolved by TEM but was strongly inferred by the Raman spectroscopy. The induction of graphitic phase was also believed to be the factor, resulting in marvelous EFE properties for these UNCD films. In the two-step MPECVD and Au-ion irradiation processes, the nano-graphite phase was presumably induced by the grain growth phenomenon. However, the most marked enhancement on the EFE properties for the UNCD films is resulted from the utilization of CH4/H2 plasma to replace the CH4/Ar plasma in MPECVD process. The nano-graphite phase was induced, forming an encapsulation layer surrounding the needle-like diamond grains. The EFE process of the nitrogen-doped UNCD films can be turned on at 8.3V/μm, achieving 2160.0μA/cm2 EFE capacities at an applied field of 18.0V/μm.

Enhanced Growth and Osteogenic Differentiation of Human Osteoblast-Like Cells on Boron-Doped Nanocrystalline Diamond Thin Films

Intrinsic nanocrystalline diamond (NCD) films have been proven to be promising substrates for the adhesion, growth and osteogenic differentiation of bone-derived cells. To understand the role of various degrees of doping (semiconducting to metallic-like), the NCD films were deposited on silicon substrates by a microwave plasma-enhanced CVD process and their boron doping was achieved by adding trimethylboron to the CH4:H2 gas mixture, the B∶C ratio was 133, 1000 and 6700 ppm. The room temperature electrical resistivity of the films decreased from >10 MΩ (undoped films) to 55 kΩ, 0.6 kΩ, and 0.3 kΩ (doped films with 133, 1000 and 6700 ppm of B, respectively). The increase in the number of human osteoblast-like MG 63 cells in 7-day-old cultures on NCD films was most apparent on the NCD films doped with 133 and 1000 ppm of B (153,000±14,000 and 152,000±10,000 cells/cm2, respectively, compared to 113,000±10,000 cells/cm2 on undoped NCD films). As measured by ELISA per mg of total protein, the cells on NCD with 133 and 1000 ppm of B also contained the highest concentrations of collagen I and alkaline phosphatase, respectively. On the NCD films with 6700 ppm of B, the cells contained the highest concentration of focal adhesion protein vinculin, and the highest amount of collagen I was adsorbed. The concentration of osteocalcin also increased with increasing level of B doping. The cell viability on all tested NCD films was almost 100%. Measurements of the concentration of ICAM-1, i.e. an immunoglobuline adhesion molecule binding inflammatory cells, suggested that the cells on the NCD films did not undergo significant immune activation. Thus, the potential of NCD films for bone tissue regeneration can be further enhanced and tailored by B doping and that B doping up to metallic-like levels is not detrimental for cells.

Enhanced electron field emission properties by tuning the microstructure of ultrananocrystalline diamond film

Synthesis of microcrystalline-ultrananocrystalline composite diamond (MCD-UNCD) films, which exhibit marvelous electron field emission (EFE) properties, was reported. The EFE of MCD-UNCD composite diamond film can be turned on at a low field as 6.5 V/μm and attain large EFE current density about 1.0 mA/cm2 at 30 V/μm applied field, which is better than the EFE behavior of the nondoped planar diamond films ever reported. The MCD-UNCD films were grown by a two-step microwave plasma enhanced chemical vapor deposition (MPECVD) process, including forming an UNCD layer in CH4/Ar plasma that contains no extra H2, followed by growing MCD layer using CH4/H2/Ar plasma that contains large proportion of H2. Microstructure examinations using high resolution transmission electron microscopy shows that the secondary MPECVD process modifies the granular structure of the UNCD layer, instead of forming a large grain diamond layer on top of UNCD films. The MCD-UNCD composite diamond films consist of numerous ultrasmall grains (∼5 nm in size), surrounding large grains about hundreds of nanometer in size. Moreover, there exist abundant nanographites in the interfacial region between the grains that were presumed to form interconnected channels for electron transport, resulting in superior EFE properties for MCD-UNCD composite films.

Tailoring nanocrystalline diamond coated on titanium for osteoblast adhesion

Diamond coatings with superior chemical stability, antiwear, and cytocompatibility properties have been considered for lengthening the lifetime of metallic orthopedic implants for over a decade. In this study, an attempt to tailor the surface properties of diamond films on titanium to promote osteoblast (bone forming cell) adhesion was reported. The surface properties investigated here included the size of diamond surface features, topography, wettability, and surface chemistry, all of which were controlled during microwave plasma enhanced chemical-vapor-deposition (MPCVD) processes using CH4-Ar-H2 gas mixtures. The hardness and elastic modulus of the diamond films were also determined. H2 concentration in the plasma was altered to control the crystallinity, grain size, and topography of the diamond coatings, and specific plasma gases (O2 and NH3) were introduced to change the surface chemistry of the diamond coatings. To understand the impact of the altered surface properties on osteoblast responses, cell adhesion tests were performed on the various diamond-coated titanium. The results revealed that nanocrystalline diamond (grain sizes <100 nm) coated titanium dramatically increased surface hardness, and the introduction of O2 and NH3 during the MPCVD process promoted osteoblast adhesion on diamond and, thus, should be further studied for improving orthopedic applications.

Orthopedic nano diamond coatings: Control of surface properties and their impact on osteoblast adhesion and proliferation

The superior mechanical and tribological properties of diamond coatings suggest their promise for improving current orthopedic implants. Therefore, understanding and controlling biological responses on diamond coatings are important and necessary for their advancement in orthopedics. For this reason, the objective of the present study was to correlate surface properties of diamond coatings with osteoblast (OB) adhesion and proliferation. Diamond coatings on silicon of variable surface features (specifically, grain size, surface roughness and surface chemistry) were fabricated by microwave plasma enhanced chemical-vapor-deposition (MPCVD). Scanning electron microscopy (SEM) as well as atomic force microscopy (AFM) was applied for topographical characterization and contact angles were measured to assess surface wettability. Results revealed that the grain size, surface roughness and wettability of diamond coatings can be controlled by adjusting H(2) plasma in the MPCVD process. Further, results showed enhanced OB adhesion on nanocrystalline diamond (ND) with grain sizes less than 100 nm whereas nanostructured diamond/amorphous carbon coatings (NDp) and microcrystalline diamond (MD) inhibited OB adhesion. H(2) plasma treated ND (NDH) also promoted OB adhesion. Similarly, OB proliferated to a greater extent on ND and NDH compared with MD and uncoated silicon controls. In summary, surface properties (including topography and chemistry) of diamond coatings can be controlled to either promote or inhibit OB functions, which implies that various forms of diamond coatings can be used to either support or inhibit bone growth in different regions of an orthopedic implant.

Excellent Field Emission Properties of Short Conical Carbon Nanotubes Prepared by Microwave Plasma Enhanced CVD Process

Randomly oriented short and low density conical carbon nanotubes (CNTs) were prepared on Si substrates by tubular microwave plasma enhanced chemical vapor deposition process at relatively low temperature (350–550 °C) by judiciously controlling the microwave power and growth time in C2H2 + NH3gas composition and Fe catalyst. Both length as well as density of the CNTs increased with increasing microwave power. CNTs consisted of regular conical compartments stacked in such a way that their outer diameter remained constant. Majority of the nanotubes had a sharp conical tip (5–20 nm) while its other side was either open or had a cone/pear-shaped catalyst particle. The CNTs were highly crystalline and had many open edges on the outer surface, particularly near the joints of the two compartments. These films showed excellent field emission characteristics. The best emission was observed for a medium density film with the lowest turn-on and threshold fields of 1.0 and 2.10 V/μm, respectively. It is suggested that not only CNT tip but open edges on the body also act as active emission sites in the randomly oriented geometry of such periodic structures.

Field emission from diamond nanotips treated with nitrogen plasma immersion ionimplantation

Diamond nanotips were synthesized on diamond/Si substrates by using a microwaveplasma-enhanced chemical vapor deposition process. The diamond nanotips are∼1 µm in height and the diameter at the bottom of a nanotip is∼200 nm. The as-grown diamond nanotips exhibit excellent field emission characteristics after treatingwith nitrogen plasma immersion ion implantation (PIII) for 10 min. A low turn-on field of3 V µm−1 and a highcurrent density of 4 mA cm−2 at 9 V µm−1 are achieved. The morphologies of diamond nanotips are unchanged after thenitrogen PIII treatment. The diamond quality reduces with treatment time. Thesp3 bonds in diamond can be broken by the nitrogen ions accelerated by the pulse voltage of−25 kV, which results in G peak broadening in Raman spectra. Several nanoscale amorphousregions are generated in a crystalline diamond nanotip, observed by the high resolutiontransmission electron microscope. The improvement of field emission properties iscorrelated to the amorphous regions and possible implantation-induced atomicdefects.

Growth of multiwalled-carbon nanotubes using vertically aligned carbon nanofibers as templates/scaffolds and improved field-emission properties

Multiwalled-carbon nanotubes (MWCNTs) are grown on top of vertically aligned carbon nanofibers (VACNFs) via microwave plasma-enhanced chemical vapor deposition (MPECVD). The VACNFs are first grown in a direct-current plasma-enhanced chemical vapor deposition reactor using nickel catalyst. A layer of carbon–silicon materials is then deposited on the VACNFs and the nickel catalyst particle is broken down into smaller nanoparticles during an intermediate reactive-ion-plasma deposition step. These nickel nanoparticles nucleate and grow MWCNTs in the following MPECVD process. Movable-probe measurements show that the MWCNTs have greatly improved field-emission properties relative to the VACNFs.

Preparation and characterization of DLC films by twinned ECR microwave plasma enhanced CVD for microelectromechanical systems (MEMS) applications

Diamond-like carbon (DLC) films have recently been pursued as the protection of MEMS against their friction and wear. Plasma enhanced chemical vapor deposition (PECVD) technique is very attractive to prepare DLC coating for MEMS. This paper describes the preparation of DLC films using twinned electron cyclotron resonance (ECR) microwave PECVD process. Raman spectra confirmed the DLC characteristics of the films. Fourier-transform infrared (FT-IR) characterization indicates the carbon is bonded in the form sp3 and sp2 with hydrogen participating in bonding. The surface roughness of the films is as low as approximately 0.093 nm measured with an atomic force microscope. A CERT microtribometer system is employed to obtain information about the scratch resistance, friction properties, and sliding wear resistance of the films. The results show the deposited DLC films have low friction and good scratch/ wear resistance properties.

Nanodiamond Thin Film Electrodes: Metal Electro‐Deposition and Stripping Processes

AbstractThe properties of a nanodiamond thin film deposit formed on titanium substrates in a microwave‐plasma enhanced CVD process, are investigated for applications in electroanalysis. The nanodiamond deposit consists of intergrown nano‐sized platelets of diamond with a high sp2 carbon content giving it high electrical conductivity and electrochemical reactivity. Nanodiamond thin film electrodes (of approximately 2 μm thickness) are characterized by electron microscopy and electrochemical methods. First, for a reversible one electron redox system, Ru(NH3)63+/2+, nanodiamond is shown to give well‐defined diffusion controlled voltammetric responses. Next, metal deposition processes are shown to proceed on nanodiamond with high reversibility and high efficiency compared to processes reported on boron‐doped diamond. The nucleation of gold is shown to be facile at edge sites, which are abundant on the nanodiamond surface. For the deposition and stripping of both gold and copper, a stripping efficiency (the ratio of electro‐dissolution charge to electro‐deposition charge) of close to unity is detected even at low concentrations of analyte. The effect of thermal annealing in air is shown to drastically modify the electrode characteristics probably due to interfacial oxidation, loss of active sp2 sites, and loss of conductivity.

Controlling Steps During Early Stages of the Aligned Growth of Carbon Nanotubes Using Microwave Plasma Enhanced Chemical Vapor Deposition

Vertically aligned carbon nanotubes (CNTs) with controllable length and diameter fabricated by microwave plasma enhanced chemical vapor deposition (MPECVD) are of continuing interest for various applications. This paper describes the role of process gas composition as well as the pre-coating catalytic layer characteristics. It is observed that nucleation of CNTs was significantly enhanced by adding nitrogen in the MPECVD process, which also promoted formation of bamboo-like structures. The very first key step toward growth of aligned CNTs was the formation of high-density fine carbon onion encapsulated metal (COEM) particles under a hydrogen plasma. Direct microscopic investigation of their structural evolution during the very early stages revealed that elongation, necking, and splitting of the COEM particles occurred accompanying the growth of CNTs, such that one of the split portions rode on the top of the growing tube while the remaining one resided on the root. Our results suggest that CNTs grow via the “tip-growth” as well as “root-growth” mechanisms.