Heteroepitaxial diamond nucleation and growth on silicon by microwave plasma-enhanced chemical vapor deposition synthesis

Heteroepitaxial diamond films were successfully nucleated and deposited on 1-inch diameter Si(001) substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD). The precursor gases for the synthesis were methane and hydrogen. Before the application of a negative d.c. bias to the substrate, an in-situ carburization pre-treatment on the silicon was found to be an indispensable step towards the heteroepitaxial diamond on the silicon. Morphologies of the films were characterized by scanning electron microscopy (SEM). Interface observations based on the cross-sectional HRTEM directly reveal the heteroepitaxial diamond nucleation phenomena in detail. No interlayers of silicon carbide and/or amorphous carbon phases were observed. Tilt and azimuthal misorientation angles between the heteroepitaxial diamond crystals and the substrate were determined by combining the Ewald sphere construction in the reciprocal lattice space and the selected area diffraction (SAD) patterns taken across the interface.     

Liquid phase growth of graphene on silicon carbide

We developed a novel method to produce graphene on silicon carbide (SiC) at a temperature as low as 1000 °C. The method is based on liquid phase growth (LPG) of graphene mediated by liquid gallium, which acts not only as a flux to store carbon dissolved from a surface of SiC when heated, but also as a catalyst to promote the formation of graphene on SiC when cooled. Our experimental results revealed that gallium-treated SiC substrates are coated with uniform and continuous graphene films. The LPG method is able to supply graphene films consisting of one to several hundreds of layers, depending on heating temperatures. Our approach can not only provide an alternative way to form graphene natively on SiC, but will also bring a technological breakthrough in industrial applications of graphene, e.g. the realization of graphene-on-insulator substrates.     

The synthesis of graphene nanowalls on a diamond film on a silicon substrate by direct-current plasma chemical vapor deposition

Graphene nanowalls have been synthesized on diamond by direct-current plasma enhanced chemical vapor deposition (CVD) on silicon substrates pre-seeded with diamond nanoparticles in gas mixtures of methane and hydrogen. Switching from diamond CVD to graphene CVD is done by increasing the methane concentration and decreasing the plasma power without breaking the vacuum. Graphene nanowalls stand on the CVD diamond film to form a 3-dimensional network. Scanning electron microscopy, high-resolution transmission electron microscopy, UV and visible Raman scattering and electrochemical cyclic voltammetry measurements are used to characterize the multi-layer turbostratic graphitic carbon nanostructure and demonstrate its electrochemical durability with a low background current in a wide electrochemical potential window.     

Two-step growth of graphene with separate controlling nucleation and edge growth directly on SiO2 substrates

Recently, we have developed a catalyst-free and direct growth approach for nanographene on various substrates by a remote-plasma assisted chemical vapor deposition. A two-step growth strategy for separately controlling the nucleation and subsequent edge growth was further developed for growing graphene sheets with adjusted nuclei density and large domain size of 500 nm. The key for tuning the growth mode from nucleation to edge growth is the growth temperature; at a specific growth temperature (∼510–545 °C), only edge growth is available while the nucleation can be largely suppressed. This fine tuning of growth process yields a continuous polycrystalline graphene film with domain size of ∼150 nm. This domain size is controllable in this tunable growth to thus giving more freedom to control the graphene film properties.     

Preparation of graphene nanowalls by a simple microwave-based method

A simple method for the preparation of graphene sheets is described which uses a conventional microwave oven to produce plasma inside a quartz tube, which is used to degrade and carbonize a carbon source (polyethylene powder) to produce graphitic nanolayers. This technique also allows deposition of the resultant graphene layers on thermally-stable surfaces in interesting morphologies. The effect of post-processing of the nanosheets by thermal treatment in air and argon was investigated.     

Carbon nanowalls decorated with silicon for lithium-ion batteries

Carbon nanowalls (CNWs) are suggested as a promising nanostructured substrate for 3D anodes of lithium-ion batteries. Silicon sputtering onto CNWs results in improved electrochemical performance due to either a large surface area of free-standing CNWs or easy adhesion of deposited silicon clusters via the SiC interface layer formation. The 3D silicon-decorated nanowall framework (SDNF) could give the possibility to minimize the lithium diffusion length and make charge collection more effective yielding better cycling performance at high rates exceeding 2000 mA h per 1 g of silicon in the range of 0.05–2.00 V at 1.5 C rate.     

Field emission study of graphene nanowalls prepared by microwave-plasma method

The field emission properties of graphitic nanowalls prepared by a simple microwave-based method are presented. The effects of deposition method on the agglomeration and field emission properties of samples were also studied. Thermal treatment of these samples at 300 °C in air and at 1000 °C in argon further increased electron emission properties by improving the graphitic order of the samples and removal of impurities.     

Diamond nucleation and growth on zeolites

In this work, we report the use of zeolites as substrates for the deposition of porous diamond films. Films were deposited in a hot-filament chemical vapor deposition (HFCVD) apparatus. The HFCVD system was fed with a mixture of methane (0.8%) with the balance being hydrogen. A series of depositions were done in the pressure range 20–120 Torr and at substrate temperature 880 °C. The morphologies of the as-deposited films were analyzed by scanning electron microscopy and show isolated diamond grains in the initial nucleation stages, which develop into a microporous film in the next stage and form a continuous film after long time deposition. Raman spectroscopy was used to investigate the crystal morphology, structure and non-diamond impurities in the films deposited at various growth conditions. The nature of the hydrogen bonding with sp³ and sp² network and the quantitative analysis were done by Fourier transform infrared spectroscopy.     

In situ observations of the nucleation and growth of atomically sharp graphene bilayer edges

Using in situ transmission electron microscopy, we observed the nucleation and growth of graphene bilayer edges (BLE) with “fractional nanotube”-like structure from the reaction of graphene monolayer edges (MLEs). Most BLEs showed atomically sharp zigzag or armchair crystallographic facets in contrast to the atomically rough MLEs with irregular shapes, suggesting that the BLEs are much more stable and crystallographically anisotropic. Our direct observations and theoretical studies (geometric models and ab initio calculations) provide important clues for tailoring the edge structure and transport properties of multi-layer graphene.     

Vertically aligned epitaxial graphene nanowalls with dominated nitrogen doping for superior supercapacitors

For graphene-based electrode materials, N doping is one of the leading approaches for enhancing the performance of supercapacitors. However, such an outstanding performance is suppressed by the agglomeration of graphene and unspecified N incorporation. Here, we demonstrate a direct growth of vertically epitaxial graphene nanowalls (GNWs) on flexible carbon cloths (CCs) via microwave plasma-enhanced chemical vapor deposition, whereby predominantly N doping was sequentially achieved by introducing in situ NH₃ plasma, to form N-doped GNWs (NGNWs). The vertically aligned three-dimensional (3D) architecture of epitaxial NGNWs and their unique selectivity to the specific N dopants make such electrodes an ideal platform, not only for enhancing the capacitive performance but also for studying the role of the CN bonding configuration in its performance. Remarkably, NGNW supercapacitors exhibit an excellent specific capacitance of 991.6 F/g (estimation based on the actively contributing component) and an apparent area-normalized capacitance of 1488.9 mF/cm², at a specific current of 14.8 A/g. This approach allows us to achieve an energy density of 275.4 Wh/kg at a power density of 14.8 kW/kg (specific current of 14.8 A/g), and a power density of 74.1 kW/kg at an energy density of 232.6 Wh/kg (specific current of 74.1 A/g) in 1 M H₂SO₄.     

Direct nucleation of cubic boron nitride on silicon substrate

Cubic boron nitride thin films were deposited on silicon substrates by plasma chemical vapor deposition, and the nanostructure of the interface between the film and substrate was investigated by cross-sectional high-resolution transmission electron microscopy. Cubic boron nitride was found to nucleate directly on the properly pretreated silicon substrates in some local areas; in particular, cBN nuclei with a size of approximately 3 nm are nucleated on (111)–(100) step surfaces with an epitaxial relationship. This suggests the possibility of direct growth of cubic boron nitride in special surface environments of silicon substrates.     

Heteroepitaxial nucleation of diamond on Si(100) via double bias-assisted hot filament chemical vapor deposition

A new process has been developed to obtain high density epitaxial diamond nucleation via a double bias-assisted hot filament chemical vapor deposition (HFCVD). In the process, a negative bias voltage is applied to the Si substrate and a positive bias voltage is applied to a steel grid placed on top of the hot filaments. With this arrangement, a stable plasma can be generated between the grid and the hot filaments. Ions in the plasma are then drawn to the substrate by a negative substrate bias voltage. The impinging rate of these ions can be easily controlled by adjusting the grid current, and the ion energy can be independently controlled by adjusting the substrate bias voltage. Hence, the energy and dosage of ion bombardment onto the Si(100) substrate can be controlled easily and independently. With the controlled ion bombardment, high density and heteroepitaxial nucleation can be achieved routinely. After the nucleation process, highly textured diamond films were grown by either the HFCVD or the microwave plasma chemical vapor deposition process (MPCVD).     

Revealing the surface and bulk regimes of isothermal graphene nucleation and growth on Ni with in situ kinetic measurements and modeling

In situ optical diagnostics are used to reveal the isothermal nucleation and growth kinetics of graphene on Ni across a wide temperature range (560 °C < T < 840 °C) by chemical vapor deposition from single, sub-second pulses of acetylene. An abrupt, two-orders of magnitude change in growth times (∼100–1 s) is revealed at T = 680 °C. Above this temperature, sigmoidal kinetics are measured and attributed to autocatalytic nucleation and growth from carbon dissolved in the bulk of the Ni film. However, for T < 680 °C fast surface nucleation and growth occurring during the gas pulse appears responsible for the drastic alteration of the kinetics of subsequent dissolution-mediated growth. A simple and general kinetic model for isothermal graphene growth is developed that includes the nucleation phase and the effects of carbon solubility in metals, describes delayed nucleation, and allows the interpretation of the competition between surface- and bulk-nucleation and growth. The easily-implemented optical reflectivity diagnostics and the simple kinetic model described here allow a pathway to optimize the growth of graphene on metals with arbitrary carbon solubility.     

Effect of oxygen on the bias-enhanced nucleation of diamond on silicon

The influence of traces of oxygen in the process gas on the bias-enhanced nucleation (BEN) of diamond on silicon has been studied in the present work. CO₂ in concentrations ranging from 0 to 3000 ppm was added during the nucleation procedure at U_bias=−200 V in microwave plasma chemical vapour deposition (MPCVD). A significant influence of CO₂ could already be detected for a concentration of 75 ppm, corresponding to a C:O ratio of 600:1. It resulted in a continuous reduction of the biasing current and a delay in the nucleation process accompanied by a decrease of the final diamond-covered substrate surface area with increasing CO₂ concentration. At 3000 ppm, the nucleation was completely suppressed. An etching of diamond nuclei by the oxygen could be excluded from in-situ growth rate measurements under bias. Instead, optical emission spectra of the H_β Balmer line indicated a decrease in electrical field strength in the plasma above the substrate. For all gas compositions allowing diamond nucleation, epitaxially aligned films could be obtained, provided that the duration of the biasing step was chosen appropriately. Thus, traces of oxygen do not completely suppress epitaxy. However, the in-plane orientation of the films as determined by X-ray diffraction measurements deteriorates with increasing oxygen concentration.     

Influence of silicon carbide interlayer evolution on diamond heteroepitaxy during bias enhanced nucleation on silicon substrates

In order to improve the crystalline quality of diamond films produced by microwave plasma assisted chemical vapour deposition (MPCVD), the structural evolution of the silicon carbide interlayer during the bias nucleation step has been investigated by reflection high energy electron diffraction (RHEED). Here we highlight the fact that the carbonisation pre-treatment induces a strong extension of the silicon carbide lattice in the direction perpendicular to the surface. This extension gives a lattice constant close to that of silicon. Then, during bias enhanced nucleation, the carbide lattice relaxes. At the same time, this modification is accompanied by an increase of the surface roughness and by a progressive polar misorientation of the silicon carbide. All these transformations could be responsible for the observed drop of the diamond epitaxial ratio when the duration of the bias step is extended. Finally, we found that a lower methane concentration in the plasma slows down this carbide transformation, allowing us to obtain a promising 37% epitaxial ratio.     

Surface treatments of silicon to enhance thermal nucleation

We have investigated the physical characteristics and means of producing silicon surfaces treated to provide reliable nucleation sites for boiling in perfluorinated coolants. Three techniques may be used to provide these sites; removal of mechanical defects by liquid etchants; selective etching of one of the phases of a eutectic alloy; and deposition of an inert, porous material such as alumina. Consistent nucleation, required to assure proper heat transfer of silicon circuit chips, does not occur on polished silicon surfaces.     

Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film

Amorphous carbon (a-C) films with high contents of tetrahedral carbon bonding (sp³) were synthesized on smooth Si(100) surfaces by cathodic arc deposition. Before diamond growth, the a-C films were pretreated with a low-temperature methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. The evolution of the morphology and microstructure of the a-C films during the pretreatment and subsequent diamond nucleation and initial growth stages was investigated by high-resolution transmission electron microscopy (TEM). Carbon-rich clusters with a density of ∼10¹⁰ cm⁻² were found on pretreated a-C film surfaces. The clusters comprised an a-C phase rich in sp³ carbon bonds with a high density of randomly oriented nanocrystallites and exhibited a high etching resistance to hydrogen plasma. Selected area diffraction patterns and associated dark-field TEM images of the residual clusters revealed diamond fingerprints in the nanocrystallites, which played the role of diamond nucleation sites. The presence of non-diamond fingerprints indicated the formation of Si–C-rich species at C/Si interfaces. The predominantly spherulitic growth of the clusters without apparent changes in density yielded numerous high surface free energy diamond nucleation sites. The rapid evolution of crystallographic facets in the clusters observed under diamond growth conditions suggested that the enhancement of diamond nucleation and growth resulted from the existing nanocrystallites and the crystallization of the a-C phase caused by the stabilization of sp³ carbon bonds by atomic hydrogen. The significant increase of the diamond nucleation density and growth is interpreted in terms of a simple three-step process which is in accord with the experimental observations.     

Electrochemical nucleation and growth of copper on chromium-plated electrodes

Nucleation and growth of copper electrodeposited on chromium plated electrodes in copper sulfate electrolytes were examined, focusing on the influence of prior Cr plating conditions on the nucleation density and growth kinetics of the copper electrodeposits. The Cr-plated electrodes were made by electrodeposition of Cr on copper sheets for 2 to 60 s at 0.1 A cm^–2 in CrO₃ 350 g L^–1 + H₂SO₄ 3.5 g L^–1. Copper was then electrodeposited onto the Cr-plated electrode under potentiostatic conditions. Copper initially nucleated and grew according to a three-dimensional diffusion controlled progressive nucleation process, and later according to an instantaneous nucleation process. The period during which copper nucleation is controlled by the diffusion controlled progressive nucleation process decreases with increasing Cr plating time. The nucleation density of copper was extremely high on the 2 s Cr-plated electrode, producing an extremely fine and uniform electrodeposit. However, on the 4 s Cr-plated electrode, the nucleation density of copper significantly reduced to one hundredth of that on the 2 s Cr-plated electrode, and then decreased slightly with increasing Cr plating time (thickness of Cr layer). These results appear to be associated with the IR drop across the Cr layer, including the surface Cr oxide/hydroxide film (termed the cathode film), which significantly reduces the driving force for the electrodeposition of copper under potentiostatic plating conditions.     

Young压力对晶核形成和晶体生长的影响

本文以Young压力和构晶时的水体扩散为基础讨论了晶核形成和晶体生长的微观机理。     

Combined growth of carbon nanotubes and carbon nanowalls by plasma-enhanced chemical vapor deposition

A technique is reported for the combined growth of carbon nanotubes (CNTs) and carbon nanowalls (CNWs) by plasma-enhanced chemical vapor deposition. The observation serves as a direct proof of a close correlation between the growth of both materials because both are obtained in a single experiment without making any changes to the growth parameters. The growth of freestanding CNTs is driven by a nickel catalyst deposited on an oxidized silicon wafer. It is assumed that the remaining carbon radicals are inserted in the sidewalls and tips of the tubes after the saturation of the catalyst by abundant carbon, thereby forming a CNW layer on top of the CNTs. A possible growth scheme, based on qualitative analysis by electron microscopy, Raman spectroscopy and X-ray diffraction, is presented. It is further shown that the CNWs easily detach by dipping the sample into water, while the CNTs remain attached to the sample.