Microwave Plasma Chemical Vapor Deposition Research Articles

Battery-like flexible supercapacitors from vertical 3D diamond/graphite composite films on carbon cloth

Flexible supercapacitors (SCs) based on carbon nanomaterials are one of the promising energy storage devices for wearable electron devices. However, their low energy density and cycle stability severely restrict further practical applications. The development of unique capacitor electrodes is expected largely to overcome these challenges. Herein, vertical 3D diamond/graphite (DG) composite films are directly grown on carbon cloth (CC) using a microwave plasma chemical vapor deposition (MPCVD) technique. Thin, straight nanosheets, consisted of diamond as core and graphite as shell, are interlaced with each other to form a 3D porous structure. As a binder- and additive-free electrode material, the DG/CC electrodes are used to construct electrical double layer capacitors (EDLCs) in diluted H2SO4 solution and pseudocapacitors (PCs) using Fe(CN)63−/4- redox electrolytes. Both EDLCs and PCs exhibit large specific capacitance and high capacitance retention. When assembled as all-solid-state flexible symmetric SCs, they offer high full-cell capacitances of 30.4 and 143.6 mF cm−2 at 10 mV s−1 for EDLCs and PCs, respectively. The power densities are 6.3 μW cm−2 and 3.0 mW cm−2 for EDLCs and PCs, together with their energy densities of 3.7 and 86.2 μWh cm−2, respectively. As flexible SCs, the specific capacitance can be completely recovered for EDLCs and has only about 6.7% loss for PCs even after the U-shape bending state. Such vertical 3D porous DG films are thus promising for the construction of high-performance battery-like SCs for an intelligent wearable energy supplier.

Scanning Deposition Method for Large-Area Diamond Film Synthesis Using Multiple Microwave Plasma Sources.

The demand for synthetic diamonds and research on their use in next-generation semiconductor devices have recently increased. Microwave plasma chemical vapor deposition (MPCVD) is considered one of the most promising techniques for the mass production of large-sized and high-quality single-, micro- and nanocrystalline diamond films. Although the low-pressure resonant cavity MPCVD method can synthesize high-quality diamonds, improvements are needed in terms of the resulting area. In this study, a large-area diamond synthesis method was developed by arranging several point plasma sources capable of processing a small area and scanning a wafer. A unit combination of three plasma sources afforded a diamond film thickness uniformity of ±6.25% at a wafer width of 70 mm with a power of 700 W for each plasma source. Even distribution of the diamond grains in a size range of 0.1–1 μm on the thin-film surface was verified using field-emission scanning electron microscopy. Therefore, the proposed novel diamond synthesis method can be theoretically expanded to achieve large-area films.

Origin, characteristics, and suppression of residual nitrogen in MPCVD diamond growth reactor

Unintentional nitrogen incorporation has been observed in a set of microwave plasma chemical vapor deposition (MPCVD)-grown samples. No abnormality has been detected on the apparatus especially the base pressure and feeding gas purity. By a comprehensive investigation including the analysis of the plasma composition, we found that a minor leakage of the system could be significantly magnified by the thermal effect, resulting in a considerable residual nitrogen in the diamond material. Moreover, the doping mechanism of leaked air is different to pure nitrogen doping. The dosage of several ppm of pure nitrogen can lead to efficient nitrogen incorporation in diamond, while at least thousands ppm of leaked air is required for detecting obvious residual nitrogen. The difference of the dosage has been ascribed to the suppression effect of oxygen that consumes nitrogen. As the unintentional impurity is basically detrimental to the controllable fabrication of diamond for electronic application, we have provided an effective way to suppress the residual nitrogen in a slightly leaked system by modifying the susceptor geometry. This study indicates that even if a normal base pressure can be reached, the nitrogen residing in the chamber can be “activated” by the thermal effect and thus be incorporated in diamond material grown by a MPCVD reactor.

Significant suppression of residual nitrogen incorporation in diamond film with a novel susceptor geometry employed in MPCVD

This work proposed to change the structure of the sample susceptor of the microwave plasma chemical vapor deposition (MPCVD) reaction chamber, that is, to introduce a small hole in the center of the susceptor to study its suppression effect on the incorporation of residual nitrogen in the MPCVD diamond film. By using COMSOL multiphysics software simulation, the plasma characteristics and the concentration of chemical reactants in the cylindrical cavity of MPCVD system were studied, including electric field intensity, electron number density, electron temperature, the concentrations of atomic hydrogen, methyl, and nitrogenous substances, etc. After introducing a small hole in the center of the molybdenum support susceptor, we found that no significant changes were found in the center area of the plasma, but the electron state in the plasma changed greatly on the surface above the susceptor. The electron number density was reduced by about 40%, while the electron temperature was reduced by about 0.02 eV, and the concentration of atomic nitrogen was decreased by about an order of magnitude. Moreover, we found that if a specific lower microwave input power is used, and a susceptor structure without the small hole is introduced, the change results similar to those in the surface area of the susceptor will be obtained, but the spatial distribution of electromagnetic field and reactant concentration will be changed.

New chemical pathway for large-area deposition of doped diamond films by linear antenna microwave plasma chemical vapor deposition

Today, boron-doped diamond (BDD) is among the key materials for electrochemical sensing and advanced oxidation of micropollutants. Therefore, a cost-effective fabrication of large-area BDD electrodes is of high interest. We present an implementation of a liquid-phase boron precursor trimethyl borate for large-area deposition of boron-doped diamond films by linear antenna microwave plasma CVD. Trimethyl borate vapors were used not only as a source of boron for doping but also as the only source of carbon and oxygen, while completely saturating the requirements for the growth of high-quality boron-doped diamond films. However, to allow for control over the doping level through maintaining the B/C and B/O ratios, carbon dioxide was employed as an additional source of carbon and oxygen. The film morphology was controllable from microcrystalline to ultra-nanocrystalline by changing the concentrations of trimethyl borate. Using this unique precursor system, we were able to grow diamond films with a doping level in range from 8 × 1017 cm−3 to 2 × 1022 cm−3 and resistivity as low as 1.16 × 10−2 Ω·cm. Low activation energies were calculated from the Arrhenius plot and growth rates as high as up to 170 nm/h for the low pressure microwave plasma CVD were reached. The investigation of the plasma emission spectrum revealed a chemical composition similar to that of hydrogen-rich plasmas with methane, and the proposed chemical reactions indicate that the diamond growth takes place via the hydroxyl radical. The results demonstrated that trimethyl borate is a suitable source of carbon and boron for the large-area growth of highly boron-doped diamond via low pressure microwave plasma CVD methods.

In-situ formation, structural transformation and mechanical properties Cr[sbnd]N coatings prepared by MPCVD

This study innovatively employed the microwave plasma chemical vapor deposition (MPCVD) technique to fabricate CrN coatings. The influence of preparation temperature on chemical composition, crystalline phase, morphology, and mechanical properties of the coatings were analyzed systematically. The increase of preparation temperature resulted in the structural transformation of the CrN coatings from the duplex of Cr2N/CrN to the single-phased Cr2N. The coatings with dense and flat surface were obtained at ≤800 °C; however, the decomposition of CrN to Cr2N and N2 led to the formation of pores at ≥850 °C and consequently an increase in friction coefficient and wear. The hardness and elastic modulus increased with increasing temperature because of the structural evolution of the CrN coating and phase transformation. 800 °C was the optimal preparation temperature as the coating exhibited a dense surface and possessed the best overall mechanical properties.

Analysis of Surface Microstructures Formed on Ir Substrate under Different Bias Conditions by Microwave Plasma Chemical Vapor Deposition

To understand the process window of bias‐enhanced nucleation of diamond, it is important to study surface modifications of Ir substrates that occur under different conditions. Herein, various microstructures formed on Ir substrates are reported. The first microstructure contains rectangular particles with a size of several hundred nanometers obtained at a relatively high pressure of 12.5 kPa and a high temperature of 880 °C; these particles consist of epitaxially grown Ir3Si species. The second microstructure includes particles oriented along the [110] direction; these particles, however, consist of etched Ir pillars with a height of 400 nm when the substrate temperature increases to 1110 °C. Both microstructures form because of Ir sputtering due to the bombardment of energetic charged ions and the transport of Ir surface atoms. This study demonstrates two types of surface modifications under different bias‐enhanced nucleation conditions and provides a method for preparing an epitaxial Ir–Si compound that can be applied in various silicon‐based electronic devices.

Study of horizontal and vertical uniformity of B-doped layer on mosaic single crystal diamond wafers by using hot-filament chemical vapor deposition

Aiming at developing inch-sized processing of diamond, B-doped layer was grown on mosaic single-crystallin diamond wafers by using hot-filament chemical vapor deposition (CVD), which is expected to have an advantage in terms of the deposition area compared with microwave plasma (MWP) CVD. Uniformity in horizontal and vertical directions is studied. It is found that the junctions of the monocrystalline diamond domains in the mosaic wafer and the direction of the crystal off-angles against to these junctions are less effective to the uniformity of the impurity concentrations. On the other hand, it is suggested that excess incorporation of W from the filament suppresses the growth and incorporation of B. It is shown that millimeter scale or more precise control of the arrangement of the wafer and the filament enables to obtain more uniform and efficient doping.

Fracture mechanics of microcrystalline/nanocrystalline composited multilayer chemical vapor deposition self-standing diamond films

Though microwave plasma chemical vapor deposition (MPCVD) diamond films exhibit extraordinary strength, the toughness improvement is still a huge challenge. The catastrophic fracture of the diamond films is undesirable in many applications, especially for the application of fabricating cutting tools. In the present study, adopting the pre-notched and unnotched cantilever bending tests, fracture behaviors of monolayer microcrystalline diamond (m-M), monolayer nanocrystalline diamond (m-N), and microcrystalline/nanocrystalline composited multilayer diamond films were observed and compared. Typical fracture mechanics, including Young's modulus (E), fracture strength (σF), and fracture toughness (KIC) of different diamond films were investigated. Effects of diamond crystal structure, intrinsic stress, and tensile-side roughness were systematically analyzed. Grain size and graphite phase content dominated the E of self-standing diamond films. Tensive-side surface roughness and intergranular bonding strength affected the σF. When the growth side in tension, E of the multilayer film (modulation period Λ = 3 h) was 16.0% lower than m-M and 26.6% higher than m-N, σF of multilayer film was 16.8% higher than m-M and 8.3% lower than m-N. At fixed Λ, doubling the total deposition period, E, σF, and KIC of the multilayer film increased 72.3%, 22.3%, and 2.7%, respectively. MCD/NCD composited multilayer architecture presented significant strengthening and toughening effects on self-standing diamond films.

Nucleation growth mechanism of diamond on 4H-SiC substrate by microwave plasma chemical vapor deposition

Diamond was deposited on mirror-like polished, grinded, and grooved 4H-SiC substrates by microwave plasma chemical vapor deposition. The nucleation process and grain morphology varied with different growth time and concentrations of methane in hydrogen were observed by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. It was shown that the nucleation density increased by three orders of magnitude on the grinded substrate, compared with mirror-like polished substrate. The concentration of methane is critical factor that affected the nucleation density, grain size and developing facet. Furthermore, the transformation from nucleation to continuous film was revealed on side wall of the grooved substrate. And the nucleation growth mechanism of diamond on the 4H-SiC substrate was discussed, and a four-stage nucleation growth mechanism has been proposed with the formation of carbon clusters by converting from sp1 to sp2 and sp3 carbon, shaping into preferred crystal planes and forming a well-defined cubo-octahedral diamond grains before producing a compact and coalesced diamond film.

Design and simulation of a novel MPCVD reactor with three-cylinder cavity

A novel microwave plasma chemical vapor deposition (MPCVD) reactor with three-cylinder cavity and outstanding tuning capability was proposed. The electric field and plasma in the reactor were simulated, and four geometric parameters were optimized. After optimization, the electric field intensity above the substrate exceeded 5×103V/m (input power of 1 W), which was 1.6 times that of the similar cylindrical reactors. Only one strong plasma formed above the substrate surface with an input power of 5 kW once the gas pressure exceeds 8 kPa. But the plasma coverage area was decreased with the increase of the gas pressure. The plasma coverage area could be restored by adjusting the height difference between the adjusting mechanism 1 and 2. The frequency detuning caused by the microwave frequency changes also could be returned by adjusting the adjusting mechanisms. The subsequent experimental results showed that uniform and high-quality diamond films could be deposited under the simulation condition (5 kW and 12 kPa). It could verify the accuracy of the simulation and the excellent performance of the new MPCVD reactor.

Effects of the electric field at the edge of a substrate to deposit a Ø100 mm uniform diamond film in a 2.45 GHz MPCVD system

In this study, uniform diamond films with a diameter of 100 mm were deposited in a 15 kW/2.45 GHz ellipsoidal microwave plasma chemical vapour deposition system. A phenomenological model previously developed by our group was used to simulate the distribution of the electric strength and electron density of plasma. Results indicate that the electric field in the cavity includes multiple modes, i.e. TM02 and TM03. When the gas pressure exceeds 10 kPa, the electron density of plasma increases and plasma volume decreases. A T-shaped substrate was developed to achieve uniform temperature, and the substrate was suspended in air from Ø70 to 100 mm, thus eliminating vertical heat dissipation. An edge electric field was added to the system after the introduction of the T-shaped substrate. Moreover, the plasma volume in this case was greater than that in the central electric field but smaller than that in the periphery electric field of the TM02 mode. This indicates that the electric field above and below the edge benefits the plasma volume rather than the periphery electric field of the TM02 mode. The quality, uniformity and surface morphology of the deposited diamond films were primarily investigated to maintain substrate temperature uniformity. When employing the improved substrate, the thickness unevenness of the Ø100 mm diamond film decreased from 22% to 7%.

CVD diamond-SiC composite films: Structure and electrical properties

Thin films of microcrystalline diamond-silicon carbide composite have been deposited by microwave plasma chemical vapor deposition (CVD) in methane-silane-hydrogen gas mixtures on Si substrates with an insulating polycrystalline diamond buffer layer. The structure and phase composition of the films were characterized by scanning electron microscopy, Raman and photoluminescence spectroscopy. The silicon carbide grains are identified to be cubic polytype 3C-SiC with only a minor admixture of hexagonal 6H-SiC. The measured electrical resistivity of the films is found to decrease with the temperature from ~10−1 Ω·cm at 300 K to ~10−2 Ω·cm at 460 K, with activation energy Ea = 0.20 eV. The conductivity is assumed to be realized via pathways in the SiC network immersed in isolating diamond matrix. The obtained results show that such diamond/SiC composites may be considered as an alternative to electrically conductive nitrogenated ultrananocrystalline diamond films for a number of practical applications.

Homoepitaxial overgrowth of (111) diamond films on Au-coated and Ni-coated substrates

Homoepitaxial growth of (111)-oriented diamond films on the Au-coated and Ni-coated diamond substrates by microwave plasma chemical vapor deposition (CVD) has been investigated. The process is involved with overgrowth of CVD diamond on Au and Ni islands formed by hydrogen plasma annealing of the 20 nm Au and Ni film coatings on diamond substrates. Cross-sectional transmission electron microscopy (TEM) observations show that both of the Au and Ni islands as single crystallites are in epitaxy with diamond substrate and no diamond growth occurs on top of those islands in the early stage of diamond deposition. The overgrowth of diamond over the metal islands results in excellent coalescence to form a continuous film with a lower defect density than the film without the metal coating. With such Au and Ni islands, the film stress can be further relaxed as shown with Raman spectroscopy.

Enhanced electrochemical kinetics and three dimensional architecture lithium iron phosphate/carbon nanotubes nanocomposites for high rate lithium-ion batteries

Three-dimensional architecture lithium –iron phosphate (LiFePO4)/carbon nanotubes (CNTs) nanocomposites with outstanding high-rate performances are synthesized by using a combination of in situ microwave plasma chemical vapor deposition (MPCVD) and co-precipitation methods. A stainless-steel mesh is adopted as the green catalyst for the in situ controllable growth of CNTs. Highly conductive and uniformly dispersed CNTs weave an effective three-dimensional (3D) conductive network, each isolate active LiFePO4 nanoparticle is fully wrapped and connected by the CNTs. The optimized electrode of LiFePO4/CNTs nanocomposites deliverer a high initial discharge capacity of 168.7 mAh g−1 at 0.1 C. Amazingly, the discharge capacities can reach 126.1, 111.2, 99.5 and 71.3 mAh g−1 even at high rates of 10 C, 20 C, 30 C and 50 C. The LiFePO4/CNTs nanocomposite displayd an excellent electrochemical performance, such as ultrahigh cyclic stability, extraordinary rate capability and smaller capacity fading at high current densities, which can be ascribed to the synergistic effects of the highly 3D CNTs conductive network and the shorter lithium ion diffusion path in the LiFePO4 nanoparticles. The electrochemical kinetics demonstrate that the insertion process was the rate-determining step at low current densities, while the transports of charges from their reservoirs towards the active particles became prevailing at high current densities. The brilliant high-rate performance of LiFePO4/CNTs nanocomposites can be ascribe that the open and highly conductive network established by CNTs allows a much more efficient ionic and electronic conduction.

Study on diamond temperature stability during long-duration growth via MPCVD under the influence of thermal contact resistance

During the process of growing diamond crystals via microwave plasma chemical vapor deposition (MPCVD), the substrate temperature will gradually increase with increasing growth time. At present, the commonly used method to compensate for the temperature increase is to reduce the deposition parameters (gas pressure and input power) or adjust the height of the susceptor. However, according to previous studies, reducing the substrate parameters will reduce the growth rate. In addition, adjusting the height of the susceptor may cause secondary plasma, which could damage the equipment. Therefore, it is necessary to find the reason for the substrate temperature increase with the growth time and eliminate it. Firstly, the effect of substrate size change on the substrate temperature was simulated dynamically, which indicated that the substrate size change was not the main reason for the continuous increase of the substrate temperature. Subsequent analysis showed that non-diamond carbon formed at the bottom of the grown diamond, which may cause a change of the thermal contact resistance. Therefore, a thermal contact resistance component was introduced into the existing model of diamond growth, and it was found that the substrate temperature increased with the roughness of the bottom of the substrate. In order to eliminate the influence of thermal contact resistance, a welding growth method was proposed with the aim of avoiding the accumulation of the non-diamond carbon at the bottom of the substrate. The growth with different welding materials showed that the grown diamond's internal stress distribution after welding growth was similar to that without welding growth, but the non-diamond carbon at the bottom was obviously restrained. However, the selection of welding materials needs to be further optimized with respect to the welding stability and growth rate.

Mini-review: Novel Graphene-based Acoustic Devices

Graphene has extremely high electron mobility, good thermal conductivity, and excellent acoustic properties, making it an ideal acoustic material. Currently, there are three main types of graphene used in acoustic devices: graphene films prepared by chemical vapor deposition (CVD), vertical graphene (VGr) and laser-induced graphene (LIG). Among them, Graphene prepared by CVD is lighter, thinner and more sensitive; VGr is grown by the microwave plasma chemical vapor deposition (MPCVD) method, which has higher ductility and sound pressure level (SPL); LIG has the advantages of low cost, simple preparation and no biological toxicity. This paper introduces the applications of these three materials in the direction of acoustic devices. In addition to the simple acoustic-electrical conversion application, an artificial throat with the intelligent recognition function is introduced. These different types of graphene acoustic devices have greatly promoted the application of graphene in the field of acoustics.

High growth rate synthesis of diamond film containing perfectly aligned nitrogen-vacancy centers by high-power density plasma CVD

Diamond film sensors with large volumes that are synthesized by microwave plasma chemical vapor deposition (MPCVD) and contain perfectly aligned nitrogen-vacancy (NV) ensembles are promising material for achieving highly sensitive quantum magnetometers. The step-flow growth mode of MPCVD on diamond (111) surface is required to realize aligned NV center ensembles. However, it is difficult to control the growth mode on diamond (111) surface due to the presence of twins and the conventional growth rate of aligned NV center ensembles is lower than 0.5 μm/h. In this study, we achieved a high growth rate (6.6 μm/h) of diamond (111) film containing perfectly aligned NV ensembles with high contrast (30%) by applying high power density plasma (103 W/cm3) and precisely controlling the gas flow rate ratios of CH4/H2 and N2/CH4. The growth mode was controlled to realize step flow growth by inducing the hydrogen etching of the nucleation with decreasing the CH4/H2. It was considered that increasing the microwave power density produces a large amount of atomic hydrogen and carbon precursor, leading to a high growth rate of perfectly aligned NV center diamond film. By controlling the N2/CH4 ratio, the nitrogen density in the diamond film could be precisely controlled to obtain a high contrast. Moreover, we measured the quality of the obtained diamond sensor films using the ODMR spectra and Ramsey sequence. We confirmed a high Rabi contrast (approximately 30%) and no significant decrease in T2∗ due to the deterioration of the crystallinity of the diamond film was observed. This result is promising for building material technology for highly sensitive quantum sensors with large sensor volumes.

Preparation and characterization of diamond-like carbon (DLC) film on 316L stainless steel by microwave plasma chemical vapor deposition (MPCVD)

The 316L stainless steel was widely used in medical, surgical instruments and human implant medical materials. The diamond-like carbon (DLC) films were deposited on 316L stainless steel by varying temperature, CH4:H2 flow rate, and deposition time in a microwave plasma chemical vapor deposition (MPCVD) system to improve their biocompatibility. The morphology and composition of deposited films were investigated using a scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscope. The results showed that with the increase in deposition temperature and the decrease in gas flow ratio, the structure of the film gradually became compact and smooth, and the graphite phase structure of the film tended to change to diamond phase. In addition, the island-like structure gradually grew into a sheet, and the defects in the films were reduced with the enhanced bonding force between the films and the stainless steel matrix. The optimum temperature and gas flow ratios were 900 °C and 10:190, respectively. Medical stainless steels containing DLC films had better bone-forming ability and biocompatibility than untreated.

Diamond-germanium composite films grown by microwave plasma CVD

We report on novel microcrystalline diamond-germanium composite films grown by microwave plasma-assisted chemical vapor deposition in CH4–H2-GeH4 mixtures on Si substrate. The structure of the films, characterized with scanning electron microscopy, X-ray diffraction, Raman and photoluminescence spectroscopy, is found to strongly depend on the deposition temperature: the Ge grains compete with diamond crystallites upon growth at 750–800 °C, but vanish at higher temperatures of 850–950 °C due to enhanced growth rate of diamond matrix. The Ge grains nucleated on the (100) oriented Si wafer at 800 °C are in an epitaxial relationship with the substrate, while Si–Ge alloy forms at intermediate temperatures 850–900 °C. As both components are transparent in the infrared spectrum, the composite shows certain optical transmission for wavelengths λ > 10 μm. The films revealed a bright optical emission of GeV color centers at 601 nm wavelength due to diamond doping with Ge. From the calculation of the effective coefficient of thermal expansion for the diamond-Ge composites, we show that it can be tuned to be close to that for SiC or Si single crystals by appropriate choice of the Ge volume fraction in the composite, facilitating the integration of the semiconductor wafers with the diamond-based film.