In situ observation of nucleation by optical emission spectra in CVD diamond

Optical emission spectroscopy (OES) was used to in situ detect the intensity variation of C₂ radical with the deposition time in the boundary layer during homo-nucleation of CVD diamond by DC Arcjet Plasma. The obvious drop and fluctuation of the optical emission intensity were found during the early growth stage. The samples grown after selected deposition time were characterized by micro-structural probes (transmission electron microscope (TEM), high resolution electronic microscope (HREM), selected area diffraction (SAD) and electron energy loss spectra (EELS)), in order to determine the occurrence of the diamond nucleation. Based on the results of the OES and the micro-structural probes, it was revealed that the variation of the optical emission intensity corresponded to the diamond nucleation. The incubation period and the lasting time of nucleation were thus deduced as 6–8 min and 20–60 s depending on the concentration of CH₄ in H₂. The incubation period decreased and the lasting time of nucleation increased with the increase of the concentration of CH₄ in H₂.     

Growth of diamond by DC Arcjet Plasma CVD: From nano-sized poly-crystal films to millimeter-sized single crystal grain

A 30 kW-powered DC Arcjet Plasma enhanced chemical-vapor deposition (CVD) system was applied to grow diamonds which included the nano-crystal free-standing film, the nano-/micro-crystal layered free-standing film, the gradient micro-crystal free-standing film and the millimeter-sized grain. The free-standing film quality, such as the roughness, the sp² content, the residual stress and the grain morphology, was studied by an atomic force microscope (AFM), Raman spectra, a scanning electron microscope (SEM) and a high resolution electron microscope (HREM). In large-sized grain deposition, as-grown deposit was obtained about 1 × 1 × 1 mm³ in size under the condition of 10 μm/h of the substrate moving speed without Nitrogen enhancement. Characterized by Raman spectra and Laue back reflection X-ray diffraction, the deposit was proven to be single crystal diamond with small grains coving its surfaces. The growth rate was about 30 μm/h. Optical emission spectrum (OES) was utilized to characterize gas phases in the plasma for diamond deposition. The mean electron temperature (Te) in the plasma was calculated based on the value of the emission intensity ratio of I_Hγ/I_Hβ. Te varied from 0.33 eV to 0.5 eV depending on the concentration of CH₄ in H₂ from 1.0% to 25%. C₂ radical was found to be the dominant carbon source compared with CH radical. The influence of the radical on the morphology of diamond was discussed. It was found that the nano-crystal could be grown when the ratio of the emission intensity, I_C2/I_CH, was larger than 8.     

Investigation of the combined effect of argon addition and substrate bias on the growth of ultrananocrystalline diamond layers

In our recent project the combined effect of argon addition and substrate bias was investigated in the microwave plasma assisted chemical vapor deposition of diamond, focused on the ultrananocrystalline phase. Over the conventional qualifying techniques, i.e., Raman and SEM studies, we have led a special in-situ mass spectrometry investigation to explore the growth mechanism of UNCD, analysing the gas composition close to the surface. To achieve this aim, ion beam mass spectrometry (IBMS) was used for in-situ, real time, mass-selective analysis of the incoming species playing an important role in the MWPECVD (Microwave Plasma Enhanced Chemical Vapor Deposition) of the ultrananocrystalline diamond. In our experiments Ar, CH₄, and H₂ gases were used as source gases in a wide range of concentrations applying different values of substrate bias to deposit different phases of diamond. By the IBMS technique we can measure the fluxes of different species: C_xH_y (x = 1–2, y = 0–2) during the phases of deposition, either under the conditions of microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) synthesis. As a result of it, we can compare the different mechanisms of layer formation: i.e.: whether C₁ species or C₂ mediated growth method takes place, or probably both C₁ and C₂ species propagate the diamond lattice. Based on the given tendency by comparing the IBMS data (i.e.: fluxes of surface species) with the growth rate, morphology, and Raman spectra of the layers we propose, that in the case of UNCD a similar (but not exactly the same) growth mechanism can be found as in the case of MCD i.e.: C₁ species are the most likely precursors.     

Characterization of the near-surface gas-phase chemical environment in atmospheric-pressure plasma chemical vapor deposition of diamond

A numerical model was developed and used to study the near-surface gas-phase chemistry during atmospheric-pressure radio-frequency (RF) plasma diamond chemical vapor deposition (CVD). Model predictions of the mole fractions of CH₄, C₂H₂, C₂H₄ and C₂H₆ agree well with gas chromatograph measurements of those species over a broad range of operating conditions. The numerical model includes a two-dimensional analysis of the sampling disturbance in the thin boundary layer above the substrate, accounts for chemistry in the gas chromatography sampling line, and utilizes a reaction mechanism that is significantly revised from a previously reported version. The model is used to predict the concentrations of H, CH₃, C₂H₂ and C at the diamond growth surface. It is suggested that methyl, acetylene and atomic carbon may all contribute significantly to film deposition during atmospheric-pressure RF plasma diamond CVD. The growth mechanism used in the model is shown to predict growth rates well at moderate substrate temperatures (∼1100 to 1230 K) but less well for lower (∼1000 K) and higher (∼1300 K) temperatures. The near-surface gas-phase chemical environment in atmospheric-pressure RF plasma diamond CVD is compared with several other diamond CVD environments. Compared with these other methods the thermal plasma is predicted to produce substantially higher concentration ratios at the surface of both H/CH₃ and C₂H₂/CH₃.     

Effect of H2/Ar plasma on growth behavior of ultra-nanocrystalline diamond films: The TEM study

Incorporation of H₂ species into Ar plasma was observed to markedly alter the microstructure of diamond films. TEM examinations indicate that, while the Ar/CH₄ plasma produced the ultrananocrystalline diamond films with equi-axed grains (~ 5 nm), the addition of 20% H₂ in Ar resulted in grains with dendrite geometry and the incorporation of 80% H₂ in Ar led to micro-crystalline diamond with faceted grains (~ 800 nm). Optical emission spectroscopy suggests that small percentage of H₂-species (< 20%) in the plasma leads to partially etching of hydrocarbons adhered onto the diamond clusters, such that the C₂-species attach to diamond surface anisotropically, forming diamond flakes, which evolve into dendrite geometry. In contrast, high percentage of H₂-species in the plasma (80%) can efficiently etch away the hydrocarbons adhered onto the diamond clusters, such that the C₂-species can attach to diamond surface isotropically, resulting in large diamond grains with faceted geometry. The field needed to turn on the electron field emission for diamond films increases from E₀ = 22.1 V/μm (J_e = 0.48 mA/cm² at 50 V/μm applied field) for 0% H₂ samples to E₀ = 78.2 V/μm (J_e < 0.01 mA/cm² at 210 V/μm applied field) for 80% H₂ samples, as the grains grow, decreasing the proportion of grain boundaries.     

Development of electrochemical cell with layered composite of the Gd-doped ceria/electronic conductor system for generation of H2–CO fuel through oxidation–reduction of CH4–CO2 mixed gases

This paper shows the recent results on the development of layered composite promoting two types of electrochemical reactions (oxidation and reduction) in one cell. This cell consisted of porous Ni–Gd-doped (GDC) ceria cathode/thin porous GDC electrolyte (50 μm)/porous SrRuO₃–GDC anode. The external electric current was flowed in this cell at the electric field strength of 1.25 and 6.25 V/cm. The mixed gases of CH₄ (30–70%) and CO₂ (70–30%) were fed at the rate of 50 ml/min to the cell heated at 400–800 °C under the electric field. In the cathode, CO₂ was reduced to CO (CO₂ + 2e^? → CO + O^2?) and the formed CO and O^2? ions were transported to the anode through the pores and surface and interior of grains of GDC film. On the other hand, CH₄ was oxidized in the anode to form CO and H₂ through the reaction with diffusing O^2? ions (CH₄ + O^2? → CO + 2H₂ + 2e^?). As a result, H₂–CO mixed fuel was produced from the CH₄–CO₂ mixed gases (CH₄ + CO₂ → 2H₂ + 2CO). This electrochemical reaction proceeded completely at 800 °C and no blockage of gases was measured for long time (>10 h). Only H₂–CO fuel was generated in the wide gas compositions of starting CH₄–CO₂ gases.     

Graphitized filament plasma enhanced CVD deposition of nanocrystalline diamond

A novel approach to the deposition of polycrystalline diamond is presented. The technique is based on the hot filament chemical vapour deposition technique (HFCVD). While it is similar to a high plasma power “bias enhanced growth” HFCVD, it relies on a graphite filament rather than on a metal one. It was found that with an appropriate choice of the growth parameters, 4–9% CH₄ in H₂, filament temperature > 2200 °C, 25 mBar gas pressure, plasma power > 500 W, a long filament lifetime can be achieved, when a simultaneous deposition of graphitic carbon on the hot graphite filament and of nanocrystalline diamond on a substrate facing the filament assembly is realized. In this paper the growth of nanocrystalline diamond films and their characterization (SEM, XRD, AFM) are presented. While the technique is promising for low cost, large area deposition of nanocrystalline diamond films, also the growth of microcrystalline diamond has been observed.     

Development of a plate-to-plate MPCVD reactor configuration for diamond synthesis

The design and performance of a microwave plasma chemical vapor deposition (MPCVD) reactor based on compressed microwave waveguides and plate-to-plate substrate holders are described. This reactor can be operated at pressures from 10 to 40 kPa with microwave power of 0.4–1.2 kW, and a high plasma power density up to 500 W/cm³ can be obtained. The single-crystal diamond (lower substrate holder) and polycrystalline diamond (upper substrate holder) have been grown by the plate-to-plate MPCVD reactor using high pressure CH₄-H₂ mixture gases. Experimental results show that high quality single-crystal diamond and polycrystalline diamond were simultaneously synthesized at a growth rate of 25 μm/h and 12 μm/h, respectively. The results indicate that our MPCVD reactor is unique for the synthesis of diamond with high efficiency.     

Study of CrNx and NbC interlayers for HFCVD diamond deposition onto WC–Co substrates

Chromium nitride (CrN_x) and niobium carbide (NbC) films were deposited by magnetron sputtering on Co-cemented tungsten carbide (WC–Co) substrates and diamond deposition was performed by using hot-filament chemical vapor deposition (HFCVD) technique. The CrN_x and NbC interlayers have been deposited at different substrate temperatures (T_S = 400, 550 and 700 °C). The stability of these interlayers for diamond deposition has been studied by a heat treatment in H₂ atmosphere for 60 h at a temperature of 765 °C in the HFCVD reactor. X-ray diffraction (XRD), scanning electron microscopy (SEM) and glow discharge optical emission spectroscopy (GDOES) confirmed that due to this heat treatment the CrN_x films transformed into porous films composed of CrN_x, Cr₃C₂, Cr₇C₃ and Co phases, accompanied by a dramatic loss of nitrogen which is replaced by carbon. It was observed that higher nitrogen contents in the CrN_x films reduce the Co diffusion through the CrN_x layer. For NbC films, deposited by non-reactive magnetron sputtering from an NbC compound target, the heat treatment in the HFCVD reactor revealed that the films are absolutely stable during the heat treatment with some relaxation of residual stresses up to a factor of about 3. Furthermore it was found that Co diffuses through the NbC films with a T_S-dependant accumulation on the NbC film surface. By HFCVD it was possible to deposit adherent diamond coatings on the CrN_x and NbC interlayers. However, a reasonable adhesion of diamond on NbC was only obtained after different pre-treatments of the WC–Co substrates. The adhesion seems to be mainly governed by the topography of the WC–Co substrates.     

Low temperature catalytic steam gasification of HyperCoal to produce H2 and synthesis gas

HyperCoal is an ultra clean coal with ash content <0.05 wt%. Catalytic steam gasification of HyperCoal was carried out with K₂CO₃ at 775–650 °C for production of H₂ rich gas and synthesis gas. The catalytic gasification of HyperCoal showed nearly four times higher gasification rate than raw coal. The major gases evolved were H₂: 63 vol%, CO: 6 vol% and CO₂: 30 vol%. Catalyst was recycled for four times without any significant rate loss. The partial pressure of steam was varied from 0.5 atm to 0.05 atm in order to investigate the effect of steam pressure on H₂/CO ratio. The H₂/CO ratio decreased from 9.5 at 0.5 atm to 1.9 at 0.05 atm. No significant decrease in gasification rate was observed due to change in partial pressure of steam. Gasification rate decreased with decreasing temperature and become very slow at 650 °C. The preliminary results showed that HyperCoal, an ash less coal, could be a potential hydrocarbon resource for H₂ and synthesis gas production at low temperature by catalytic steam gasification process.     

Superconducting properties of homoepitaxial CVD diamond

Superconductivity was achieved above 10 K in heavily boron-doped diamond thin films deposited by the microwave plasma-assisted chemical vapor deposition (CVD) method. Advantages of the CVD method are the controllability of boron concentration in a wide range, and a high boron concentration, compared to those obtained using the high-pressure high-temperature method. The superconducting transition temperatures of homoepitaxial (111) films are determined to be 11.4 K for T_C onset and 8.4 K for zero resistance from transport measurements. In contrast, the superconducting transition temperatures of (100) films T_C onset = 6.3 K and T_C zero = 3.2 K were significantly suppressed.     

Improved microwave plasma cavity reactor for diamond synthesis at high-pressure and high power density

Microwave plasma assisted synthesis of diamond is experimentally investigated using high purity, 2–5% CH₄/H₂ input gas chemistries and operating at high pressures of 180–240 Torr. A microwave cavity plasma reactor (MCPR) was specifically modified to be experimentally adjustable and to enable operation with high input microwave plasma absorbed power densities within the high-pressure regime. The modified reactor produced intense microwave discharges with variable absorbed power densities of 150–475 W/cm³ and allowed the control of the discharge position, size, and shape thereby enabling process optimization. Uniform polycrystalline diamond films were synthesized on 2.54 cm diameter silicon substrates at substrate temperatures of 950–1150 °C. Thick, freestanding diamond films were synthesized and optical measurements indicated that high, optical-quality diamond films were produced. The deposition rates varied between 3 and 21 μm/h and increased as the operating pressure and the methane concentrations increased and were two to three times higher than deposition rates achieved with the MCPR operating with equivalent input methane concentrations and at lower pressures (≤ 140 Torr) and power densities.     

Preparation of superhard BxCy fibers by microvortex-flow hyperbaric laser chemical vapor deposition

A novel class of freestanding B_xC_y fibers was prepared by hyperbaric-pressure laser chemical vapor deposition. Utilizing mixtures of diborane and helium with hydrocarbons, such as methane, ethene, and pentane, B_xC_y-alloy fibers were prepared at axial rates of up to 12.2 μm/s. Regions of kinetically-limited and transport-limited growth were identified, and the activation energy for deposition from B₂H₆ + C₅H₂₀ + He mixtures (at relative concentrations of 1:25:10) was found to be 197 ± 27 kJ/mol, while the rate constant was approximately 1810 μm/s. Compositions ranged from B_0.4C_0.6 to B_0.03C_0.97 depending on the gas mixture and laser powers employed; axial and radial fiber compositions/microstructure were explored using Auger spectroscopy and electron microscopy. Glassy B_xC_y fibers with Vickers hardnesses of 42–45 GPa were grown at laser powers below 150 mW. The growth kinetics of pure boron fibers was also investigated from BCl₃ + H₂, BF₃ + H₂, and B₂H₆ + H₂ gas mixtures, producing fine-grained α-boron and large single-crystals of β-boron. Micro-scale vortices in the gas flow emanating from the reaction zone were observed using particle image velocimetry; such vortices enhance axial fiber growth rates through rapid gas mixing. Arrays of fibers were also grown in-parallel using diffractive optical elements.     

The role of inert gas in MW-enhanced plasmas for the deposition of nanocrystalline diamond thin films

Nanocrystalline diamond thin films have been deposited using microwave plasma enhanced deposition with gas mixtures of composition H₂/CH₄/X, where X was one of the inert gases He, Ne, Ar and Kr and typically constituted > 90% of the total gas flow. The diamond films obtained with each gas mixture deposited at approximately the same rate (0.15–0.5 µm h^? 1), and all showed similar morphologies and average grain sizes, despite very obvious differences in the appearance and gas temperatures of the respective plasmas. These plasmas were probed by optical emission and cavity ring-down spectroscopy, and results from companion 2D chemical kinetic modelling of the Ar/H₂/CH₄ and He/H₂/CH₄ plasma were used to guide interpretation of the experimental observations. We conclude that the inert gas, though acting primarily as a buffer, also has significant effects on the thermal conduction of the gas mixtures, the electron temperature and electron energy distribution, and thereby changes the main channels of ionization and input power absorption. As a result, inert gas dilution elevates the electron and gas temperatures, enhances the hydrogen dissociation degree and affects the H/C mixture composition and deposition mechanisms.     

Chemical modification of diamond surface with X–(C6H4)–COOH (X = F,Cl, Br,I) using benzoyl peroxide

The chemical reactivity of a hydrogenated diamond surface with X–(C₆H₄)–COOH (X = F, Cl, Br, I) when using benzoyl peroxide was investigated in this study. After the reaction processes the shapes of the IR spectra changed. It was confirmed from the XPS measurements that halogen atoms existed on the samples after the reaction process. The position of the IR peak at ca. 700 cm^? 1 depended on the kind of halogen in X–(C₆H₄)–COOH. Moreover, the peak position depended on the kind of constitutional isomer, that is, ortho-, meta-, or para-. It was confirmed from the experimental results of this study that halogen-containing organic functional groups can be introduced onto a diamond surface.     

Processes and parameters of diamond films deposition in AC glow discharge

We report about a new PACVD reactor construction for diamond films deposition from the alternating high-current glow discharge plasma. Argon–hydrogen–methane gas mixture was used as the precursor gas. Argon percent in the gas mixture could reach 75%. The discharge was generated between two tungsten electrodes with the total area no more than 1 cm². The electrode temperature was about 2000 °C. The discharge was in the form of plasma line and could reach 20 cm lengthwise and more. The discharge voltage was varied from 90 to 700 V depending on the argon partial pressure, the discharge current and the interelectrode spacing used. The current of discharge reached 30 A. The maximum growth rate of optical grade films was about 2 μm/h. OES spectrometry showed that hydrogen activation degree decreases from the center of plasma line to its edges. X-ray diffractometry and SEM showed the high quality of diamond films. The uniformity of the films in the transverse direction was measured.     

Using fluorine and chlorine in the diamond CVD process

We deposited diamond films at low substrate temperatures T_sub using the halogen containing precursor gases CHF₃ and C₂H₅Cl with an abundance of hydrogen. Diamond film growth was possible down to T_sub=370 °C using a hot-filament chemical vapor deposition process. The possibility of low temperature growth of diamond could be correlated with an increase of radical density in the gas phase caused by the halogen addition. These radicals, especially atomic hydrogen, fluorine and chlorine are responsible for the creation of active surface sites on the diamond surface. Chlorine especially is able to break surface bonds even at low substrate temperatures. Recent secondary ion mass spectroscopy measurements revealed that halogens are involved in surface reactions and that fluorine and chlorine were incorporated in the deposited films especially at low T_sub. Along with the creation of active surface sites the surface diffusion is important for the diamond growth, which is strongly limited by reduction of the substrate temperature. We succeeded in good quality diamond growth on glasses and aluminium substrates at T_sub=490 °C. A further decrease of T_sub leads to a decrease of diamond film quality and a poor adhesion of the diamond films to the substrate.     

Effect of total reaction pressure on electrical properties of boron doped homoepitaxial (100) diamond films formed by microwave plasma-assisted chemical vapor deposition using trimethylboron

Boron was doped into diamond films which were synthesized homoepitaxially on polished (100) diamond substrates by means of microwave plasma-assisted chemical vapor deposition (MPCVD) using trimethylboron as the dopant at a constant substrate temperature of 1073 K. The morphologies and electrical properties of the synthesized diamond films were dependent on the total reaction pressure. A maximum Hall mobility, 760 cm² V⁻¹ s⁻¹, was obtained for the film synthesized at 10.7 kPa. The values of Hall mobility were comparable with those obtained for B₂H₆-doped films at corresponding hole concentrations.     

High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method

Diamond film on titanium substrate has become extremely attractive because of the combined properties of these two unique materials. Diamond film can effectively improve the properties of Ti for applications as aerospace and biomedical materials, as well as electrodes. This study focuses on the effects of process parameters, including gas composition, substrate temperature, gas flow rate and reactor pressure on diamond growth on Ti substrates using the hot-filament chemical vapor deposition (HFCVD) method. The nucleation density, nuclei size as well as the diamond purity and growth tendency indices were used to quantify these effects. The crystal morphology of the material was examined with scanning electron microscopy (SEM). Micro-Raman spectroscopy provided information on the quality of the diamond films. The growth tendency of TiC and diamond film was determined by X-ray diffraction analysis. The optimal conditions were found to be: CH₄:H₂ = 1%, gas flow rate = 300 sccm, substrate temperature T_sub = 750 °C, reaction pressure = 40 mbar. Under these conditions, high-quality diamond film was deposited on Ti with a growth rate of 0.4 μm/h and sp² carbon impurity content of 1.6%.     

Hydrogen formation from a real biogas using electrochemical cell with gadolinium-doped ceria porous electrolyte

The electrochemical cell consisting of a gadolinium-doped ceria (GDC, Ce_0.9Gd_0.1O_1.95) porous electrolyte, Ni–GDC cathode and Ru–GDC anode was applied for the dry-reforming (CH₄+CO₂→2H₂+2CO) of a real biogas (CH₄ 60.0%, CO₂ 37.5%, N₂ 2.5%) produced from waste sweet potato. The composition of the supplied gas was adjusted to CH₄/CO₂=1/1 volume ratio. The supplied gas changed continuously into a H₂–CO mixed fuel with H₂/CO=1/0.949–1/1.312 vol ratios at 800 °C for 24 h under the applied voltage of 1–2 V. The yield of the mixed fuel was higher than 80%. This dry-reforming reaction was thermodynamically controlled at 800 °C. The application of external voltage assisted the reduction of NiO and the elimination of solid carbon deposited slightly in the cathode. The decrease of heating temperature to 700 °C reduced gradually the fraction of the H₂–CO fuel (61.3–18.6%) within 24 h. Because the Gibbs free energy change was calculated to be negative values at 700–600 °C, the above result at 700–600 °C originated from the gradual deposition of carbon over Ni catalyst through the competitive parallel reactions (CH₄→C+2H₂, 2CO→C+CO₂). The application of external voltage decreased the formation temperature of carbon by the disproportionation of CO gas. At 600 °C, the H₂–CO fuel based on the Faraday's law was produced continuously by the electrochemical reforming of the biogas.