Microwave Plasma-assisted Chemical Vapor Deposition Research Articles

Oxidation Resistance of Bare and Pt-Coated Electrically Conducting Diamond Powder as Assessed by Thermogravimetric Analysis

A corrosion-resistant electrocatalyst support was prepared by overcoating high surface-area diamond powder ( diameter, ) with a thin layer of boron-doped ultrananocrystalline diamond (B-UNCD) by microwave plasma-assisted chemical vapor deposition. This core-shell approach produces electrically conducting and high surface-area diamond powder (B-UNCD-D). Accelerated degradation testing was performed by thermogravimetric analysis (TGA) to assess the oxidation resistance (i.e., corrosion resistance) of powder in the absence and presence of nanoscale Pt. The oxidation onset temperature for B-UNCD-D powder decreased with the Pt loading from . However, compared with the bare powder, the rate of carbon consumption was significantly greater for Pt-(XC-72) as compared to the platinized diamond powder. For example, the temperature of the maximum carbon consumption rate, , occurred at for Pt-(XC-72) (20% Pt/C), which was lower than the for bare XC-72. In contrast, for Pt-(B-UNCD-D, 20% Pt/C) was ; a temperature that was only lower than that for bare diamond. Isothermal oxidation at for produced negligible weight loss for Pt-UNCD-D (20% Pt/C) while a 75% weight loss was observed for Pt-(XC-72) (20% Pt/C). The results clearly demonstrate that platinized diamond is more resistant to gas phase oxidation than is platinized Vulcan at elevated temperatures.

Diagnostics and modeling of CH4–CO2 plasmas for nanosmooth diamond deposition: Comparison to experimental data

Microwave plasma-assisted chemical vapor deposition of very smooth diamond coatings is an important process for various applications including mechanical and micromechanical systems and acoustic wave devices. Nanosmooth coatings have been deposited from CH4–CO2 gas mixtures at moderate temperature, the order of 600 °C. In order to increase the knowledge of the process and the control of the final characteristics of the films, a modeling of these plasmas is necessary. This has been carried out here from the prior determination of the plasma parameters. Optical emission spectroscopy was used in order to determine the gas kinetic temperature. Microwave interferometry and Langmuir double probe were used to determine the electron density and the electron temperature, respectively. All these experimental data have been obtained for a wide range of external parameters, such as the inlet composition, the pressure, the gas flow rate, and the power injected in the plasma. Then modeling of CH4–CO2 plasmas was developed by coupling chemical kinetics with a two-dimensional description of hydrodynamics and a surface-wall recombination of main radicals. The kinetic description of the CH4–CO2 plasmas was done by combining a specific mechanism of dissociation by electrons to a slightly modified version of a combustion mechanism for neutral-neutral interactions. This model has been validated by comparing the calculated species concentrations and the experimental results obtained by molecular beam mass spectrometry as a function of various external parameters. The influence of the inlet composition at three microwave power densities has been especially emphasized here. The calculations are in good agreement with the experimental results. It is shown that among the various parameters that influence the diamond growth from CH4–CO2 plasmas, the power density injected in the plasma is very important as it changes strongly the degree of completion of the chemical system and then the deposition conditions.

Phosphorus incorporation and activity in (100)‐oriented homoepitaxial diamond layers

AbstractIn this work, we present a study about the homoepitaxial growth of phosphorus‐doped diamond on (100) substrates. The growth was performed by microwave plasma assisted chemical vapor deposition (MPCVD) adding an organic precursor for phosphorus (tertiarybutylphosphine: TBP) in the gaseous phase. We show that phosphorus is incorporated in (100) chemical vapor deposition (CVD) diamond as proved by secondary ion mass spectrometry (SIMS). The recombination of excitons bound to phosphorus donors is observed by cathodoluminescence (CL) spectroscopy. The influence of the growth parameters on the phosphorus donor activity is investigated. We show that the [C*]/[H2] ratio is a key parameter for controlling the P‐donor activity when diamond is grown on (100) surfaces.

Sequential analysis of diamond nucleation on silicon (001) with bias enhanced nucleation using X‐ray photoelectron spectroscopy and reflection high energy electron diffraction investigations

AbstractDiamond synthesis by microwave plasma assisted chemical vapour deposition (MPCVD) with a bias enhanced nucleation (BEN) step constitutes the most efficient method to obtain heteroepitaxy and is also useful to reach high nucleation densities. In this study, sequential reflection high‐energy electron diffraction (RHEED) and X‐ray photoelectron spectroscopy (XPS) investigations enabled us to highlight the formation of amorphous carbon and crystalline silicon carbide on the surface before diamond growth. By varying the bias voltage, we underlined an optimized value corresponding to a higher quantity of amorphous carbon. This quantity is strongly correlated to the diamond nucleation density. As for the quality of the SiC texture, this seems to be directly linked to bias voltage too: low bias voltages enable us to obtain high oriented 3C‐SiC whereas high bias voltages lead to a completely misoriented polycrystalline SiC film.

Microwave engineering of plasma-assisted CVD reactors for diamond deposition

The unique properties of CVD diamond make it a compelling choice for high powerelectronics. In order to achieve industrial use of CVD diamond, one must simultaneouslyobtain an excellent control of the film purity, very low defect content and a sufficientlyrapid growth rate. Currently, only microwave plasma-assisted chemical vapour deposition(MPACVD) processes making use of resonant cavity systems provide enough atomichydrogen to satisfy these requirements.We show in this paper that the use of high microwave power density (MWPD)plasmas is necessary to promote atomic hydrogen concentrations that are highenough to ensure the deposition of high purity diamond films at large growth rates.Moreover, the deposition of homogeneous films on large surfaces calls for theproduction of plasma with appropriate shapes and large volumes. The production ofsuch plasmas needs generating a fairly high electric field over extended regionsand requires a careful design of the MW coupling system, especially the cavity.As far as MW coupling efficiency is concerned, the presence of a plasma load represents amismatching perturbation to the cavity. This perturbation is especially important at highMWPD where the reflected fraction of the input power may be quite high. This mismatchcan lead to a pronounced heating of the reactor walls. It must therefore be taken intoaccount from the very beginning of the reactor design. This requires the implementation ofplasma modelling tools coupled to detailed electromagnetic simulations. This is discussed insection 3.We also briefly discuss the operating principles of the main commercial plasma reactorsbefore introducing the reactor design methodology we have developed. Modelling results fora new generation of reactors developed at LIMHP, working at very high powerdensity, will be presented. Lastly, we show that scaling up this type of reactorto lower frequencies (915 MHz) can result in high density plasmas allowing forfast and homogeneous diamond deposition on up to 160 mm diameter surfaces.

Adhesion and Wear Behaviour of NCD Coatings on Si<SUB>3</SUB>N<SUB>4</SUB> by Micro-Abrasion Tests

Nanocrystalline diamond (NCD) coatings offer an excellent alternative for tribological applications, preserving most of the intrinsic mechanical properties of polycrystalline CVD diamond and adding to it an extreme surface smoothness. Silicon nitride (Si3N4) ceramics are reported to guarantee high adhesion levels to CVD microcrystalline diamond coatings, but the NCD adhesion to Si3N4 is not yet well established. Micro-abrasion tests are appropriate for evaluating the abrasive wear resistance of a given surface, but they also provide information on thin film/substrate interfacial resistance, i.e., film adhesion. In this study, a comparison is made between the behaviour of NCD films deposited by hot-filament chemical vapour deposition (HFCVD) and microwave plasma assisted chemical vapour deposition (MPCVD) techniques. Silicon nitride (Si3N4) ceramic discs were selected as substrates. The NCD depositions by HFCVD and MPCVD were carried out using H2-CH4 and H2-CH4-N2 gas mixtures, respectively. An adequate set of growth parameters was chosen for each CVD technique, resulting in NCD films having a final thickness of 5 microm. A micro-abrasion tribometer was used, with 3 microm diamond grit as the abrasive slurry element. Experiments were carried out at a constant rotational speed (80 r.p.m.) and by varying the applied load in the range of 0.25-0.75 N. The wear rate for MPCVD NCD (3.7 +/- 0.8 x 10(-5) mm3 N(-1) m(-1)) is compatible with those reported for microcrystalline CVD diamond. The HFCVD films displayed poorer adhesion to the Si3N4 ceramic substrates than the MPCVD ones. However, the HFCVD films show better wear resistance as a result of their higher crystallinity according to the UV Raman data, despite evidencing premature adhesion failure.

Low-temperature thermionic emission from nitrogen-doped nanocrystalline diamond films on n-type Si grown by MPCVD

Temperature-dependent emission current–voltage measurements were carried out for nitrogen (N)-doped nanocrystalline diamond (NCD) films grown on n-type Si substrates by microwave plasma-assisted chemical vapor deposition (MP-CVD). Low threshold temperature (~ 260 °C) and low threshold electric field (~ 5 × 10 − 5 V/µm) were observed. Both the temperature dependence and the electric field dependence have shown that the obtained emission current was based on electron thermionic emission from N-doped NCD films. We have also studied the relation between nitrogen concentration and the saturation emission current. The saturation current obtained was as high as 1.4 mA at 5.6 × 10 − 3 V/µm at 670 °C when the nitrogen concentration was 2.4 × 10 20 cm − 3 . Low value of effective work function (1.99 eV) and relatively high value of Richardson constant (~ 70) were estimated by well fitting to Richardson–Dushman equation. The results of smaller φ and larger A′ suggest that N-doped NCD has great possibility of being a highly efficient thermionic emitter material.

The effect of boron doping and gamma irradiation on the structure and properties of microwave chemical vapor deposited boron-doped diamond films

We investigated the effects of gamma irradiation doses of 50, 100, and 103 kGy on boron-doped diamond (BDD) thin films synthesized using microwave plasma-assisted chemical vapor deposition with varying boron concentrations of [B]/[C]gas = 100, 1000, 2000, and 4000 ppm. The diamond thin films were characterized prior to and post-irradiation and the influence was assessed in terms of morphology, structure, and physical properties using scanning electron microscopy, atomic force microscopy, x-ray diffraction, vibrational spectroscopy (Raman and IR), x-ray photoelectron spectroscopy, and electrical measurement techniques. The results clearly showed that the response of gamma irradiation on BDD films was distinctive compared to those of undoped diamond films with changes in electronic behavior from metallic (>1019 to 1020 cm–3) to semiconducting (≤1019 cm–3), especially in the case of heavily boron-doped diamond films demonstrated by micro-Raman spectroscopy and electrical property characteristics. In fact, this modification in electrical property behavior induced by “gamma conditioning” can be effectively used to fine control boron doping in chemically vapor deposited diamond much needed for various electronic devices. The “gamma conditioning” refers to material processing by radiation which helps to passivate electrically active boron and defects with hydrogen migration thus fine tunes the boron acceptor concentration albeit that this is difficult to achieve during BDD film deposition. In addition, the results also indicate that almost all of the BDD films studied hereby tend to reach a state of damage saturation when submitted to gamma irradiation of 103 kGy. We discuss our novel findings in terms of the interplay of boron-hydrogen in diamond with possible multiple scenarios: (i) the generation of point/Frenkel defects due to Compton scattered electrons, (ii) the formation of a nonmetallic boron-rich borocarbide (e.g., B13C2) phase, (iii) the passivation of electrically active acceptor (B) sites due to the invariable presence of H in the grains and at the grain boundaries, (iv) the desorption of H from the diamond surface, and (v) the “priming or pumping” effect, which improves the electronic properties by compensating for the shallow boron acceptors to produce semiconducting/insulating material, verified by the cold neutron depth profiling technique.

Hopping conductivity in polycrystalline diamond films

The results of the experimental study of the conductivity of a polycrystalline diamond film by dielectric spectroscopy are presented. A diamond film about 200 μm thick was grown by microwave plasma-assisted chemical vapor deposition for 180 h. Two pronounced local peaks in the frequency dependence of the conductivity were observed. The relevant frequencies were temperature-dependent. These data permit us to hypothesize that the hopping mechanism of conductivity occurs in the polycrystalline diamond film. Two types of electrically active centers with different activation energies and relaxation times are involved in the conductivity.

Deposition of thick boron-doped homoepitaxial single crystal diamond by microwave plasma chemical vapor deposition

The deposition of high quality single crystal boron-doped diamond is studied. The experimental conditions for the synthesis of 1–2 mm thick boron-doped diamond are investigated using a high power density microwave plasma-assisted chemical vapor deposition reactor. The boron-doped diamond is deposited at a rate of 8–11.5 μm/h using 1 ppm diborane in the feed gas as the boron source, and the capability to overgrow defects is demonstrated. The experimental study also investigates the deposition of diamond with both 10 ppm diborane and 2.5–500 ppm of nitrogen added to the feedgas. Synthesized material properties are measured including the electrical conductivity using a four-point probe and the substitutional boron content using infrared absorption.

Growth and Applications of Nanocrystalline-Diamond Films

We have developed low temperature and large area nanocrystalline-diamond growth methods using microwave plasma-assisted chemical vapor deposition sustained by surface waves. Highly transparent and smooth nanocrystalline-diamond films were successfully grown on borosilicate glass, aluminum and other substrate materials by this method. Properties of our films were examined by using UV-excited Raman spectroscopy, X-ray diffraction, atomic force microscopy, and other techniques. Possible applications of our nanocrystalline-diamond films are discussed.

Single Crystal Boron-Doped Diamond Synthesis

AbstractThe electrical characteristics of high quality single crystal boron-doped diamond are studied. Samples are synthesized in a high power-density microwave plasma-assisted chemical vapor deposition (CVD) reactor at pressures of 130-160 Torr. The boron-doped diamond films are grown using diborane in the feedgas at concentrations of 1 to 50 ppm. The boron acceptor concentration is investigated using infrared absorption and a four point probe is used to study the conductivity. The temperature dependent conductivity is analyzed to determine the boron dopant activation energy.

Superlattices from Diamond

AbstractDiamond superlattices were fabricated by producing multilayer structures of isotopically pure carbon-12 (12C) and carbon-13 (13C), which confine electrons by a difference in band-gap energy. Secondary ion mass spectrometry (SIMS) measurements were employed to characterize the isotopic composition of the diamond superlattices. Layers between 2 nm and 350 nm in thickness can be designed and fabricated using a microwave plasma-assisted chemical vapor deposition technique.

Ex situ variable angle spectroscopic ellipsometry studies on chemical vapor deposited boron-doped diamond films: Layered structure and modeling aspects

We report the optical property measurements on boron-doped diamond (BDD) films which were synthesized by microwave plasma-assisted chemical vapor deposition technique on Si (100) using methane in high hydrogen dilution and trimethylboron as precursors with varying boron concentration such that [B]∕[C]gas=100, 500, 1000, 2000, 4000, and 6467ppm. These BDD films were investigated using a rotating analyzer variable angle spectroscopic ellipsometry (SE) from the near IR to UV range (830–193nm). By applying the conventional Bruggeman effective medium approximation and linear regression analyses to the raw SE data that is, [ψ(λi),Δ(λi)] and pseudodielectric function (⟨εr(λi)⟩,⟨εi(λi)⟩), we determined the most appropriate model fit. The SE modeling was performed through the normal and point-by-point fit methods combined with the coupled and uncoupled bulk and surface layer approaches providing the details about the thin films’ microstructure in terms of the (a) multilayer (component and surface) structure and component layer thickness of the films, (b)volume fraction of constituents [fsp3C, fsp2C and void (fv) in the component layer], (c) inhomogeneity of the structure along the growth axis and its variation with boron concentration, and (iv) surface roughness layer thickness (ds) with dimensions less than the optical wavelength that is not otherwise available. A simplified three-layer structural model consisting of an interfacial layer, an intermediate (or bulk) layer, and a top surface roughness layer has been proposed, which simulates the ellipsometry data reasonably well with coupled point-by-point method. An estimator, i.e., mean squared error (χ2), is used to assess the accuracy of the model fit. The results (surface roughness and constituents’ fraction) obtained through SE modeling are also compared with those from atomic force microscopy (AFM) and Raman spectroscopy to validate the layered model employed. Typically, high surface roughness values around 6nm were found for films grown under different boron concentrations which is almost five times smaller than determined from AFM. In this context, we determined an approximate linear relationship between these two variables. The relatively smaller surface roughness for BDD films indicates the combined role of boron-hydrogen (B, H) in diamond (C) while keeping the substrate temperature constant. We also discussed the variation of (fv and fnd) for the bulk and surface layers with increasing boron concentration.

Chemically vapor deposited diamond-tipped one-dimensional nanostructures and nanodiamond–silica–nanotube composites

Composite thin films of nanodiamond and silica nanotubes were synthesized by means of microwave plasma assisted chemical vapor deposition (MPCVD) on silica nanotube matrix that was seeded with nanodiamond particles. SEM, Raman spectroscopy, and EDX were used to analyze the composite. Wet chemical etching was applied to selectively remove exposed silica from the composites for further revealing the nanostructure of the composites. Nanodiamond grew around silica nanotubes and filled the space left between silica nanotubes to form a continuous film. When appropriately selected sizes of nanodiamond particles were used as diamond seeds, silica nanotubes capped with CVD-grown diamond crystals were also obtained. Potential applications and implication of composites of nanodiamond and 1-D nanostructures will be discussed.

The Physicochemical and Electrochemical Properties of 100 and 500 nm Diameter Diamond Powders Coated with Boron-Doped Nanocrystalline Diamond

There is a need for advanced, corrosion-resistant electrocatalyst support materials for use in fuel cells. To this end, electrically conducting diamond powder was prepared by depositing a layer of boron-doped nanocrystalline diamond on 100 and diamond powders. The doped layer was deposited by microwave plasma-assisted chemical vapor deposition using an -rich source gas mixture. After coating, the doped diamond powder had a specific surface area of and an electrical conductivity of . The doped diamond powder had a specific surface area of and an electrical conductivity of after coating. The specific surface area of both powders decreased by ca. 50% after diamond coating due mainly to particle–particle fusion. The electrical measurements provided conclusive evidence for a doped diamond overlayer as the uncoated powders possessed no electrical conductivity. Furthermore, the fact that the electrical properties were unaltered by acid washing confirmed that the conductivity arises from the doped diamond overlayer and not any adventitious carbon impurity on the particle surface, which is removed by such chemical treatment. Scanning electron microscopy images and Raman spectroscopy yielded further evidence in support of a nanocrystalline diamond overlayer. Both powders exhibited electrochemical responses for , , and that were comparable to typical responses seen for high-quality, boron-doped nanocrystalline diamond thin-film electrodes. The electrochemical behavior of the powders was assessed using a pipette electrode that housed the packed powder with no binder. The doped diamond powder electrodes were more plagued by ohmic resistance effects than were the powder electrodes because of reduced particle contact. Importantly, it was found that the doped diamond powder electrodes are dimensionally stable and corrosion-resistant during anodic polarization at vs in at . In contrast, glassy carbon powder polarized under identical conditions underwent significant microstructural degradation and corrosion.

Multiple substrate microwave plasma-assisted chemical vapor deposition single crystal diamond synthesis

A multiple substrate, microwave plasma-assisted chemical vapor deposition synthesis process for single crystal diamond (SCD) is demonstrated using a 915MHz reactor. Diamond synthesis was performed using input chemistries of 6–8% of CH4∕H2, microwave input powers of 10–11.5kW, substrate temperatures of 1100–1200°C, and pressures of 110–135Torr. The simultaneous synthesis of SCD over 70 diamond seeds yielded good quality SCD with deposition rates of 14–21μm∕h. Multiple deposition runs totaling 145h of deposition time added 1.8–2.5mm of diamond material to each of the 70 seed crystals.

Performance analysis of poly-, nano- and single-crystalline diamond-based photocathodes

The results of the external Quantum Efficiency (QE) measurements, in the range 150–210 nm, of Poly-, Nano- and Single-Crystalline Diamond (PCD, NCD and SCD) film photocathodes (PCs) are reported and discussed. The diamond films, grown by Microwave Plasma Assisted Chemical Vapour Deposition (MPACVD) technique, have been analyzed with the aim to define which characteristics enhance their QE. It has been found that NCD film PCs, generally showing a lower QE with respect to PCD ones, enhance their photoemission if a graphitic component is present in the film. Surface hydrogenation effects on the photoemissivity of PCs have been investigated. Comparative studies between the QE of an untreated and a hydrogenated-treated diamond film PC show that the hydrogenated sample exhibits Negative Electron Affinity (NEA) properties. An ageing test of 24 h air exposure shows a stability of the hydrogenated diamond PC higher than that of the CsI one. Moreover, the PC conditioning by means of H 2 plasma allows to recover almost completely the loss of PC QE due to ageing effect caused by the air exposure of a previously hydrogenated sample. The sensitivity of PCs depends strongly on the topography of diamond films.

High speed continuous and interrupted dry turning of A390 Aluminum/Silicon Alloy using nanostructured diamond coated WC–6 wt.% cobalt tool inserts by MPCVD

Nanostructured diamond films were grown to a thickness of approximately 35 µm by a 30 kW, 915 MHz, microwave plasma-assisted chemical vapor deposition (MPCVD) on chemically treated WC–6 wt.% Co tool inserts. Rockwell indentation tests were performed to evaluate the adhesion of the films and compared to that of traditional microcrystalline diamond. A series of high speed dry turning tests on high-silicon (18 wt.% Si) aluminum alloy A390 under continuous and interrupted modes were performed and comparisons were carried out to investigate the wear behavior on tool inserts that were uncoated, coated with nanostructured diamond, and commercial PCD (polycrystalline diamond cutter) ones. The tests showed that nanostructured diamond coatings demonstrated excellent durability against the highly abrasive A390 aluminum–silicon alloys in high speed dry turning. Ultra fine grain structure of this coating produces workpiece surface finish comparable or even better than PCD tools in the range we studied. Excellent coating adhesion of nanostructured diamond on WC–6% Co substrates leads to reliable wear behavior. For the first time, we evaluated the performance of nanostructured diamond film coated insert under high speed interrupted turning mode. A “self-cleaning” mechanism was observed which can significantly improve the performance of nanostructured diamond films. Micro-Raman spectra were taken on tested tools to study the wear mechanism of the coating.

Chemical Analysis of Impurity Boron Atoms in Diamond Using Soft X-Ray Emission Spectroscopy

To analyze the local structure and/or chemical states of boron atoms in boron-doped diamond, which can be synthesized by the microwave plasma-assisted chemical vapor deposition method (CVD-B-diamond) and the temperature gradient method at high pressure and high temperature (HPT-B-diamond), we measured the soft X-ray emission spectra in the CK and BK regions of B-diamonds using synchrotron radiation at the Advanced Light Source (ALS). X-ray spectral analyses using the fingerprint method and molecular orbital calculations confirm that boron atoms in CVD-B-diamond substitute for carbon atoms in the diamond lattice to form covalent B-C bonds, while boron atoms in HPT-B-diamond react with the impurity nitrogen atoms to form hexagonal boron nitride. This suggests that the high purity diamond without nitrogen impurities is necessary to synthesize p-type B-diamond semiconductors.