Synthesis Of Diamond Films Research Articles

Formation of nanostructured carbon films in gas-discharge plasmas

Deposition of carbon materials from methane-hydrogen gas mixtures in a DC gas discharge is investigated. Parameters ensuring stable discharge conditions and synthesis of diamond and graphite-like films are determined. Optical emission spectroscopy is used to analyze the composition of the activated gas phase in the course of carbon film deposition. Synthesis of graphite-like carbon nanotubes and nanocrystallites is shown to correlate with the presence of C2 dimers in the plasma. A noncatalytic mechanism of synthesis of nanostructured graphite in a carbon-containing gas phase is proposed.

Reemission Intensity and Energy Spectrum Measurements of Slow Positron Beams for Various Moderators

Various characteristics of moderators for slow positron beam experiments using a 22Na source in non-ultra-high vacuum (UHV) were studied. Relative reemission intensities of slow positrons for various moderators: tungsten (W) annealed at various temperatures, rubbed by the polishing material SiC powder, etched chemically, covered by evaporation films of Cu metals, iridium (Ir) films and synthesized diamond films were relatively measured. Measurements of the beam energy spectrum of reemitted positrons just after annealing and after a long time in use are presented. By measurement of the energy spectrum, the value of the positron work function for W was found to be 2.1±0.3 eV. Intensity and spectrum studies for the ribbon, plate and mesh type W moderators were also carried out. An Ir moderator shows 20% stronger intensity than a W-ribbon moderator, but is not convenient for practical use. A W-mesh moderator is the best choice for gas scattering experiments in non-UHV. The scanning transmission microscope (STM) and atomic force microscope apparatus (AFM) were used to observe the surface state of the W moderator for slow positrons. The relationship between the surface state and the reemission intensity was analyzed in the atomic and sub-micron scales.

Synthesis of boron-doped diamond films by DC plasma CVD using a CH 4+CO 2+H 2 gas mixture at lower substrate temperature and formation of an n-Si/p-diamond heterojunction

Diamond films were synthesized by direct current plasma chemical vapour deposition using a CH 4+CO 2+H 2 gas mixture on Si substrates. The optimum deposition conditions were determined. It was found that 0.4 A/cm 2 current density, at applied voltage of 1 kV, resulted in good-quality diamond films. The substrate temperature was 750 K which is considerably lower than the conventional requirement of ∼1100 K. Boron doping was achieved by passing a portion of the gas mixture through boric acid dissolved in methanol. The boron-doped p-type diamond films were deposited on an n-type single crystalline Si substrate and an n-Si/p-diamond heterojunction was fabricated. The p–n junction was characterized in terms of current–voltage ( I– V) and capacitance–voltage ( C– V) measurements.

Thin diamond films for integrated circuits

We propose a technology for the synthesis of thin diamond films, which allows integrated emission elements of the vacuum triode type to be created on a substrate surface. The process includes three main stages: (i) deposition of a thin diamond film onto a silicon substrate; (ii) lithographic procedure with an aluminum mask; and (iii) post-growth in a regime ensuring required emission properties. An emission triode design is presented that can be realized using the proposed technology.

Comparison of pulsed and CW regimes of MPACVD reactor operation

This work is a study of possible advantages of the pulsed regime of microwave plasma-assisted CVD (MPACVD) reactor operation for diamond film synthesis. The influence of the pulsed regime as compared to the CW regime is studied with respect to changes in the deposition rate and changes in the temperature of the deposition surface. The results of two series of experiments in reactors based on a cylindrical cavity MPACVD reactor are presented. Results show a constant mean specific microwave power and constant substrate temperature in a thermally floating reactor configuration can be maintained in the pulsed regime by increasing the pressure as the duty cycle decreases. Furthermore, results indicate that at a fixed mean specific microwave power and substrate temperature the deposition rate increases in the pulsed regime as the pressure increases. Hence, for substrates that require a thermally floating reactor configuration (such as irregular-shaped, non-flat substrates and low thermal conductivity substrates, for which no active substrate cooling or heating can be used) the pulsed regime can be used to increase the deposition pressure and hence the deposition rate without overheating the substrate.

Spectroscopic Characterization of the Atomic Hydrogen Energies and Densities and Carbon Species during Helium−Hydrogen−Methane Plasma CVD Synthesis of Diamond Films

Polycrystalline diamond films were synthesized on silicon substrates for the first time without diamond seeding by a very low power (38 W) microwave plasma chemical vapor deposition reaction of a mixture of helium-hydrogen-methane (48.2/48.2/3.6%). The films were characterized by time-of-flight secondary ion mass spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, and X-ray diffraction. It is proposed that He + served as a catalyst with atomic hydrogen to form an energetic plasma. CH, C 2 , and C 3 emissions were observed with significantly broadened H α, β, y, and δ lines. The average hydrogen atom temperature of a helium-hydrogen-methane plasma was measured to be 120-140 eV versus 3 eV for pure hydrogen. Bombardment of the carbon surface by highly energetic hydrogen formed by the catalysis reaction may play a role in the formation of diamond. Then, by this novel pathway, the relevance of the CO tie line is eliminated along with other stringent conditions and complicated and inefficient techniques which limit broad application of the versatility and superiority of diamond thin film technology.

Synthesis and characterization of sulfur-incorporated microcrystalline diamond and nanocrystalline carbon thin films by hot filament chemical vapor deposition

The synthesis of microcrystalline and nanocrystalline carbon thin films using sulfur as an impurity addition to chemical vapor deposition (CVD) was investigated. Sulfur-incorporated microcrystalline diamond (μc-D:S) and nanocrystalline carbon (n-C:S) thin films were deposited on Mo substrates using methane (CH4), hydrogen (H2), and hydrogen sulfide (H2S) gas feedstocks by hot-filament CVD. These films were grown under systematically varied process parameters, while the methane concentration was fixed at 0.3% and 2% for μc-D:S and n-C:S, respectively, to study the corresponding variations of the films’ microstructure. Through these studies we obtained an integral understanding of the materials grown and learned how to control key material properties. The nanocrystalline nature of the material was proposed to be due to the change in the growth mechanisms in the gas phase (continuous secondary nucleation). The growth rate (G) was found to increase with increasing TS and [H2S] in gas phase, thus following the chemisorption model that describes the surface reactions. One of the propositions for the increase was that H2S increases the production rates of methane and consequent methyl radicals without much of its own consumption, which is almost negligible and increases the carbon-containing species. This is analogous to the increase of G with increasing methane concentration, but for the relatively high S/C ratio used here, there is a possibility of its incorporation in the material, however small. This particular conjecture was verified. In this context, the results are discussed in terms of the decomposition of reactant gases (CH4/H2/H2S) that yield ionized species. The inferences drawn are compared to those grown without sulfur to study the influence of sulfur addition to the CVD.

Influence of SnO 2 over layer on nucleation and growth of diamond films on silicon substrate

The influence of tin oxide (SnO 2) over layer on nucleation and growth of diamond films on silicon substrates has been investigated. In this work, the SnO 2 over layer was deposited on virgin and pre-treated silicon substrates by spray pyrolysis technique and the synthesis of diamond films was accomplished using hot filament chemical vapor deposition (HFCVD) system. The deposition characteristics of diamond films were investigated by scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffraction measurements (XRD). It is clearly observed that the SnO 2 overlayer facilitates the nucleation and growth of diamond, although tin is a non-carbide forming element. With SnO 2 overlayer, the nucleation density is found to increase irrespective of whether the substrate surface is pre-treated or not, prior to SnO 2 deposition.

Influences of Additions of Hydrogen and Methane on the Combustion Synthesis of Diamond Films.

An experimental study has been conducted in order to elucidate influences of additions of hydrogen and methane on the combustion synthesis of diamond films. Differences between the flat flame and the conical flame are first examined, and it is reconfirmed that the velocity gradient is one of the dominant parameters in the combustion synthesis of diamond films. Additions of hydrogen and/or methane are examined on the growth rates of diamond films, crystal sizes, and morphology. These results are also confirmed by conducting the similar experiments with a welding torch, instead of the flat flame burner. It is found that an addition of hydrogen reduces the growth rate and crystal size, while that of methane enhances those although homogeneity of the diamond films is reduced.

Growth of carbon nanotubes by microwave plasma chemical vapor deposition using CH 4 and CO 2 gas mixture

Carbon nanotubes (CNTs) were grown vertically and aligned on Fe catalytic nanoparticles which were deposited on a Si substrate at low temperature using CH 4 and CO 2 gas mixtures. A dynamic form of optical emission spectroscopy was used to detect the species in the plasma. These data show the dominant species in gas phase reaction. The composition of plasma significantly affects the reaction mechanism of CNTs growth and diamond film synthesis. The growth quality of CNTs is better than a conventional reaction in a gas mixture of hydrogen and hydrocarbons. However, the highest yield of CNTs of approximately 70% was obtained by microwave plasma chemical vapor deposition for CH 4–CO 2 gas mixture at a flow rate of CH 4/CO 2 at 96%, power supply of 300 W, reaction time of 20 min and bias voltage of 150 V. In summary, the oxygen containing in the CH 4–CO 2 gas system can increase the amount of C 2. In the C 2-rich plasma the excited C 2 emission at a higher intensity is beneficial to graphite deposition, which enhances CNTs synthesis on catalyst-deposited surface quality.

Synthesis of Thick Diamond Films by Direct Current Hot-CathodePlasma Chemical Vapour Deposition

The method of direct currenthot-cathode plasma chemical vapour deposition has been established. Along-time stable glow discharge at large discharge current and highgas pressure has been achieved by using a hot cathode in thetemperature range from 1100°C to 1500°C andnonsymmetrical configuration of the poles, in which the diameter ofthe cathode is larger than that of anode. High-quality thick diamondfilms, with a diameter of 40-50 mm and thickness of 0.5-4.2 mm,have been synthesized by this method. Transparent thick diamond filmswere grown over a range of growth rates between 5-10 µm/h. Mostof the thick diamond films have thermal conductivities of10-12 W/K·cm. The thick diamond films with high thermalconductivity can be used as a heat sink of semiconducting laser diodearray and as a heat spreading and isolation substrate of multichipmodules. The performance can be obviously improved.

Synthesis and tribological characteristics of nanocrystalline diamond film using CH4/H2 microwave plasmas

Nanocrystalline diamond film was synthesized on a silicon substrate using CH4/H2 microwave plasmas. The synthesis was carried out for 20 h at 640–680 °C in 10% CH4–H2 gas mixtures at 32 mbar using a coaxial antenna-type microwave plasma chemical vapor deposition (MWPCVD). From the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, the obtained film was found to grow directly as continuous and/or rectangular grains of approximately 5–15 nm from the substrate. The full width at half maximum (FWHM) of the film at 43.9° (2θ) was also approximately 0.35° from the X-ray diffraction (XRD) analysis. In the UV Raman spectrum, the somewhat sharp peak of sp3-bonded carbon attributed to diamond at approximately 1333 cm–1 and broad peak of sp2-bonded carbon attributed to graphite at 1580 cm−1 were observed. The friction coefficient, which was measured with a SiC ball of 6.35 mm diameter with 0.5 N load, was estimated to be approximately 0.05–0.07 in dry air. The hardness measured with a nanoindenter was in the range of approximately 97–104 GPa.

Diagnostics of plasma ball formed in high pressure microwave plasma for diamond film synthesis

A high pressure hydrogen microwave plasma for diamond film deposition has been experimentally studied. Spatial distributions of plasma parameters and electric field intensity inside the plasma reactor were investigated in detail to understand the mechanism of plasma ball formation under the discharge conditions; a filling pressure is typically 10–50 torr and an input microwave power is 1.3 kW. In order to study the spatial distributions of the high-energetic electrons, we carried out the measurements using the optical emission measurements with the periscope technique, together with the electron saturation current profile measurements with the electrostatic probe. Numerical analysis using the finite difference time domain (FDTD) method shows a fairly good agreement with the experimental results of the electric field distributions and power dissipation inside the plasma ball.

Microwave plasma enhanced chemical vapor deposition diamond synthesis from high carbon content source

To synthesize diamond films by microwave plasma enhanced chemical vapor deposition (MPECVD), the methane concentration (CH4/H2) plays a crucial role. It is well-known that there always exists a critical methane concentration (≤0.6%) only below which a good quality diamond film can be obtained. In this study, however, the phenomena of diamond synthesis resulting from high carbon concentration conditions were observed. The molten metals, e.g., Ag, Cu, were used as the deposition substrates on which crystalline diamonds can be achieved from a methane content of CH4/H2≥6% or even from solid carbon sources. These results suggest that there may exist a low methane content boundary layer (<0.6%) in the proximity of molten metal surface on which suitable species, CH, CH+, Hαa and Hβ are composed for the diamond nucleation and growth similar to the condition as in the conventional low methane contents. The molten metal inclines to dissolve other forms of carbonaceous materials other than diamond, and thus keeps a much higher steady supply of carbon atoms that enhances the quality as well as the growth rate of the forming diamonds.

Influence of Equivalence Ratio, Velocity Gradient, and Surface Roughness of Substrate on the Combustion Synthesis of Diamond Films Using a Flat Flame Burner.

An experimental study to synthesize diamond films has been conducted under a well-defined flow field to make clear the mechanism of combustion synthesis of diamond films. Using a flat flame burner, the homogeneous diamond films are synthesized on substrates, above which flat, premixed acetylene/hydrogen/oxygen flames are established. The effect of equivalence ratio and velocity gradient is examined on the growth rates of diamond films, crystal sizes, and their morphology. It is found that not only the maximum growth rate but also the maximum crystal size can be obtained when the equivalence ratio is about 2.50, and that the growth rate, crystal size, and/or morphology are nearly the same when the velocity gradient is kept constant. These results are confirmed by a conical flame with a welding torch. In addition, the scratching treatment for the substrates by diamond powder is unnecessary, the maximum growth rate of diamond films can be obtained when the surface roughness is about 0.11 μm, and the nuclei of diamond appear in scratches on the substrates.

Diamond deposition on Si (111) and carbon face 6HSiC (0001) substrates by positively biased pretreatment

The growth of textured diamond films on Si (111) and carbon face 6HSiC (0001) has been achieved by positive bias-enhanced-nucleation in microwave plasma chemical vapor deposition. A bias voltage of +100 to +300 V, 3% CH4 concentration in bias step, and 0.33% CH4 concentration in growth step were used to synthesize diamond films on Si and carbon face 6HSiC. Highly oriented diamond films were observed by scanning electron microscopy and X-ray diffraction patterns. Cross-sectional transmission electron microscopy showed that the interface between diamond and Si substrate was smooth, while it was rough between diamond and SiC. It also showed that diamond films were directly grown on both substrates.

Synthesis of multilayer diamond film and evaluation of its mechanical properties

When diamond is used as a wear-resistant material for tools, its unique wear resistance cannot be fully exploited due to the possibility of brittle fracture. In general, diamond films synthesized by the vapor phase method are polycrystalline exhibiting columnar crystal growth. In the polycrystalline structure, once cracks occur on the surface of the film, they tend to propagate through the columnar particles, leading to a decrease in toughness. In this study, using the hot-filament chemical vapor deposition (CVD) method, diamond secondary nuclei are grown on a substrate by applying bias current to the substrate repeatedly and intermittently; multilayer diamond films, in which the continuity of grain growth is suppressed, are synthesized. The interfaces formed by the multilayer structure are expected to prevent crack propagation. To confirm this effect, mechanical characteristics, such as the bending strength of the multilayer diamond film are evaluated. The results indicate that the bending strength of the multilayer diamond film is approximately 30% higher than that of a conventionally-produced diamond film.

Temperature-dependent conduction of W-containing composite diamond films

We report on the synthesis and electrical characterizations of composite W-containing diamond films. These layers have been produced by a hybrid chemical-vapor-deposition/powder flowing technique that allows control of the dispersion of the foreign W phase inside a diamond matrix. The electrical behavior of such material in the 25–500 K temperature range is found characterized by a nonlinear response, typical of metal/insulator composite materials, with conductivity reaching the maximum value of 95 Ω−1 cm−1 at 148 K. Structural and morphological investigations performed by reflection high-energy electron diffraction, scanning electron microscopy, and dispersive x-ray spectrometry indicate that the presence of a percolating network of metallic grains did not perturb the quality of the diamond lattice.

Synthesis and characterization of highly-conducting nitrogen-doped ultrananocrystalline diamond films

Ultrananocrystalline diamond (UNCD) films with up to 0.2% total nitrogen content were synthesized by a microwave plasma-enhanced chemical-vapor-deposition method using a CH4(1%)/Ar gas mixture and 1%–20% nitrogen gas added. The electrical conductivity of the nitrogen-doped UNCD films increases by five orders of magnitude (up to 143 Ω−1 cm−1) with increasing nitrogen content. Conductivity and Hall measurements made as a function of film temperature down to 4.2 K indicate that these films have the highest n-type conductivity and carrier concentration demonstrated for phase-pure diamond thin films. Grain-boundary conduction is proposed to explain the remarkable transport properties of these films.

Large area high quality diamond film deposition by high power DC arc plasma jet operating at gas recycling mode

Principles of a high power DC arc plasma torch with rotating arc roots and the semi-closed gas recycling system are presented. Recent results on large area high quality free standing diamond film deposition by a high power DC arc plasma jet CVD system operating at gas recycling mode is discussed. It was shown that the uniformity of diamond film deposition over a large substrate area was closely dependent on the uniformity and symmetry of the plasma, which can be adjusted by proper manipulation of the magnetic field and the effects of fluid dynamics. Effect of gas recycling on the quality of diamond films is discussed. By proper design of the whole system and process optimization, thick uniform diamond wafers of Φ60–110 mm in diameter with a thickness up to 2 mm and transparent diamond wafers of Φ30–60 mm in diameter has been deposited successfully. It is demonstrated that gas recycling can be used even for high quality diamond film synthesis. This is of technical and economical importance in the development of high power DC arc plasma jet CVD systems for economical fabrication of high quality diamond wafers.