Methane-hydrogen Gas Mixture Research Articles

Aspects of diamond deposition by anomalous pulsed d.c. glow-discharge-activated chemical vapor deposition

Diamond was deposited onto SiAlON substrates by glow-discharge-activated chemical vapor deposition (CVD) using hydrogen-methane gas mixtures. The formation of diamond depends strongly on the plasma parameters (current, voltage, pressure, gas mixture) and the substrates used. Diamond deposition occurred in a d.c. glow discharge at a pressure of 30 mbar, a voltage of 660 V and current density of 0.5 A cm 2, and a series of bright and dark striations in the Faraday dark space was observed. As the presence of illumination in the interelectrode space is apparently important for successful diamond deposition, its dependence on the various luminous effects, and the voltage distribution in the interelectrode space are discussed.

(110)-oriented diamond films synthesized by microwave chemical-vapor deposition

Bilayer diamond films were deposited on Si substrates by microwave-plasma chemical-vapor deposition (CVD) using a methane-hydrogen gas mixture. The first layer was deposited for 3 h using a reaction gas which was composed of 2.5 vol. % methane and 97.5 vol.% hydrogen. The deposited film consisted of very weakly (110)-oriented microcrystalline diamonds as well as amorphous carbon and graphite. In order to remove non-diamond carbons from the film surface, the specimen was treated in hydrogen plasma for 1 h. Finally, a second layer was deposited on the first layer for 14 h using a methane concentration of between 0.2 and 1.6 vol.%. It was found that the x-ray intensity of the (220) diffraction of the bilayer films was much greater than that of the (111) diffraction, indicating that the diamond grains in the second layer were strongly oriented with their crystallographic (110) planes parallel to the substrate surface. X-ray diffraction spectra of bilayer films in which the second layer was deposited for 7, 14, 21, and 35 h using two different methane concentrations, 0.3 and 1.2 vol.%, showed that within periods of up to 21 h, the (220) intensity increased with the deposition time much more quickly than the (111) intensity, indicating that the degree of (110) orientation was further enhanced as the second layer thickness increased. However, the (220) intensity decreased after 21 h, presumably due to thermal randomization. Results of scanning electron microscopy, electron diffraction, and Raman spectroscopy of the bilayer films are also presented.

Electron microscopy of vapor phase deposited diamond

Thin carbon films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced CVD have been examined by electron microscopy. Previously reported transmission electron microscopy (TEM) of the diamond films has shown that the majority of diamond crystals have a very high defect density comprised of {111} twins, {111} stacking faults, and dislocations. In this study, high resolution electron microscopy (HREM) has been utilized to lattice image individual defects in these polycrystalline diamond films. Interpretation of the images from these defects is not trivial and reported image simulations have been utilized to understand further these defects. Fivefold multiply twinned particles have also been examined and it was found that the 7.5° misfit present in such particles has been accommodated at the twin boundaries rather than by elastic deformation. This creates a twin boundary coincident with a low angle grain boundary which has been termed a “tilted twin boundary”. The density of defects in these particles is generally high; however, a dramatic reduction in the defect density near the twin boundaries was observed. This defect reduction is significant because if its origin can be determined, this information may be useful in producing higher quality diamond films.

The analysis of defect structures and substrate/film interfaces of diamond thin films

Diamond is an excellent candidate material for use in selected electronic and wear resistant coating applications due to its superior hardness, strength and thermal conductivity as well as its high electron drift velocity, chemical and thermal stability, radiation hardness and optical transmission. Electronic devices of particular interest include those having high-power, -frequency and -temperature applications, as well as those for chemically harsh and/or high radiation flux environments. The recent development of techniques for growth of crystalline diamond films at low pressures using common hydrocarbon and H2 gases has created the potential for growing thin films for such devices or wear resistant coatings and a host of related applications. In this research, diamond thin films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced chemical vapor deposition (CVD) have been examined by various transmission electron microscopy (TEM) techniques including bright and dark field, high resolution (HREM), selected area diffraction (SAD) and electron energy loss spectroscopy (EELS). Columnar growth of polycrystalline grain structure, twins, stacking faults, dislocations and intermediate layers were characteristic of the diamond films. No sp2 bonding character in the grains, defects or grain boundaries was detected by EELS.

Characterization of filament-assisted chemical vapor deposition diamond films using Raman spectroscopy

Results are presented on a Raman study of diamond films prepared by the filament-assisted chemical vapor deposition technique from a methane-hydrogen gas mixture. It is shown that Raman spectroscopy is a useful probe of both the phase of carbon present and of the quality of the diamond component in the films. The study entailed varying the total gas pressure, but holding the ratio of CH4 to H2 constant. It is clearly demonstrated that as the pressure is decreased, the density of defects within the diamond phase increases. The increase in defect density as the pressure is lowered leads eventually to disordered forms of carbon. This result can be understood in terms of the relatively higher electron and ion fluxes at the substrate for films prepared at low pressure.

Characterization of diamond thin films: Diamond phase identification, surface morphology, and defect structures

Thin carbon films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced CVD have been examined by Auger electron spectroscopy, secondary ion mass spectrometry, electron and x-ray diffraction, electron energy loss spectroscopy, and electron microscopy. They were determined to be similar to natural diamond in terms of composition, structure, and bonding. The surface morphology of the diamond films was a function of position on the sample surface and the methane concentration in the feedgas. Well-faceted diamond crystals were observed near the center of the sample whereas a less faceted, cauliflower texture was observed near the edge of the sample, presumably due to variations in temperature across the surface of the sample. Regarding methane concentration effects, threefold {111} faceted diamond crystals were predominant on a film grown at 0.3% CH4 in H2 while fourfold {100} facets were observed on films grown in 1.0% and 2.0% CH4 in H2. Transmission electron microscopy of the diamond films has shown that the majority of diamond crystals have a very high defect density comprised of {111} twins, {111} stacking faults, and dislocations. In addition, cross-sectional TEM has revealed a 50 Å epitaxial layer of β3–SiC at the diamond-silicon interface of a film grown with 0.3% CH4 in H2 while no such layer was observed on a diamond film grown in 2.0% CH4 in H2.

Separation of hydrogen-methane gas mixtures by permeation under pressure through porous cellulose acetate membranes

Separation of hydrogen-methane gas mixtures by permeation under pressure through porous cellulose acetate membranes

Thermodynamics and solubility of carbon in ferrite and ferritic Fe-Mo alloys

A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of α-Fe, Fe-0.48 wt pct Mo and Fe-1.16 wt pct Mo specimens which were equilibrated with a series of methane-hydrogen gas mixtures. The equilibria investigated involved the ferrite phases of the alloys between 682 and 848‡C. The experimental results permitted direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of other thermodynamic functions, without the necessity for any correction factors. The results have been compared with the experimental measurements of a number of other investigators. In ferrite, the partial molar enthalpy and entropy of solution of carbon are found to be 26,800 cal/mole, (112, 130 J/mole), and 30.59 cal/K-mole, (127.99 J/K-mole) respectively at temperatures below about 727‡C. Above this temperature, the values are 25,200 cal/mole and 29.13 cal/K-mole, respectively. The addition of molybdenum results in an increase in these properties below 727‡C and a decrease in the values above 753‡C, and the changes are found to be proportional to the molybdenum content. The solubility of carbon in α-Fe is found to be 0.0176 wt pct at the eutectoid temperature. Molybdenum increases the solubility relative to the Fe-C system at temperatures above the eutectoid and decreases it below the eutectoid.

Thermodynamics of carbon in austenite and Fe-Mo austenite

A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of iron, iron-0.48 wt pct molybdenum and iron-1.16 wt pct molybdenum specimens which were equilibrated with a series of methane-hydrogen gas mixtures of constant composition. The equilibria investigated involved the austenite phases of the alloys at 783, 813 and 848‡C. The experimental results permit direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of the enthalpy and entropy of solution of carbon. The results are compared with the experimental measurements of a number of other investigators. The results are in excellent agreement with those of Smith and Schenck and Kaiser for the Fe-C system at 800‡C, and indicate -HC/M values of 9700 ± 500 cal/mole for pure Fe, 10,030 ± 500 cal/mole for an Fe-0.48 wt pct Mo alloy, and 10,150 ± 500 cal/mole for an Fe-1.16 wt pct Mo alloy. The effect of molybdenum in austenite is to decrease the activity coefficient of carbon in austenite.

Thermodynamic properties of carbon in b.c.c. and f.c.c. iron-silicon-carbon solid solutions

The equilibrium between hydrogen-methane gas mixtures and Fe-Si-C solid solutions has been investigated both as a function of temperature and carburizing gas composition. The thermodynamic properties of the carbon atoms in both b.c.c. and f.c.c. solid solution have been derived from the equilibrium measurements. The results found have been compared with those of earlier investigations and with the predictions of recent theoretical models on ternary solid solutions containing both substitutional and interstitial solute atoms.