Measurements of the mass of gas enclosed in a quartz tube confining a burning hydrogen microwave plasma and the emitted intensities of atomic lines with axial or radial dependence are described. The microwave power was varied between about 450 and 1300 W, the gas pressure between 10 and 150 mbar. The experimental results are compared with numerical calculations. The simulations lead to three-dimensional distributions of electron densities, electron engines, field strengths, temperatures and gas compositions, and the expected mass of gas enclosed in the plasma tube. Besides some input parameters taken from the literature there remains one important fittable constant, characteristic for hydrogen, in the calculations, namely the microwave power Theta needed to sustain one electron-ion pair in the discharge. Agreement between experiments and calculations is good. The radial dependence of the light emission is used to discuss the physics behind the emission process. It is concluded that the main process is recombination of ions and electrons forming excited atoms and not excitation due to collisions with fast electrons (under the conditions investigated). Similar results were achieved in O2, H2/Ar and H2/CO.
We performed diamond deposition experiments from a gas phase containing H2, CH4, and sometimes CO, using a microwave plasma ball reactor operating at 400 mbar pressure. The molybdenum substrates were stamped with a suitable tool to form a number of flattened cones on its surface. A strong preference for crystal growth on top of the cones was observed. Numerical calculations were used to solve the underlying thermal conduction and diffusion problems. At the substrate, the flow of the active species entering by diffusion from the bulk of the gas phase was balanced by those leaving the system due to incorporation in the crystals. Comparison with the experiments showed that at least 10% of the active species striking the surface are incorporated. Thus, the limitation of diamond growth in our investigation lies in gas phase transport and not in incorporation difficulties at the growing surface.
Diamond growth experiments were performed in a microwave plasma ball reactor on silicon wafers or on a molybdenum sheet provided with cones (stamped into the sheet with a punch). All substrates had been treated by scratching with diamond powder in advance. The gas mixture used was CH4/H2, sometimes with the addition of CO. Substrate temperatures ranged from 953 to 1428 K, pressures from 100 to 400 mbar, and microwave powers from 250 to 700 W. A strong preference of diamond growth was observed on the cones in the molybdenum substrates. This is interpreted as being caused by gas transport hindrance. The resulting deposition coefficient of the “active” species is about 0.1 under all conditions investigated. The deposition experiments on silicon substrates are numerically modeled in two steps. In the first step, temperature fields and electron density and energy distributions in pure hydrogen are calculated following the method described previously. The output of this first simulation step is taken as input data for the second step. The condition is applied that chemical reaction rates due to thermal or electronic activation and diffusional flows compensate each other at every point of the reactor. In this way stationary concentrations of the 13 species in 29 elementary reactions are computed and, from these, the expected deposition profile of diamond on the silicon substrate, assuming one of the carbon-containing species to be the “active” one. When the experimental deposition profiles are compared with the calculated ones, C2H as the “active” species gives the best match to all the experimental results. CH3 and C2H2 (and perhaps others) might contribute to the diamond growth to a limited extent only.
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