Mole fractions of H, CH3, and other species during filament-assisted diamond growth

Molecular beam mass spectrometry has been used to determine the concentrations of several free-radical and stable gaseous species in a hot-filament reactor under diamond-growth conditions. At a filament temperature of 2600 K, the H-atom concentration in the proximity of the growth surface was found to decrease with increasing addition of methane to the feed gas, dropping by more than an order of magnitude when the methane percentage was increased from 0.4% to 7.2%. Concurrent with this decrease, large changes in the concentration of the hydrocarbon species were observed. All numerical modeling results that have been reported to date do not include heterogeneous reactions on the filament and have failed to predict or explain these observations. Also from the measured mole fractions, the reaction H+CH4↔H2+CH3 was found to be in nonequilibrium near the surface, with the reverse rate ranging from approximately three to sixteen times larger than the forward rate. We attribute the cause of this nonequilibrium to heterogeneous reactions on the surface of the substrate.