Atomic hydrogen concentration profiles at filaments used for chemical vapor deposition of diamond

The quantitative determination of atomic hydrogen concentrations cn in the vicinity of hot filaments is performed with two-photon laser-induced fluorescence. The measurements yield atomic hydrogen concentration profiles up to 28 mm from the filament surface with a spatial resolution of about 0.5 mm. The nonequilibrium nature of the hydrogen dissociation on the filament surface results in a saturation of hydrogen concentration profiles cH(r) for gas pressures above 10 mbar. Atomic concentrations in immediate vicinity of the filament are significantly lower than expected from thermodynamical calculations and depend on the filament diameter. Addition of methane results in a decrease of cn by more than 30% near the filament and a steeper en(r) dependence, demonstrating the accelerated consumption of H atoms by the presence of hydrocarbon species. H concentration profiles for Ta, Ir, and W filaments show a dependence on filament materials which might be taken into account when selecting filament materials for chemical vapor deposition of diamond.