Effects of NH3 and N2 additions to hot filament activated CH4/H2 gas mixtures

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Abstract
Resonance enhanced multiphoton ionization and cavity ring down spectroscopies have been used to provide spatially resolved measurements of relative H atom and CH 3 radical number densities, and NH column densities, in a hot filament (HF) reactor designed for diamondchemical vapor deposition and here operating with a 1% CH 4 /n/ H 2 gas mixture—where n represents defined additions of N 2 or NH 3 . Three-dimensional modeling of the H/C/N chemistry prevailing in such HF activated gas mixtures allows the relative number densitymeasurements to be placed on an absolute scale. Experiment and theory both indicate that N 2 is largely unreactive under the prevailing experimental conditions, but NH 3 additions are shown to have a major effect on the gas phase chemistry and composition. Specifically, NH 3 additions introduce an additional series of “H-shift” reactions of the form NH x + H ⇌ NH x−1 + H 2 which result in the formation of N atoms with calculated steady state number densities >10 13 cm −3 in the case of 1% NH 3 additions in the hotter regions of the reactor. These react, irreversibly, with C 1 hydrocarbon species forming HCN products, thereby reducing the concentration of free hydrocarbon species (notably CH 3 ) available to participate in diamond growth. The deduced reduction in CH 3 number density due to competing gas phase chemistry is shown to be compounded by NH 3 induced modifications to the hot filament surface, which reduce its efficiency as a catalyst for H 2 dissociation, thus lowering the steady state gas phase H atom concentrations and the extent and efficiency of all subsequent gas phase transformations.