Effects of NH 3 and N 2 additions to hot filament activated CH 4/H 2 gas mixtures

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 diamond chemical 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 density measurements 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 13cm -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. © 2002 American Institute of Physics.