On the mechanism of CH3 radical formation in hot filament activated CH4/H2 and C2H2/H2 gas mixtures

Abstract

Resonance enhanced multiphoton ionization spectroscopy has been used to determine relative number densities of CH3 radicals in a hot filament chemical vapour deposition (HF-CVD) reactor designed for diamond growth, as a function of process gas (i.e. both CH4/H2 and C2H2/H2 gas mixtures), position (d), filament temperature (Tf) and local gas temperature (Tg). The similar CH3 radical number density profiles observed upon activation of the two feedstock gas mixtures suggest that CH3 radical formation in both cases is dominated by gas phase chemistry, in contradiction of the current consensus which invokes surface catalysed hydrogenation as the means of inducing the necessary CC bond fission in the case of C2H2/H2 gas mixtures. Three body addition reactions involving C2H2 (and C2H4), together with H atoms and H2 molecules, are identified as probable reactions requiring further study in order to provide a proper description of diamond CVD using a C2H2/H2 gas feed.