On the mechanism of CH_3 radical formation in hot filament activated CH_4/H_2 and C_2H_2/H_2 gas mixtures

摘要

Resonance enhanced multiphoton ionization spectroscopy has been used to determine relative number densities of CH_3 radicals in a hot filament chemical vapour deposition (HF-CVD) reactor designed for diamond growth, as a function of process gas (i.e. both CH_4/H_2 and C_2H_2/H_2 gas mixtures), position (d), filament temperature (T_f) and local gas temperature (T_g). The similar CH_3 radical number density profiles observed upon activation of the two feedstock gas mixtures suggest that CH_3 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 C_2H_2/H_2 gas mixtures. Three body addition reactions involving C_2H_2 (and C_2H_4), together with H atoms and H_2 molecules, are identified as probable reactions requiring further study in order to provide a proper description of diamond CVD using a C_2H_2/H_2 gas feed.