Modelling of the gas phase chemistry during diamond CVD: the role of different hydrocarbon species

The CHEMKIN suite of computer programs has been used to model the concentra tion profiles of different hydrocarbon species present within a hot filamen t CVD reactor during diamond growth, and the calculated values are compared with those obtained by direct measurements using an in situ molecular beam mass spectrometer. Different hydrocarbon gases (CH4, C2H2, C2H4 and C2H6) were used as the carbon source in the input gas mixture, ensuring that the ratio of C:H-2 remained constant at 1%. Calculations for when C2H6 is used as the precursor gas, after reaction and thermal equilibrium is realised, y ield similar CH4:C2H2 mole fraction ratios in the reactor under growth cond itions to those obtained using CH4, and to those measured experimentally. H owever, simulations using C2H4 or C2H2 as input gases do not reproduce the experimentally observed ratio of CH4:C2H2 mole fractions. This suggests tha t the conversion of unsaturated C-2 species to C-1 species is not a straigh tforward gas phase process, and there must be one or more reactions occurri ng within the chamber that are not present in the standard models for hydro carbon reactions. We suggest that these neglected reaction(s) probably invo lve surface-catalysed hydrogenation, which in this case, is most likely occ urring on the surface of the filament. (C) 1999 Elsevier Science S.A. All r ights reserved.