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 concentration profiles of different hydrocarbon species present within a hot filament 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:H₂ remained constant at 1%. Calculations for when C₂H₆ is used as the precursor gas, after reaction and thermal equilibrium is realised, yield similar CH₄:C₂H₂ mole fraction ratios in the reactor under growth conditions to those obtained using CH₄, and to those measured experimentally. However, simulations using C₂H₄ or C₂H₂ as input gases do not reproduce the experimentally observed ratio of CH₄:C₂H₂ mole fractions. This suggests that the conversion of unsaturated C₂ species to C₁ species is not a straightforward gas phase process, and there must be one or more reactions occurring within the chamber that are not present in the standard models for hydrocarbon reactions. We suggest that these neglected reaction(s) probably involve surface-catalysed hydrogenation, which in this case, is most likely occurring on the surface of the filament.