Chemical kinetics in carbon depositing d.c.-arc jet CVD reactors

Abstract

Experimental and theoretical studies of the behaviour of hydrocarbon species in d.c.-arc jet chemical vapour deposition reactors are reported, as a function of carbon source gas flow rate. CH(X) and C 2(a) radical number densities have been measured in absorption (by cavity ring-down spectroscopy) and via their optical emission in an arc jet plume operating with a standard CH 4/H 2/Ar feedstock gas mixture. The C 2(a) radical number density is seen to exhibit a linear (or sub-linear) dependence on CH 4 flow rate, in accord with previous findings (J. Appl. Phys. 82 (1997) 2072) for both C 2(a) and C 3(X) radicals in a lower power d.c.-arc jet. The present findings, together with the comprehensive set of earlier experimental data on gas velocity and gas temperature measurements (Diam. Relat. Mater. 7 (1998) 165; Plasma Sources Sci. Technol. 10 (2001) 595) have been used in developing a model of the plasma plume. The present calculations suggest that the observed high diamond growth rates (>50–100 μm/h) are most probably related to atomic C, which is present at concentrations as high as 10 14 cm −3, though C 2 species are calculated to be present at comparable abundance and thus may also contribute to growth. The high temperatures (∼3200 K) and large H 2 dissociation fraction (tens of percent) result in fast conversion of the input CH 4 into C atoms as a result of H-shifting reactions of the form: CH x +H⇌CH x−1 +H 2. The plasma–chemical reaction mechanism and thermochemical data developed here goes some way to unravelling the complex inter-conversion mechanisms linking C 1, C 2 and C 3 hydrocarbons and changing the extent of H-saturation via series of H-shifting reactions of the form: C x H y +H⇌C x H y−1 +H 2, ( x=1–3). The simplified model for carbon source gas incorporation into the free stream introduced in this work is shown to overestimate the C 2(a) density but to provide a very reasonable description of CH densities measured in the Bristol d.c.-arc jet, and to reproduce well the C and CH radical absorbances and broadband absorbance at 248 nm reported in an expanding cascaded arc jet reactor operating with Ar/C 2H 2 at Eindhoven University of Technology.