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

摘要

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{sub}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{sub}4/ H{sub}2/Ar feedstock gas mixture. The C{sub}2(a) radical number density is seen to exhibit a linear (or sub-linear) dependence on CH{sub}4 flow rate, in accord with previous findings (J. Appl. Phys. 82 (1997) 2072) for both C{sub}2 (a) and C{sub}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{sup}14 cm{sup}(-3), though C{sub}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{sub}2 dissociation fraction (tens of percent) result in fast conversion of the input CH{sub}4 into C atoms as a result of H-shifting reactions of the form: CH{sub}4 + H →← CH{sub}(x-1) + H{sub}2. The plasma-chemical reaction mechanism and thermochemical data developed here goes some way to unravelling the complex inter-conversion mechanisms linking C{sub}1, C{sub}2 and C{sub}3 hydrocarbons and changing the extent of H-saturation via series of H-shifting reactions of the form: C{sub}xH{sub}y + H →← C{sub}xH{sub}(y-1) + H{sub}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{sub}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{sub}2H{sub}2 at Eindhoven University of Technology.