Ar doping of ch4 plasmas for carbon film deposition

We have modeled the gas-phase chemistry of a typical radio frequency CH4/Ar plasma used for the deposition of diamond and diamond-like carbon films. Our simulations show that the most abundant carbon containing radical is CH3 in pure methane discharges, but it is the carbon dimer C-2 in discharges of methane highly diluted by argon. Thus we propose that the gaseous precursor of the film is CH3 in methane plasmas, and C-2 in CH4/Ar plasmas. This proposal resolves outstanding discrepancies and is consistent with recent experiments demonstrating the deposition of diamond from hydrogen deficient plasmas.