A 13C isotopic tracing study of a-C:H thin-film growth

The growth mechanisms of amorphous carbon (a-C:H) thin films obtained by the decomposition of methane gas in a multipolar plasma set up were studied using 13C as a tracer. Hydrogenated carbon films first grown in a methane plasma with natural isotopic composition (12C) were further grown in highly enriched 13C methane discharge (experiments were also carried out with the reverse order). 13C profiles were obtained by resonant nuclear reaction depth profiling using the narrow (FWHM<100 eV) resonance at 1747.6 keV, in the 13C(p, γ)14N nuclear reaction. Plasma conditions were varied so as to vary the energy of particles arriving at the sample surface from 30 eV to 500 eV. In all cases, a system of two layers was observed: a pure 13C film overlaying a pure 12C film (or the reverse order) with a rather sharp interface between the two layers. These results suggest layer by layer amorphous hydrogenated carbon film growth with minimal mixing or interdiffusion of the isotopic layers induced by the ionic bombardment during the growth.