Fourfold to threefold transition in diamondlike amorphous carbon films: A study of optical and electrical properties

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Abstract
Hard, diamond-like amorphous carbon films (a-C) were prepared as a function of sputtering power density by employing dc planar magnetron sputtering of a graphite target in pure argon. Films deposited at 300 K and at increasing sputtering power density in the range 0.25–25 W cm−2 show a transition of optical and electrical properties, with the room temperature electrical conductivity increasing from 5×10−4 to 5 Ω−1 cm−1 and the optical gap decreasing from 0.74 to 0.40 eV. The imaginary part ε2 of the complex dielectric function is determined in the photon energy range 0.5–7 eV and shows a clear dependence on sputtering power density. The ε2 spectra for a a-C films possess features typical of amorphous semiconductors. neff, the effective number of valence electrons per carbon atom taking part in optical transitions, is determined via a sum rule operation on ε2. Comparisons of neff for each a-C film with that for crystalline graphite allow the average coordination of the carbon atom to be determined. We observe a systematic four-fold to three-fold transition, with the ratio of carbon atoms having four-fold sp3 configuration (diamond-like) to carbon atoms with three-fold sp2 configuration (graphite) varying from 3:1 to 1:1.