Properties of hydrogenated amorphous carbon films and the effects of doping

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Abstract
Amorphous carbon films have been prepared in a r.f. glow discharge from various hydrocarbon gases. Electrical conductivity and optical absorption measurements show a strong dependence on the preparation gas and on the substrate temperature T d. Particularly for specimens prepared from ethylene, varying T d from 230 to 300°C increased the conductivity by ten orders of magnitude and decreased the optical gap from 2·0 to 0·8 eV. The hydrogen content of the specimens, measured by the thermal evolution technique, lies between 30% and 45% and shows no systematic dependence on T d or the preparation gas. Results are also presented for films which have been prepared from gas mixtures containing small predetermined amounts of phosphine, diborane or nitrogen. The addition of each dopant gas increases the specimen conductivity by between one and three orders of magnitude, depending on T d. It is concluded that for both undoped and doped specimens, conduction is predominantly by hopping in a region of fairly high density of localized states. Increasing T d or doping modifies the density of localized states in a way that moves the conduction path closer to the Fermi level, which remains fixed with respect to the valence states.