The electrical and optical properties of amorphous carbon prepared by the glow discharge technique

Abstract

Amorphous thin films of carbon have been prepared by the decomposition of acetylene in a radio frequency glow discharge. The specimens were deposited on substrates held at temperatures T _d between 400 and 650 K. Measurements of the optical absorption show that glow discharge carbon has an optical gap of the order of 2 eV which tends to decrease with increasing deposition temperature. The room temperature electrical conductivity of these specimens, about 10⁻¹² (ohm cm)⁻¹, is up to 12 orders of magnitude lower than that of evaporated material. Above 500 K, conduction is likely to take place in extended states but this changes to hopping transport at lower temperatures. The results are discussed in terms of a structural model in which glow discharge carbon consists of regions of graphically and tetrahedrally bonded carbon atoms.