Disorder and Urbach energy in hydrogenated amorphous carbon: A phenomenological model

We develop a phenomenological model describing the structural and topological effects of the disorder in hydrogenated amorphous carbons (a-C:H), through the analysis of the Raman G-peak width and the optical absorption spectra, providing information on the densities of electronic pi ad pi(*) states (piDOS). We show that the Urbach energy is not related to topological disorder but to the Gaussian width (sigma(pi)) of the piDOS, peaked at +/-E(pi) energies above/below the Fermi level. sigma(pi), on its turn, is not related in a straightforward manner to the disorder. The disorder is better represented by the sigma(pi)/E(pi) ratio, expressing the disorder-induced narrowing of the Tauc optical gap. (C) 2004 American Institute of Physics.