Asymmetry of 'valence' and 'conduction' Gaussian π bands in a-C:H and a-C thin films and its origin

By taking into account the peculiar structure of a-C(:H) thin films and, in particular, the tendencies of C-sp(2) atoms to cluster, we have developed, by modifying one of the current assumptions of the Huckel model for molecular orbitals (namely the negligible effect of the overlap between nearest neighbours atomic orbitals), a model able to describe the density-of-states of a-C(:PI) films. It is shown that the bonding (('valence') and anti-bonding ('conduction') a-bands have asymmetric shapes. The imaginary part of the dielectric constant is evaluated on the basis of such model, by considering as well the role of many-body effects. The model properly describes the optical properties of a large set of different a-C(:H) films. (C) 2000 Elsevier Science S.A. All rights reserved.