Asymmetry of 'valence' and 'conduction' Gaussian pi 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 molecu lar orbitals (namely the negligible effect of the overlap between nearest n eighbours 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-bondi ng ('conduction') a-bands have asymmetric shapes. The imaginary part of the dielectric constant is evaluated on the basis of such model, by considerin g as well the role of many-body effects. The model properly describes the o ptical properties of a large set of different a-C(:H) films. (C) 2000 Elsev ier Science S.A. All rights reserved.