Optical and electronic properties of amorphous carbon materials

摘要

The optical and electronic properties of amorphous carbon (a-C) films are studied with spectroscopic ellispometry in the extended energy region from IR to vacuum ultraviolet (VUV). The results are discussed in view of electronic structure of diamond and graphite and the density of states of the a-C states. It is demonstrated that the investigation of a-C films in the UV to VUV energy regions can reveal not only their diamond-or graphite-like character, but also their local electronic structure and configuration, based on the fact that the strong sigma->sigma~* transition in carbon takes place at high energies. For example, the strong absorption peak in sp~3 bonded C occurs at 9.3 eV, whereas in a-C films the sp~2 bonded C peaks of pi and sigma excitations varies with the fraction of sp~3 sites. We also dicuss the criteria based on which the calculation of the bulk dielectric function epsilon(omega) of films becomes feasible beyond the measured energy range. Based on the above, calculation of the effective dielectric constant epsilon_eff due to the electronic transitions, and the effective number of electrons per C atom n_eff, provides valuable information on the type of bonding in carbon materials.