PHOTOLUMINESCENCE IN A-C-H FILMS

Amorphous hydrogenated carbon films show a characteristic photoluminescence behavior that is of great interest for the study of their electronic structure. In the present paper we develop a model of the photoluminescence process in a polymerlike hydrocarbon on the basis of a mixed sp2-sp3 hybridization of the carbon atoms in this material. The sp2 phase is assumed to be confined in clusters embedded within a sp3 matrix. We assume excitation and recombination to take place within a single cluster. Thus the overall photoluminescence signal consists of the contribution of all single clusters, having different energy-gap values Eg, due to their various sizes and/or shapes. Applying the model to experimental data, we deduce a material-characteristic function f(Eg) that turns out to be representative of the distribution of the different energy-gap values Eg of the sp2 clusters in the film. The good agreement with experimental data suggests that the classical Tauc gap ETauc in a-C:H films is a value averaging over all single-cluster gap values Eg. Moreover, our model reproduces all the peculiar features of the photoluminescence behavior of a-C:H films, such as the nonactivated dependence upon temperature. © 1995 The American Physical Society.