Photoluminescence ina-C:H films

Abstract

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 ${\mathit{sp}}^{2\mathrm{-}}$ ${\mathit{sp}}^3$ hybridization of the carbon atoms in this material. The ${\mathit{sp}}^2$ phase is assumed to be confined in clusters embedded within a ${\mathit{sp}}^3$ 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 ${\mathit{E}}_{\mathit{g}}$, due to their various sizes and/or shapes. Applying the model to experimental data, we deduce a material-characteristic function f(${\mathit{E}}_{\mathit{g}}$) that turns out to be representative of the distribution of the different energy-gap values ${\mathit{E}}_{\mathit{g}}$ of the ${\mathit{sp}}^2$ clusters in the film. The good agreement with experimental data suggests that the classical Tauc gap ${\mathit{E}}_{\mathrm{Tauc}}$ in a-C:H films is a value averaging over all single-cluster gap values ${\mathit{E}}_{\mathit{g}}$. Moreover, our model reproduces all the peculiar features of the photoluminescence behavior of a-C:H films, such as the nonactivated dependence upon temperature.