Thermal stability of the optical properties of plasma deposited diamond-like carbon thin films

In present paper we studied the optical constants of the diamond-like carbon (DLC) films and their changes with annealing. The multisample modification of combined variable angle spectroscopic ellipsometry and near normal spectroscopic reflectometry was used. The optical constants of the DLC films were simulated by our recently published six-parameter dispersion model employing a parameterization of the density of electronic states (DOS). Based on the dispersion model parameters the density of π and σ electrons were evaluated. We showed that from our model and the independently determined hydrogen atomic fraction of the films before and after annealing the ratio between momentum matrix elements of π → π* and σ → σ* transitions and the correct sp3-to-sp2 carbon bonding configuration ratio can be calculated. It is worth to notice that the first quantity is usually assumed to be equal to unity but we showed that this assumption may cause a significant error in the determination of the sp3-to-sp2 ratio. Therefore, our suggested method represents a novelty in this field.