Spectroscopic ellipsometric study of tetrahedral amorphous carbon films: optical properties and modelling

We present a spectroscopic ellipsometric (SE) study of ta-C and ta-C:N film s obtained in a filtered cathodic vacuum are system with an improved S-bend filter that yields high values of sp(3) content and mass density. The film s consist of an ambient/roughness/film/interface/substrate structure, as de monstrated by X-ray reflectivity (XRR) and SE. The optical properties of th e films have been derived by using four different approaches for the optica l function: (i) Cauchy's absorbent dispersion formula; (ii) the contributio n of three classical oscillators; (iii) the Forouhi-Bloomer model; and (iv) the Tauc-Lorentz model. The interface appears to be a mixture of a-Si and ta-C material, due to an amorphisation process originated by the carbon ion bombardment. The comparison of SE viith XRR results, as well as the depend ence of the fitted parameters with mass density, allows the Tauc-Lorentz mo del to be applied with confidence. (C) 2001 Elsevier Science B.V. All right s reserved.