Optical properties of nanogranular and highly porous TiO2 thin films

We present a systematic investigation of the optical properties of cluster-assembled TiO2 films (20-150 nm nominal thickness) deposited onto Si wafers and processed with two types of ex-situ oxidation treatments. Spectroscopic ellipsometry (0.7-5 eV energy range), supported by x-ray photoelectron spectroscopy and atomic force microscopy measurements, allowed us to obtain information on the depth-dependent morphology of the films in a non-destructive mode. The characteristic grainy and porous structure of the films was modelled by means of a flexible effective-medium approach, allowing us to obtain a reliable estimate of the amount and distribution of voids into the films and their accessibility to foreign liquids. The absorption-edge broadening and significant optical absorptions within the gap were attributed to the intrinsically nanogranular morphology and to the interrelated defective structure/stoichiometry of the particles, also in relation to the occurrence of poorly accessible interstices or pores in the inner parts of the film.