Critical thickness and nanoporosity of TiO 2 optical thin films

This work reports on the porosity and refraction index of TiO 2 thin films as a function of the film thickness. Samples were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a microwave electron cyclotron resonance (MW-ECR) reactor at room temperature using titanium tetra-isopropoxide (TTIP) as precursor. Experimental parameters such as plasma gas composition (pure oxygen and argon/oxygen mixtures) and pressure (either ECR conditions or >normal> pressure, i.e. 10 -4 or 10 -3 torrs correspondently) were varied. The evolution of the thin film microstructure, porosity and optical properties is critically studied by AFM, SEM, water adsorption isotherms, ellipsometry and UV-Vis transmittance and the existence of a certain critical thickness (t c) demonstrated. The porosity of the films with thicknesses ranging from several tens of nanometers up to half a micrometer is evaluated by QCM-isotherms at room temperature. The dependency of this critical thickness with the plasma conditions is evaluated experimental and theoretically. Thus, the microstructure change at t c is attributed to a transition from a surface diffused dominated growth mechanism for t < t c to another where shadowing is predominant. Dynamic scaling analysis of the two regimes and their Monte Carlo simulation complete the reported study. © 2012 Elsevier Inc. All rights reserved.We thank the Junta de Andalucía (projects P09-CTS- 5189, TEP5283 and FQM-6900) and the Ministry of Science and Innovation (CONSOLIDER CSD2008-00023, MAT2010-21228 and MAT2010-18447) for financial support.Peer Reviewe