Optical reflectance from anodized Al-0.5 wt % Cu thin films: Porosity and refractive index calculations

Abstract

Porous anodic alumina (PAA) films with periodically arranged horizontal and vertical nanopores have been applied as templates or platforms upon which optical sensors can be based. While the porosity and the refractive index of these 3D PAA can be tuned by varying the anodization parameters, the determination of these parameters from reflectance data has not been explored so far. In this study, we estimated the porosity and the refractive index of thin anodized Al-0.5 wt% Cu from the alumina film thickness and the interference pattern composed of the incident and reflected light beam in the alumina film. The 3D PAA films were prepared by anodization of Al-0.5 wt% Cu thin film deposited on TiN/Si substrate. Anodization was performed in three electrolytes at different concentrations (1 M sulfuric acid, 0.3 M oxalic acid and 0.75 M phosphoric acid) within the voltage range of 10–90 V. The structural characteristics showed that both pore size and inter-pore distance increased with increased anodization voltage and time while the pore density decreased exponentially. The results from reflectance spectra clearly showed that the effective refractive index of the samples decreased with voltage and anodization time, while porosity (total, vertical and horizontal) increased. The density of the samples was found to be inversely proportional to porosity.