High performance TiO2 nanotubes antireflection coating

The present study analyzes the use of TiO2 layer as antireflection coating. TiO2 thin films were prepared on silicon substrate by electrochemical anodization process with thicknesses of 130 and 170 nm. The films exhibit a uniform nanotubular structure. The ellipsometry measurement data were analyzed using a multi-layer model, the bottom layer is a dense layer of TiO2 and the top layers are porous; they are formed by a mixture of TiO2 and void. The values of refractive index are obtained by the Bruggeman effective medium approximation. The refractive indexes of the bottom and top TiO2 sublayers were about 2.3 and 1.6, respectively. The lower values of reflectance are obtained with the thinnest film. For these samples the reflectance decreases about 90% compared to silicon and in some cases the reflectivity is less than 5%. The thickness influences of TiO2 thin films and their synthesis parameters on the optical constants are discussed.