Dielectric properties of SiO2-TiO2-Al2O3 thin films prepared by sol-gel method for humidity sensors

The influence of the composition of precursor sols and water chemisorption on the dielectric properties of SiO2-TiO2-Al2O3 films prepared by the sol-gel method was studied. The compositions of the prepared layers, expressed in terms SiO2:TiO2:Al2O3 ratios were 0:0.95:0.05; 0.32:0.63:0.05; 0.475:0.475:0.05; 0.63:0.32:0.05 and 0.95:0:0.05. Chemisorption was carried out by controlled cooling of prepared sensors in environments with relative humidities of 0.13%, 52.9% and 97.6%. The dielectric properties of the films were evaluated using impedance spectroscopy. The best fitting of the experimental data was achieved with a resistor (R) in series with a parallel resistor (R), and capacitor (C) circuit element, in series with a parallel constant phase element (CPE) and resistor (R) circuit element, i.e. an -R-(R/C)-(R/CPE)-model. Dielectric properties of samples were described by the dependence of the imaginary part of the normalized complex electric susceptibility on frequency. As the basis for the calculations, results gained from nonlinear regression analysis were used. The presence of atmospheric humidity during cooling of SiO2-TiO2-Al2O3 films prepared by the sol-gel method has a significant influence on their dielectric properties.