Electrical humidity response of sol-Gel processed undoped and alkali-doped TiO2-Al2O3 thin films

Al2O3-TiO2thinfilms were prepared by a sol-gel processing. Sols were prepared having different TiO2/Al2O3 molar ratios, i.e., 9:1, 8:2, and 6:4, without and with the addition of 10 at% of K ions. The films were prepared by dipping the substrates (silicon wafers, alkali-free glass or alumina with comb-type Au electrodes) in the sols. The films were fired in air for 1 h at 300, 500, 650 and 800°C. The films were amorphous, at any composition, up to the firing temperature of 500°C. Crystallization of the films was inhibited by larger contents of Al2O3 and K. The humidity-sensitive electrical properties of the thinfilms were studied using d.c. and a.c. measurements. The addition of K dramatically improved the relative humidityresponse of the films.