NO2 sensing characteristics of Nb doped TiO2 thin films and their electronic properties

Thin films of Ti–Nb alloy were deposited on an Al2O3 substrate by sputtering from multi-targets of constituent metals, followed by annealing at 1000°C in 1% O2/N2. The films consisted of rutile-type TiO2 structure and showed fairly good sensing characteristics to NO2 over the range from 100 to 300 ppm at 600°C. According to the Arrhenius plot between the conductivity and the reciprocal temperature, the conductivity of the fairly Nb doped films were high and showed a saturated range among 300–600°C, on the other hand, that of undoped or slightly doped films were low and no saturation range was observed. After the temperature programmed desorption (TPD) measurement, no differences were observed on the desorption characteristics of NO2 between different amount of Nb-doped powder samples. A numerical analysis taking account of carrier concentration and mobility indicated that the majority electronic carrier has been alternated from p-type to n-type according to the amount of doped Nb, and also reproduced some complicated behavior of slightly doped film. As a result, NO2 sensing characteristics of the Nb doped TiO2 films seem to mainly depend on the electronic properties of the film.