TiO2 and Bi2O3 Thin Film Oxygen Sensor Materials

The electrical conductivity of polycrystalline TiO2 and Bi2O3, deposited on sapphire substrate by various methods, has been determined as a function of the film thickness, the oxygen partial pressure and the temperature. As a rule, TiO2 films with d ≲ 30 nm are incoherent after heat treatment, thereby restricting the miniaturization of the planar sensor device. Larger films essentially behave like bulk material with fully ionized oxygen defects V2•o (or Ti4•i interstitials) as major intrinsic defects. In case of extrinsic p‐type films, the conductivity contribution of the TiO2/sapphire substrate and TiO2/gas phase interfaces is clearly enhanced with respect to the bulk contribution, mainly due to segregation effects. Doubly ionized oxygen vacancies V2•o and M'Ti defects (trivalent acceptor atoms at Ti‐sites) appear to be the main intrinsic and extrinsic defects involved in the segregation process and in the oxygen exchange reaction. In contrast to TiO2, thin films of Bi2O3 react strongly with the substrate ultimately forming a new stable phase, Al4Bi2O9. Along with this structural change, the conductivity decreases by several orders of magnitude, classifying Bi2O3 as an unsuitable material for such thin film sensor devices.