|Ph.D Student||Rothschild Avner|
|Subject||TiO2 Thin Films for Gas Sensors|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Emeritus Yigal Komem|
Thin films of TiO2 are prospective materials for integrated gas sensors. The goal of this work was to study the properties of TiO2 thin films as gas sensing materials and to investigate the role of grain boundaries in the response mechanism to O2 and CO.
Gas sensor prototypes were produced by reactive sputtering of TiO2 films (70¸200 nm) on oxidized Si substrates with embedded Au electrodes. The specimens were annealed in vacuum 24 h at 450ºC to crystallize the films and stabilize their microstructure. While the thicker films (200 nm) had a dense microstructure, the thinner films (70 nm) were discontinuous with many voids between the crystallites.
Under vacuum conditions the electrical conductivity of all the specimens was quite high due to donor-like oxygen vacancies. Exposure to O2 at T £ 380ºC decreased the conductivity drastically, partially due to annealing of some of the vacancies but mostly due to electron capture in chemisorption-induced trapping states at the free surfaces and grain boundaries.
Exposure to traces of CO in air at 300ºC increased the conductivity of the thinner TiO2 films, which had high surface to volume ratio, but did not affect the conductivity of the thicker films with relatively low surface to volume ratio. Consequently, it was concluded that the interactions with CO molecules, which removed pre-adsorbed oxygen adions and liberated the trapped electrons, occurred only at the free surfaces that were directly exposed to the gas phase. This conclusion has a very important implication on the design of integrated gas sensors from metal-oxide thin films, namely the thickness of such films should be below 100 nm in order to obtain high sensitivity to gases.