Structural and CO sensing characteristics of Ti-added SnO2 thin films

Thin films of SnO2 with Ti additives were deposited on silicon wafers by co-sputtering from a Sn and a Ti metal targets in an Ar+O2 mixture. The Ti contents of the films were controlled by the d.c. power (10–90 W) applied on Ti target. All SnO2–Ti films were annealed in flowing oxygen at 900 °C for 5 h. The phases, composition, microstructure and chemical bonding states of the SnO2–Ti films were examined. Resistances of the SnO2–Ti samples under the atmosphere of various CO concentrations (1500–2500 ppm) were measured to determine the gas sensitivity. As-deposited SnO2–Ti films are amorphous and exhibit zero sensitivity to CO gas. SnO2–Ti films transformed to polycrystalline structure of rutile SnO2 phase after annealing at 900 °C. The quantity of Ti additives increases with increasing sputtering power on Ti. In the mean time, the Sn concentration decreases with increasing Ti, which results in the reduction of oxygen vacancies in the films. Sensitivity measurements on the annealed SnO2–Ti films reveal that the SnO2–Ti film prepared with 50 W of d.c. power on Ti target has the highest sensitivity to CO. The effects of Ti additives on the material characteristics and sensing properties of SnO2–Ti films are discussed.