Sensing Behavior of TiO₂ Thin Film Prepared by r.f. Reactive Sputtering

TiO₂ thin films were fabricated by the r.f. reactive sputtering method at a deposition rate of approximately 1 nm/min. As-deposited thin films were amorphous and they crystallized to anatase and rutile after annealing at two different temperatures, 800 and 1000 °C, respectively. The film was determined to be continuous and dense based on SEM observation. Films of thickness of 90 nm, 300 nm, and 500 nm were deposited on an alumina substrate with Au electrodes to investigate the relationship between the thickness and the sensitivity. Gas sensing tests were performed on these films at two different temperatures (550 and 650 °C) and under various CO concentrations (250–1000 ppm). The maximum sensitivity was observed for a thickness comparable to the Debye length (∼300 nm). The proposed simple model agrees with the trend in the experimental results.