High gas sensitivity of tin oxide ultrathin films deposited on glasses and alumina substrates

Abstract

Tin oxide ultrathin films were deposited on optically flat Pyrex glass, optically flat quartz glass and sintered alumina substrates by the ion-beam sputtering method. Films with thickness varying from about 1 to 500 nm were annealed in air at 500 °C for 50 h. The gas sensing properties of these films were investigated at 300 °C against synthetic air containing 0.5% hydrogen. A small substrate dependence of sensor behaviour was observed; however, an overwhelmingly important thickness dependence occurred in all substrates tried. Sensitivities higher than ∼104 or 105 were obtained in films in a narrow thickness range of about 3 to 20 nm, and films thicker or thinner than this were relatively insensitive.