REACTIVELY SPUTTERED INDIUM TIN OXIDE POLYCRYSTALLINE THIN-FILMS AS NO AND NO2 GAS SENSORS

Indium tin oxide polycrystalline thin films (90%In2O3 + 10%SnO2 (by weigth)) have been grown by r.f. reactive sputtering from a target of the same compound. The films exhibit columnar structure with a preferential (400) orientation with a dispersion of 8° around the normal of the film plane, as obtained by rocking curve measurements. These films exhibit an increase in resistivity when exposed to small concentrations of NO or NO2 in air. At an operating temperature of 590K they have a sensitivity SNO,NO, defined as SNO,NO = (Iair - INO,NO )/INO,NO for NO and NO2 chemisorption, of 100 and 30 respectively for gases at room temperature and atmospheric pressure at a concentration of 1000 ppm. Four zones for oxygen ion adsorption and desorption can be distinguished in the plot of conductivity activation energy vs. temperature. The absence of long-term drift and a satisfactory reproducibility even at high NO and/or NO2 concentrations are attributed to the excellent crystallographic properties of these films and should allow the development of detectors for nitrogen oxides at concentrations in the range 1-2000 ppm. © 1990.