Superior positive relative humidity sensing properties of porous nanostructured Al:ZnO thin films deposited by jet-atomizer spray pyrolysis technique

A highly responsive resistive type humidity sensor is fabricated from porous aluminium-doped ZnO (Al:ZnO, AZO) thin films deposited by using jet-atomizer spray pyrolysis technique. The deposited thin films are calcined at various temperatures, ranging from 450 to 650 °C, to improve the crystallinity, electrical and optical properties. Electrical resistivity of the films is measured by Van der Pauw technique. Lowest resistivity of 13 Ω·cm is obtained for 3 at.% AZO samples calcined at 500 °C. Humidity sensing properties like responsivity, sensitivity, repeatability, response time and recovery time of the AZO samples are investigated at room temperature in the range of 10–90% of relative humidity (RH). Among all the samples, 5 at.% AZO thin films show the highest responsivity of 733% at 90% RH. In contrast to other ZnO based sensors, it is found that the resistance of the AZO sensors, prepared in our method, increases with the increase of relative humidity. A sensing mechanism of the AZO and ZnO thin films is proposed to explain the water adsorption technique and the cause of increase in resistance of the sensors during water adsorption.