Improvement in Sensing Responses to Ammonia Gas for Gas Sensors With Separately Designed Sensing Element Using ALD-Grown ZnO Nanoparticles and Read-Out Element of Top-Gate In-Ga-Zn-O Thin-Film Transistor

Abstract

Ammonia (NH3) gas sensors with unique oxide semiconductor thin-film transistor (TFT) configuration using ZnO nanoparticles (NPs) as gas-sensing element were fabricated and their gas-sensing responses were evaluated. The ZnO NPs composing the sensing elements were synthesized using atomic layer deposition (ALD). The optimum ALD conditions for the particle-like island-growth of ZnO were established by systematically investigating the ALD temperature and cycle conditions from 140 °C to 160 °C and from 5 to 30, respectively. Controlled devices of the sensor TFTs were prepared with different ALD conditions, in which sensing and read-out elements were designed to be physically separated. This unique device configuration provided us both benefits of improvement in gas-sensing property and stability of the device characteristics. The device showed instant gas responses as well as stable device behaviors at different operating temperature even with repeated gas measurements. Sufficiently good response to NH3 were obtained at 150 °C, which was significantly lower than the operating temperature of previously reported NH3 gas sensors. Based on the results the combination of novel TFT configuration and ALD-prepared ZnO NPs could be an effective method for the improvement in device characteristics of the gas sensors.