Electrical Impedance Analysis of ZnO Thin Films for Ammonia Gas Sensors

The electrical impedance analysis of the ZnO films has been performed using complex impedance spectroscopy in the frequency range from 100 Hz to 1 MHz with temperature change from 70 to 175°C. Combined impedance and modulus plots have been used to analyse the sample behaviour as a function of frequency at different temperatures. Temperature dependence of ac conductivity indicates that the electrical conduction in the material is a thermally activated process. The frequency dependence of the ac conduction activation energy is found to obey a mathematical formula. It is concluded that the conductivity mechanism in the ZnO sensor is controlled by surface reaction. The operating temperature of the ZnO gas sensor is 175°C. The impedance spectrum also exhibited a decreased semicircle radius as the ammonia concentration is increased from 50 to 500 ppm. In addition, the impedance spectrum also exhibited a decreased semicircle radius with the exposure time increase from 0 to 20 min thereafter slightly increased. Impedance spectroscopy analysis has shown that the resistance variation due to grain boundaries significantly contributed to the gas sensor characteristics.