Study of electrical, structural and gas sensing characteristics of ZnO thick solid films with different dopants

Abstract

Solid state gas sensors have proved to be very promising for monitoring the emission of air pollutant and toxic gases because they are a low cost element. Zinc oxide (ZnO) is II-VI compound semiconductor with a wide direct band gap of 3.37eV and a hexagonal structure. ZnO is often used in photovoltaic, gas sensor application, varistors, surface acoustic wave devices, electric transducers, piezoelectric materials and room temperature ultraviolet lasing. The objectives of this project are to study the structural, electrical and gas sensing properties of undoped ZnO thick films, to study the relationship of doping with structural, electrical and gas sensing properties of doped ZnO films with Cr2O3, Nb2O5, Al and Cu as a dopant. The experimental work included preparation of thick films on alumina substrate by screen printing technique and their characterization. The structural characterization was performed by XRD, SEM and EDAX technique to determine the variation of microstructure, crystallite size, orientation and chemical composition of the undoped and doped ZnO thick films. Electrical characterization was based on measurement of resistance of the films by half bridge method at different operating temperatures. From measured resistance values, the resistivity, activation energy and TCR were determined. Gas sensing properties of the films were studied in static gas sensing system by exposing the films to the different gases as CO2, H2S, NH3, Ethanol, LPG and NO2 at different operating temperatures. The optimum operating temperature, sensitivity, selectivity, response and recovery time and the active region of detected gas were determined. Results showed that the doping alters the orientation of microstructure, grain size, composition, electrical and gas sensing characteristics of the films. From XRD pattern, different phases and preferred orientation were observed and grain size was determined.