Ultrasonically sprayed ZnO films: optical, electrical and gas sensing properties

Highly transparent, polycrystalline ZnO thin films were deposited by ultrasonic spray pyrolysis. The phase and chemical composition of the films were characterized by X-ray analysis (XRD) and X-ray photoelectron spectroscopy (XPS). According to the XRD analysis the films possess wurtzite structure with preferential (002) orientation. The morphology of the ZnO films was investigated by scanning electron microscopy (SEM). Optical transmittance higher than 80% is found in the visible region. The influence of both the substrate and postdeposition annealing temperatures on the dark conductivity and its activation energy were investigated. It has been found that the conductivity increases with increasing of the substrate and annealing temperatures while the activation energy decreases. These effects have been related to an increase of the density of oxygen vacancies. The sensing properties of the samples were tested in the presence of vapours of water, ammonia, ethanol and acetone. It has been revealed that the substrate temperature influences the gas sensitivity. The films deposited at 200 degrees C have shown the highest sensitivity with the best results registered in the presence of ammonia vapours.