Undoped ZnO thin films deposited by ultrasonic spray technique: effect of the water content in the starting solution on the physical properties

Undoped ZnO thin films were deposited on sodocalcic glass substrates by the spray pyrolysis technique. Atomization of the starting solution was made by ultrasonic excitation. The effect of the water content in the starting solution on the structural, optical, and morphological properties of the films was studied. All films showed mirror-like finish with good adherence to the substrate. A preferential (002) growth was observed in all the films, although the increase in the water content leads to a slight increase in the (101), (102), (103) peaks intensity. Surface morphology changes slightly with water content variation in the starting solution, since films deposited with low water content show hexagonal shaped grains with round edges and a wide distribution in the average size; whereas well-defined rod-shaped grains are present in films manufactured from a starting solution with high water content. From photoluminescence, PL, measurements at room temperature it was found that, as the water content in the starting solution increases, both the UV and the green emission increase at a constant deposition temperature of 425 degrees C. PL measurements as a function of the substrate temperature for the optimal water content show that, at a low temperature the green PL peak intensity shows an increase with the temperature, reaching a maximum at 425 degrees C; however, at higher temperatures the PL response decreases.