Nanocrystalline ZnO thin film growth by ultrasonic spray from a non-aqueous solution

Abstract

In the present work, zinc oxide (ZnO) thin films are deposited by the ultrasonic spray method. In order to study the influence of the nature of the precursor solution, zinc acetate and zinc nitrate salts dissolved in methanol are used respectively to prepare two non-aqueous starting solutions. The influence of substrate temperature on the structural growth kinetics and optical properties of ZnO films is also investigated. For this purpose, the substrate temperature is varied in the range of 250—400°C. X-ray diffraction (XRD) analysis reveals that at low temperature the films that are grown contain nanocrystallites with a privileged orientation depending on the nature of the starting precursor; these privileged orientations correspond to the plane (100) for the films deposited with zinc acetate and (002) for films prepared with zinc nitrate precursor. However, with increasing deposition temperature, the privileged orientation disappears. The growth kinetics is closely related to the substrate temperature and to the nature of the starting solution. The scanning electron microscopy (SEM) observations suggest that the liquid droplets react once they reach the substrate surface when zinc acetate precursor is used. However, in the case of zinc nitrate solution, the droplets spread first on the substrate surface followed by the subsequent surface reaction. This difference is explained in terms of the dissociation enthalpy of both solutions.