Abstract: ZnO thin films were grown on Si (100) substrate by the modified ultrasonic spray pyrolysis (USP). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to analyze the
crystalline and microscopic structure of the films. The effect of the substrate temperature on the growth mechanism and the microstructure of ZnO films were studied. Results show that the as-deposited
ZnO films are dense, smooth and homogeneous with uniform spherical crystallites, consistent with the hexagonal wurtzite polycrystalline structure. The preferential orientation of (002) is for all deposited
films. The minimum crystallite size of ZnO films under the optimal growth condition is 40 to 50 nm. The temperature of the substrate has significant effects on the deposition process. With the increase
of the temperature of the substrate, there lies a maximum deposition rate of ZnO films, which can be interpreted by different growing mechanism, and the preferential orientation along $c$ axis
is augmented in a marked degree and the crystalline grain size is refined as well.

Key words: Zinc oxide, thin film, ultrasonic spray pyrolysis （USP）