Impact of Spray Cycles on Crystallinity, Structural Defects, and Optical Performance of ZnO Thin Films Deposited by Chemical Spray Pyrolysis

Abstract

ZnO thin films were deposited on glass substrates by chemical spray pyrolysis using number of spray cycles (20, 25, and 30), designated as S1, S2, and S3, respectively. The influence of the number of spray cycles on the structural, morphological, and optical properties of the films was investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and UV-Vis spectroscopy. XRD analysis confirmed the formation of single phase polycrystalline ZnO with a hexagonal wurtzite structure. Increasing the number of spray cycles enhanced the crystallinity, resulting in an increase in crystallite size from 27.7 to 34.1 nm, while the microstrain and dislocation density decreased from 4.45 × 10^-3 to 3.48 × 10^-3 and from 1.31 × 10¹⁵ to 0.86 × 10¹⁵ m^-2, respectively. FESEM observations revealed grain growth from approximately 160 nm to 350 nm with improved film compactness and surface homogeneity. AFM results showed an increase in the maximum surface height from 61.57 to 84.26 nm. Optical measurements indicated that absorbance and absorption coefficient increased with increasing number of spray cycles, whereas transmittance decreased from about 60% to 43%. The optical band gap decreased from 3.10 eV for S1 to 2.93 eV for S3, accompanied by an increase in the extinction coefficient. The results demonstrate that controlling the number of spray cycles provides an effective route for tailoring the structural and optical performance of ZnO thin films. The S3 film exhibited the best overall characteristics, making it a promising candidate for optoelectronic and photodetector applications.