Effect of Boron-doping and annealing on the structure, morphological and optical properties of ZnO films prepared by spray pyrolysis method

Abstract

Abstract This study investigates the influence of boron doping from boric acid (H3BO3) on the structural, morphological, and optical properties of ZnO thin films deposited on glass substrates via spray pyrolysis at a deposition temperature of 250 °C. Subsequently, they undergo annealing in air at varying temperatures (200, 300, and 400 °C) for 45 min. The research reveals a significant dependence of all investigated properties on both annealing temperature and boron doping. Both pristine ZnO and boron-doped ZnO (BZO) films exhibit a polycrystalline hexagonal wurtzite structure. At the highest annealing temperature (400 °C), both films demonstrate high optical transmittance exceeding 70%, with crystallite diameters ranging from 25.8 to 28.5 nm for ZnO and 15.96 to 24.80 nm for BZO. Additionally, both films exhibit pronounced absorbance up to 400 nm. Scanning electron microscopy (SEM) analysis confirms a uniformly thick film surface. As the annealing temperature increases, the crystallite size grows, particle morphology evolves, and the overall film thickness increases. Fourier transform infrared (FTIR) spectroscopy validates the successful incorporation of boron atoms into the ZnO lattice structure. No contaminant peaks are observed in the FTIR spectra, indicating film purity. Furthermore, the study finds that BZO films possess a narrower optical band gap compared to pristine ZnO films. The band gap of the as-deposited and annealed BZO films is measured to be 3.04 eV and 2.9 eV, respectively, which is lower than the 3.23 eV band gap observed for the ZnO film. In general, the band gap exhibits a decreasing trend with increasing annealing temperature.