Structural enhancement and optical band gap modulation of ZnO thin films: Effect of precursor concentration and substrate temperature

Undoped ZnO thin films were successfully deposited by spray pyrolysis technique by varying the precursor molarity (0.05–0.125 M) and substrate temperature (350–410 °C) in order to investigate their influence on structural and optical properties. X-ray diffraction analysis revealed that all films crystallized in the hexagonal wurtzite structure with a strong preferential orientation along the (002) plane. The diffraction peak located at 2θ ≈ 34.49° corresponding to the (002) plane became sharper and more intense with increasing precursor concentration and substrate temperature, indicating improved crystalline quality and preferential c-axis growth. An increase in precursor concentration and substrate temperature significantly improved the crystalline quality, as evidenced by the enhancement of diffraction peak intensity and the increase in crystallite size. The crystallite size increased from 28.45 to 34.71 nm with increasing precursor molarity, confirming enhanced crystallite size growth and reduced structural defects. FTIR spectroscopy confirmed the formation of Zn O bonding vibrations in all samples. Optical investigations showed high transparency in the visible region, reaching up to 83%. The transmittance increased from approximately 44% to 83% as a result of improved crystallinity and reduced defect-related scattering. The optical band gap energy (Eg) was found to vary between 3.15 and 3.29 eV depending on deposition parameters. Increasing precursor molarity and substrate temperature led to improved transparency and reduced structural disorder, as confirmed by the decrease in Urbach energy (Eu). Furthermore, the decrease in Urbach energy from 0.20 to 0.12 eV at higher substrate temperatures indicates a reduction in localized defect states and band tail disorder. The results demonstrate that careful control of deposition parameters enables tuning of crystallinity and optical properties of ZnO thin films, making them suitable for optoelectronic applications. Future work may focus on doping effects and the integration of ZnO thin films into photovoltaic and photocatalytic devices.