Spin-Coated ZnO Thin Films: Optical Bandgap Analysis Based on Rotational Speed and Dislocation Density Modeling

Abstract Zinc oxide (ZnO) thin films were successfully synthesized on glass slide substrates via the sol–gel spin coating method using a zinc acetate-propanol-MEA precursor (Monoethanolamine). The impact of rotational speed on the structural and optical properties was systematically examined by varying the rotational speed between 350 and 950 rpm, followed by annealing at 500°C. X-ray diffraction (XRD) confirmed the wurtzite hexagonal phase and revealed clear variations in crystallite size, dislocation density, and lattice strain as a function of deposition speed. UV-Visible spectroscopy showed that the optical bandgap energy was strongly influenced by the film microstructure, with the best balance of high transparency and low defect density achieved at 800 rpm (Eg ≈ 3.229 eV). To provide predictive insight, two mathematical models were established: a quadratic function linking Eg to rotational speed and a logarithmic decay describing its relationship with dislocation density. Both models reproduced the experimental results with very small relative errors, ranging between 0.04–1.35% and 0.11–1.15%, respectively, confirming their robustness. The optimized ZnO thin film at 800 rpm exhibits properties well suited for ultraviolet photodetectors, where controlled defect density and tunable bandgap are essential. This dual-model approach provides a powerful framework for guiding the process-driven design of ZnO-based optoelectronic devices.