Time-dependent evolution of structural, optical, mechanical and electrical characteristics in ZnO thin films

Abstract

ZnO thin films were deposited on glass substrates by RF magnetron sputtering with deposition time varied from 0.5 to 2 h while maintaining constant deposition parameters. The influence of deposition time on the structural, microstructural, optical, mechanical and electrical properties of the films was systematically investigated. X-ray diffraction analysis revealed polycrystalline wurtzite ZnO with increasing peak intensity and progressive crystallite growth from similar to 6.6 to similar to 9.9 nm as deposition time increased, indicating improved crystalline ordering. FESEM observations confirmed grain coalescence and enlargement with increasing film thickness. Optical measurements showed high transparency in the visible region (74-81%), while the optical band gap decreased from 3.26 to 3.18 eV with increasing deposition time due to combined effects of thickness evolution, strain relaxation and defect redistribution. Photoluminescence spectra exhibited enhanced near-band-edge and visible emissions with increasing deposition time, reflecting improved crystallinity and reduced non-radiative recombination at grain boundaries. Nanoindentation measurements revealed a maximum hardness of 12.08 +/- 1.2 GPa for the film deposited for 1 h, corresponding to an optimal crystallite size governed by the transition between Hall-Petch and inverse Hall-Petch strengthening mechanisms. Impedance spectroscopy indicated thermally activated electrical transport dominated by grain and grain-boundary contributions, with the lowest overall resistance observed for the 1 h film. Overall, the results demonstrate that deposition-time-induced microstructural evolution strongly governs the multifunctional performance of ZnO thin films, with the 1 h deposition condition providing the most balanced combination of optical transparency, mechanical strength and electrical conductivity.