Effect of film thickness and annealing temperature on the structural and optical properties of ZnO thin films deposited on sapphire (0001) substrates by sol–gel

ZnO thin films were prepared on sapphire (0001) substrates by the dip-coating sol–gel technique. Then, ZnO thin films with thicknesses of 50 nm, 150 nm, 250 nm, and 350 nm were annealed at 800 °C and with thickness of 250 nm were annealed at 600 °C, 700 °C, 800 °C, and 900 °C, respectively. The effect of film thickness and annealing temperature on the structure and optical properties of sol–gel ZnO thin films on sapphire substrates were investigated by XRD, SEM, RT-PL. All the sol–gel ZnO thin films show polycrystalline hexagonal wurtzite structure and a high preferential c-axis orientation. The XRD and SEM results show that the better structural quality, high c-axis preferred orientation, uniform, compact sol–gel ZnO thin films 250 nm thick were obtained when annealed at 700 °C in air. Room-temperature PL spectrum of sol–gel ZnO thin films can be divided into the UV emission and the visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the visible broadband emission can be ascribed to the deep level emissions (DLE). The visible emission is suppressed with the annealing temperature of 600–700 °C. By analyzing our experimental results, we concluded that the deep-level emission corresponds to oxygen vacancies (VO). The biggest ratio of the PL intensity of UV emission to that of visible emission (INBE/IDLE) is observed from sol–gel ZnO thin films 250 nm thick annealed at 700 °C. Therefore, we suggest that film thickness of 250 nm and annealing temperature of 700 °C are the most suitable conditions for obtaining high quality, high c-axis preferred orientation, uniform, compact sol–gel ZnO thin films with good luminescence performance.