Effect of annealing temperature on the evolution of structural, microstructural, and optical properties of spin coated ZnO thin films

Zinc oxide (ZnO) thin films were sol-gel spin coated on glass substrates and annealed at various temperatures from 300-500 °C. Zinc acetate dihydrate (ZAD), monoethanolamine (MEA), and 2-methoxyethanol were used as the starting materials, stabilizer and solvent, respectively. The effect of annealing temperature on the structural and optical properties of the ZnO thin films was investigated by X-ray diffractometer (XRD), atomic force microscope (AFM), UV-VIS spectrophotometry and ellipsometry. The XRD results showed the films to have a preferential c-axis orientation, whereas the AFM results confirmed a columnar structure. The surface roughness increased with the increase in annealing temperature. Parameters such as ratio of free charge carrier concentration to effective mass (N/m*) and plasma frequency (ωp) were determined from the transmittance graph using the Wemple di Domenico model. Both N/m* and ωp were noticed to reduce with the increase in annealing temperature. Band gap decreased with the increase in the annealing temperature indicating absorption edge shift towards the red region.