Microstructural evolution and optical dispersion of sputtered ZnO thin films at low annealing temperature

In an attempt to understand microstructural evolution and its effect on optical properties, nanocrystalline ZnO thin films are deposited by pulsed DC magnetron sputtering and then heat treated at a low temperature for several hours. X-ray diffraction experiments confirm that texture and residual stress are sensitive to morphological changes from globular to plate-like grains. Nanocrystalline ZnO thin films are highly transparent to visible light but opaque to UV light below 350 nm. The transparency in the UV region is limited by its fundamental absorption below 400 nm. The spectral dependence of optical constants demonstrates normal dispersion and the films deposited at 70 W of sputtering power and then heat treated have higher values of refractive indices and significantly high extinction. The dispersion parameters evaluated under the purview of a single oscillator model propose vital linkages of optical properties with the microstructure of ZnO thin films. Dispersion parameters viz. plasma frequency can be attuned by controlling the microstructure of ZnO thin films via simply varying process parameters and post-deposition heat treatment at low temperature. This study confirms that high tensile residual stress and texture are favorable for the development of novel nanocrystalline ZnO thin films.