Spectroscopic-ellipsometry measurement of the optical properties of zinc oxide thin films prepared by sol–gel method: coating speed effect

In this work, thin films of zinc oxide were prepared by sol–gel spin coating method with different coating speeds of 3600, 4800 and 6000 rpm. Spectroscopic ellipsometry analysis was used to extract the optical constants such as refractive index n, and extinction coefficient k, in the wavelength range of 300–800 nm. The knowledge of both n and k allows the calculation of the important properties such as real and imaginary parts of the dielectric function, the absorption coefficient and the optical bandgap energy. Ellipsometric measurements were collected at an incidence angle of 70°. Also, a computer programming of Kramers–Kronig (KK) relations was used to calculate the optical properties. The ellipsometric results were compared with the parameters obtained by the KK method. It was found that the coating speed in sol–gel method, noticeably affect optical properties of the films. The films were found to exhibit high transmittance and low absorbance in the visible region. The direct optical bandgap of the films was obtained in the range of 3.4–3.7 eV using the ellipsometric method. A decrease in the void fraction of layers with an increment in the coating speed had been seen.