Near-infrared optical constants and optical polarization properties of ZnO thin films

The optical characteristics of ZnO thin films deposited on Si (111) and quartz at temperature < 50 °C using reactive RF sputtering deposition were examined within the near-infrared (NIR) region. From the X-ray diffraction observations, it was found that the films show a polycrystalline structure. X-ray photoelectron spectroscopy analysis shows the chemical bonding states of zinc and oxygen in the surface of the films and confirms the formation of Zn-O bonds. The field emission scanning electron microscopy and the atomic force microscopy images of the films shown an almost uniform distribution and the surfaces are smooth with grains of the order of 40-60 nm. The average surface roughness was in the range of 3.15-24.33 nm. Over a wavelength range 700-2500 nm, the optical constants of the films were obtained by analysis of the measured ellipsometric spectra using the Cauchy-Urbach model. Refractive indices and extinction coefficients of the ZnO films were determined to be in the range n = 1.65-1.73 and κ = 0.0060-0.029, respectively. Also, the s- and p-polarized optical properties (transmittance and reflectance) of the films have been measured at different angles of incidence in the UV-Vis-NIR spectral range. In addition, calculations have been carried out in order to determine the optical absorptance by using the conservation of energy. The films reveal a high transmittance (85%-95%), low reflectance (5%-15%), and very low absorptance (< 1%) at normal incidence. This means that the sputtered polycrystalline ZnO films could be good candidates for antireflection (AR) optical coatings.