Investigation of structural, morphological and nonlinear optical properties of ZnO thin films using reflective Z-scan technique

Zinc Oxide is an n-type direct bandgap (3.2 eV) material having large exciton binding energy of (60 meV) and strong optical nonlinearity. Zinc Oxide thin films plays crucial role in revolutionizing performance of optical and electrical devices. In this study, laser-induced third-order optical nonlinearity in Zinc oxide thin film samples are investigated. This method is used to investigate the nonlinear refraction using the high-intensity source. Zinc oxide has been deposited on p-type silicon substrate using sol–gel techniques with various substrate temperature of room temperature (RT), 300 °C, and 500 °C. The samples have been characterized by X-ray diffraction studies. And the deposited zinc oxide film shows hexagonal wurtzite structure and the lattice parameters change with substrate temperature. Surface morphology of Zinc oxide thin film samples are analysed using FE-SEM and branching like urchin morphology is observed and the size of rods and crystallites of thin films increases, when temperature is varied. A pulsed laser beam having a wavelength of 532 nm has been used as the excitation source. Reflective Z-scan (RZ-scan) technique of open aperture was performed, since zinc oxide thin film samples are opaque. Normalized reflectance is fitted to experimental data to calculate nonlinear reflective index, n2 of the sample and the values of n2 estimated for room temperature (RT), 300 °C, and 500 °C are 1.5 × 10–9 cm2/W, 2.9 × 10–9 cm2/W and 25.3 × 10–9 cm2/W.