Significant effect of film thickness on morphology and third-order optical nonlinearities of Cd_1−xZn_xO semiconductor nanostructures for optoelectronics

Abstract

The work presented here reported the thickness dependent structural, linear and nonlinear optical properties of nanostructured Cd_1−xZn_xO thin films. Thin films were prepared with two different thickness (≈ 0.5 µm and 1 µm) by employing a spray pyrolysis (SP) technique for different Zn-do** levels (Cd_1−xZn_xO with the x value of 0.00, 0.01, 0.05 and 0.1). X-ray diffraction studies confirm the polycrystalline nature having a cubic crystal structure. In terms of an aspect ratio of the columnar structure and dispersion in hexagonal (1 1 1) basal plane orientation, a thickness dependency of structural evolution was discussed. The Scherrer rule was employed to determine the crystallite size and found to be decreased. FESEM images indicate grains which are uniform and grain size slightly increased with an increase in dopant concentration, annealing and thickness of the films, respectively. The optical energy band gap (E_g) of the prepared films was found to be increased from 2.50 to 2.67 eV. The NLO parameters of the samples were measured from the Z-scan data under DPSS continuous wave laser excitation at 532 nm and the results reveal that reverse saturable absorption (RSA) and self-defocusing natures are the attributed and observed nonlinearity of the nanostructures. The third-order NLO components such as β, $n_{2}$ and χ⁽³⁾ are found to be enhanced with one order of magnitude higher with the influence of thickness from 1.25 × 10^–4 to 2.47 × 10^–3 (cm W⁻¹), 7.08 × 10^–9 to 3.35 × 10^–8 (cm² W⁻¹) and 4.06 × 10^–7 to 1.96 × 10^–6 (esu) respectively. The inspiring results of NLO parameters are also due to the increasing localized defect states on grain boundaries as the film thickness increases, suggesting the prepared films are a promising material for nonlinear photonic device applications.