Synthesis of multiple layers of alternating ZnO and TiO2 using atomic layer deposition and their optical characterization

Abstract

We report the optical characterization of bi-layer and quadri-layers of TiO2/ZnO transparent oxide films having a net thickness of 100 nm. The atomic layer deposition (ALD) was used to grow these multiple-layered films. We examined their optical properties to understand the impact of the number of layers. The grazing incidence X-ray diffraction (GIXRD) was used to assess the microstructure of the films, which showed that the ZnO layers were polycrystalline, while the TiO2 layers were (X-ray) amorphous. The optical transmission and reflection spectra of the investigated films were recorded in the range of 150–2500 nm at room temperature. The refractive index (n), extinction coefficient (k), real and imaginary parts of the dielectric constant, optical conductivity, and loss factor agree well with those of other amorphous oxides. It was established that the indirect allowed optical transitions are favorable and the optical gap (Eg) increases from 3.03 eV to 3.14 eV as the number of layers increases. The contribution of polarizability (ξ) and electrical susceptibility (χe) to the dielectric constant was also discussed. The relation between the relative density (D) and the porosity (Pro) with the crystalline quality, layer thickness, and the number of layers was also estimated. The results obtained from the study of TiO2/ZnO multilayer films indicate that multilayer thin films improve the quality of film crystallinity and optical properties with the increase of the layers numbers, and could guide the development of potential applications.