Enhancement of crystallinity and optical properties of bilayer TiO ₂/ZnO thin films prepared by atomic layer deposition

Bilayer and multilayer thin films are becoming increasingly important in the development of faster, smaller and more efficient electronic and optoelectronic devices. One of the motivations of applying bilayer or multilayer structures is to modify the optical properties of materials. Atomic layer deposition (ALD) is a variant of Chemical Vapour Deposition that can produce uniform and conformal thin films with well controlled nanostructures. In this study, we have demonstrated new findings of the use of ALD fabricated bilayer TiO ₂/ZnO thin films with enhanced crystallinity and optical properties. TiO ₂ films have been deposited at 300 °C for 1000 (51 nm in thickness) or 3000 (161 nm in thickness) deposition cycles onto glass and Si substrates. ZnO films are subsequently deposited on the TiO ₂ layers at 280 °C for 500 deposition cycles (55 nm). The crystallinity and optical properties of the TiO ₂/ZnO thin films have been analysed by X-ray diffraction, photoluminescence, UV-Vis spectroscopy, Atomic Force Microscopy and Scanning Electron Microscopy. XRD diffraction pattern confirmed the presence of ZnO with wutrtize crystal structure and TiO ₂ with anatase structure. It shows that the crystallinity of the TiO ₂ films has been improved with the deposition of ZnO. The intensity of UV luminescence has increased by almost 30% for TiO ₂/ZnO bilayer as compared to the single layer TiO ₂. The possible mechanism for the enhancement of the optical properties of bilayer TiO ₂/ZnO thin films will be discussed.