Enhancement of crystallinity and optical properties of bilayer TiO2/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 TiO2/ZnO thin films with enhanced crystallinity and optical properties. TiO2 films have been deposited at 300 degrees 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 TiO2 layers at 280 degrees C for 500 deposition cycles (55 nm). The crystallinity and optical properties of the TiO2/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 TiO2 with anatase structure. It shows that the crystallinity of the TiO2 films has been improved with the deposition of ZnO. The intensity of UV luminescence has increased by almost 30% for TiO2/ZnO bilayer as compared to the single layer TiO2. The possible mechanism for the enhancement of the optical properties of bilayer TiO2/ZnO thin films will be discussed.