Atomic layer epitaxy of ZnO and TiO_2 thin films on c-plane sapphire substrate for novel oxide soft x-ray mirrors

摘要

Atomic layer epitaxy of zizc oxide (ZnO) and titanium dioxide (TiO_2) have been applied to compose superlattice film devices such as soft x-ray multilayers. The reason why oxide films are chosen, is that oxygen of oxide films has transparency for the "water-window"(λ=2.332-4.368 nm) wavelengths. Actually, we researched the fabrication of TiO_2/ZnO multilayer mirrors and then found that these multilayer films provided high reflection in the wavelength region. Our theoretical calculation indicated that multilayer mirror could have the high reflectance of nearly 50% at the wavelength of 2.73 nm and at the incidence angle of 18.2° from the normal incidence.TiO_2/ZnO films were grown on the c-plane (0001) sapphire substrate by means of atomic layer epitaxy (ALE) technique, which involved the alternate reactions of Zn(CH_2CH_3)_2 (DEZ) and H_2O, and Ti(Cl)_4 (TCT) and H_2O.Our experimental results indicated that thin Wurtzite ZnO (0001) and Rutile TiO_2(200) films were grown epitaxially on c-plane (0001) sapphire substrates at 450°C with self-limiting mechanism. The 10-bilayer TiO_2/ZnO multilayer indicated high soft X-ray reflectivity of around 30%.