Influence of annealing on polymeric precursor derived ZnO thin films on sapphire

Zinc oxide (ZnO) thin films on c-sapphire substrates were synthesized by spin-coating aqueous polymeric precursors. The effects of annealing at 1000 °C on crystallinity, surface morphology, and optical properties of ZnO thin films, with varying thicknesses, were studied. Single-layered ZnO thin films are polycrystalline with wurtzite structure and preferentially oriented along the (002) plane. X-ray diffraction pattern also reveals the presence of spinel zinc aluminate (ZnAl2O4) peaks. ZnO films have highly faceted granular morphology. Multilayered ZnO films, annealed twice at 1000 °C, do not exhibit any ZnO peaks but only ZnAl2O4 peaks. Moreover, the surface morphology was smooth with ridges. These films do not exhibit the band gap or ultra-violet emission photoluminescence characteristics of ZnO. On annealing, there is an interfacial reaction between ZnO and sapphire resulting in ZnAl2O4.