Observation of Titania and Titanate Phase Changes in Oxidation-Controlled ZnO/TiN and HfO2/TiN Thin Films: An X-ray Absorption Spectroscopy Study

Abstract

We investigated the local electronic structures of oxidation-controlled TiN thin films for preparation of photocatalytic titania using soft X-ray absorption spectroscopy. It is shown that the TiN layers on top of oxides, such as HfO2 or ZnO, are easily oxidized by heat during postdeposition annealing (PDA) so as to form nitrogen-incorporated titania (N-TiO2). The local structures of the oxidized films evolved significantly depending on the bottom oxide and the PDA conditions; when TiN was deposited on HfO2, which is less reactive than ZnO, PDA at 700 degrees C stabilized a mixture of rutile and anatase phases under almost any gas (N-2 or O-2) environments. On the other hand, when TiN was deposited on ZnO, which is reactive enough to oxidize the TiN layers substantially, the PDA resulted in a rich phase diagram according to the gas environment: under N-2 environment, an anatase local structure is dominant, whereas under O-2 environment, a rutile or yet another local structure with a high symmetry, for example, perovskite or ilmenite ZnTiO3, becomes dominant. As for the origin of the phase changes, the abundance of the oxide phases is found to be strongly correlated with the averaged grain sizes in the films. The changes in the local structures resulted in the blue- or red-shifts of conduction band, implying that we can engineer the electronic properties in the N-TiO2 films by properly choosing the bottom oxide.