Phase Tuning, Thin Film Epitaxy, Interfacial Modeling, and Properties of YSZ-Buffered TiO<sub>2</sub> on Si(001) Substrate

We have studied systematically the influence of pulsed laser deposition variables on microstructure and properties of TiO<sub>2</sub> epitaxial thin films where integration with Si(100) substrate was achieved by cubic yttria-stabilized-zirconia (c-YSZ) buffer layer. Details of crystallographic and atomic arrangements across the interfaces are discussed in the light of the domain matching epitaxy paradigm. The single crystalline rutile films were obtained at higher substrate temperatures and lower oxygen pressures; whereas, the growth of epitaxial anatase films was promoted by decreasing the temperature and increasing the oxygen pressure. We showed that crystallographic structure of the TiO<sub>2</sub> films is determined mainly by the termination structure of the c-YSZ layer and the bonding characteristics across the TiO<sub>2</sub>/c-YSZ interface. Using 2θ and φ scans of XRD, the epitaxial relationship between Si(100) substrate and the zirconia buffer layer was shown to be cube-on-cube: (001)[100]<sub>YSZ</sub>∥(001)­[100]<sub>Si</sub>. Furthermore, the epitaxial relationships at the rutile/zirconia and the anatase/zirconia interfaces were determined as (100)[01̅1]<sub>rutile</sub>∥(001)­[010]<sub>YSZ</sub> and (001)[110]<sub>anatase</sub>∥(001)­[100]<sub>YSZ</sub>, respectively. The proposed crystallographic arrangements as well as the epitaxial growth were confirmed by high resolution TEM diffraction and imaging. XPS and XRD studies showed less defect content and favorable crystallinity in the films grown at higher temperatures. AFM results revealed that the finest domain size and the smoothest surface were obtained at the intermediate deposition temperatures. Based on the four-point electrical measurements, the heterostructures deposited at 500 °C were more conductive than those grown at 300 and 700 °C. Photocatalytic activity of the films was studied through decomposition of 4-chlorophenol under UV illumination. The maximum photocatalytic reaction rate constants were determined as 0.0124 and 0.0087 min<sup>–1</sup> for the anatase and the rutile films grown at 500 °C.