Structural properties of Pt/TiO2/Pt heterostructure grown on sapphire substrate-Influence of annealing processes

Simple gas sensors based on resistivity change of TiO2 thin films using combined top and bottom metallic contacts are very promising. In this work influence of ex situ annealing in ambient air on structure of TiO2 thin film stacked between two platinum contact layers has been studied. The layers were deposited using DC magnetron sputtering on unheated c-cut sapphire substrates. For lowering of the Schottky barrier at the Pt-TiO2 interfaces and for improved crystalline stability, ex situ annealing at 600 °C in air was carried out. In order to study separately influence of top and bottom platinum layers on crystal structure, also reference samples Pt/TiO2/Al2O3 and TiO2/Pt/Al2O3 have been prepared. Non-ambient X-ray diffraction measurement during annealing process and X-ray pole figures after annealing has been measured. Near epitaxial relationship was observed for bottom Pt layer grown on c-cut sapphire substrate: Pt(1 1 1)[11¯0]||Al2O3(0 0 0 1)[11¯00]. Inner titania layer shows randomly oriented both TiO2-rutile (R) and anatase (A) phases with the volumetric ratio of R/A ∼ 2.7. If prepared without top Pt contact layer, the TiO2 transforms during annealing to random single anatase phase. The TiO2 layer overgrown with only single Pt top contact layer shows randomly oriented both rutile and anatase phases with volumetric ratio R/A ∼ 2.3. The top Pt layer on TiO2 shows filamentary uniaxial orientation Pt(1 1 1)||Al2O3(0 0 0 1). The interdiffusion at the Pt-TiO2 interfaces extends to several nm.