Structural properties of Pt/TiO_2/Pt heterostructure grown on sapphire substrate-Influence of annealing processes

摘要

Simple gas sensors based on resistivity change of TiO_2 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 TiO_2 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-TiO_2 interfaces and for improved crystalline stability, ex situ annealing at 600 ℃ in air was carried out. In order to study separately influence of top and bottom platinum layers on crystal structure, also reference samples Pt/TiO_2/Al_2O_3 and TiO_2/Pt/Al_2O_3 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(111)[11-bar0]||Al_2O_3(0001)[11-bar00]. Inner titania layer shows randomly oriented both TiO_2-rutile (R) and anatase (A) phases with the volumetric ratio of R/A ～ 2.7. If prepared without top Pt contact layer, the TiO_2 transforms during annealing to random single anatase phase. The TiO_2 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 TiO_2 shows filamentary uniaxial orientation Pt(111)||Al_2O_3(0001). The interdiffusion at the Pt-TiO_2 interfaces extends to several nm.