XAFS Study of Epitaxial Co_xTi_1-xO_2-x Anatase

Co doped TiO₂-anatase is a promising candidate for a room-temperature ferromagnetic semiconductor. XAFS measurements have been used to investigate the local Co environment and Co valence for several Co-anatase films. The samples were grown on LaAlO₃(001) by oxygen plasma assisted molecular beam epitaxy and on SrTiO₂ by atomic oxygen assisted MBE. Co concentrations were about 5%. The measurements were made at the PNC-CAT bending magnet and undulator beamlines at the Advanced Photon Source. For the films on LaAlO₃, the near edge clearly shows the presence of only Co(2+), and no evidence for metallic Co, while the films on SrTiO₂ showed significant metallic Co. Analysis of the extended fine structure for the LaAlO₃ films finds that the Co substitutes for Ti with some distortion of the lattice. Both in-plane and out-of-plane Co-O bonds are expanded from the Ti-O bonds in anatase. The in-plane bonds are expanded approximately twice as much. A deficit in the oxygen coordination number suggests a correlation of oxygen vacancies with Co sites