Structural, Optical And Magnetic Properties Of Polycrystalline Ti_1–xM_xO_2–δ (M=Fe, Cr) Films

Wide band gap semiconductor TiO₂ has potential applications in the fields of optical coatings, photocatalysts and diluted magnetic semiconductors (DMS). Even though the incorporation of transition metal in TiO₂ has been extensively investigated, there is a considerable controversy, such as the reasons for lower or higher photocatalytic activity and the origin of ferromagnetic order at room temperature, etc. In order to further understand these issues, Fe and Cr doped polycrystalline TiO₂ thin films were fabricated by facing-target reactive sputtering. The composition, microstructure, optical and magnetic properties of the films were investigated systematically.In Fe and Cr doped anatase films, the charge state of iron and chromium is determined to be +3. While for Fe doped TiO₂ films with mixed anatase and rutile phases, the Fe dopant is mainly assigned to be Fe3+ with small amount of Fe²⁺. No secondary phase is observed in all the samples. The grain size of the anatase Ti_1–xFe_xO_{2–δ} films reaches the maximum value when the Fe doping content is 0.017. The incorporation of Cr atoms does not change the anatase structure. The introduction of Fe and Cr atoms into the TiO₂ films with mixed phases in which the rutile is the dominant phase, promotes the transition from anatase to rutile.The absorption edge of the polycrystalline TiO₂ films shifts to long wavelength side with increasing Fe and Cr contents. The direct and indirect band gaps of pure anatase TiO₂ film are 3.73 and 3.39 eV, respectively, and decrease with the Fe and Cr doping. The photoluminescence (PL) spectra of Fe doped anatase indicate that direct and indirect band-to-band transitions coexist in the films. The indirect band-to-band radiative recombination can be regarded as a coupling emission process of two phonons and one photon. The emissions from direct and indirect transition recombination shift to low energy side with the increasing Fe concentration. The energy position of the emission from the oxygen vacancies does not shift with Fe content, but its intensity enhances. The refractive index (atλ=550 nm) of the Fe doped anatase films increases with Fe content.All Fe and Cr doped polycrystalline TiO₂ films exhibit ferromagnetism at room temperature. The saturation magnetization of the Fe doped anatase films primarily increases with the Fe concentration and then decreases after reaching its maximum. The oxygen vacancy plays an important role in mediating the ferromagnetism which can be explained on the basis of F–center model. The saturation magnetization of the Fe and Cr doped TiO₂ with mixed phases is larger than that of anatase, implying that structure defects and/or oxygen vacancies are intimately related to the ferromagnetism of the films.