| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 98 === Cobalt doped bismuth ferrite were synthesized by both solid state reaction and RF magnetron sputtering. In the solid state sintering process, the formation of BiFeO3 phase was often accompanied by the appearance of some secondary phases like Bi2Fe4O9 and Bi25FeO40. To avoid the problem, various sintering conditions were tried in order to get the most pure phase possible and then, the remaining trace of any secondary phase was washed away using some acid solution. It was found that the relative amount of the secondary phase, Bi2Fe4O9, was reduced by the Co doping. Similar sintering process was used to make the Co doped BiFeO3 targets for the RF magnetron sputtering of thin film samples. Under the optimum growth parameters, the films of pure BiFeO3 phase could be grown on the Pt coated silicon and the conductive FTO glass substrates, without the presence of any secondary phase.
The absorption spectra of Co-doped BiFeO3 films were measured by the UV-Vis spectrometry, from which the band gap was calculated to be 2.79 eV, almost exactly the same as the theoretical value for the un-doped pure BiFeO3. Photoluminescence spectra were also measured and a number of broad emission peaks were observed over the wavelength range 380-489 nm, among which the 450 nm emission came from the inter-band transition and the 469 nm emission was probably due to the transition from the Bi2+ defect level to the valence band. The origin of other emissions is not clear yet. In the electric conduction measurements, the resistivity of the BiFeO3 samples was found to decrease with the increasing temperature. The Arrhenius plots of conductivity vs. temperature showed that there existed at least two activation energies, arising from the defect or dopant energy levels inside the band gap. However, these two energy levels seemed not to be relevant to the unknown emissions observed in the photoemission spectra. In the magnetic measurements, the vibration sample magnetometry was employed to measure the M-H curves, which showed that both the saturation magnetization and the coercivity increased with the Co doping level. Thermal gravity analysis under applied magnetic field showed that the magnetization of the Co doped samples disappeared at 400 ?C, manifesting that it could not possibly come from the magnetic secondary phases such as CoFe2O4 (TC= 520 ?C), Fe3O4 (Tc= 585 ?C) and ?-Fe2O3 (Tc= 590 ?C). It was more likely that the Co doping somehow modified the spin arrangements in BiFeO3.
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