| As an important giant dielectric constant material, Ba(Fe1/2Nb1/2)O3 has drawn much scientific attention. The physical nature of its dielectric behavior and the ambiguous ferroelectricity are the key issues for this material. In the present thesis, complex perovskite Ba(Fe1/2Nb1/2)O3-based thin films were successfully grown on the Pt/TiO2/SiO2/Si substrates by pulsed laser deposition. The influences of the deposition conditions on the crystal structures, surface morphologies and physical properties of the thin films were investigated systematically. The study on the local crystal structure and symmetry by Raman scattering demonstrated the structure origin for the weak ferroelectricity observed in Ba(Fe1/2Nb1/2)O3 thin films, and this might provide a new aspect for understanding the dielectric behavior of Ba(Fe1/2Nb1/2)O3.XRD data suggested larger lattice constants of Ba(Fe1/2Nb1/2)O3 thin films than that of the bulk ceramic. With decreasing the deposited oxygen pressure, the lattice constant of the thin film increased. The standard error of the average lattice constant calculated from the different diffraction peaks increased as well, which implied the possible crystal structure derivation from the original cubic symmetry, especially for the ones deposited with lower oxygen pressures. The surface morphologies of thin films investigated by scanning probe microscopy showed great dependence on the deposition conditions. Both increasing growth temperature and decreasing deposited oxygen pressure produced larger grains.Obvious P-E hysteresis loops and butterfly-shaped C-V curves were observed at room temperature, which suggested the ferroelectricity in Ba(Fe1/2Nb1/2)O3 thin films. With increasing the deposition oxygen pressure, the ferroelectricity was depressed. The same effect was observed after annealing in oxygen. At 123 K, nearly saturated P-E hysteresis loops were observed. The corresponding J-E curves exhibited a broad peak, which proved the contribution from the polar domain switching.The Raman spectra of Ba(Fe1/2Nb1/2)O3-BaTiO3 solid solutions and Ba(Fe1/2Nb1/2)O3 thin films provided the evidence for local symmetry breaking and composition fluactuation, and these are just the structural origins for the ferroelectricity in Ba(Fe1/2Nb1/2)O3 thin films. The obvious band splitting in the high wavenumber Raman bands of Ba(Fe1/2Nb1/2)O3 could be caused by the composition fluctuation in the material. The existence of the Nb-rich regions and as well as the Fe-rich regions could cause the local symmetry breaking and result in weak ferroelectricity in the thin films. Besides, the Raman spectra of Ba(Fe1/2Nb1/2)O3 thin films deposited with various oxygen pressure showed the FWHM of the high wavenumber Raman band decreased with decreasing deposited oxygen pressure, which proved that the oxygen deficiencies played a positive role in composition fluctuation.Ba[(Fe1/2Nb1/2)0.1Ti0.9]O3 thin films exhibited three dielectric abnormalities in the temperature range of 200 to 350 K. A weak dielectric relaxation was observed at about 315 K. With decreasing temperature, the thin film showed abrupt increase in both dielectric constant and loss, suggesting the diffused ferroelectric phase transition. The Curie temperature of the thin film shifted to the higher temperature for about 30 K compared to the bulk ceramic. Around 250 K, a dielectric relaxation with frequency dispersion was observed, which could be related to the existence of the polar regions with different polarization directions. The temperature dependence of the P-E hysteresis loops suggested the saturated polarization at low temperature. Besides, the P-E loops were still observed at 323 K. The Raman spectrum of the thin film at room temperature demonstrated the local tetragonal symmetry. Besides, two extra Raman modes centered around 452 and 610 cm-1 were inditifed in thin films, which indicated extra structure distortion.
|
PDF Full Text Request