Interfaces And Crystal Defects In Bismuth Ferrite Thin Films

Bismuth ferrite (BiFeO3) possesses a great potential in multiferroic materials owing to its high ferroelectric Curie temperature and antiferromagnetic Neel temperature, as well as its giant polarization and outstanding magnetoelectric coupling. The availability of electric-field control of lattice, charge, orbital and spin degrees of freedom enables BiFeO3 to be a promising candidate for next generation of nanoelectronics. In multifunctional heterojunctions, the performance of film depends largely on interfaces, especially lattice defects, epitaxy and element diffusion. In this paper, the effects of substrate temperatures on the microstructures and stoichiometry of BiFeO3/SrRuO3/SrTiO3 heterostructure were investigated by high-resolution transmission electron microscopy (HRTEM) and X-ray energy dispersive spectroscopy (EDS). Then the stacking faults, precipitates and dislocations in a mixed-phase BiFeO3 thin film grown on LaAlO3 substrate were observed by Cs-corrected high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). At last, element diffusion and oxygen vancancies will be detected among the interface of BTO/SRO/STO heterojunction. The main conclusions are as follows:1. Effects of substrate temperature on the microstructure and stoichiometry of BiFeO3/SrRuO3/SrTiO3 heterostructure.A series of BiFeO3 thin films were fabricated on SrRuO3 buffered SrTiO3 (001) substrates at 500,600 and 700℃ using pulsed laser deposition (PLD). A large number of islands were observed on the surface of BiFeO3 thin films grown at 500℃, while the other two exhibited relative flat morphologies. They were identified as Bi2O3 by HRTEM and EDS analyses. The interfaces of BiFeO3/SrRuO3 showed perfect epitaxy, free of dislocations. However, an amorphous-like layer was sandwiched at the SrRuO3/SrTiO3 interface, especially in the films grown at 600 and 700℃. The Bi/Fe ratios were quantified as 0.80,0.91 and 0.87, respectively. The lack of Bi were probably caused by evaporation of Bi at high temperature and formation of Bi2O3 precipitates.2. Atomically visualization of stacking faults and precipitates in mixed-phase BiFeO3/LaAlO3 heterojunction.A tetragonal (T) and romhbohedral (R) mixed-phase BiFeO3 thin film was grown on LaAlO3 (001) substrate by PLD, which shows electric controllable ferromagnetism. Using HAADF-STEM, the stacking faults, accompanying with precipitates of Bi2FeO6-x and β-Bi2O3 were observed at the vicinity of BiFeO3/LaAlO3 interface. These precipitates were intrinsically related in microstructure. First, Fe ions could be replaced by Bi ions, forming the defects of stacking faults. If the concentration of bismuth increased further, Bi2FeO6-x would emerge. At last, Fe ions of Bi2FeO6-x could be substituted by Bi, producing a new type of β-Bi2O3 precipitate. Furthermore, the precipitates and BiFeO3 matrix were bridged via dislocations, which played a role in relaxing the mismatch stress between BiFeO3 and LaAlO3. Based on these detailed investigations, a possible mechanism of coexistence of T-like and R-like phases of BiFeO3 was proposed.3. Element diffusion within SrRuO3 electrode layer in BaTiO3/SrRuO3/SrTiO3 epitaxial heterostructure.The BaTiO3 film was grown on SrRuO3 electrode buffered on SrTiO3 (001) substrate using PLD method. The microstructures of BaTiO3/SrRuO3/SrTiO3 heterojunction were observed at atomic scale using HAADF-STEM. The BaTiO3 and SrTiO3 layers exhibited uniform contrasts while SrRuO3 electrode displayed bright and dark contrast. The intensity profiles revealed inhomogeneous contrasts of Sr and Ru columns in the SrRuO3 electrode, and the intensities of several Sr columns approached to those of Ba columns in BaTiO3 film. The diffusion of Ba and Ti into the SrRuO3 layer was characterized by EELS linescan. Meanwhile a non-negligible concentration of oxygen vacancies were introduced into the SrRuO3 layer. The Ti-L2,3 energy-loss near-edge structures (ELNES) revealed Ti+3 in the SrRuO3 layer, different from the Ti+4 in the BaTiO3 film and the SrTiO3 substrate. The resistance of SrRuO3 increased due to the doping effects of elemental diffusion, giving rise to a much longer screening length and a critical thickness of BaTiO3 ferroelectric barrier.