Anisotropy Of BiFeO₃-Based Films Deposited On Different Substrates

Ferroelectricity has been one of the most used and studied phenomenon in both scientific and industrial communities. Properties of ferroelectric materials make them particularly suitable for a wide range of applications, ranging from sensors, actuators to optical and memory devices. Presently, lead titanate series materials are widely used in intelligent ferroelectric material system. However, the toxicity of lead can induce seriously environment pollution. Under this given situation, lead free Bi Fe O3?BFO? is a potential candidate to replace of lead oxide based ferroelectrics. BFO is a multiferroic with rich and intricate physical properties. Among the currently studied single-phase compounds, BFO is also the only known material exhibiting ferroelectric and antiferromagnetric orders at room temperature. It has been intensively investigated because of its superior ferroelectric, piezoelectric and pyroelectricity properties in the applications of high-density ferroelectric random access memories, microelectromechanical and pyroelectric imaging systems.For crystals, their properties are strongly dependent on crystallographic directions, which has been paid much close attentions, especially for epitaxial thin films. Both the ferroelectric and dielectric properties of BFO films are orientation dependent, and it provides us with a valuable shortcut to optimize the electrical properties of BFO thin films. However, pure BFO thin films are confronted with serious leakage current problem resulting from poor sample quality, which hinders its practical applications. Therefore, in order to investigate the anisotropy of BFO-based films, leakage problem must be resolved first. It was reported that site-engineering concept is an efficient way to improving the insulating property in BFO films. Different doping ions have different effect on the leakage behavior of BFO films. Thus, in this paper, we prepared a series of doping modified BFO-based films by a chemical solution deposition method?CSD? with sequential annealing process. The effect of annealing temperature, electrodes and substrates on the crystal orientation are studied systematically and orientation dependence of the electrical properties?insulating, ferroelectric and dielectric properties? are also discussed.The main contents and conclusions are summarized as follows:1. The effect of organic solvent on the crystallinity and surface morphology of BFO-based thin film are characterized. It is found that the isopropanol is in favour of forming denser crystal structure. Compared with the Nd-doped and Ti-doped BFO films, Nd and Ti ions co-doped BFO film shows the smallest leakage current, highest dielectric constant and lowest dielectric loss.2. The crystal structure can be modified by the annealing temperature, different preferred orientation growth can be observed in polycrystalline Bi_0.97Nd_0.03Fe_0.98Ti_0.02O₃?BNFTO? films deposited on the ITO/glass substrates under different annealing temperature. The BNFTO thin film prepared at 525°C shows a?110?-preferred orientation, while film annealing at 550 °C has a?012?-preferred orientation. The ferroelectric hysteresis loops?P-E? and capacitance-voltage?C-V? curves of highly?012?-and?110?-oriented BNFTO thin films strongly depends on the applied voltages and measured frequencies. All the P-E and C-V curves are shifted to the positive electric field direction due to the presence of internal field.3.?100?-preferred orientation is more readily formed by using the La Ni O3 electrode because of the good lattice matching. The BNFTO film of 500 nm is relatively dense with columnar structure due to its moderated-layer thickness, while the thicker film of 1.3 ?m exhibits a multilayer structure with evident layer interface. Meanwhile, one can find an oscillation phenomenon in the leakage current curves under a low electric field, which can be explained by the domain backswitching in the film. Compared with polycrystalline films,?100?-oriented BNFTO film displays a higher dielectric constant.4.?100?-,?110?-, and?111?-preferred Sm-doped BFO thin films are fabricated on the single crystal LaAlO₃ substrates. We found that the orientation of the substrates has a strong influence on not only the crystal structure but also the ferroelectric properties. All films display a single P-E hysteresis loop rather than double loops reported in the literature. This may be related to the low crystallizing degree and the preparation method.