Study On The Preparation And Properties Of BiFeO₃ Thin Film And Devices

By definition, a single phase multiferroic is a material that simultaneously possesses two or more of the so-called'ferroic'order parameters-ferroelectricity, ferromagnetism, and ferroelasticity. Magnetoelectric coupling typically refers to the linear magnetoelectric effect or the induction of magnetization by an electric field or polarization by a magnetic field. The promise of coupling between magnetic and electronic order parameters and the potential to manipulate one through the other has captured the imagination of researchers worldwide. The ultimate goal for device functionality would be a single phase multiferroic with strong coupling between ferroelectric and ferromagnetic order parameters making for simple control over the magnetic nature of the material with an applied electric field at room temperature. BiFeO₃ (BFO) is well known to be the only prototypical material that possesses both high ferroelectric Curie temperature TC = 1143 K and high antiferromagnetic Néel temperature TN = 643 K, so it attracts much more attention. But the synthesis of pure BiFeO₃ samples is quite subtle because it is necessary to take both kinetic and thermodynamic properties into account. The work to control Growth conditions of BiFeO₃ becomes a big challenge. Moreover, a high leakage current, often observed in BiFeO₃ bulk and thin films, which might limites its applications. On the other hand, for applications in invisible electronic circuits high quality epitaxial ferroelectric heterostructures are also demanded.In this thesis pulsed laser ablation method was used to deposit thin films. Epitaxial ferroelectric capacitors were successfully fabricated by employing some conductive oxide as electrodes. Through changing the temperature and oxygen pressure we grew a series of ferroelectric capacitors. The effects in structure, ferroelectric and leakage properties of temperature and oxygen pressure were studied. Found that below 700℃the structure was improved with increasing temperature and kept temperature at 680℃films'structure was improved with decreasing oxygen pressure. Ferroelectric and leakage test reveals that the film grew at 680℃7Pa had well-saturated polarization-electric field hysteresis-loops and the lowest leakage current compared with other films; and we further studied the mechanism of the leakage current. Stimulatingly, we grew several heterojunctions, the I-V curves reveal that all junctions exhibited typical rectifying effect; and as add bias voltage at a forward direction the curves switched towards the high voltage sides compared with the curves got when add bias at a backward direction, it is believed that ferroelectric properties caused this phenomena. At last, the effects of Mn-doping on BiFeO₃ films were investigated. Leakage current was greatly suppressed with Mn-doping, as a result the ferroelectric properties were improved.The whole thesis consists of four chapters:Chapter 1: The general review of the history and present research situation of the mulitferroic material is given and we sort these materials into several classes. Particularly, we introduced the structure properties of BiFeO₃ which was ferroelectric and antiferromagnetic at room temperature. Some related properties, such as ferroelectric and multiferroic thin film devices, crystal structure of perovskite, ferroelectric and magnetic properties of BiFeO₃, magnetoelectric effect in BiFeO₃, are introduced. Then we sum up the methods of fabricating BFO bulk and thin films. In the end the problems in the research of BFO were brought forward, doping might be the way to improve the properties.Chapter 2: Under different temperature and oxygen pressure, two series of BiFeO₃ thin film capacitors have been grown on (LaAlO₃)0.3(Sr2AlTaO6)0.7(001) (LSAT) substrates with employing La0.7Sr0.3MnO₃ (LSMO) as bottom electrode by the pulsed laser deposition method, and their structure, ferroelectric and leakage properties were investigated. Below 700℃the structure was improved with increasing temperature and also improved with decreasing oxygen pressure at 680℃. We could get polarization-electric field hysteresis-loops at samples fabricated at 700℃, 680℃, 650℃with the same oxygen pressure and at 10Pa, 7Pa, 5Pa with the same temperature, but well-saturated polarization-electric field hysteresis-loops only observed in the sample fabricated at 680℃, 7Pa. The current density versus electric field curves showed that the lowest leakage current appeared in the capacitor grew at 680℃, 7Pa. All results respects that 680℃, 7Pa is the best condition of fabricating BFO capacitor.Chpter 3: By using highly conductive p-type LaNiO₃ (LNO) and n-type La_0.05Sr_0.95SnO₃ (LSSO), Nb-SrTiO₃ (NSTO) as electrodes, all-oxide BiFeO₃ (BFO) heterojunctions, BFO/LNO, BFO/LSSO, and BFO/NSTO, have been grown epitaxially on SrTiO₃(001) substrates. All heterojunctions exhibited rectifying current-voltage effects with some other interesting results. When add bais at a forward direction the I-V curves switched towards the high voltage side, which might be related to the ferroelectric property of BFO. The band structure of BFO/NSTO heterojunction analyzing agrees well with the ferroelectric results. In our assumption, the rectifying effects in the BFO/LNO p-p heterojunction might be ascribed to the interfacial barrier.Chapter 4: Mn-doped BiFeO₃ films have been grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7(001) substrates by the pulsed laser deposition method, and the effects of Mn-doping on their leakage and ferroelectric properties were investigated. The current density versus electric field curves measured from the Mn-doped and undoped BFO epitaxial films indicate that the Mn-doping can effectively suppress the leakage current. As a result, we got well-saturated polarization-electric field hysteresis-loops in 5% Mn-doped BFO film at room temperature. XPS spectra reveal that this improvement could be attributed to the greatly reduced Fe²⁺ in the films due to the Mn-doping.