Investigations On Orientation And Properties Of Solution Derived BiFeO₃ Thin Films

Bismuth ferrite?BiFeO₃,BFO?as an ideal room-temperature single phase multiferroics has been widely investigated in recent years because of its magnetic and ferroelectric properties as well as potential application to its maximum polarization at room-temperature.In the past decade or more,extensive research has been devoted to BFO-based materials in a variety of different forms,including ceramic bulks,thin films and nanostructures.Due to the large leakage current characteristics of BFO,it is necessary to control the defects to obtain excellent ferroelectric and magnetoelectric coupling properties.Previous works indicate that defects can be effectively controlled in the BFO thin films.Moreover,the BFO thin films are needed with the development of micro-electric technology and devices.Chemical solution deposition?CSD?,as a kind of thin film preparation method,has the advantages of low-cost,precursors mixed at an atomic level,suitable for films forming in complex shape substrates and large-scale preparation of large-area films.In recent years,the CSD method for BFO thin films has significantly improved.However,the solution-derived BFO thin films show poor properties as comparable with physical vapor deposition?PVD?prepared BFO films.Additionally,the CSD-derived BFO thin films are usually polycrystalline with random orientation.It is difficult to obtain oriented or epitaxial BFO thin films with excellent properties,which limit the applications.In this dissertation,oriented and epitaxial BFO thin films are successfully prepared on Si-based substrates and LaAlO₃?LAO?single crystal substrates respectively by CSD method.The growth mechanisms and relative properties of the prepared BFO thin films are systematically investigated.The main contents are concluded as follows:1.Preparation and properties of BiFeO₃?lll?/Pt/Ti/SiO₂/Si thin films.BFO thin films were directly deposited on Pt/Ti/Si02/Si substrates by CSD method.The effects of annealing temperature on the microstructures and properties of BFO thin films were investigated.The results show that the?111?-oriented BFO films can be only achieved when Bi and Fe acetates are used as raw materials and propionic acid?PPA?as solvent and the annealing atmosphere is N2.The BFO thin films with good ferroelectric properties can be obtained when the annealing temperature is within 500-600 ?.The lattice constant,the grain/crystal size and remnant polarization increase with the increase of annealing temperature,however,the leakage current and coercive field also increase.The BFO thin film annealed at lower than 500 ?cannot be crystallized,while serious Bi volatilization and secondary phase occurr in BFO thin film annealed at higher than 650 ?.2.Preparation and properties of BiFeO₃?001?/LaNiO₃/Si thin films.An all-solution approach is used to deposit strongly?001?-oriented BFO/LNO/Si thin films.The effects of annealing temperature on microstructures and electrical properties are investigated.The results show that the optimal annealing temperature is between 500-600 ?.The remnant polarization of BFO thin films are improved with the increase of annealing temperature,while the coercive field is nearly unchanged.The interface reactions will occur between BFO and LNO interface when the BFO thin films are annealed at temperature above 600 ?,which will lead to the deterioration of the microstructures and ferroelectric properties.3.Preparation and properties of epitaxial BiFeO₃ thin films.Epitaxial BFO thin films with different orientations are deposited on solution-derived SrRuO₃?SRO?bottom electrodes buffered?001?,?110?and?111?oriented LAO single crystal substrates by CSD.The epitaxial growth mechanisms of BFO thin films and the orientation dependent electrical properties are investigated.The experimental results show that the BFO thin films heteroepitaxially grow along the substrates.The ferroelectric properties of BFO thin films are orientation dependent,and the?111?oriented BFO thin film shows the largest remnant polarization and the largest coercive electric field;while the?001?oriented BFO thin film has the largest dielectric constant.4.Mn-doped epitaxial BiFeO₃ thin films.Substitution of Fe with Mn element can effectively improve the magnetic and ferroelectric properties of BFO thin films.The results show that the leakage current gradually increases with the increase of Mn doping concentration.At the same time,the conduction mechanism is also changed.For the undoped BFO thin films,the leakage at low electric fields can be described by the Ohmic behavior,while the Fowler-Nordheim tunneling behavior is suitable at high electric fields.In the case of Mn doping,the interface between the electrode and the film with Schottky contact controls the charge injection,resulting in the Schottky barrier domination in the conduction.In addition,the crystal/grain sizes of BFO thin films gradually increase with the Mn doping.Moreover,the Mn doped can induce Jahn-Teller distortion in BFO thin films,resulting in the improvements of magnetic and ferroelectric properties.In summary,oriented and epitaxial BFO-based thin films with excellent ferroelectric properties can be successfully prepared via solution approaches.The results will provide experimental and theoretical information to prepare the oriented and epitaxial perovskite ferroelectric thin films with excellent ferroelectric properties.