Resistive Switching Characteristics And Mechanisms Of BiFeO₃ Based Heterostructures

As the only known room-temperature multiferroic material simultaneously exhibiting ferroelectric and ferromagnetic properties, bismuth ferrite(BiFeO3, BFO) has attracted immerse attention in recent years due to its excellent ferroelectric and magnetoelectric properties. It has shown great potential in applications such as novel non-volatile memories, multifuntionalized sensors, spintronics devices, photovoltaic devices, et al. Resistive switching memory devices based on BFO materials, which combines the advantages of ferroelectric and resistive switching random access memories, are based on the modulation effects of ferroelectric polarization reversal on the interfacial barrier profile and charge transport characteristics induced resistive switching between HRS and LRS. This resistive switching mechanism is based on the intrinsic properties of BFO films, not defect-mediated resistive switching mechanisms,exhibiting unique advantages. This dissertation proposes two resistive switching structures namely BFO/metal and BFO/ semiconductor heterostructures and thoroughly investigates the fabrication methods of BFO films and devices and resistive switching behaviors of both structures, as well as the resistive switching mechanisms, physical models and modulation factors of the resistive switching heterostructures.High-quality and uniform BFO films with excellent ferroelectric properties are obtained on single-crystalline silicon and SrTiO3 substrates by sol-gel and pulsed laser doposition methods. Film defects and leakage paths are greatly reduced by introducing oxide buffer layers, chemical doping, controllable optimization of oxygen pressure and deposition temperature, resulting in dense and uniform BFO films with excellent ferroelectric properties and low leakage current density. Meanwhile, BFO is doped with dual metal elements to obtain much improved magnetic properties with some ferromagnetism; BFO nanostructures doped with non-magnetic Mg atoms are also fabricated with simultaneously exisiting ferromagnetism and paramagnetism.The Au/BFO/SRO resistive switching heterostructure is designed and fabricated,which shows bipolar resistive switching behaviors with self-compliance and high reliability properties after no high voltage forming process. Highly controllable resistive states with different states and high stability are obtained by setting different sweepingvoltages, compliance current and sweeping methods, indicating its multi-level data storage abilities. BFO based resistive switching mechanisms are analysed from the aspects of electrode materials, ferroelectric polarization and material defects. Combing the above analyse with the dependence of temperature and measurement ambient on the resistive switching behaviors, the modulation effects of ferroelectric polarization reversal on the interfacial barrier profile and charge transport characteristics induced resistive switching mechanisms are proved to be responsible for the resistive switching behaviors, where the competition mechanisms between ferroelectric polarization reversal and migration of oxygen vacancies are proposed and the modulation factors are also obtained. Moreover, the self-compliance resistive switching properties and multi-level data storage properties are also explained.The BFO/ZnO heterostructure is also fabricated and investigated which exhibits bistable HRS and LRS switching behavior and rectifying current behaviors. The modulation effects of film thickness and chemical doping on the current behaviors are also studied in detail. The physical model responsible for the BFO/ZnO resistive switching behaviors are proposed and perfected where the interfacial carrier depletion or accumulation are modulated by the interfacial polarization coupling effects.Considering the ferroelectric polarization effects into the energy band diagrams, the corresponding energy band diagrams are proposed and the interfacial barrier profile are obtained. Moreover, the modulation methods and key modulation factors of BFO/ZnO heterostructure are also proposed.