Study On The Oritentation Control Of Ferroelectric Field Polarization Of Low-dimensional Complex Oxide Structure

Ferroelectric materials are actively explored in both fundamental science and new functional materials research such as sensors,non-volatile memory devices,and solar cells,owing to their promising properties.There is a dipole pinning effect in the ferroelectric/oxide metal interface,that is,ion displacement occurs in the oxide metal near the interface,and dipole further interact with multiple charges to form an abundant physical property even at nominally simple interfaces.Furthermore,the physical properties of ferroelectric and other materials superlattices may differ dramatically from simple solids and can be tailored by varying the layering sequence,aroused the interest of material scientists.Based on the ferroelectric superlattice can be analogous to ferromagnetic/paramagnetic/ferromagnetic multilayer films.By controlling the parallel and anti-parallel magnetization directions of the ferromagnetic layer,the minimum and maximum resistance states can be obtained respectively,namely the giant magnetoresistance effect?GMR?,which greatly improves the storage capacity of the disk and is widely used in high-density disk storage devices.The GMR is modulated by magnetic exchange coupling and intermediate paramagnetic layer thickness.For the ferroelectric/oxide metal/ferroelectric heterostructure,the resistance of the oxide metal can be modulated by controlling the polarization direction of the ferroelectric layer.In order to promote the low-dimensional structure of ferroelectric oxide to the development of new devices,it is first necessary to characterize the ferroelectric coupling and the critical thickness of the paraelectric layer.Here,we make BaTiO₃/SrRuO₃/BaTiO₃ by Laser-MBE.X-ray diffraction?XRD?,electric transport,piezoelectric force microscopy?PFM?and photovoltaic I-V means are used to characterize the BTO ferroelectric field coupling interaction and to analyze the factors of influencing the BTO polarization direction.And we achieve to control the spontaneous polarization direction of BTO though regulating growth oxygen pressure.Firstly,for the BTO/SRO/BTO system,it is found that when the thickness of the SRO is sufficiently thin,there is the pinning effect of the polarization directions of bilateral BTO,that is,the BTO coupling interaction.Combined that metal oxides will occur ionic polarization to shield the ferroelectric field in ferroelectric materials,it is proposed that the bilateral BTO coupling interaction is transmitted through the ionic displacement in SRO.Furthermore,the characterization of the difficulty of reversing the polarization direction of the unilateral BTO by the photovoltaic transport method shows that the coupling effect of bilateral BTO tends to make the polarization direction opposite;the intensity of the coupling interaction gradually increases with the decrease of SRO thickness,and corresponding SRO critical thickness?dc?is 8ML.Combined with that BTO ferroelectric field will inhibit SRO transport,it is found that when the SRO thickness is less than dc,SRO electrical transport property is controlled by the polarization direction of the ferroelectric layers,and get a relatively large switching ratio??250%?at room temperature.Nextly,based on the resistance state of the system dependent on the polarization direction of the ferroelectric layers,therefore,it is necessary to analyze the factors affecting the polarization direction of BTO.On the other hand,ferroelectrics spontaneously polarization will introduce a new degree of freedom to design ferroelectric heterostructure and corresponding devices,further leading to fascinating new phenomena.The presence of oxygen vacancies is found to have a huge impact on the orientation of spontaneous polarization.Two different BTO polarization directions are obtained by controlling the BTO growth oxygen pressure,the two BTO samples with oxygen pressure from low to high and from high to low.Combined with the model analysis,the influence of oxygen vacancy on the polarization direction is obtained.Finally,in BTO/SRO/BTO/STO system,the effects of compressive epitaxial strain,oxygen vacancy,and bilateral BTO coupling interaction are clarified on the BTO polarization direction and establishes a systematic understanding of the factors affecting the polarization direction.Finally,it is expected to obtain a system with a higher switching ratio by replacing SRO with other oxides metal.ReO₃ is chosen because it has a simple cubic structure,high conductivity,and great application potential in the liquid crystal devices,solid-state batteries and electrochromic.Because the rhenium oxide has a complicated phase diagram,there have been no reports of high-quality ReO₃film in the experiment.Here ReO₃ bulk is made by direct sintering the pressed chips of ReO₃powders under the ambient pressure.We obtain the best sintering conditions of ReO₃is 300?-4h,through XRD,scanning electron microscope?SEM?and electric transport.The cubic structure,high crystalline homogeneity,and dense ReO₃ bulk is obtained.After preliminary experimental attempts,the ReO₃ bulk can be used as the target of Laser-MBE,providing a foundation for high-quality ReO₃ films preparation.