The Study Of Atomic And Electronic Structure Of Perovskite Ferroelectric Oxide Interface

In strong correlated oxide systems,various degrees of freedom including charge,spin,orbital and lattice coexist and interplay cooperatively,and the correlation between these degrees of freedom can produce a series of competing phases and physical phenomena,including high temperature superconductivity,metal-insulator transition,colossal magnetoresistance,?anti-?ferromagnetism,?anti-?ferroelectricity,piezoelectricity,and multiferroic.Those new phenomena have led to extensive research on bulk and thin films of complex oxide materials,for which the purpose is to increase our understanding of the physical nature of existing material systems,and to help design various new materials with potential applications in spintronic devices,ReRAM,SOFCs,LED and solar cells.In view of the rapid development of thin film deposition techniques,routine synthesis of atomically flat heterointerface of perovskites has been practical.Therefore,one can use lattice mismatch,stress and polarization field to manipulate the interfacial physic properties.Perovskite oxides have the potential of being used to fabricate high integration density devices,which will continuously broaden our understanding of solid state physics,and open up a wider space for the design and research of new electronic devices.It is very important to study the atomic structure and electronic structure of ferroelectric oxide heterointerface and interfacial dislocation region for optimizing properties of material system and better understanding of physical mechanism.The development of spherical aberration-corrected transmission electron microscope,monochromator and high speed EDS detectors provides a powerful tool for studying materials structure and electronic information at atomic scale.PbTiO₃ and other perovskite ferroelectric oxides,have been the focus of researches on ferroelectricity and piezoelectricity.Normally the substrate of PTO-based heteroepitaxial thin films is multi-domain state,there is few researches on tuning the physical properties by using the electrostatic force from single domain PTO substrate.Therefore,many scientific problems need to be solved on this subject:whether there exists an electronic reconstruction and electron transfer on the single-domain PTO/STO heterointerface;how is the depolarization field screened when STO thin film is epitaxially grown;and where does the screen charge come from.Meanwhile,there has been no experimental literature on the growth of?010?PbTiO3/SrTiO3 heterointerface due to the 6.4%misfit between two materials.Whether there exist unrelived strain at the?010?PTO/STO interface and how does the strain affect the charge distribution and local phase structure deserve to be explored deeply.In this dessertation,we selected PbTiO₃/SrTiO₃ heterostructure using single domain PbTiO₃ substrate as the research subject,and we used atomic resolution scanning transmission electron microscopy high angle annular dark fied?STEM-HAADF?,integrated differential phase contrast?iDPC?,electron energy loss spectra?EELS??atomic-scale image quantitative analysis and other TEM analysis tools,and explored the atomic struture,phase structure and electronic structure in?001?and?010?interface of PbTiO₃/SrTiO₃.The main contents of the dessertation is listed as follows.?1?We selected the single domain PbTiO₃ substrated?001?PbTiO₃/SrTiO₃heterointerface as the subject.The atomic resolution STEM-HAADF imaging and EDS analysis results show that the interface is atomically abrupt,and no element diffusion can be seen.The atomic position detremination and displacement quantitative analysis show that the PTO substrate maintains its single-domain state and the spontaneous polarization direction is along[001]to the interface.The first 5-6 unit cells of PTO near the interface shows lower ferroelectric displacement,which is probably related to incompletely screened ferroelectric depolarization field.The EELS analysis shows that there exists large amount of Ti³⁺at the 2 unit cells near the interface on PTO side,which probably exhibits a two dimensional distribution and could be interfacial conductive state.Afterwards,we analysed the oxygen K edge fine structure,and found that the concentrated electrons at the interface comes from the electron transfer from the other polarization suface,and that the origin is oxygen vacancies.This system is different from the classical two dimensional electron gas?2DEGs?LaAlO₃/SrTiO₃,the interfacial electronic changes in this system come from the electrostatic force of single domain substrate.This system has potential applications in interfacial conductivity and magnetism properties in the future.?2?We selected the single domain PbTiO₃ substrated?010?PbTiO₃/SrTiO₃heterointerface as the subject.There has been no experimental literature on the growth of?010?PbTiO₃/SrTiO₃ heterointerface due to the 6.4%misfit between two materials.We studied the atomic structure and charge distribution near the periodic misfit dislocation cores at?010?PbTiO₃/SrTiO3 heterointerface,by the methods of aberration corrected scanning transmission electron microscope?STEM?and electron energy-loss spectroscopy.We found that there exists an aggregation of lower valence Ti ions near the a[001]dislocation cores,which is probably caused from the aggregation of electrons,and could increase the electron conductivity along the dislocation line.The results of this chapter can play an important role in exploring the influence of the interfacial dislocations on the physics at ferroelectric heterointerfaces.?3?The?010?PbTiO₃/SrTiO₃ heterointerface is further studied.By using iDPC and atomic position determination and displacement analysis,we found that in a small region below the misfit dislocation,the polarization direction rotates from the ordinary[001],and rotation angle is about 45°,which is in agreement with the[100]projection of rhombohedral phase PbTiO_3.Furthermore,we conducted unit-cell by unit-cell analysis of strain distribution,and found that the compressive strain on c axis and tensile strain on a axis can possibly cause the local region transition of from tetragonal PbTiO₃to rhombohedral.The T-R phase transition or T-R coexistence could be related to the morphotropic phase boundary,and still need further exploration.