【作者】 陈星；

【导师】 张泽； 田鹤；

【作者基本信息】 浙江大学 ， 材料学， 2020， 博士

【摘要】 在强关联复杂氧化物体系中,电荷、自旋、轨道以及晶格等自由度共存且相互作用,产生了一系列相互竞争的新奇物理响应,包括高温超导、金属-绝缘体转变、巨磁阻效应、（反）铁磁以及多铁等物理性质。研究铁电氧化物异质界面以及界面位错区域的原子结构与电子结构,对于优化改进材料体系以及加深对材料物理机理的理解是非常重要。而球差校正透射电镜以及单色器、高速能谱探头等技术的发展,为原子尺度下研究材料结构信息和电子信息提供了一个强有力的工具。PbTiO₃为代表的钙钛矿铁电氧化物,是铁电、压电材料研究中的热点关注对象。常规的PTO基异质外延薄膜衬底为多畴态,对于利用单畴PTO衬底存在的静电力进行性能调制的研究目前还较少。因此,单畴PTO/STO异质界面处是否会由于静电力的作用发生电子重构与电荷转移等现象,外延生长了STO薄膜以后基底的退极化场如何屏蔽,屏蔽电荷的来源等,都是该体系十分值得研究的一系列科学问题。此外,（010）晶面PTO与STO由于6.4%失配度的存在,尚未有文献报道该晶面异质结的生长。该异质界面是否会存在未完全释放的应变,该应变又将对于界面附近材料的电荷分布与局域相结构产生怎样的影响,都非常值得去深度探究。本论文立足于上述出发点,选取了单畴PbTiO₃为基底的PbTiO₃/SrTiO₃异质结体系为研究对象,通过原子分辨率高角环形暗场像（STEM-HAADF）、积分差分相位衬度像（iDPC）、电子能量损失谱（EELS）、原子图像定量分析等透射电子显微分析方法为主要手段,分别探索了该体系中PbTiO_3（001）//SrTiO_3（001）与PbTiO_3（010）//SrTiO_3（010）两种界面的原子结构、相结构、电荷分布等信息。具体研究内容及成果如下:（1）选取单畴PbTiO₃为基底静电力驱动生长的PbTiO_3（001）//SrTiO_3（001）界面为研究对象。原子位置拟合以及原子位移定量分析结果发现,PTO内部维持其单畴状态,极化沿（001）指向界面,而在靠近界面的PTO一侧5-6个单胞区域,铁电极化位移减小,表现为一个抑制区域,这与铁电退极化场的未完全屏蔽有关。之后的电子能量损失谱（EELS）分析得出,在PTO一侧最接近界面的2个单胞,存在局域三价Ti离子组分较高的现象,说明有可能形成导电态的二维电子气。之后的EELS O精细结构以及定量分析表明,界面上集聚的电子来源于从另一侧表面（负极化面）开始的氧空位提供电子之后的电荷转移。该体系与经典LaAlO₃/SrTiO₃二维电子气体系不同,是利用单畴衬底所具有静电力诱导产生的界面电子结构变化,有望在其界面电导率、磁性等特性方面获得器件层面上更有效的应用。（2）选取单畴PbTiO₃为基底的PbTiO_3（010）//SrTiO_3（010）界面为研究对象。由于（010）晶面PTO与STO由于6.4%失配度的存在,此前尚未有文献报道该晶面异质结的生长,这也是PTO/STO体系（010）界面在实验上第一次成功的制备。在该界面上发现周期存在的失配位错,博格斯矢量为a[001]。同时采用高能量分辨率STEM-EELS谱学表征,发现在位错核心区域存在较低价态的Ti离子,以及一个氧空位耗尽区,这可能是由于电子富集造成的。而该富集电子的来源则是PTO体内的氧空位。这种一维分布的负电荷富集现象,对该绝缘体系在位错线上的电子电导率可能会有提高作用。（3）继续选取单畴PbTiO₃为基底PbTiO_3（010）//SrTiO_3（010）界面为研究对象。通过几何相位分析（GPA）的方法,分析了在上述周期性出现的失配位错核心的下方小局部区域的应变,发现了局域的c轴压应变。参考现有理论计算文献推测该压应变有可能会诱导产生四方相-菱方相的相变。继而,通过差分相位衬度像（iDPC）与原子位移定量分析,在上述区域发现了偏离正常（001）方向角度45°的铁电极化位移分布,符合菱方相PbTiO₃铁电极化在[100]晶带轴上的投影。之后结合逐个单胞的应变分布统计结果,发现在c轴的压应变与a轴的拉应变可能会造成PbTiO₃从正常的四方相向菱方相的转变。铁电材料具有的巨压电效应等都往往出现在准同型相界附近,本文发现的T-R转变或者说T-R共存很可能与PTO单晶的准同型相界有关,对于进一步探索界面应变对于异质结体系相结构的影响有指导意义。更多还原

【Abstract】 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.更多还原