| The rapid development of thin film growth technology makes it possible to grow ultra-thin films on a layer-by-layer basis.Research groups around the world have grown in a variety of artificial thin films and superlattice structures,including semiconductors and oxides.In the field of oxide thin film research,researchers have worked together to develop a number of epitaxial oxide heterojunction structures grown on a single crystal oxide substrate,which have novel interfacial functions at the interface.Such as spin filter function,enhanced electrical resistance,magnetoresistance and so on.These novel properties often depend on the structure of the thinfilm,especially at the interface,such as interface sharpness,atomic mixing,defects and strain.Therefore,it is necessary to understand the relationship between the function of these thin film oxide heterojunctions and its interfacial microstructures,characterization with atomic resolution is necessary.Transmission electron microscopy(TEM)is a powerful research tool that provides high-resolution characterization methods that provide us with atomic-level characterization resolution.Scanning transmission electron microscopy(STEM)can provide us with atomic resolution of real space images,while it has a direct interpretation of the atomic number contrast.The electron energy loss energy spectrum(EELS)used in conjunction with the STEM can detect the local chemical composition of the tiny sample and the local electronic structure of the transition metal element and the oxygen element.The energy resolution can be greatly improved in the case of monochromators to observe the fine structure of spectrums.With the emergence of spherical correction,the electron beam probe size can be smalled as 1A or less,so that the techniques described above have been significantly improved.The spherical aberration correction makes it possible to precisely analyze a single atom column and to detect the electronic structure of the material at the atomic scale.The combination of STEM and EELS technology allows us to understand the relationship between the properties of the oxide thin film heterojunction structure and its microstructure and electronic structure.In this paper,the spatially resolved electron energy loss spectroscopy combined with aberration-corrected scanning transmission electron microscopy is employed to measure the compositional profiles across the interfaces of different layers with atomic resolution.In order to obtain the results,a lot of sample preparation work was carried out to explore the optimal sample preparation method,this part is described in the fouth chapter.The main findings of this paper are as follows:(1)The high angle annular dark field image of LaNiO3/Pr0.sCa0.2MnO3/BaTiO3/La0.7Sr0.3MnO3 heterojunction structure was obtained by scanning electron microscopy with spherical aberration correction.Heterojunction structure is cristallized well,there is no obvious defects and strain.At the same time,the c/a ratio of the ferromagnetic barrier layer was obtained by statistical method.(2)Two-dimensional elemental imaging with atomic resolution is demonstrated,and the influences of the interface sharpness,the terminal layer,and cation intermixing are investigated.An asymmetric sublattice intermixing at the Pr0.8Ca0.2MnO3/BaTiO3/La0.7Sr0.3MnO3 interfaces is observed,which can affect the local Mn valence and coupling.(3)EELS experiment with monochromator was used to obtain the EELS energy spectrum of transition metal Mn and O elements with high energy resolution,and the white line ratio method was used to analysis the Mn valence.It was found that the Mn valence is different at the two interfaces which is associated with the different intermixing at the two interfaces.The detrimental effect on the function of the heterojuntion as a memory coused by this intermixing phenomenon is also demonstrated.We also demonstrate the detrimental effect on the function of the heterojuntion as a memory,which is caused by the intermixing phenomenon. |
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