"利用掃描穿透式電子顯微鏡結合電子能量損失能(La,Sr)MnO3/SrTiO3氧化物異質介面之研究"

Abstract

隨著對高空間解析度分析材料結構需求增加，掃描穿透式電子顯微鏡(Scanning transmission electron microscope, STEM)為不可或缺的重要研究工具。利用掃描穿透式電子顯微鏡結合電子損失能譜(Electron energy-loss spectroscopy, EELS)，可以同時得到材料的電子結構與原子級影像。 在本研究中，我們應用掃描穿透式電子顯微鏡結合電子損失能譜研究La0.7Sr0.3MnO3(10u.c.)/SrTiO3與La0.7Sr0.3MnO3(5u.c.)/SrTiO3，在室溫下分別為金屬性與絕緣性。原子級尺度的電子結構分析顯示二維電子存在於La0.7Sr0.3MnO3(10u.c.)/SrTiO3。然而，La0.7Sr0.3MnO3(5u.c.)/SrTiO3沒有電荷累積於介面上。此外，我們也討論La0.7Sr0.3MnO3/SrTiO3結構的特性與蕭基特位能障(Schottky barrier ) La0.7Sr0.3MnO3(10u.c.)/SrTiO3之間的關聯性。

With the increasing demand in structural and electronic characterizations at high spatial resolution, atomically-resolved scanning transmission electron microscope (STEM) has become an indispensable tool in modern materials research. When used in combination with electron energy-loss spectroscopy (EELS) that reflects the electronic features of unoccupied density of states, a simultaneous tackling of the structural and electronic characters at atomic resolution had been proven possible and this conjunct STEM-EELS technique is most suitable for addressing the physics at a reduced dimension. In this thesis, we apply the STEM-EELS to the heterostructural system of La0.7Sr0.3MnO3/SrTiO3 with the La0.7Sr0.3MnO3 thickness of 5 and 10 unit cells, respectively. Notably, the 5 (10) unit-cell La0.7Sr0.3MnO3 film is insulating (conductive) at room temperature, whereas the corresponding bulk is characteristically metallic. The atomic-scale electronic characterization revealed the existence of a two-dimension electron density in the conductive 10-unit-cell La0.7Sr0.3MnO3/SrTiO3. In comparison, the 5-unit-cell counterpart displays a missing charge density as expected for an insulating interface. The profound structure-property interplay in the heterostructures were discussed and the reported Schottky barrier in a metallic La0.7Sr0.3MnO3/SrTiO3 heterostructure was also tackled.