利用掃描穿透式電子顯微鏡結合電子能量損失能譜於SrTiO3/LaAlO3/SrTiO3氧化物異質介面之研究

Abstract

在過渡金屬氧化物之鋁酸鑭(LAO)與鈦酸鍶(STO)異質介面發現二維電子氣存在後，介面上獨特的物理性質接連被發現。近來又發現若於LAO/STO雙層結構之上再覆蓋STO後，上下介面處分別會累積二維電洞氣與二維電子氣。然而，對於此介面累積二維電子氣的成因仍舊尚未明確，更何況二維電洞氣。此研究將系統性地探討上下介面的異同之處與上層STO所扮演的角色。我們利用掃描穿透式電子顯微鏡搭配電子能量損失能譜，以具有原子級解析度的影像與能譜從微觀的角度研究此三層超晶格結構材料。由原子晶格結構的變化與化學成分組成比例的變動，推斷出於每層單位晶格中的電荷載子分布與內建極化場大小。進而指出極化不連續理論與介面累積電荷並無顯著的關聯，而是上下介面兩側分別所具有的尾對尾及頭對頭的內建極化場誘使二維電洞氣與二維電子氣聚積於介面處。在發現單一種過渡金屬氧化物異質介面可共同存有二維電洞氣與二維電子氣的現象之後，必然可繼續挖掘出背後所隱含更多新奇的物理現象與其應用之處。

The discovery of the two-dimensional electron gas (2DEG) at the heterointerface between transition metal oxides, LaAlO3(LAO) and SrTiO3(STO), has led to further observation of a number of emergent electronic properties. The capping of an additional STO layer on the LAO/STO gives rise to the onset of the two-dimensional hole gas (2DHG) and 2DEG on the top and bottom interfaces, respectively. However, the extensive studies on the origin of the 2DEG why there should be the 2DEG remain ambiguous, not to mention the origin of the 2DHG. Here, we present a systematic investigation on the interfaces and the role of the STO-capped layer. We resolve this long-standing puzzle through atomically resolved images and spectrums using scanning transmission electron microscopy in conjunction with electron energy-loss spectroscopy. Based on the atomic-scale chemical and structural scrutinization, the charge distributions and the lattice polarizations across the interfaces can be resolved in a layer-by-layer manner. Notably, the results indicate that polar catastrophe has no significant impact on the formation of the 2DHG and 2DEG and the interfaces are decorated by tail-to-tail and head-to-head ferroelectric-like polarizations across the respective top and bottom interfaces. The coexistence of the 2DHG and 2DEG in a single oxide heterostructure may open the avenue to the novel interfacial phenomena by design.