掃描穿透式電子顯微鏡結合電子能量損失能譜儀於氧化物界面物理之研究

絕緣體氧化物異質界面LaAlO3/SrTiO3 (LAO/STO) 存在高密度的二維電子氣，在氧化物界面提供了全新的視野與期待。LAO/STO界面的二維電子氣擁有超導、鐵磁和絕緣鐵電性等豐富的物理特性。在豐富的物理特性背後，絕緣體材料界面LAO/STO的二維電子氣扮演關鍵的主因。然而其來源機制目前仍尚未明確。在這個研究中，我們利用具有原子解析的掃描穿透式電子顯微鏡搭配電子能量損失能譜解決了這個長久以來的未解問題。透過系統性地研究絕緣性界面與導電性界面的LAO/STO，明確地指出二維電子氣的來源與極化不連續導致極化災難並無關聯，也排除了陽離子在界面相互擴散與氧空缺導致的二維電子氣可能性。取而代之的是，異質界面應力誘發晶體結構的扭曲而產生的頭對頭的極化場，誘發電子氣聚集在STO界面以屏蔽此極化場。此外我們也發現這個界面不僅有電子氣，界面的LAO亦存在等量電子氣密度的互補局域電洞氣。電洞的存在，支持了LAO薄膜內存在內部極化場。因為鐵電材料具有可翻轉極化方向的特性，所以藉由它可以調控鄰近界面的電子結構。因此我們成長了具有不同極化方向(P–與P+)的鐵電薄膜Pb(Zr0.2Ti0.8)O3(PZT)在LAO/STO上面，形成PZT/LAO/STO的三明治結構，進而探討不同極化方向的鐵電薄膜對LAO/STO界面的電子結構的影響。從實驗結果發現，不同極化方向的鐵電薄膜確實改變了LAO/STO界面的電子結構：不同極化方向的鐵電薄膜使得二維電子氣位在STO的深度不同（P–的二維電子氣聚集位置離LAO/STO界面三個單位晶格，而P+的二維電子氣聚集位置離LAO/STO界面一個單位晶格）；在LAO中的局域電洞氣的密度也不同（P–:10^14 cm-2，P+:10^13 cm^-2）。除此之外，不管在P–或P+樣品，我們均量測到電子密度存在PZT薄膜，這與薄膜內的氧的空缺有關。不同極化方向的鐵電薄膜確實改變了LAO/STO界面的電子結構，但背後的機制仍然尚未被解決。未來，我們會盡可能地釐清它。

關鍵字

掃描穿透式電子顯微鏡；電子能量損失能譜；氧化物界面；鋁酸鑭；鈦酸鍶；鋯鈦酸鉛；鐵電薄膜；頭對頭極化場

並列摘要

The two-dimensional electron gas (2DEG) between insulating LaAlO3 (LAO) and SrTiO3 (STO) opens up the field of oxide interface and harbors properties absent in the parent bulks such as superconductivity, ferromagnetism, and sensitivity to electromechanical stimuli like insulating ferroelectrics. The most celebrated unresolved problem of this model system is the origin of the 2DEG formation. In this thesis, we resolved this long-standing puzzle of the interfacial metallicity by thorough microscopic and spectroscopic investigations using scanning transmission electron microscopy in conjunction with electron energy-loss spectroscopy. For conductive interface, the epitaxial strain rejuvenates the hidden ferroelectric-like lattice instabilities in LAO and STO, which accompany with the head-to-head ferroelectric-like polarizations across the interface. The divergent depolarization fields of the head-to-head polarizations cast the interface into an electron reservoir, forming screening electron gas, that is, 2DEG in STO with LAO hosting complementary localized holes. Ferroelectric oxides characterize switchable polarized directions, which are conventionally considered that it is capable to modulate the electronic structures of adjacent interface as an active layer. The P–- and P+-PZT/LAO/STO systems with different polarized states, indeed, show the distinct electronic structures. Between the two states, the 2DEG is buried in the different depth within STO and the hole density displays difference within LAO (P– ~ 10^14 cm^-2, P+ ~ 10^13 cm^-2). In addition, for both states, the electron charge density always appears in PZT, which indicate the oxygen vacancy in PZT. Nonetheless, the mechanism of how the polarized states of PZT change the electronic structure at LAO/STO interface still puzzle us. We will clarify it soon.

並列關鍵字

scanning transmission electron microscopy ； electron energy-loss spectroscopy ； oxide interface ； LAO ； STO ； PZT ； head-to-head polarization

參考文獻

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掃描穿透式電子顯微鏡結合電子能量損失能譜儀於氧化物界面物理之研究

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