二維極限下凡德瓦系統之電子結構－－分子束磊晶薄膜成長與臨場角解析光電子能譜之應用

Electronic Structures of van der Waals Systems in Two-Dimensional limit–Application of Molecular Beam Epitaxy Growth and In-Situ Angle-Resolved Photoemission Spectroscopy

自石墨烯的發現，近十年來凡德瓦二維材料的相關研究有著飛躍性的發展，這些特殊的材料除了有助於縮減電子元件尺度外，也因為量子效應的涉入，材料特殊的電子結構擴展了工業應用的可能性。在二維材料中，凡德瓦材料因為層間的作用微弱，易於以分子束磊晶、機械撥離等方式製備少數層及單層的樣品，甚或是不同材料間堆疊的異質結構。儘管界面間的作用微弱，凡德瓦材料的電子結構仍有機會受到外層電子軌域耦合、晶格匹配等效應進一步做出改變；換言之，提供了一個調控材料特性的自由度。對於材料電子結構的瞭解，角解析光電子能譜是唯一可直接量測電子結構的實驗技術，此技術同時具備很高的表面敏感度，非常適合用於研究低維度材料；然而也因此對於待測樣品的品質有著極高的需求。為了取得高品質的樣品以及電子譜，分子束磊晶與臨場角解析光電子能譜的整合系統在此類研究上有著其必要性。本文將結合筆者的研究經驗，並以中研院物理所的整合系統為例，說明分子束磊晶與臨場角解析光電子能譜整合系統在物理研究上的應用與價值。

Since the discovery of graphene, the investigation toward van der Waals two-dimensional materials has increased rapidly in the past 10 years. The finding of these materials not only assists the size reduction of electronic devices, but also strongly advances electronic designs due to their unusual electronic properties originated from quantum size effects. Because of the weak bonding nature, single/few layer(s) and artificial stacking heterostructures can be fabricated by techniques such as molecular beam epitaxy (MBE), and mechanical exfoliation. Although the interlayer interaction is generally thought to be weak, coupling of electronic states, and lattice matching might still modulate the physical properties of the system investigated. In other words, a new degree of freedom of electronic tuning is generated by the control of interlayer interaction. Angle-resolved photoemission spectroscopy (ARPES) is a surface sensitive technique and is the only experimental technique that directly measures the electronic structures of materials, which is suitable for investigating low-dimensional systems. However, it is because of the surface sensitivity, fabrication of high-quality sample is demanded. MBE and in-situ ARPES integrated system is of great importance for the experimental propose. In this article, we will review some of our research works on electronic structures of van der Waals systems and will take the MBE and in-situ ARPES integrated system at the Institute of physics, Academia Sinica, as an example to show the capability and application of the in-situ experimental setup.