鉍系拓樸絕緣體的組成依賴性表面摻雜效應之研究

Abstract

隨著科學蓬勃發展下，更多新穎材料漸漸被挖掘，而拓撲絕緣體（TI；Topological Insulator）就是其中之一。拓樸絕緣體有別於傳統絕緣體的基本特性，其中三維拓樸絕緣體，因為表面態特殊性質，不僅讓塊材的表面具導電性，也讓傳導的電子具自旋極化，因此對於控制自旋極化流有很大的潛力。在過去的十幾年中，拓撲絕緣體引起凝態物理、自旋電子學、量子電腦等領域廣泛的關注。近年來開始轉向結合磁性物質來探討交互作用下的基本特性及量子反常霍爾效應。鐵磁金屬元素與拓樸絕緣體的主題引起我們好奇，進行超薄膜以及少鍍量的面向研究。 本論文著重材料間的介面效應，利用少量鈷在Bi2Se3及Bi2Te3這兩種拓樸絕緣體探討基本特性變化。使用分子束磊晶在兩種TIs：Bi2Se3和Bi2Te3上沉積鈷。使用掃描穿隧顯微鏡在78 K下研究鈷成長後不同TI的表面形態，並比較成長鈷後的表面樣貌及電子密度分布之差異。蒸鍍不同少量的鈷後，在Bi2Se3及Bi2Te3發現鈷引發不同缺陷，而隨鈷厚度相依的掃描穿隧能譜於不同拓撲絕緣體上亦有不同的結果：與原本的拓樸絕緣體比較發現，Bi2Se3系列的曲線平移較Bi2Te3系列來得顯著。此說明蒸鍍少量鈷後，Bi2Se3的電子結構被鈷影響較大。進一步使用X射線光電子能譜來研究鈷與拓樸絕緣體之介面關係，了解鈷於拓樸絕緣體間的交互作用。比較蒸鍍少量鈷在不同基板後，得到與掃描穿隧能譜互相應證之結果：在成長鈷後，Bi2Se3中各元素與鈷有明顯的耦合；與之相較，鈷對Bi2Te3的各元素則沒什麼影響。總結以上實驗結果得到一致的結論，鈷在Bi2Se3比在Bi2Te3介面有較強化學鍵結產生，電子結構上呈現鈷對Bi2Se3的n型摻雜效應亦較Bi2Te3明顯。未來在整合鐵磁金屬與拓樸絕緣體應用時，鈷於不同硫族元素之鉍系拓樸絕緣體之影響將可盡一份貢獻。

With the burgeoning science, more novel materials are gradually explored, and one among them is the topological insulator (TI) that subverts the traditional insulators in the basic features. Because of the topological surface states (TSS), the three-dimensional TIs not only possess conductive surfaces, but also spin-polarized electrons in the surface states. Due to the great potential to control spin-polarized currents, in the last ten years, TIs have attracted wide attention in the fields of condensed physics, spintronics, quantum computers, and so on. Based on well-studied basic properties of TIs, there come combinations of magnetic materials and TIs for exploration of the interaction coupling and quantum abnormal Hall effect (QAHE). This topic of the coupling of ferromagnetic metal and TIs arouses our curiosity, which can be divided into ultra-thin film and low-plating method to investigate. The thesis studied the variety of properties of sub-monolayer Co on two kinds of TIs: Bi2Se3 and Bi2Te3. In this work, using molecular beam epitaxy (MBE), we investigated the morphology and electron density states of different TIs after Co decoration by scanning tunneling microscopy (STM) at 78 K. Several types of Co-induced defects were observed in the samples, and the thickness-dependent scanning tunneling spectroscopy (STS) curves showed comparative results. The Co-induced band shift in Bi2Se3 is obviously larger than in Bi2Te3, which means the electronic structure of Bi2Se3 is easily modified by Co. Further, X-ray photoelectron spectroscopy (XPS) was used to study the interface and chemical interaction between Co and TIs. In the dependence on TI types and Co thickness, there existed TI XPS peak shifting consistent with STS results. The results indicated that Co was more chemically reactive with Bi2Se3. The chemical bonding between Bi2Se3 surfaces and Co is stronger than Bi2Te3. In electronic structures, the n-type doping effect in Bi2Se3 with cobalt is also more obvious than Bi2Te3. In the future, this work that integrates ferromagnetic materials and TIs could benefit designs and applications of TI-based spintronics.