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標題: | 研究雙層hBN封裝單層WSe2元件中的界面傳輸性質及通過非局域自旋閥量測石墨烯中的電子自旋傳輸 Interfacial transport properties in bilayer hBN-encapsulated monolayer WSe2 and nonlocal spin-valve measurements via electron spin transport in graphene |
作者: | Gaurav Pande 高洛夫 |
指導教授: | 林敏聰(Minn-Tsong Lin) |
關鍵字: | 二維材料,過渡金屬硫屬化物,二硒化鎢,能谷-pseudo自旋,單層,激子,自旋軌道耦合,蕭基特能障,鐵磁接觸,自旋,石墨烯,場效電晶體, Two-Dimensional Materials,Transition Metal Dichalcogenides,WSe2,Valley-Pseudospin,Monolayers,Excitons,Spin-Orbit Coupling,Schottky Barrier,Ferromagnetic Contacts,Spin,Graphene,Field-Effect Transistors, |
出版年 : | 2020 |
學位: | 博士 |
摘要: | 近幾年,層狀二維材料出現,引起了學界大量關注,其中探究具有半導體特性的層狀過渡金屬硫化物 (TMDCs)內部量子自由度及其應用於低維電子元件的主題也如火如荼的在學界中進行。這些量子自由度是電子的內稟自旋、層-pseudo自旋和能谷pseudo自旋。其中令人感到興趣的是當上述材料透過適當的方法分離出單層時,將成為擁有直接能隙的低維半導體,顯現出其應用於新穎半導體元件的潛力。新的控制電子自旋和pseudo自旋的方法是基於與貝瑞相位(Berry phase)相關的物理性質,直接的反應出了這些材料在實空間與動量空間的對稱性及其強自旋軌道耦合。前者造就了能谷自旋的多樣控制,而後者造成了自旋和pseudo自旋之間的交互作用。此外,具有單原子層結構的石墨烯,顯示出極其出色的自旋傳輸特性,例如在室溫下最長的自旋擴散長度(spin diffusion length)可達到30微米。這是由於固有的自旋軌道耦合較小,導致相對較慢的自旋弛豫率(spin relaxation rates),及相對較高的遷移率(mobility)。然而,較弱的自旋軌道耦合也導致了較難實現對電子自旋在元件上的控制及操縱。
自旋電子學已經發展為一個廣泛的研究領域,涵蓋了不同種類的材料、磁性系統和元件。本研究試圖觸及這些層面,而這些在自旋電子學上的最新進展,將可能影響信息技術領域和微電子學上的發展。我們確立了四個主要研究方向:可柵極控制的二硒化鎢能谷電晶體(WSe2 Valley-Transistor );透過激子在單層二硒化鎢中擴散研究拓撲地驅動的激子霍爾效應(Exciton Hall-effect);透過雙層六方氮化硼(bilayer Hexagonal Boron Nitride, Bi-hBN)作為WSe2-FET的無針孔穿隧勢壘(pinhole free tunnel barrier)再利用底層較厚h-BN一起進行封裝,最後得到超低蕭特基勢壘;在石墨烯自旋閥進行非局域自旋閥和Hanle效應的量測。希望藉由上述研究方向能夠整合新穎材料與自旋電子元件於主流微電子元件中。我們討論這些領域的最新發展以及將要面臨的機會和挑戰,而這些挑戰涵蓋單層TMDCs的半導體自旋電子元件製備及量測系統的精進。在量測方法上有許多種設置,可以在不同的材料中創造和測量自旋積累(spin accumulation) 和傳輸。本文還將介紹兩種常見的量測技術,即局域、非局域磁阻量測設置。同時也介紹一些關於擴散長度和擴散時間相關的一些分析技術,上述都將在本文中使用與討論。這些量測設計使我們能夠研究這些單層系統中的各種自旋傳輸過程,而這些傳輸機制也激發了與之相關引人入勝的物理學研究。 The recent emergence of two-dimensional layered materials in particular the transition metal dichalcogenides (TMDCs)- provides a new laboratory for exploring the internal quantum degrees of freedom of electrons and their potential for new electronics. These degrees of freedom are the real electron-spin, the layer-pseudospin, and the valley-pseudospin. The monolayers of these materials are particularly exciting showing a direct band gap. New methods for the quantum control of the spin and these pseudospins arise from the existence of Berry phase-related physical properties which is a direct reflection of the symmetry properties in these materials and strong spin-orbit coupling. The former leads to the versatile control of the valley pseudospin, whereas the latter gives rise to an interplay between the spin and the pseudospins. On the other hand, there is Graphene. It exhibits excellent properties for spin transport including the long spin diffusion lengths at room temperature (up to 30 μm). This results from a small intrinsic spin-orbit coupling, leading to relatively slow spin relaxation rates, in combination with relatively high mobilities. The low spin-orbit coupling, however, also has some downsides, mainly that the electrical control/manipulation of spin is difficult to achieve. Spin-based electronics has evolved into a major field of research that broadly encompasses different classes of materials, magnetic systems, and devices. This research has tried to touch on these aspects which marks recent advances in spintronics that have the potential to impact key areas of information technology and microelectronics. We identify four main axes of research: Ultralow Schottky barriers via-bilayer Hexagonal Boron Nitride as pinhole free tunnel barrier in WSe2 based field-effect transistors (FETs), spin transport measurements design via Non-Local Spin valve and Hanle Effects in Graphene spin valves, WSe2 based gate tunable Valley-Transistors, and Topologically driven Exciton Hall-effect via Exciton diffusion in monolayer WSe2, in order to integrate novel spintronic functionalities and materials in mainstream microelectronic platforms. We discuss state-of-the-art developments in these areas as well as opportunities and challenges that will have to be met, both at the device and system level keeping in mind the future attempts at monolayer TMD based semiconductor spintronics. There are various configurations in which spin accumulations and transport can be created and measured electrically in different materials. This thesis also describes two common techniques i.e. Local Magnetoresistance measurement setup and Non-Local setup, along with some analysis techniques related to diffusion lengths and timescales which will be used and discussed in the thesis. These measurement designs allow us for studying different kinds of spin transport processes in these monolayer systems enriched with the fascinating physics lying underneath it. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/71516 |
DOI: | 10.6342/NTU202004344 |
全文授權: | 有償授權 |
顯示於系所單位: | 物理學系 |
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