博碩士論文 102282001 詳細資訊



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姓名 林侑鋌(Yu-Ting Lin)  查詢紙本館藏   畢業系所 物理學系
論文名稱
(Nanoelectronics and temperature dependent transport properties of two-dimensional materials)
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摘要(中) 二碲化鎢(WTe2)是一種新穎且具有前瞻性的二維材料。它的晶格結構具有高異相性且已經被證實在低溫的環境下(低於10 K)有著超巨大磁阻(extremely large magnetoresistance, XMR)。近來，雙層與多層的WTe2單晶晶體在室溫(300 K)的環境下，已被證實有著顯著的鐵電性(Ferroelectricity, FE)。此外，單層的WTe2單晶薄膜在100 K的環境下，也被發現存在著量子自旋霍爾效應(Quantum spin Hall effect, QSHE)。這些發現強烈地指出，WTe2在下一個世代的電子元件與自旋電子元件中有著巨大的潛力。因此，為了能夠整合它的磁傳輸性質與上述新穎的特性，研究其磁阻與溫度的關係與在高溫環境下實現大的磁阻 (large MR) 是必要的。然而，迄今為止大部分WTe2相關的研究主要是在機械剝離 (mechanical exfoliation) 的樣品中被實現，且溫度相關的磁傳輸性質的研究仍然非常稀少。在這個實驗中，藉由化學氣象沉積法(chemical vapor deposition, CVD)，高品質且厚度可調控的WTe2單晶晶體被成功地合成。在高溫環境下(高於80 K)，大的磁阻被首次實現於合成的WTe2單晶晶體中。此外，藉由調控材料中的載子濃度，其磁阻可以被進一步地調控與增強。
摘要(英) Tungsten ditelluride (WTe2) is a kind of low symmetry two-dimensional (2D) materials and exhibits extremely large magnetoresistance (XMR) below 10K. Recently, understanding of novel properties in the materials is highlighted to induce unique properties at elevated temperature. Ferroelectricity (FE) of the bilayer WTe2 is experimentally observed at 300K and quantum spin Hall effect (QSHE) of the monolayer WTe2 is demonstrated at 100K, suggesting diverse possibilities for next generation electronics and spintronics. Hence, study of the temperature dependent magnetoresistance (MR) of the WTe2 is essential for real applications and deep insights. However, most reported studies on fundamental issues are mainly achieved with exfoliated WTe2 for reduced disorders and hexagonal boron nitride (h-BN) encapsulation for ideal interfaces. In this thesis, the enhanced performances of the synthesized WTe2 single crystals are demonstrated. Tunable thickness and high crystallinity of the WTe2 are achieved using KCl-assisted chemical vapor deposition (CVD). A large MR of the WTe2 above 80K is experimentally realized for the first time, and the MR is engineerable by tuning the carrier configuration.
關鍵字(中) ★ 二維材料
★ 過渡金屬硫族化合物
★ 電子傳輸性質
★ 磁傳輸性質		 關鍵字(英) ★ two dimensional materials
★ transition metal dichalcogenides
★ electronic transport
★ magneto-transport
論文目次 1 Introduction 1
2 Background 5
2.1 Atomic structure of the WTe2 5
2.2 Band structure of the WTe2 8
2.3 Extremely large and non-saturating magnetoresistance 10
2.4 Negative magnetoresistance 16
2.5 Temperature-induced Lifshitz transition 19
2.6 Quantum spin Hall effect 22
2.7 Ferroelectricity 27
2.8 Weak antilocalization 29
2.9 Motivation and challenges 34

3 Experimental procedure and methods 36
3.1 Material synthesis 37
3.2 Material characterization 38
3.2.1 Surface morphology 38
3.2.2 Optical characterization 39
3.2.3 Chemical configuration 39
3.3 Device fabrication 40
3.4 Electronic transport and magneto-transport measurements 42

4 Results and discussion 45
4.1 Synthesis of WTe2 flakes with tunable thickness 45
4.2 Characterization of the as-grown few-layer WTe2 flakes 47
4.2.1 Surface morphology 47
4.2.2 Optical characterization 48
4.2.3 Chemical configuration 50
4.3 Electronic transport properties of the few-layer WTe2 flakes 51
4.3.1 Ohmic contact confirmation 51
4.3.2 Thickness dependent resistivity and residual resistivity ratio 53
4.4 Magneto-transport properties of the few-layer WTe2 flakes 55
4.4.1 Temperature dependent magneto-transport properties 55
4.4.2 Field dependent magneto-transport properties 56
4.4.3 Thickness dependent magneto-transport properties 58

4.5 Benchmarks of the temperature-dependent MR of the WTe2 samples 62
4.6 The effect of doping on the magneto-transport properties of the few-layer WTe2 flakes 65
4.6.1 Doping strategy 65
4.6.2 Temperature dependent magneto-transport properties 67
4.6.3 Field dependent magneto-transport properties 68

5 Conclusions 70
6 References 74
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指導教授 陳永富 李奕賢(Yung‐Fu Chen Yi-Hsien Lee) 審核日期 2020-1-21
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