拓撲絕緣體Bi2-xSbxTe1.7Se1.3 奈米薄片與奈米線之物性研究

The physical properties of topological insulator Bi2-xSbxTe1.7Se1.3 nanoflakes and nanowires

熊德智(Te-Chih Hsiung)

指導教授 ： 李定國

共同指導教授 ： 陳洋元

國立臺灣大學/理學院/物理研究所/博士(2015年)

https://doi.org/10.6342/NTU.2015.02191

摘要

拓撲絕緣體（TI）是一個新的量子材料存在獨特的表面態為其特徵。這獨特的表面態或稱之為狄拉克表面態是沒有帶隙的，而且是被時間反衍對稱性所保護使其存在於絕緣塊材的能帶之中，這使得電荷載子在拓撲絕緣體樣品表面或邊緣傳遞而無後向散射。拓撲絕緣體其自旋與動量成90度垂直的特性提供獨特的機會於自旋電子學，量子計算，和熱電應用。然而，大多數拓撲絕緣體的報導其表面載子遷移率都非常低，如此無法提供可靠的表面電(導)性量測。因為他們的費米能階落在能帶裡。例如，空缺（Bi2Se3）或錯位（Bi2Te3）所造成的金屬性導電行為，將使得對樣品表面的測量有所誤差。因此，對拓撲絕緣體和熱電材料的電性應用，塊材達到高度絕緣是一個關鍵先決條件。在本論文中我選擇Bi2-xSbxTe1.7Se1.3系統而非Bi2Se3或Bi2Te3作為我研究的起始材料，原因在於它已經被確認是一個塊材達到高度絕緣的拓撲絕緣體以及其費米能階位於帶隙之中。
前三個章節描述了基本概念和物理背景。第1章和第2章給出歷史前景，介紹拓撲絕緣體，以及熱電的背景訊息。第3章介紹在本文中使用的儀器。第4章，一系列Bi1.5Sb0.5Te1.7Se1.3（BSTS）的薄片80奈米至140微米的厚度被製作來探討它們的金屬表面態。我們發現了這些拓撲絕緣體BSTS奈米片他們的電性傳輸是被表面所主導著。由表面所主導的電性傳輸行為可以被歸咎於高表面載子遷移率(~3000 cm2/V s)與低塊材載子遷移率(12 cm2/V s)。從厚度相依電導率以及磁阻Shubnikov-de Hass振盪發現於200奈米BSTS薄片的結果我們可以估算出拓撲絕緣體表面貢獻占所有電性傳輸的百分之九十。拓撲絕緣體的表面態存在也透過weak anti-localization effect來證實。
最近發現的三維拓撲絕緣體例如Bi2Se3和Bi2Te3也是良好的熱電材料，因為它們具有類似的特徵，如重元素和小的帶隙。在發現拓撲絕緣體的新型表面態之後，其對熱電材料性能有何影響成為一個有趣的科學問題。由於奈米線表現出比塊材和奈米薄片具有更多的表面態貢獻，因此，我們將有一個更好的機會來觀察拓撲絕緣體 BSTS奈米線的獨特熱電性質。
第5章，我們報導所觀察到的熱電優質係數於拓撲絕緣體 BSTS奈米線(zT=0.36)相較於其塊材(zT=0.028) 在300 K提升一個數量級。優質係數的提升，主要是由於表面主導電性的行為使的電導率提升了一個數量級和奈米線中存在的熱激發載子。磁阻分析揭示狄拉克電子的存在而且也確定費米能階靠近導帶邊緣附近。對於無需額外加柵極偏壓，使得拓撲絕緣體的費米能階在能帶中的熱電量測，這是一個嶄新的研究並且可能沒有被報導過，並提供研究表面態對電導率的貢獻，且無需考慮能帶彎曲所帶來的複雜影響。

關鍵字

拓撲絕緣體 ； 奈米薄片 ； 奈米線 ； 熱電 ； 半導體

並列摘要

Topological insulator (TI) is a new quantum material characterized by the existence of distinct surface state. The gapless Dirac surface state resides in bulk insulating band gap and is protected by time-reversal symmetry, which enables charge carriers to propagate on the surface or along the edge of a TI without backscattering. The spin-momentum locking mechanism in TI renders a unique opportunity for applying TI in spintronics, quantum computations, and thermoelectric (TE) applications. However, the reported surface carrier mobility of most TIs is very low for reliable detection of surface conductance because of their Fermi levels locate in the bands. For example, the metallic bulk conduction caused by vacancies (Bi2Se3) or antisite defect (Bi2Te3) will smear the surface sensitive probe measurement. Thus, achieving a high-insulating bulk state is a crucial prerequisite for the transport applications of TI and TE materials. In this dissertation, I select Bi2-xSbxTe1.7Se1.3 system instead of Bi2Se3 or Bi2Te3 as my starting materials, due to it has been confirmed to be a high-insulating bulk TI as well as their Fermi levels locate in the gap.
The first three chapters describe fundamental concepts and physical background. Chapter 1 and Chapter 2 give the history prospect and introduction of TI as well as thermoelectric background information. Chapter 3 introduces instruments used in this dissertation. Chapter 4, a series of Bi1.5Sb0.5Te1.7Se1.3 (BSTS) flakes 80-nm to 140-μm in thickness was fabricated to investigate their metallic surface states. We report the observation of surface-dominated transport in these topological insulator BSTS nanoflakes. The achievement of surface-dominated transport can be attributed to high surface mobility (~3000 cm2/V s) and low bulk mobility (12 cm2/V s). Up to 90% of the total conductance from the surface channel was estimated based on the thickness dependence of electrical conductance and the result of the Shubnikov-de Hass oscillations in a 200-nm BSTS. The nature of nontrivial Dirac surface states was also confirmed by the weak anti-localization effect.
The recently discovered 3D TI Bi2Se3 and Bi2Te3 are also good TE materials because of their similar characteristics, such as heavy elements and a small band gap. After discovery the novel surface state in TI, what is the effect of the nontrivial topology on the thermoelectric performance becomes an interesting scientific question. Since nanowires exhibit much more surface states than those of bulk and nanoflake, thus we will have a better opportunity to observe the novel thermoelectric properties effect on TI BSTS nanowires.
Chapter 5, we report an observation of an order of magnitude enhancement of the thermoelectric figure of merit (zT=0.36) in topological insulator Bi1.5Sb0.5Te1.7Se1.3 nanowires at 300 K as compared with its bulk specimen (zT=0.028). The enhancement was primarily due to an order of magnitude increase of electrical conductivity of the surface-dominated transport and thermally activated charge carriers in the nanowires. Magnetoresistance analysis revealed the presence of Dirac electrons and indicated the Fermi level near the conduction band edge. This might be the first thermoelectric measurement of samples with a chemical potential in the gap of topological insulator without gate tuning and provides an opportunity to study the contribution of surface states to electric conductivity without concern for the complex effect of band bending.

並列關鍵字

topological insulator ； nanoflakes ； nanowires ； thermoelectric ； semiconductor

參考文獻

[1] C. Kane and J. Moore, Topological insulators. Physics World, 24, 32 (2011).

[2] C. L. Kane and E. J. Mele, Quantum Spin Hall Effect in Graphene. Phys. Rev. Lett. 95, 226801 (2005).

[3] C. L. Kane and E. J. Mele, Z2 Topological Order and the Quantum Spin Hall Effect. Phys. Rev. Lett. 95, 146802 (2005).

[4] B. A. Bernevig, T. L. Hughes, and S.-C. Zhang, Quantum Spin Hall Effect and Topological Phase Transition in HgTe Quantum Wells. Science 314, 1757 (2006).

[5] M. König, S. Wiedmann, C. Brüne, A. Roth, H. Buhmann, L. W. Molenkamp, X.-L. Qi, S.-C. Zhang, Quantum Spin Hall Insulator State in HgTe Quantum Wells. Science 318, 766 (2007).