拓撲絕緣體Bi2-xSbxTe1.7Se1.3 奈米薄片與奈米線之物性研究

Abstract

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.