利用掃描式穿隧顯微技術研究三元碲鉍化物的晶體結構與能帶變化

Abstract

拓撲絕緣體的研究在近十年來受到各研究領域的關注，不僅是因為有別於傳統絕緣體的物理特性，近年來已經有許多在三維拓撲絕緣體的研究，利用角解析光電子能譜或是掃描式穿隧顯微鏡的研究都有指出Bi2Se3和Bi2Te3都擁有相對大的能隙以及狄拉克錐，因此拓撲絕緣體具有極高的潛力可控制自旋偏振電流，然而晶體內部的缺陷會使得材料具有N型或P型的特性，並導致表面態與內部導電態重疊，使得費米能接橫跨的是內部價帶或導電帶而不是表面態，造成在自旋電子元件應用上的困難。 本論文的主題即是研究利用原子代換產生的合金三元碲鉍化物(Bi2Te3-xSex)探討是否能改善拓撲表面態的電性，掃描式穿隧顯微鏡是一種高精密的表面電性感測系統，可以量測材料表面形貌以及電子的局部態密度，利用掃描式穿隧顯微鏡以及掃描式穿隧能譜的技術可以幫助解析三元碲鉍化物的晶體結構與呈現能帶結構在不同碲/硒比例的變化，發現碲與硒之間的替換會導致五層結構中的最外層與中間層有原子雜亂排列的結構，這個雜亂排列的結構不但影響到三元碲鉍化物的能帶分布，也抑制原本在Bi2Se3和Bi2Te3內部缺陷的產生，進而降低內部導電性增進三元碲鉍化物的電性傳輸能力，更進一步的原因與解釋分析都會在此論文中提及。

Topological insulators get wide attention in these 10 years not only because of its non-trivial physics properties, but also high potential of application in spintronic devices. There have been many researches on 3D topological insulators such as Bi2Se3 and Bi2Te3 using angle-resolved photoemission spectroscopy (ARPES) or scanning tunneling microscopy and spectroscopy (STM and STS) indicating most of them host big bulk energy gaps and a spin-helical Dirac cone. Therefore, topological insulators have powerful potential for controlling spin polarized electrons. However, uncontrolled defects in crystal render materials as p-type or n-type or make surface states overlap with bulk band states. That means the Fermi level crosses over valance band or conduction band which is disadvantageous for applying spin-polarized electrons. In order to exploit the ideal Dirac cone and use spin-polarized electrons more efficiently, the Fermi level has to lie at topological surface states without any other spin independent state. Doping metal atoms or substitution could be the solution. In this work, ternary tetradymite Bi2Te3-xSex which are between p-type Bi2Te3 and n-type Bi2Se3 have been investigated by STM. STM is a very powerful surface electron sensitive tool which is suitable for probing surface morphology and local density of states. The techniques of STM and STS were used to investigate the crystal structures and demonstrate the evolution of band structure under different ratio of selenium (Se) and tellurium (Te). The substitution of Se and Te causes disorder in the outer layer or the center layer of the quintuple layer (QL). The random substitution affects the electronic structure and suppress inducing bulk conductance defects that may improve electronic properties very well. And further, the reasons for and result of substitution influences on band structure are also discussed in this work.