衬底和应变对Bi2Se3类拓扑绝缘体薄膜的费米能级和Dirac锥的调控研究

Bi2Se3-type materials are strong three dimensional topological insulators (TIs) with one single Dirac cone in the surface states. For the application of the extraordinary properties of the topological surface states, it is necessary that the Fermi level should reside in the gap and cross or approach the Dirac point. However, it is observed experimentally that the Fermi level of the high quality films of Bi2Se3 and Bi2Te3 are still appreciably higher than the Dirac point. In particular, the Dirac point of Bi2Te3 is deeply buried into the valence bands. These unfavorable properties hinder the study and application of TIs. In order to solve these problems, we will use first-principles methods in this project to study the effect of the substrate on the electronic properties of TI films, especially the Fermi level of TIs. Based on these studies, we will design proper substrates which can move the Fermi level to the Dirac point. We will further study the strain effect on the dispersion of the electronic bands near the valence band maximum (VBM). By application of different strains, we will find a proper scheme to modulate the shape of the VBM at gamma point so that the Dirac point is moved up into the gap and above the VBM. This method is expected to be able to solve the similar problems in other TIs. Our project will provide a good basis for design and tuning of the TIs and suggest new schemes to realize the application of the topogical states in spintronic devices.

Bi2Se3类材料是表面拥有单个狄拉克锥的三维强拓扑绝缘体。为利用其拓扑表面态的奇异特性，费米能级必须位于带隙中，且穿过或接近表面态的狄拉克点。实验上制备的高品质拓扑绝缘体Bi2Se3和Bi2Te3薄膜的费米能级都明显高于狄拉克点，且Bi2Te3薄膜的狄拉克点深埋于价带，这些严重影响该类拓扑绝缘体的研究和应用。在本项目中，我们将针对这些问题，用第一性原理方法研究衬底对Bi2Se3类薄膜电子态的影响，特别是对它们费米能级的影响规律，并根据这些规律设计合适的衬底将费米能级调控到狄拉克点。进一步研究应变调制Bi2Te3薄膜狄拉克点附近的能带形状，使狄拉克点从价带移到带隙中，这既可解决Bi2Te3材料的狄拉克点与价带交叠的问题，还可为调控其它有同类问题的拓扑绝缘体的狄拉克锥提供思路。研究结果将为设计和调控拓扑绝缘体材料提供基础，为实现本征拓扑表面态输运和其在自旋电子学器件中的应用提供新的方案。

Bi2Se类材料是表面拥有单个狄拉克锥的三维强拓扑绝缘体。为了利用其表面态的奇异特性，费米能级必须位于带隙中，且穿过或接近表面态的狄拉克点。实验上制备的高品质拓扑绝缘体Bi2Se3和Bi2Te3薄膜的费米能级都明显高于狄拉克点，且Bi2Te3薄膜的狄拉克点深埋于价带，这些严重影响该类拓扑绝缘体的研究和应用。在本项目中，我们针对这些问题，用第一性原理方法研究了衬底对Bi2Se3类薄膜电子态的影响，特别是对它们费米能级的影响规律，并根据这些规律设计合适的衬底把费米能级调控至狄拉克点附近。进一步研究了应变调制Bi2Te3材料的狄拉克点与价带交叠的问题。由于衬底较小的功函数，其在Bi2Se3/graphene,Bi2Te3/Si(111)复合体系中起着贡献电子的角色，从而使拓扑绝缘体薄膜的费米能级上升。较厚的拓扑绝缘体薄膜在费米能级附件态密度较大，从衬底转移相同数量的电子，导致费米能级上移的幅度较小，与实验观测一致。所以，我们建议通过选择高功函数的衬底生长拓扑绝缘体薄膜来平衡由于生长过程中缺陷导致的拓扑绝缘体薄膜费米能级的上移。我们的研究发现F-graphene衬底和F-Si（111）衬底的功函数大于Bi2Se3类拓扑绝缘体薄膜的功函数，可以使Bi2Se3类拓扑绝缘体薄膜的费米能级调控至狄拉克点或附近。