Construction and manipulation of topological insulators from III-V heterostructures

拓扑绝缘体(TIS)是一种新的物质状态，其特征是具有绝缘体但无间隙和反向传播的表面或边缘状态。在大量的TI材料中，基于III-V材料的复合量子阱(QWS)异质结构，即InAs/GaSb，结合了III-V半导体先进的外延和器件制造程序，提供了前所未有的器件功能。这些复合量子阱异质结构由于其丰富的相图，可通过能带和对称性工程以及外加电场来访问，从而使它们的性质能够以独特的灵活性进行修改，因此对二维(2D)TI特别有吸引力。然而，尽管有许多研究，但对于这种材料体系中的拓扑绝缘相螺旋边缘状态的完全令人信服的证明仍然是难以捉摸的。因此，该项目提出首先克服在这些材料中观察到所谓的量子自旋霍尔效应(QSHE)的主要障碍，例如这些不对称结构中固有的小能隙。此外，自从2D TIS被发现以来，在不同的凝聚态系统中也发现了大量的三维(3D)TIS和半金属(SM)态。然而，观察到这些拓扑态的材料不允许其能带结构或拓扑性质的任何连续可调。因此，无论是为了更好地探测其物理性质，还是为了考虑实际应用，具有许多可调参数的柔性平台仍然是非常需要的。通过控制InAs/GaSb超晶格中的晶体对称性和多个能带反转，可以产生所有已知的拓扑量子态，如3D TIS、Dirac和Weyl Sms，但也可以产生具有前所未有丰富相图的所谓高阶TIS(HOTIs)。与“低阶”情况不同，HOTI的边态至少比系统的边态小两维。因此，该项目的目的一方面是建立一个灵活的基于QWS和III-V超晶格的2D和3D拓扑态的研究平台，另一方面在优化的生长结构和工艺过程中证明和控制QSHE。第一个目标是实现技术突破：控制双门高性能场效应器件，允许通过外部电压而不是结构变化来观察从微不足道到拓扑的相变。第二个目标是观察三层量子波结构中边缘态的量子化传导。第三个目标是验证3D InAs/Ga(In)Sb超晶格中存在的各种拓扑态，如3D TIS、Dirac和Weyl SM以及3D HOTI。

Topological insulators (TIs) are new state of matter, characterized by an insulating bulk but gapless and counter-propagating surface or edge states. Among the vast amount of TI materials, composite quantum wells (QWs) heterostructures based on III-V materials, i.e., InAs/GaSb, provide unprecedented device functionality in combination with the advanced epitaxial and device fabrication routines of III-V semiconductors. These composite QW heterostructures are particularly attractive two-dimensional (2D) TIs thanks to their rich phase diagram accessible by band and symmetry engineering and by the application of external electric fields, allowing the modification of their properties with unique flexibility. However, despite many investigations, a fully convincing demonstration of helical edge states in the topological insulating phase in this material system is still elusive. This project therefore proposes first to overcome the main obstacles to the observation of the so-called quantum spin Hall effect (QSHE) in these materials, such as the small energy gap inherent in these asymmetric structures. Moreover, since the discovery of 2D TIs, a large variety of three-dimensional (3D) TIs and semi metallic (SM) states have also been identified in different condensed matter systems. However, the materials in which these topological states have been observed do not allow any continuous tunability of their band structure or topological properties. Therefore, a flexible platform with many adjustable parameters is still very much in demand, both to better probe their physical properties and to consider practical applications. By controlling crystal symmetries and multiple band inversions in InAs/GaSb-based superlattices, it becomes possible to create all known topological quantum states such as 3D TIs, Dirac and Weyl SMs, but also so-called higher order TIs (HOTIs) with a phase diagram of unprecedented richness. Unlike the "lower order" case, edge states of HOTIs are at least two dimensions smaller than that of the system. The project therefore aims on the one hand to create a flexible platform for the study of 2D and 3D topological states based on QWs and III-V superlattices and on the other hand to evidence and control the QSHE in structures of optimized growth and technological processes. The first objective is the realization of a technological breakthrough: the control of dual gated high-performance field effect devices, allowing the observation of the trivial-to-topological phase transition by external voltage rather than structural change. The second objective is to observe quantized conduction of edge states in three-layer QW structures. The third objective is to validate various topological states existing in 3D InAs/Ga(In)Sb superlattices, such as 3D TIs, Dirac and Weyl SMs, and 3D HOTIs.