Straintronics with van der Waals Ferroelectrics

铁电性是固体物理中的一个基本现象，铁电器件在许多应用和器件中是不可缺少的。当在货车德瓦尔斯（vdW）同质和异质结构中堆叠二维半导体时，铁电性出现在堆叠平面之间的明确定义的扭转角的界面处。这种界面铁电性是由两层之间的反转对称性的破坏导致层间电荷转移引起的。这种转移反过来又产生了一个强大的自发的平面外电偶极矩，表现在铁电畴图案本身。该项目的首要目标是探测和操纵界面铁电性的半导体VDW结构采用静态电场和动态应变的组合。动态应变以表面声波（SAW）的形式产生，频率高达几千兆赫。在这个项目中，我们将开发混合VDW-SAW器件的模拟和纳米加工技术。利用这些器件，我们主要进行了两个方面的研究：1）首先，在LiNbO_3和LiTaO_3 SAW芯片上制备非铁电（H-堆叠）栅控vdW双层器件。在这些vdW双层中，层内（电子和空穴在同一层中）和层间（电子和空穴在不同层中）激子形成，其可以由提供全局准静态调谐参数的栅极电压优雅地控制。这样的编程激子物种的动态控制叠加SAW光束产生的可编程的声学电位，并读出光学使用时间和位置分辨光谱。2)其次，我们将制作铁电（R-堆叠）门控VDW双层器件上的铌酸锂和钽酸锂SAW芯片。这两种基板允许以瑞利和剪切水平（SH）SAW的形式产生动态法向应力和剪切应力。铁电极化、畴壁和磁滞将由动态法向应力和剪切应力控制，并由光学和电学光谱检测。

Ferroelectricity is a fundamental phenomenon in solid-state physics and ferroelectric devices are indispensable in many applications and devices. When stacking two-dimensional semiconductors in van der Waals (vdW) homo- and heterostructures, ferroelectricity emerges at the interface for well-defined twist angles between the stacked planes. This interfacial ferroelectricity is caused by the breaking of the inversion symmetry between the two layers which leads an interlayer charge transfer. This transfer in turn creates a strong spontaneous out-of-plane electric dipole moment which manifests itself in ferroelectric domain patterns. The overarching goal of this project is to probe and manipulate interfacial ferroelectricity in semiconducting vdW structures employing a combination of static electric fields and dynamic strain. The dynamic strain is generated in the form of surface acoustic waves (SAWs) at frequencies up to a few gigahertz. In this project we will develop the simulation and nanofabrication technology of hybrid vdW-SAW devices. Using these devices, we pursue two main research tracks: 1) First, non-ferroelectric (H-stacked) gated vdW bilayer devices will be fabricated on LiNbO3 and LiTaO3 SAW-chips. In these vdW bilayers, intralayer (electron and hole in the same layer) and interlayer (electron and hole in different layers) excitons form, which can be elegantly controlled by a gate voltage which provides a global, quasi-static tuning parameter. The such programmed excitonic species are dynamically controlled in programmable acoustic potentials generated by superimposed SAW beams and read out optically using time- and position-resolved optical spectroscopy. 2) Second, we will fabricate ferroelectric (R-stacked) gated vdW bilayer devices on LiNbO3 and LiTaO3 SAW chips. These two substrates allow for the generation of dynamic normal and shear stress in the form of Rayleigh and shear horizontal (SH) SAWs. Ferroelectric polarization, domain walls and hysteresis will be controlled by dynamic normal and shear stress and detected by optical and electrical spectroscopy.