Straintronics with van der Waals Ferroelectrics

铁电性是固体物理中的一种基本现象，铁电器件在许多应用和器件中都是不可或缺的。当在范德华(VDW)同质和异质结构中堆积二维半导体时，在界面处出现铁电性，使堆积面之间有明确的扭转角。这种界面铁电性是由于两层之间的反转对称性的破坏而导致的层间电荷转移。这种转移反过来又产生了一个强烈的自发离面电偶极矩，它表现为铁电畴图案。该项目的主要目标是利用静电场和动态应变的组合来探测和操纵半导体VDW结构中的界面铁电性。动态应变是以频率高达几千兆赫的表面声波(SAW)的形式产生的。在这个项目中，我们将开发混合VDW-SAW器件的模拟和纳米制造技术。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芯片上制作非铁电(H-叠层)栅控VDW双层器件。在这些VDW双层中，层内(同一层中的电子和空穴)和层间(不同层中的电子和空穴)形成激子，可以通过提供全局准静态调谐参数的栅极电压来精确地控制激子。这种程序化的激子物种被动态地控制在由叠加的声表面波束产生的可编程声势中，并使用时间和位置分辨光学光谱来光学读出。2)其次，我们将在LiNbO_3和LiTaO_3 SAW芯片上制作铁电(R叠层)栅控VDW双层器件。这两种衬底允许以瑞利和剪切水平(SH)锯的形式产生动态法向应力和剪应力。铁电极化、磁畴壁和磁滞将由动态法向应力和动切应力控制，并通过光电光谱进行检测。

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.