Straintronics with van der Waals Ferroelectrics

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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.