Real-Time 3D Imaging of Nanoscale Ferroelectric Domain Wall Dynamics in Lithium Niobate Single Crystals under Electric Stimuli: Implications for Domain-Wall-Based Nanoelectronic Devices

Abstract

Challenges in the fields of renewable energy harvesting, data storage, and nanoelectronics have resparked interest in ferroelectric domain walls (DWs) as tunable, nanoscale elements. However, the study of such structures has mostly relied on 2-dimensional, rather slow imaging techniques such as scanning probe microscopy. Therefore, Cherenkov second harmonic generation (CSHG) microscopy has been established as a technique suitable for the nondestructive imaging of ferroelectric DWs and their 3-dimensional (3D) evolution. Here, we report on the real-time and in situ 3D DW kinetics when inspecting electrical-field-biased 200 μm thick lithium niobate (LNO) single crystals. A linear electric field increase up to +4.0 kV/mm (antiparallel to the direction of spontaneous polarization PS) resulted in the collapse of laser-poled hexagonal domains into cone-like structures. The average inclination was measured to rise up to 2.5°. Head-to-head (h2h) domain walls dominated. Simultaneously, the domain wall current (DWC) was recorded in situ. It increased by 4 orders of magnitude to 1 μA. The DW mobility increased dramatically as a function of depth. Moreover, a significant asymmetry was found, as DW mobility was much higher along crystallographic (Y+) directions. Subsequently, the electric field was reversed and swept to −3.6 kV/mm. While the hexagonal domain shapes were restored for moderate electric fields, the domains separated into many nucleating spike domains when exceeding a critical threshold of −3.5 kV/mm. DWC increased dramatically through this process, reaching magnitudes of up to 1 mA. The understanding of DW dynamics upon electric stimulation was used to realize a two-port DW-based nanoswitch. Alternating positive and negative electric fields were applied to a neutral hexagonal domain contacted purely via solid electrodes. The field strengths were kept well below the critical threshold of spike domain formation. The electrical conductivity of such a device could be tuned over 4 orders of magnitude, i.e., deliberately switched on and off. Our findings support the development of future DW-based nanoelectronic devices.