Charge–Ferroelectric Transition in Ultrathin Na0.5Bi4.5Ti4O15 Flakes Probed via a Dual‐Gated Full van der Waals Transistor

Abstract Ferroelectric field‐effect transistors (FeFETs) have recently attracted enormous attention owing to their applications in nonvolatile memories and low‐power logic electronics. However, the current mainstream thin‐film‐based ferroelectrics lack good compatibility with the emergent 2D van der Waals (vdW) heterostructures. In this work, the synthesis of thin ferroelectric Na 0.5 Bi 4.5 Ti 4 O 15 (NBIT) flakes by a molten‐salt method is reported. With a dry‐transferred NBIT flake serving as the top‐gate dielectric, dual‐gate molybdenum disulfide (MoS 2 ) FeFETs are fabricated in a full vdW stacking structure. Barrier‐free graphene contacts allow the investigation of intrinsic carrier transport of MoS 2 governed by lattice scattering. Thanks to the high dielectric constant of ≈94 in NBIT, a metal to insulator transition with a high electron concentration of 3.0 × 10 13 cm −2 is achieved in MoS 2 under top‐gate modulation. The electron field‐effect mobility as high as 182 cm 2 V −1 s −1 at 88 K is obtained. The as‐fabricated MoS 2 FeFET exhibits clockwise hysteresis transfer curves that originate from charge trapping/release with either top‐gate or back‐gate modulation. Interestingly, hysteresis behavior can be controlled from clockwise to counterclockwise using dual‐gate. A multifunctional device utilizing this unique property of NBIT, which is switchable electrostatically between short‐term memory and nonvolatile ferroelectric memory, is envisaged.