Van der Waals Ferroelectrics—From Materials Characterization to Device Application

Ferroelectric properties, traditionally studied in bulk materials, feature spontaneous electrical polarizations that are controllable via external electric field, thus holding enormous potential for non-volatile electronic devices application with high-density, high-speed, high energy-efficiency. Yet, the promise of conventional ferroelectrics has yet to materialize, primarily due to lattice mismatch and interfacial issues, resulting in deficient compatibility with silicon complementary metal-oxide-semiconductor (CMOS) technology and/or insufficient device reliability. Van der Waals (vdW) ferroelectrics, owing to their unique advantages including atomic thicknesses, dangling-bond-free surfaces, and weak interlayer couplings enabling construction of artificial structures, have recently triggered extensive research interests owing to their potential in addressing the challenges confronting their traditional counterparts, providing an unprecedented platform for implementing ultrathin ferroelectric devices as key hardware components in the post-von Neumann computing era. In this talk, I will introduce the state of the art in vdW ferroelectrics with a focus on their exploration in non-volatile ferroelectronics. A few directions that are worthy of research endeavors to fully unleash their potential will be discussed. Particularly, I will present our latest research on α-In₂Se₃, a representative vdW ferroelectric semiconductor, including the use of advanced electrical mode atomic force microscopy (AFM) techniques for investigating its intriguing physical properties combining ferroelectricity and semiconductivity on the material, MOS structure, and device levels towards non-volatile device application. As a typical example, I will introduce the application of scanning microwave impedance microscopy (SMIM) in visualizing α-In₂Se₃ flakes of varying thicknesses and in probing the change in its resistance state in ferroelectric semiconductor field effect transistors (FeSFETs) subject to gate voltage stimuli. The capacitive SMIM signal dependence on α-In₂Se₃ flake thickness is in consistency with finite element simulation, and the in-situ SMIM characterization results correlate well with electrical transport properties of α-In₂Se₃ FeSFET, manifesting the feasibility of SMIM as a convenient complementary technique in studying vdW ferroelectric materials and devices.