Anomalous ionic conduction in ferroelectric semiconductor junctions comprising multistate CuInP₂S₆

Van der Waals ferroelectric CuInP₂S₆ (CIPS) has emerged as a compelling candidate for multifunctional electronic devices, attributed to its intrinsic multiple polarization states and the coupling of these states with highly active Cu⁺ ions migration. Such a “ferroionic” feature of CIPS opens a new paradigm for the next-generation multifunctional ferroelectric devices. Here, we present a comprehensive investigation of n⁺⁺-Si/CIPS/MoS₂ ferroelectric semiconductor junction (FSJ) devices, focusing on the interplay between ferroelectric polarization and ionic migration. By employing high-resolution piezoresponse force microscopy and conductive atomic force microscopy imaging and spectroscopy methods, we revealed the distinctive dual and quadruple polarization states of the FSJ, which give rise to distinct memristive and rectifying electronic behaviors, respectively. The dual-polarization FSJ exhibits voltage- and frequency-dependent current-voltage hysteresis, while the quadruple-polarization FSJ achieves a rectification ratio exceeding 10⁴, which can be attributed to Cu⁺ ion migration correlated with ferroelectric polarization modulation. This work establishes an effective strategy for leveraging ferroelectric-ionic coupling to achieve multifunctional device performance, paving the way for advanced electronic systems through interfacial engineering in future non-volatile memories and neuromorphic computing applications.