A Scanning Microwave Impedance Microscopy Study of -In₂Se₃ Ferroelectric Semiconductor

Van der Waals ferroelectric semiconductors, which encompass both ferroelectricity and semiconductivity, have garnered intensive research interests for developing novel non-volatile functional devices. Previous studies focus on ferroelectricity characterization and device demonstration, with little attention paid to the fundamental electronic properties of these materials and their functional structures, which are essential for both device design and optimization. In this study, scanning microwave impedance microscopy (sMIM) is utilized to investigate the ferroelectric semiconductor of alpha-phase indium selenide (alpha-In2Se3) and its synaptic field effect transistors. alpha-In2Se3 nanoflakes of varying thicknesses are visualized through capacitive signal detection, whose responses are consistent with finite element simulations manifesting dependence on both flake thickness and its semiconductor property. sMIM spectroscopy performed on alpha-In2Se3-based metal-oxide-semiconductor (MOS) structures reveals typical MOS capacitance-voltage characteristics, with additional hysteresis arising from the ferroelectric switching of alpha-In2Se3. The local conductance state changes of synaptic alpha-In2Se3 ferroelectric semiconductor transistors (FeSFET) in response to gate voltage stimuli are effectively detected by in situ sMIM, in good agreement with electrical device transport properties. This work deepens the understanding of ferroelectric semiconductor physics toward their practical device application.