A Scanning Microwave Impedance Microscopy Study of α-In2Se3 Ferroelectric Semiconductor

A Scanning Microwave Impedance Microscopy Study of 𝜶-In2Se3 Ferroelectric Semiconductor

Lin Wang, Han Chen, Mingfeng Chen, Yinfeng Long, Kai Liu, Kian Ping Loh

Abstract 

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 α-phase indium selenide (α-In2Se3) and its synaptic field effect transistors. α-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 α-In2Se3-based metal-oxide-semiconductor (MOS) structures reveals typical MOS capacitance-voltage characteristics, with additional hysteresis arising from the ferroelectric switching of α-In2Se3. The local conductance state changes of synaptic α-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.

Summary of the paper

This study employs scanning microwave impedance microscopy (sMIM) to investigate the electronic properties of the van der Waals ferroelectric semiconductor α−In2​Se3​ and its synaptic ferroelectric semiconductor field-effect transistors (FeSFETs).

Key findings include:

For α−In2​Se3​ nanoflakes, sMIM' s capacitive signal (sMIM-C) shows a quasi-inverse correlation with thickness below 100 nm (saturating beyond this) — consistent with finite element simulations, reflecting thickness-dependent carrier depletion and semiconductor properties.

sMIM spectroscopy on α−In2​Se3​-based metal-oxide-semiconductor (MOS) structures reveals classic MOS capacitance-voltage (C-V) behavior, with additional hysteresis attributed to ferroelectric switching of α−In2​Se3​.

In situ sMIM effectively detects local conductance state changes (low resistance state, LRS; high resistance state, HRS) in α−In2​Se3​ synaptic FeSFETs under gate voltage stimuli. The sMIM signals align well with device transport data, quantifying LRS/HRS conductivities and validating sMIM' s utility for non-invasive monitoring of synaptic states.

This work advances understanding of ferroelectric semiconductor physics and establishes sMIM as a powerful tool for optimizing α−In2​Se3​-based neuromorphic devices.

      在本篇文章中，扫描微波阻抗显微镜SMIM具体的应用场景和作用如下：

1. 表征 α-In₂Se₃纳米片的厚度与半导体特性：通过 sMIM 电容信号（sMIM-C）检测不同厚度 α-In₂Se₃纳米片，发现 100nm 以下时信号与厚度呈准反比关系，超过 100nm 后饱和，该结果与有限元模拟一致，反映了纳米片的厚度依赖性载流子耗尽特性及其半导体属性。
   
2. 分析 α-In₂Se₃基 MOS 结构的电容特性：对 α-In₂Se₃基金属 - 氧化物 - 半导体（MOS）结构进行 sMIM 光谱表征，观测到典型的 MOS 电容 - 电压（C-V）特征，且因 α-In₂Se₃的铁电开关效应额外产生滞后现象，同时通过 d (sMIM-C)/dV 信号捕捉到铁电开关对应的偏压峰值，明确半导体特性与铁电开关的协同作用。
3. 原位监测 α-In₂Se₃突触 FeSFET 的电导状态：对 α-In₂Se₃铁电半导体场效应晶体管（FeSFET）进行原位 sMIM 表征，清晰区分低电阻态（LRS）和高电阻态（HRS）的 sMIM-C 信号差异，信号变化与器件输运特性一致；结合模拟推导得 LRS/HRS 电导率分别约为 10 S/m 和 10⁻² S/m，实现对突触器件局部电导状态的非侵入式定量监测。