I. INTRODUCTION

Two-dimensional (2D) atomic-layered tin selenide (SnSe) has garnered significant attention in nanoelectronics and optoelectronics due to its exceptional properties, including inplane ferroelectricity, high thermoelectric coefficients, and nonlinear optical characteristics [1]. Recent advancements in low-pressure physical vapor deposition have enabled the synthesis of high-quality 2D SnSe crystals with controllable thicknesses [1]. However, conventional material characterization methods such as optical microscopy and atomic force microscopy (AFM) often fail to provide detailed information about the electric properties of 2D SnSe at the nanoscale.

In comparison, SMM has emerged as a versatile tool for both structural and electrical characterization of semiconductors, dielectrics, and biological cells [2]- [9]. This study aims to examine the feasibility of 2D SnSe characterization by SMM, especially in identifying lateral polarization domains and mapping their resistivity at the nanoscale. Once the feasibility is established, SMM can be used to advance the development of SnSe for various nanoelectronics and optoelectronic applications.

II. METHOD

A. SnSe Synthesis

2D SnSe flakes are synthesized using low-pressure physical vapor deposition [1]. The synthesis begins with placing SnSe powder precursor in a crucible at the center of a single-zone 979-8-3503-6838-3/25/$3 l .00 ©2025 IEEE tube furnace. A freshly exfoliated mica substrate, annealed at 400 °C in air for 10 min, is positioned 10 cm downstream from the precursor. The furnace is then evacuated to 10 mTorr and heated to 420 °C for a deposition of approximately 40 min. After deposition, the furnace is quickly cooled to room temperature [1]. Finally, thin SnSe flakes are transferred using polymethyl methacrylate from mica to a Si substrate precoated with a 300-nm-thick SiO2 layer. The synthesized SnSe flakes are up to 23-µm wide with a thickness from 2 to 15 nm (Fig. 1 ).

B. SMM Setup and Analysis

Fig. 2. illustrates the SMM setup. It is based on a Keysight Technologies 7500 AFM equipped with an N9545C SMM nose cone [2]. A Rocky Mountain 25Pt300A platinum probe, featuring a spring constant of 18 N/m and a tip radius of 25 nm, serves as the dual-function AFM/SMM probe. This probe is affixed to a platinum cantilever 300-µm long, 60-µm wide, and 2-µm thick. The cantilever is connected to a Keysight Technologies E8062B vector network analyzer (VNA) via coaxial cables. For impedance matching around 3 GHz and its harmonics, the cables are shunted with a 50 Q resistor [2]. The VNA generates a 0-dBm signal at 15.7 GHz, which incidents from the probe to the sample. This power level is selected to provide an adequate signal-to-noise ratio (SNR) while minimizing sample destruction. The 15.7-GHz frequency is chosen to maximize image contrast, because the higher the frequency, the higher the sensitivity to small capacitances [ 6]. Frequencies higher than 15.7 GHz cannot be used because the setup has a bandwidth of only 18 GHz. The VNA detects the signal reflected from the sample following its near-field interaction with the probe and records the reflection coefficient Sil. The intermediate frequency bandwidth of the VNA is set to 500 Hz, balancing SNR and scan rate. Under these conditions, a 15 µm x 15 µm area can be scanned at a resolution of256 x 256 pixels in approximately 4 min, corresponding to a scan rate of 200 pixel/s. During the same scan, SMM and AFM signals are simultaneously detected and recorded.

C. SMM Data Analysis

The measured S11 is converted to the probe-sample interaction impedance Z by one-port calibration [10]. The calibration is based on measuring S11 as the probe approaches the conducting substrate between the SnSe flakes, while measuring the probe-sample capacitance C using the AFM in the mode of electrostatic force microscopy [2]. By acquiring both Sll(z) and C(z) as functions of the probe-sample distance Detector Array Coaxial Cables ... Vector Network Analyzer Fig. 2. Schematics of the AFM-based SMM.

z, the error-correction coefficients eoo, e01 , and e11 are extracted for converting S11 to Z. Additionally, the probe geometry, such as the tip radius, the cone angle, and the cone height are extracted by fitting the C(z) curves. This in situ calibration enables quantitative and non-destructive mapping of the local conductance and capacitance of the 2D SnSe flakes with nanoscale resolution.

After S11 is converted to Z, it is used to extract the resistivity ofSnSe flakes using a simple equivalent circuit model validated by numerical simulations [2]. To this end, three-dimensional finite-element electromagnetic simulations are performed using the COMSOL AC/DC module. The equivalent circuit model, tailored to the present sample structure, consists of three elements: C1, representing all dielectric layers above the conductive doped Si substrate with their respective dielectric constants and thicknesses; C2, accounting for the fringing capacitance between the probe body and the sample; and Go = 1/RsHEET, where RsHEET is the sheet resistance of the SnSe flakes.

Therefore, the probe-sample interaction admittance Y can be expressed as

(1)

where w is the angular frequency of the incident signal. Note that Y jwC2 when the SMM is scanning over a bare area of the substrate not covered by any SnSe. Therefore, the admittance contrast between a SnSe flake and a bare area of the substrate is

Thus, the conductance contrast ti G and capacitance contrast !:!..Care

Ill. RESULTS AND DISCUSSION

The 2D SnSe flakes are characterized by optical microscopy, AFM, and SMM. Fig. 3 shows that the optical microscopy sorts the flake thickness by its color, with green for flakes thinner than 10 nm and yellow for flakes thicker than 10 nm. AFM provides detailed flake topography. However, it fails to resolve the polarization domains. Similarly, optical microscopy can barely resolve the domains, not to mention their electrical properties.