A van der Waals material exhibiting room temperature broken inversion symmetry with ferroelectricity

TL;DR

This work reports a novel $β^{p}$ phase of $In_2Se_3$ in large-area van der Waals films grown by MBE on sapphire, characterized by a zigzag quintuple-layer that breaks inversion symmetry and underpins room-temperature ferroelectricity. Through STEM, SHG, and FET measurements, the authors show that the biased film contains a higher fraction of the noncentrosymmetric $β^{p}$ phase, indicating a bias-driven phase transition accompanied by enhanced second-harmonic generation. The combination of scalable synthesis, structural elucidation of noncentrosymmetric motifs, and electrical tunability positions the $β^{p}$ phase as a promising platform for high-density, low-power ferroelectric devices in 2D vdW materials. Overall, this study expands the class of room-temperature ferroelectric vdW semiconductors and demonstrates a controllable pathway to manipulate ferroelectric states via electric bias in large-area thin films.

Abstract

Since the initial synthesis of van der Waals two-dimensional indium selenide was first documented in 1957, five distinct polymorphs and their corresponding polytypes have been identified. In this study, we report a unique phase of indium selenide via Scanning Transmission Electron Microscopy (STEM) analysis in the synthesized large-area films -- which we have named the $β^\text{p}$ phase. The quintuple layers of the $β^\text{p}$ phase, characterized by a unique zigzag atomic configuration with unequal indium-selenium bond lengths from the middle selenium atom, are distinct from any other previously reported phase of indium selenide. Cross-sectional STEM analysis has revealed that the $β^\text{p}$ layers exhibit intralayer shifting. We found that indium selenide films with $β^\text{p}$ layers display electric-field-induced switchable polarization characteristic of ferroelectric materials, suggesting the breaking of the inversion symmetry. Experimental observations of nonlinear optical phenomena -- Second Harmonic Generation (SHG) responses further support this conclusion. This study reports a $β^\text{p}$ phase of indium selenide showing ferroelectricity over large areas at room temperature in a low-dimensional limit.

Figures (15)

• Figure 1: Characterization of synthesized large area indium selenide thin films. (a) AFM and (b) SEM image. (c) (i) and (ii) illustrate the optical images showing the synthesized large area $\upbeta$-indium selenide thin film and the bare sapphire substrate, respectively, demonstrating the extensive coverage. (d) Raman spectra comparing the synthesized film to various other phases, including $\upalpha$, $\upgamma$, and $\upkappa$ phases. The synthesized indium selenide film exhibits a distinct Raman peak position that is characteristic of the $\upbeta$ phase, differentiating it from other investigated phases. (e) XPS analysis. The Se/In stoichiometry, determined using Scofield RSFs (In/Se RSF ratio = 9.8), is $\sim$1.4. (f) XRD characterization reveals the synthesized indium selenide film primarily consists of the $\upbeta$ phase, as evidenced by corresponding (002), (004), (006), (008), (0010), (0012), and (0014) peaks.
• Figure 2: Characterization of FETs fabricated using the synthesized indium selenide film as the channel material. (a) Schematic of the device structure. (b) Optical image. (c) Energy-dispersive x-ray spectroscopy mapping of the device cross-section. (d) Transfer characteristics illustrate electric field-induced clockwise hysteresis loops. (e) An increase in the hysteresis loop size with an increase in gate bias indicates tunable switching window. (f) Output characteristics. Hysteresis loops at various sweep rates for different device sizes (g) 5 $\upmu$m $\times$ 100 $\upmu$m and (h) 4 $\upmu$m $\times$ 100 $\upmu$m. The loops maintain consistency at different sweep rates from 96 mV/s to 420 mV/s with an increment of 15 mV/s, indicative of polarization-induced hysteresis. (i) SHG response observed at 400 nm wavelength with an incident wavelength of 800 nm. (j) The SHG response increases with the power of the incident laser; The inset intensity-versus-power shows a slope of 2, further confirming the noncentrosymmetry.
• Figure 3: STEM image analysis of the optimized indium selenide films. (a) Top-view STEM-ADF image shows that the films primarily consist of the nominally $\upbeta$ phase, scale bar 1 nm. (b) Cross-sectional STEM-ADF image reveals that the film is composed of layers of the $\upbeta$ phase with a 3R polytype configuration. Surprisingly, it also shows unique zigzag atomic layers of indium selenide (designated as the $\upbeta^\text{p}$ phase), scale bar 2 nm. Distinct van der Waals bonds are observed between the phase layers. (c) HAADF-STEM images of the $\upbeta$ phase and the zigzag $\upbeta^\text{p}$ phase, scale bar 2 nm. Sum of intensity for each atomic column in the (d) $\upbeta$ layers, (e) zigzag $\upbeta^\text{p}$ layers. Interatomic column distances in the (f) $\upbeta$ phase, (g) zigzag $\upbeta^\text{p}$ phase. The red and blue circles represent the centers of In and Se atoms, respectively. In the $\upbeta$ phase, the In–Se–In interatomic distance is 0.228 nm (yellow lines), and the Se–In distance is 0.171 nm (green line). In $\upbeta^\text{p}$ phase, the left Se–In interatomic distance is 0.151 nm, and the distance between the two right Se–In atomic columns is 0.132 nm.
• Figure 4: Cross-sectional STEM images of (a) a pristine film with $\upbeta$ phase (blue shaded region) and the $\upbeta^\text{p}$ phase (cyan shaded region); (b) a biased film, demonstrating an increased presence of zigzag $\upbeta^\text{p}$ phases. (c) False-colored STEM image of $\upbeta^\text{p}$ quintuple layer. The $\upbeta^\text{p}$ phase exhibits sliding of the In atoms within the quintuple layer, as highlighted by the red arrow. The centers of the purple and green circles represent In and Se atoms, respectively. This displacement contributes to breaking the local structural mirror symmetry and is therefore the origin of the ferroelectricity. (d) The transition region between the $\upbeta$ and $\upbeta^\text{p}$ phases, scale bar 2 nm. (e) Raman spectra show a slight shift in the Raman peak after bias. (f) Comparison of SHG response in pristine and biased films. The increased proportion of the noncentrosymmetric $\upbeta^\text{p}$ phase under bias results in approximately 1.5-fold enhancement of the SHG signal.
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