[Paper Review] A switchable diode based on room-temperature two-dimensional ferroelectric {\alpha}-In2Se3 thin layers

Siyuan Wan, Yue Li+4 more|arXiv (Cornell University)|Mar 13, 2018

2D Materials and Applications43 references65 citations

TL;DR

This study demonstrates a switchable diode based on atomically thin, room-temperature 2D ferroelectric α-In2Se3 layers (as thin as 5 nm), where external voltage switches the out-of-plane electric polarization, thereby tuning the Schottky barrier at the graphene/α-In2Se3 heterojunction. The device exhibits a switchable diode effect with an on/off ratio of ~10⁴, proving the feasibility of 2D ferroelectric heterostructures for non-volatile nanoelectronic devices.

ABSTRACT

Nanoscaled room-temperature ferroelectricity is ideal for developing advanced non-volatile high-density memories. However, reaching the thin film limit in conventional ferroelectrics is a long-standing challenge due to the possible critical thickness effect. Van der Waals materials, thanks to their stable layered structure, saturate interfacial chemistry and weak interlayer couplings, are promising for exploring ultra-thin two-dimensional (2D) ferroelectrics and device applications. Here, we demonstrate a switchable room-temperature ferroelectric diode built upon a 2D ferroelectric {\alpha}-In2Se3 layer as thin as 5 nm in the form of graphene/{\alpha}-In2Se3 heterojunction. The intrinsic out-of-plane ferroelectricity of the {\alpha}-In2Se3 thin layers is evidenced by the observation of reversible spontaneous electric polarization with a relative low coercive electric field of ~$2 X 10^5 V/cm$ and a typical ferroelectric domain size of around tens ${\mu}m^2$. Owing to the out-of-plane ferroelectricity of the {\alpha}-In2Se3 layer, the Schottky barrier at the graphene/{\alpha}-In2Se3 interface can be effectively tuned by switching the electric polarization with an applied voltage, leading to a pronounced switchable double diode effect with an on/off ratio of ~$10^4$. Our results offer a new way for developing novel nanoelectronic devices based on 2D ferroelectrics.

Motivation & Objective

• To demonstrate intrinsic out-of-plane ferroelectricity in ultrathin α-In2Se3 at room temperature.
• To achieve nanoscale domain engineering in 2D ferroelectrics using electric field switching.
• To realize a functional ferroelectric diode with switchable rectifying behavior in a graphene/α-In2Se3 heterostructure.
• To establish a platform for non-volatile, low-power nanoelectronic devices based on 2D ferroelectrics.

Proposed method

• Mechanical exfoliation of bulk α-In2Se3 to produce few-layer and monolayer flakes on SiO2/Si substrates.
• Optical microscopy and atomic force microscopy (AFM) for thickness and morphology characterization.
• Piezoresponse force microscopy (PFM) to image and switch out-of-plane ferroelectric domains with 180° phase contrast.
• Raman spectroscopy to confirm the α-phase crystal structure and identify ferroelectric signatures at 266 cm⁻¹.
• Fabrication of graphene/α-In2Se3/metal heterostructures via dry transfer and e-beam evaporation for device integration.
• I-V measurements with source meter to characterize rectifying behavior and observe hysteresis, confirming switchable diode function.

Experimental results

Research questions

• RQ1 Can intrinsic out-of-plane ferroelectricity be stabilized in α-In2Se3 at the 2D limit (down to 5 nm) and at room temperature?
• RQ2 Can the ferroelectric polarization in α-In2Se3 be reversibly switched with an external electric field at the nanoscale?
• RQ3 Can the Schottky barrier at a graphene/α-In2Se3 heterojunction be effectively modulated by polarization switching to enable a switchable diode?
• RQ4 What is the on/off ratio and hysteresis behavior of the resulting ferroelectric diode under DC bias?

Key findings

• The α-In2Se3 thin layers exhibit intrinsic out-of-plane ferroelectricity with a coercive field of ~2 × 10⁵ V/cm, enabling reversible polarization switching.
• PFM measurements show 180° phase contrast between antiparallel domains, confirming stable ferroelectric polarization at room temperature.
• Domain engineering was achieved via single-point poling, successfully writing and erasing a box-in-box polarization pattern.
• The graphene/α-In2Se3 heterostructure exhibits a switchable diode effect with a distinct I-V hysteresis and an on/off ratio of ~10⁴ under DC bias.
• The energy band diagram shows that polarization charges at the α-In2Se3 interface modulate the Schottky barrier, enabling rectification control.
• The thinnest observed α-In2Se3 layer was 1.1 nm, confirming the successful exfoliation of monolayer α-In2Se3.