Memristors are promising candidates for artificial synapses in neuromorphic computing. Current state-of-the-art memristors primarily rely on conductive filament formation (e.g., valence change mechanism (VCM) and electrochemical metallization (ECM)) within insulating layers. However, the stochastic nature of filament formation leads to poor switching uniformity. While attempts have been made to improve this using single-crystalline materials like SiGe, high growth temperatures hinder CMOS integration. Two-dimensional (2D) materials offer a low-temperature alternative, with several vertical memristors based on 2D materials demonstrated. These often rely on native defects or metal filament formation, achieving some synaptic plasticity emulation. However, their vertical structure limits multi-terminal applications. Lateral memristors, such as those based on MoS2 and utilizing SBH modulation via sulfur vacancy motion, show promise for reducing variability. ReS2, with its weak interlayer coupling, soft Re-S bonds, and low sulfur vacancy formation energy, is a potential candidate for improved memristor performance. This research aims to demonstrate a ReS2-based lateral memristor using electron-beam irradiation (EBI) to introduce sulfur vacancies, investigating its resistive switching characteristics and exploring its potential as an artificial synapse for neuromorphic computing.

The authors review existing memristor technologies, highlighting the limitations of filamentary switching mechanisms based on VCM and ECM. They discuss the stochastic nature of conductive filament formation, resulting in poor temporal and spatial switching uniformity. Previous work on epitaxial random access memory (epiRAM) based on SiGe is mentioned, noting its improved voltage variation but persistent temporal inconsistencies. The advantages of 2D materials for back-end-of-line (BEOL) compatibility and low-temperature growth are emphasized. The authors review existing vertical memristors based on 2D materials, discussing their switching mechanisms and limitations. The advantages of lateral memristors for multi-terminal applications are highlighted, particularly focusing on prior research with MoS2-based devices that demonstrated resistive switching through SBH modulation via sulfur vacancy motion. The superior properties of ReS2, including its low sulfur vacancy formation energy, are presented as a motivation for this study.

The study fabricated a two-terminal lateral ReS2-based memristor. Few-layer ReS2 flakes were exfoliated onto a silicon wafer with silicon oxide. Electron beam irradiation (EBI) was used to introduce sulfur vacancies into the ReS2. After EBI, Ti/Au electrodes were patterned using electron beam lithography (EBL) and deposited via electron beam evaporation, followed by a lift-off process. Finally, a PMMA passivation layer was applied. The devices were characterized using a semiconductor parameter analyzer for current-voltage (I-V) measurements under various DC sweep conditions. Pulse measurements were performed using a different semiconductor analyzer. Low-temperature measurements were conducted in a cryogenic probe station. Material characterization included atomic force microscopy (AFM), Raman spectroscopy, photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) to analyze the impact of EBI on ReS2's structure and composition. Selective EBI on control samples was also performed to investigate the role of sulfur vacancies in the resistive switching.

Transmission electron microscopy (TEM) confirmed the creation of lattice disorders in the ReS2, consistent with the introduction of sulfur vacancies. Atomic force microscopy (AFM), Raman spectroscopy, and photoluminescence (PL) spectroscopy further supported the formation of defects, with their density increasing with higher EBI dosage. X-ray photoelectron spectroscopy (XPS) analysis confirmed a reduction in the S/Re atomic ratio, directly indicating the formation of sulfur vacancies. The fabricated ReS2 memristors exhibited forming-free, gradual resistive switching. The resistive switching mechanism is attributed to the voltage-bias-induced motion of sulfur vacancies, modulating the Schottky barrier height (SBH) at the metal/ReS2 contacts. The devices showed a small cycle-to-cycle variation of transition voltages (6.3% for B-F transition and 5.3% for F-B transition), significantly lower than typical filamentary memristors. The devices successfully emulated various aspects of biological synaptic plasticity, including long-term potentiation (LTP), long-term depression (LTD), paired-pulse facilitation (PPF), paired-pulse depression (PPD), spike-amplitude-dependent plasticity (SADP), and spike-timing-dependent plasticity (STDP). The resistive switching ratio increased with the voltage sweep range. Optimal EBI dosage and ReS2 thickness were determined to maximize the resistive switching ratio. The response time of the device was in the microsecond range, comparable to biological synapses.

The findings demonstrate a novel ReS2-based memristor with significantly improved switching uniformity compared to existing filamentary memristors. The low transition voltage variation is crucial for reliable neuromorphic computing applications. The successful emulation of a wide range of synaptic plasticity behaviors further highlights the device's potential as an artificial synapse. The identified mechanism of resistive switching through SBH modulation via sulfur vacancy movement provides a new pathway for developing highly uniform and reliable memristors. This work demonstrates a significant advance in the field of neuromorphic computing, offering a pathway towards energy-efficient and high-performance artificial neural networks.

This research successfully demonstrated electron-beam-irradiated ReS2-based memristors with exceptionally low variability in resistive switching characteristics. The mechanism is attributed to voltage-induced sulfur vacancy motion modulating the Schottky barrier height. The device successfully emulated diverse synaptic plasticity phenomena. Future work could focus on large-scale integration of these devices into neuromorphic computing systems, exploring further optimization of the EBI process and exploring other 2D materials with similar properties.

The study primarily used mechanically exfoliated ReS2 flakes, limiting the control over flake thickness and size, which could affect the variability of device performance. The long-term stability of the devices over extended operational periods requires further investigation. The spatial (device-to-device) variation was not comprehensively investigated due to the challenges in controlling the properties of mechanically exfoliated flakes.