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Unveiling the Potential of HfO2/WS2 Bilayer Films: Robust Analog Switching and Synaptic Emulation for Advanced Memory and Neuromorphic Computing

  • Muhammad Ismail
    Muhammad Ismail
    Division of Electronics and Electrical Engineering, Dongguk University, Seoul 04620, Republic of Korea
  • Maria Rasheed
    Maria Rasheed
    Department of Advanced Battery Convergence Engineering, Seoul 04620, Republic of Korea
  • Sunghun Kim
    Sunghun Kim
    Division of Electronics and Electrical Engineering, Dongguk University, Seoul 04620, Republic of Korea
    More by Sunghun Kim
  • Chandreswar Mahata
    Chandreswar Mahata
    Division of Electronics and Electrical Engineering, Dongguk University, Seoul 04620, Republic of Korea
  • Myounggon Kang*
    Myounggon Kang
    Department of Electronics Engineering, Korea National University of Transportation, Chungju-si 27469, Republic of Korea
    *[email protected] (M. Kang).
  • , and 
  • Sungjun Kim*
    Sungjun Kim
    Division of Electronics and Electrical Engineering, Dongguk University, Seoul 04620, Republic of Korea
    *[email protected] (S. Kim).
    More by Sungjun Kim
Cite this: ACS Materials Lett. 2023, 5, 11, 3080–3092
Publication Date (Web):October 23, 2023
https://doi.org/10.1021/acsmaterialslett.3c00600
Copyright © 2023 American Chemical Society

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    Abstract

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    Nonvolatile memories using two-dimensional materials and high-k oxides have gained attention for their potential to achieve robust analog switching, easy memristive device integration, and low-energy consumption. In this study, we fabricated Pt/TiN/HfO2/WS2/Pt memristive devices. To implement these devices, a WS2 film was thermally evaporated under high vacuum conditions followed by HfO2 growth using atomic layer deposition at 400 °C. Detailed analysis using high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy revealed diffusion of W and S atoms within the HfO2 layer and extraction of oxygen by W atoms, thus resulting in a multilayer structure (HfWOySx, Wx–1OySx, and W1–xOySx) with varying ratios of oxygen, tungsten, and sulfur atoms (x and y). The fabricated devices demonstrated consistent and stable analogue switching over numerous cycles, with exceptional endurance (2000 cycles) and retention (103 s). They exhibited high cycle-to-cycle consistency, as evidenced by the low-coefficient of variation (3.5% and 4.0% for the set and reset voltages, respectively). By modulating the reset stop voltage, we achieved five-level resistance states, thus making these devices capable of being used in artificial synapses. Furthermore, we observed analog switching with gradual resistance changes under different current compliance conditions by incrementally adjusting the reset–stop voltage. The memristor-based artificial synapses exhibited fundamental synaptic functions, such as long-term potentiation, long-term depression, paired-pulse facilitation, paired-pulse depression, and spike-timing-dependent plasticity for long-term and short-term plasticity. Moreover, we employed a three-layer artificial neural network for image recognition, achieving 94% accuracy using identical pulse amplitudes. These findings highlight the potential of HfO2/WS2 bilayer films, enable controllable analogue switching, and simulate synaptic functions. They hold promise for future data storage memory and neuromorphic computing systems.

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmaterialslett.3c00600.

    • Comparative performance of memristive devices; I–V characteristics, and statistical distribution of electroforming voltages; Analog and digital I–V curves under different ICC’s; D2D variability of memristor; HRS and LRS of multiple memristors; P/D with nonidentical and identical pulses; Schematic of pulse scheme for amplitude-dependent synaptic behavior; Schematic diagram of STDP measurement (PDF)

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