Low-Voltage Control of Synaptic Behavior in WO₃ based Two Terminal Protonic Devices

Artificial neuromorphic devices are capable of processing information by consuming much lower power in comparison with traditional computing technology [1]. Among diverse synaptic structure, conductance switching in metal/insulator/metal type memristive devices are promising candidate owing to their analogous working mode. Most of the conventional memristive devices are filamentary type, where electrical control of migration of ions (Li⁺, Na⁺, K⁺, Ca²⁺ etc.) or oxygen defects result in the change of conductance state. Due to the formation of random filament and relatively low diffusion capability of such ions, these devices suffer from poor reproducibility and relatively high voltage. To circumvent the issue, proton (H⁺) could be a promising candidate owing to its lowest ionic size and high diffusability [2]. Moreover, most of the conventional protonic devices reported are 3-terminal type having an insulating barrier layer, and therefore, their operating voltage is still over 1-Volt range [3-6]. To avoid the issues of power consumption, further reduction of operating voltage should be verified. In this concern, we propose a 2-terminal protonic device in the stacked geometry without any barrier layer. Herein, we have demonstrated the ultra-low voltage switching of conductance state in a 2-terminal protonic device comprising an amorphous WO₃ layer as proton conductor.<br/>Heterostructure of Si (Sub.)/SiO₂ (500nm)/Ti (3nm)/Pt (30nm)/WO₃ (120nm)/Pd (30nm) was fabricated by using e-beam evaporation and pulsed laser deposition. Microstructural measurements confirm the fabricated WO₃ layer is amorphous and atomically flat having roughness ≈1.5nm. Additionally, WO₃ film is highly porous having density 5.76 gm/cm³ which is less than its bulk value 7.16 gm/cm³. In order to define the chemical potential of hydrogen, proton transport measurements were performed under Ar+H₂(5%) mixture gas with a fixed pressure 6.86 kPa. To understand the hydrogen incorporation process in WO₃, conductance was measured as a function of H₂ exposer time with different applied voltages. The result exhibits a gradual increase in conductance up to 3-order of magnitude over time, and the increasing rate is dramatically changed as a function of the applied voltage. At negative voltages, WO₃ still maintains its semiconducting properties while application of positive voltages induces a large change from semiconducting to conducting state. The results clearly indicate the hydrogen incorporation is strongly affected by the applied voltage, where the H-ions from Pd reservoir progressively drifts into WO₃ channel through the electrochemical reactions in the two-terminal device. We also investigated the conductance change from the viewpoint of hydrogen chemical potential rather than the applied voltage, where H₂ pressure was varied while the applied voltage was kept constant at 0.5 volt. The result shows that the device response time is largely delayed by decreasing the H₂ pressure, corroborating the underlying electrochemical reaction. Based on these thorough understandings, the reversible conductance change as like potentiation and depression in biological synapses was achieved by switching the polarity of input voltage from -0.2V to +0.2V. Despite this low operation voltage, we have obtained the conductance modulation of 22% at room temperature. These findings will pave a way to the design of low power consumption protonic neuromorphic devices.<br/>This work was supported in part by JST CREST Grant Number JPMJCR19K2.<br/>[1] D. Liu et al., Adv. Intell. Syst. 3 (2021) 200150.<br/>[2] T. Yajima et al., AIP Adv. 8 (2018) 115133.<br/>[3] M. Onen et al., Nano Lett. 21 (2021) 6111.<br/>[4] X. Yao et al., Nat. Comm. 11 (2020) 3134.<br/>[5] J-T. Yang et al., Adv. Mater. 30 (2018) 1801548.<br/>[6] Y. Van De Burgt et al., Nat. Mater. 16 (2017) 414.