Enhanced Long-Term Synaptic Plasticity of Self-Rectifying Artificial Synapse Device with Pt/Ga₂O₃/NbO_x/ZnO/Pt Structure

Neuromorphic computing, which mimics neuron-synapse structure of human brain, has been explored extensively to overcome the limitation of Von-Neumann system. Neuromorphic systems require synapse devices that enable analog or multi-state weight update and stable long-term plasticity for energy-efficient training and inference operations. For the synapse device, memristor device which shows analog or multi-state resistance change has been investigated to mimic the biological synaptic weight update. Although the weight update behaviors have been demonstrated in a variety of memristors, it is still challenging to achieve long-term plasticity with good retention of weight states. In addition, self-rectifying characteristics of memristor synapse devices are highly beneficial to construct high-density crossbar array of synapse device because sneak path current problem can be effectively alleviated even without an additional selector device.^[1]

In this study, high self-rectification ratio and long-term synaptic plasticity with good retention was demonstrated with Pt/Ga₂O₃/NbO_x/ZnO/Pt device. In this device, ZnO, a well-known n-type semiconductor, was employed to achieve large analog conductance change with respect to the redistribution of oxygen vacancies. Ga₂O₃ had a role to present self-rectifying characteristics owing to its ultrawide-bandgap of about 4.9eV.^[2] NbO_x was used as a good oxygen ion reservoir due to its high oxygen binding energy. Ga₂O₃ and ZnO were deposited by RF magnetron sputtering in Ar condition, with the thickness of 10nm and 15nm, respectively. NbO_x was deposited by reactive sputtering using Nb target in Ar and O₂ condition, with the thickness of 18nm. Both top and bottom electrodes were deposited by E-beam evaporator. The device had circular shape with diameter of 100mm.

The current-voltage (I-V) sweep measurement showed that the conductance increased when the positive bias was applied and decreased upon applying negative bias. The current at +3V was about 10⁴ times larger than the current level at -3V, exhibiting high self-rectifying characteristic of the device. In addition, pulse measurement showed analog conductance change with dynamic range of about 10 by applying 30 times of +3V pulses with a width of 0.64ms. Analog conductance modulation originated from the oxygen ion migration between ZnO and NbO_x. The increased conductance retained stably to have a long-term synaptic plasticity, remaining 10 times higher current level compared to the initial state after 30 min. This good retention properties came from the captured oxygen ions within NbO_x layer that has strong binding energy to oxygen. The performance of the device was evaluated with pattern recognition accuracy using CrossSim:Training.

The comparative study with two reference devices, i.e., Pt/NbO_x/ZnO/Pt and Pt/Ga₂O₃/ZnO/Pt, revealed that Pt/NbO_x/ZnO/Pt device had about 1000 times smaller self-rectification ratio, indicating that Ga₂O₃ layer was responsible for the self-rectifying property. Also, from much poorer retention of Pt/Ga₂O₃/ZnO/Pt with current which decayed to the initial current level in short time, it was confirmed that NbO_x layer was responsible for the long-term retention property as oxygen ion reservoir.

References
[1] Shi, L., Zheng, G., Tian, B., Dkhil, B., & Duan, C. (2020). Research progress on solutions to the sneak path issue in memristor crossbar arrays. Nanoscale Advances, 2(5), 1811-1827.
[2] Saikumar, A. K., Nehate, S. D., & Sundaram, K. B. (2019). RF sputtered films of Ga₂O₃. ECS journal of solid state science and technology, 8(7), Q3064.