Dual-Mode Functionality in Analog-Weight-Tunable TiO_x-Based RRAM: Exploiting Optical and Electrical Stimuli for Synaptic Behaviors

In this study, an RRAM with a crossbar array structure incorporating an oxygen-deficient TiO_ylayer on top of an oxygen-rich TiO_xand Al₂O₃tunnel barrier was developed. The Ti top electrode underwent oxidation reactions with the oxygen-rich TiO_x(x = 1.78) layer, resulting in the formation of an oxygen-deficient TiO_ylayer. The TiO_y/TiO_xstructure performed functions of switching and an oxygen reservoir, enhancing switching stability and inducing analog weight modulation effects, which further reduced the current level. The oxygen ion drift and diffusion effect of the proposed device led to conductance relaxation, which also means short-term memory (STM) properties. It demonstrated stable uniformity and endurance during DC sweep measurements, and the successful implementation of multilevel characteristics was achieved by using the time-dependent weight expression of the proposed device. Reservoir computing functions were executed through pulse measurements, enabling the realization of an MNIST pattern recognition system. Additionally, synaptic functionalities were emulated, successfully demonstrating the potential for neuromorphic computing system implementation. We also demonstrated dual-mode operation by simultaneously utilizing both electrical and optical stimulation, with optical stimulation emulating synaptic function.