Plasma-Engineered Au/TiO x /Ti Memristive Synapses Enabling Forming-Free Operation and Highly Tunable Multifunctional Bio-Inspired Synaptic Plasticity

Abstract

Functional oxide-based memristive synapses are emerging as promising building blocks for next-generation brain-inspired neuromorphic computing systems. However, achieving highly tunable synaptic functionalities with low power consumption in a single oxide-based memristor remains challenging. Here, we present a forming-free Au/TiOx/Ti memristive synaptic device fabricated using an oxygen plasma-based ion implantation strategy. This approach directly transforms a metallic Ti layer into a functional TiOx synaptic layer, enabling simultaneous oxide formation and oxygen-vacancy engineering through a simple fabrication process. Detailed electrical investigations reveal highly tunable synaptic responses controlled by pulse amplitude, width, interval, and number of pulses, enabling long-term potentiation (LTP), long-term depression (LTD), spike-rate-dependent plasticity (SRDP), short-term-to-long-term memory transition, and the Bienenstock–Cooper–Munro (BCM) learning rule. The device operates at a low voltage range of 0.6–1.2 V and demonstrates gradual analog conductance modulation without abrupt current switching, exhibiting stable and reproducible synaptic characteristics associated with controlled oxygen-vacancy dynamics. The BCM synaptic-weight reversal is achieved over a stimulation frequency range of 5–71 Hz, comparable to the frequency range associated with biological synaptic plasticity. The proposed plasma-based oxygen ion implantation strategy provides a simple, cost-effective, and scalable route for realizing forming-free, adaptive, and low-power neuromorphic devices.