Demonstration of synaptic and resistive switching characteristics in W/TiO₂/HfO₂/TaN memristor crossbar array for bioinspired neuromorphic computing

In this study, resistive random-access memory (RRAM)-based crossbar arrays with a memristor W/TiO₂/HfO₂/TaN structure were fabricated through atomic layer deposition (ALD) to investigate synaptic plasticity and resistive switching (RS) characteristics for bioinspired neuromorphic computing. X-ray photoelectron spectroscopy (XPS) was employed to explore oxygen vacancy concentrations in bilayer TiO₂/HfO₂ films. Gaussian fitting for O1s peaks confirmed that the HfO₂ layer contained a larger number of oxygen vacancies than the TiO₂ layer. In addition, HfO₂ had lower Gibbs free energy (ΔG°=-1010.8 kJ/mol) than the TiO₂ layer (ΔG°=-924.0 kJ/mol), resulting in more oxygen vacancies in the HfO₂ layer. XPS results and ΔG° magnitudes confirmed that formation/disruption of oxygen-based conductive filaments took place in the TiO₂ layer. The W/TiO₂/HfO₂/TaN memristive device exhibited excellent and repeatable RS characteristics, including superb 10³ dc switching cycles, outstanding 10⁷ pulse endurance, and high-thermal stability (10⁴ s at 125 °C) important for digital computing systems. Furthermore, some essential biological synaptic characteristics such as potentiation-depression plasticity, paired-pulse facilitation (PPF), and spike-timing-dependent plasticity (STDP, asymmetric Hebbian and asymmetric anti-Hebbian) were successfully mimicked herein using the crossbar-array memristive device. Based on experimental results, a migration and diffusion of oxygen vacancy based physical model is proposed to describe the synaptic plasticity and RS mechanism. This study demonstrates that the proposed W/TiO₂/HfO₂/TaN memristor crossbar-array has a significant potential for applications in non-volatile memory (NVM) and bioinspired neuromorphic systems.