Sodium-Controlled Interfacial Resistive Switching in Thin Film Niobium Oxide for Neuromorphic Applications

The growth of data-intense computational workloads has highlighted the need to improve the energy efficiency of hardware materials. Memristive devices are promising candidates due to the potential for high-density integration and analogue processing functionality, which can be controlled by voltage-induced redox changes. However, conventional filamentary memristive systems suffer from low uniformity due to the stochastic nature of the switching mechanism, which results in limited training accuracy and efficiency in neuromorphic computing. Interfacial mechanisms involve homogenous, voltage-dependent migration of ionic species towards a thin film barrier, enabling the fine-tuning of the device's electronic conductivity in a non-volatile, reversible fashion. Ionically-driven mechanisms have been shown to result in gradual switching performances and high uniformity, similarly to artificial synaptic plasticity.<br/><br/>Orthorhombic T-Nb₂O₅ is a well-studied anode material with fast cation intercalation, yet its unique ionic transport properties have not been explored for ionically driven interfacial neuromorphic applications. In this study, we report neuromorphic performance based on interfacial switching modulated by the voltage-controlled motion of Na⁺ ions in a NaNbO₃/Nb₂O₅ solid-state 2-terminal device. The accumulation/depletion of Na+ ions at the Schottky barrier drives non-volatile modulation of resistance states, evidenced by <i>in operando</i> Raman and <i>ex situ</i> spectroscopy methods. We report spike-amplitude dependent plasticity, yielding 80 distinct non-volatile resistance states. Moreover, paired-pulse facilitation and spike-timing dependent plasticity (STDP) measurements show that the modulation of synaptic weight is a function of applied pulse spike timing, which enables potential applications towards Hebbian learning. Compared to widely studied small cations (oxygen vacancies, H⁺ and Li⁺), Na⁺ ions are key biomarkers that play a significant role in biological synapses, with potential applications in smart wearables, and a larger ionic radius that can improve retention dynamics. This study opens new possibilities in neuromorphic devices with sensory functions, where ions from the perceived surroundings actively drive changes in synaptic plasticity.