Effect of asymmetric thin TiN buffer layer for switching in NbOx layer

Abstract

In the coming years, threshold switching based on insulator-metal phase transition (IMT) devices is expected to be applied in selector devices for reducing sneak currents and building blocks for neuromorphic computing. In this work, we fabricated IMT devices using NbOx as an insulator layer and compared the device performance for two cases with metal electrodes: an asymmetric electrode device of stacked Pt/Ti/TiN/NbOx/Pt films, and a symmetric electrode device of Pt/Ti/TiN/NbOx/TiN/Ti/Pt. We changed the atomic ratio of Nb and O in NbOx films by controlling the argon-to-oxygen flow ratio during reactive sputtering. In the case of the asymmetric device, we observed a clear hysteresis loop in the current-voltage characteristics, indicating threshold switching only when a positive voltage was applied to the top electrode. We proposed a model in which a domain of the NbO2 phase is present in contact with a filament with oxygen vacancies, with its size changing depending on the direction of the electric field. On the other hand, in the case of the symmetrical device, nearly symmetric hysteresis loops were observed for both positive and negative voltage sweeps.