All HfO_x-Resistive Switches with the Conducting Oxygen Vacancy Exchange Layer and Self-Limited Oxide Layer

Resistive random-access memory based on hafnia is a potential candidate for next-generation memories. However, there are some key challenges to overcome, including the stochastic nature of the filament formation during electroforming and SET operation, which results in large cycle-to-cycle and device-to-device variability. Here, we propose a device setup with an artificial virtual electrode layer of conducting oxygen-deficient HfO_x and a well-defined self-limited oxidized switching layer to reduce the stochasticity of filament growth. We have realized this concept in TiN/HfO_x/HfO₂/Pt device stacks by adjusting the oxygen content in the artificial virtual electrode during growth in a molecular beam epitaxy system and subsequent self-limited surface oxidation in an oxygen-rich atmosphere. The artificial virtual electrode functions as a conducting oxygen vacancy exchange layer (COVEL) due to the inherent low oxygen content, providing an unlimited oxygen vacancy reservoir that stabilizes the filament in the thin oxidized switching layer. Detailed electrical characterization demonstrates that the COVEL, combined with the self-limited oxidized switching layer, can significantly reduce electroforming and SET voltage stochasticity and thus contribute to improving the switching reliability.