Electrical evidence of atomic-size effects in the conduction filament of RRAM

Many resistive switching memory devices are based on the formation of a small-area conducting filament (CF). Understanding the conduction and switching properties of the CFs is crucial for their industrial application. In this work we report electrical evidence of atomic-size effects in metal-insulator-metal structures showing memory switching. In particular, we report the observation of abrupt changes of the order of the quantum of conductance G_o=2e²/h during the RESET transition of Pt/HfO₂/Pt structures. Statistical analysis of the CF conductance during the RESET experiments further confirm the appearance of preferred atomic-size configurations of the CF with a well-defined structure of conductance peaks of the order of G_o. Based on these results, we suggest that the quantum of conductance represents a natural boundary between the ON and OFF resistive switching CF states. Temperature dependence of the conduction as a function of the CF conductance further supports this conclusion. Once again, our results emphasize the atomic-scale size of the CF constrictions and the need to use first-principle calculations and molecular dynamic simulations to understand key aspects related to performance and reliability. First-principle calculations performed in both crystalline and amorphous HfO₂ confirm that one dimensional paths of oxygen vacancies can support quantum transport channels and might give rise to the experimental results. On the other extreme of the modeling hierarchy, a compact model for the CF conduction based on the quantum point contact concept is shown to provide good fitting of the CF conduction both in the ON and the OFF states, further supporting the quantum-wire picture.