Regulated Conductive Filament Formation and Evolution for Improved Switching Uniformity in Embedded Metal-oxide Based Memristor - A Phase Field Study

The extreme device-to-device variation of switching performance is one of the major obstacles preventing the applications of metal oxide-based resistive random-access memory (RRAM) in large-scale memory storage and resistive neural network. Recent experimental works have reported that embedding highly ordered metal nano-islands (NIs) can effectively improve the uniformity of the RRAM devices, but the underlying role of the ordered metal NI is not fully understood. In this study, to address this specific problem, we develop a physical model to understand the origin of the variability and how embedding metal NIs within the HfO₂ oxide can improve the electroforming and resistive switching performances of RRAMs by reducing this variability. We find that due to dimensional confinement, introducing NIs increases electric fields in their vicinity, leading to high vacancy concentrations even at low forming potentials, and hence a more deterministic formation of the CF from its vicinity, in contrast to the random growth of CFs in the absence of embedded NIs. This deterministic vacancy nucleation in the vicinity of the embedded metal NIs is not only found to reduce the initial electroforming but also the subsequent reset/set voltages, as well as enhance the uniformity of these operation voltages and current ON/OFF ratio. We further demonstrate that modifying the shapes of the metal NIs can modulate the field strengths/distributions around the NIs, and that choosing NI metals that chemically facilitate vacancy formations can further optimize the CF morphology, reduce operation voltages, and improve switching performance. Our work thus provides a fundamental understanding of how embedded metal NIs improve the resistive switching performance in oxide-based RRAMs, and could potentially guide the selection of embedded metal NIs to realize a more uniform RRAM that also operates at higher efficiency than present materials.