Improved Switching Stability and the Effect of an Internal Series Resistor in HfO 2 /TiO x Bilayer ReRAM Cells

Bipolar redox-based resistive random-access memory cells are intensively studied for new storage class memory and beyond von Neumann computing applications. However, the considerable variability of the resistance values in ON and OFF state as well as of the SET voltage remains challenging. In this paper, we discuss the physical origin of the significant reduction in the switching variability of HfO 2 -based devices achieved by the insertion of a thin TiOx layer between the HfO 2 layer and the oxygen exchange metal layer. Typically, HfO 2 single layer cells exhibit an abrupt SET process, which is difficult to control. In contrast, self-compliance effects in the HfO 2 /TiO x bilayer devices lead to an increased stability of SET voltages and OFF-state resistances. The SET process is gradual and the RESET becomes abrupt for higher switching currents. Comparison of the experimental data with simulation results achieved from a physics-based compact model for the full description of the switching behavior of the single layer and bilayer devices clearly reveal three major effects. The TiO x layer affects the temperature distribution during switching (by modifying the heat dissipation), forms an additional series resistance and changes the current conduction mechanism in the OFF state of the bilayer device compared to the single layer device

Variability-Aware Modeling of Filamentary Oxide based Bipolar Resistive Switching Cells Using SPICE Level Compact Models

Bipolar resistive switching (BRS) cells based on the valence change mechanism show great potential to enable the design of future non-volatile memory, logic and neuromorphic circuits and architectures. To study these circuits and architectures, accurate compact models are needed, which showcase the most important physical characteristics and lead to their specific experimental behavior. If BRS cells are to be used for computation-in-memory or for neuromorphic computing, their dynamical behavior has to be modeled with special consideration of switching times in SET and RESET. For any realistic assessment, variability has to be considered additionally. This study shows that by extending an existing compact model, which by itself is able to reproduce many different experiments on device behavior critical for the anticipated device purposes, variability found in experimental measurements can be reproduced for important device characteristics such as I-V characteristics, endurance behavior and most significantly the SET and RESET kinetics. Furthermore, this enables the study of spatial and temporal variability and its impact on the circuit and system level