Electroforming Kinetics in HfO_x/Ti RRAM: Mechanisms behind Compositional and Thermal Engineering

A critical issue affecting filamentary resistive random access memory (RRAM) cells is the requirement of high voltages during electroforming. Reducing the magnitude of these voltages is of significant interest, as it ensures compatibility with complementary metal-oxide-semiconductor (CMOS) technologies. Previous studies have identified that changing the initial stoichiometry of the switching layer and/or implementing thermal engineering approaches has an influence on the electroforming voltage magnitude, but the exact mechanisms remain unclear. Here, we develop an understanding of how these mechanisms work within a standard a-HfO_x/Ti RRAM stack through combining atomistic driven kinetic Monte Carlo (d-KMC) simulations with experimental data. By performing device-scale simulations at atomistic resolution, we can precisely model the movements of point defects under applied biases in structurally inhomogeneous materials, which allows us to not only capture finite-size effects but also understand how conductive filaments grow under different electroforming conditions. Doing atomistic simulations at the device level also enables us to link simulations of the mechanisms behind conductive filament formation with trends in experimental data for the same material stack. We identify a transition from primarily vertical to lateral ion movement dominating the filamentary growth process in substoichiometric oxides and differentiate the influence of global and local heating on the morphology of the formed filaments. These different filamentary structures have implications for the dynamic range exhibited by formed devices in subsequent SET/RESET operations. Overall, our results unify the complex ion dynamics in technologically relevant HfO_x/Ti-based stacks and provide guidelines that can be leveraged when fabricating devices.