Resistive random access memories with a bilayer TaOx/HfO2 stack structure have shown unique multilevel resistive switching capabilities. However, the physical processes governing their behavior, and specifically the atomistic mechanisms of forming, remain poorly understood. In this work, we present a detailed analysis of the forming mechanism at the atomic level using molecular dynamics simulations. An extended charge equilibration scheme, based on a combination of the charge transfer ionic potential formalism and the electrochemical dynamics with implicit degrees of freedom method, is employed to model the localized effects of applied voltage. Our simulations reveal that tantalum ions exhibit the highest displacement under applied voltage, followed by hafnium ions, while oxygen ions respond only minimally. This results in the formation of a tantalum-depleted, oxygen-rich zone near the positive top electrode, and the clustering of oxygen vacancies near the negative bottom electrode, where the conductive filament nucleates. This ionic segregation partially shields the bulk dielectric from the applied electric field, hindering further migration of ions in the vertical direction. We find that a minimum threshold voltage is required to initiate vacancy clustering. Filament growth proceeds through a localized mechanism, driven by thermally activated generation of oxygen vacancy defects, which are stabilized near the interface between the dielectric matrix and the nucleated filament at the bottom electrode.
Lahkar et al. (2026) studied this question.