Thickness-optimized multilevel resistive switching of silver programmable metallization cells with stacked SiO₂

Multilevel resistive switching (RS) characteristics of silver programmable metallization cells (Ag-PMCs) with stacked SiO_x/SiO₂ solid electrolytes have been investigated. Combined with conventional high/low resistance states and additional two middle resistance states (MRS1/MRS2), a multilevel cell operation of stacked-solid-electrolyte Ag-PMCs is achieved and optimized by the film thickness. Furthermore, the RS mechanism at middle resistance states has been proposed to be locally discontinuous Ag conductive filament (Ag-CF) within the stacked solid electrolytes by examining the carrier transportation and two-frequency calibrated capacitance. The stacked silicon oxide layers can prevent the Ag-CF from regeneration during the multilevel retention test, contributing to the superior retention properties to more than 10⁴ s at 125 °C. In addition, a sequentially multilevel cycling test of more than 10³ times with a resistance ratio of two orders of magnitude between each resistance state is realized by the stacked-solid-electrolyte Ag-PMCs, suitable for future high-density nonvolatile memory applications.