Atomic‐Scale Mechanisms of Multi‐Resistance States in HfO _x ‐Based RRAM: Evolution of Atomic Electric Fields and Oxygen Vacancies

The stability of multilevel resistive switching in HfO_x-based RRAM is crucial for enhancing matrix-vector multiplication efficiency in computing-in-memory architectures, yet precise control over conductive filament formation is limited by an incomplete understanding of oxygen vacancy dynamics. Using advanced transmission electron microscopy (TEM) techniques, this study reveals key correlations among crystallographic orientation, oxygen vacancy distribution, and resistive switching mechanisms. Distinct m-phase orientations govern resistance states: the High Resistance State (HRS) is characterized by a [101] orientation without oxygen vacancies; the Medium and Low Resistance States (MRS/LRS) exhibit a [011] orientation with selectively formed vacancies. Thermally driven m-phase urotation ([101] ↔ [011]) facilitates oxygen vacancy migration, with vacancies preferentially occupying specific lattice sites. This alters the atomic electric fields around Hf atoms and modifies electron transport pathways. The distribution of vacancies directly controls conduction mechanisms: Schottky emission in HRS, Poole-Frenkel emission in MRS, and Ohmic conduction in LRS, corresponding to increasing vacancy concentrations. These findings demonstrate that resistive states emerge from coupled processes: crystallographic rotation and vacancy formation reconstruct atomic electric fields, which in turn determine macroscopic conduction. This framework establishes design principles for RRAM optimization by demonstrating that precise control of thermal conductivity and voltage modulation can regulate vacancy dynamics, ensuring reliable multilevel switching.