Epitaxial Bi ₂ O ₂ Se/Bi ₂ O ₅ Se Thin Films: Revealing Electric‐Field‐Driven Oxidation and Resistive Switching Dynamics for Advanced Memory Devices

ABSTRACT Resistive random‐access memory (RRAM) is a promising technology for nonvolatile applications and neuromorphic computing. 2D bismuth oxyselenide (Bi ₂ O ₂ Se) exhibits high air stability, carrier mobility, and process compatibility. Moreover, Bi ₂ O ₂ Se naturally oxidizes into a high‐κ insulating oxide, Bi ₂ O ₅ Se, making it an ideal candidate for fabricating a dielectric layer in RRAM devices with tunable resistive properties. In this study, RRAM devices based on Bi ₂ O ₂ Se/Bi ₂ O ₅ Se bilayer dielectric thin films were epitaxially grown on (001) Nb‐doped SrTiO ₃ substrates. The devices exhibited an exceptional nonvolatile performance, including an endurance of 3 × 10 ⁶ pulse cycles, retention exceeding 10 ⁴ s, and stable multilevel resistance states. Additionally, the synaptic‐like behavior was explored by applying pulses to the devices. High‐resolution in situ transmission electron microscopy and aberration‐corrected scanning transmission electron microscopy were used to examine the bilayer structural evolution before and after electric‐field application. The electric‐field‐driven oxidation of Bi ₂ O ₂ Se was demonstrated, and a new oxygen‐deficient phase was explored during switching. These results established a resistance‐switching mechanism in the Bi ₂ O ₂ Se/Bi ₂ O ₅ Se platform for fast and reliable RRAM and neuromorphic operations.