Nature of Cu Interstitials in Al2O3 and the Implications for Filament Formation in Conductive Bridge Random Access Memory Devices

摘要

Resistive random access memory (RRAM) is a prime candidate to replace Flash memory. Of the two classes of RRAM, conductive bridge RAM (CBRAM) is favoured over that based on filaments of oxygen vacancies because of its larger on/off resistance ratio. The nature of the filament in Cu/Al₂O₃-based CBRAM is analysed using density functional theory. The defect and binding energies of Cu interstitials and clusters in Al₂O₃ are calculated. The binding energy per Cu interstitial is shown to significantly increase with increasing Cu coordination, whereas the binding per oxygen vacancy only slightly increases with vacancy concentration. This explains why metal filaments in CBRAM devices tend to be denser than oxygen vacancy filaments. Using three different filament models, we discover that the strong binding between Cu interstitials drives filament formation, resulting in Al ions being driven out of the Cu-rich environment. This leads to the formation of densely packed metallic Cu filaments with bonding similar to Cu metal, as confirmed by electronic structure calculations.