Parallel conducting filaments in resistive switching ZnO thin films

This study examines resistive switching in a Cu/ZnO/ITO structure, uncovering an anomalous phenomenon that provides insights into the mechanisms of parallel conducting filaments in ZnO thin films. The current-voltage (I-V) characteristics exhibit a sharp switch at a positive threshold voltage around 2 V, transitioning from a high resistance pristine state to a low resistance state, interpreted as the formation of a Cu filament via electrochemical metallization. However, after this forming process, the device remains in the low resistance state and cannot reset to a high resistance state in either polarity of the applied voltage, suggesting the presence of a strong, unbreakable Cu filament after the forming process. What makes this phenomenon anomalous is the observed weak bipolar resistive switching in the cycles following the forming cycle, despite the presence of the Cu filament. The I-V characteristics of forward- and reverse-bias sweeps suggest that the weak bipolar resistive switching results from an additional filament formed in parallel with the existing unbreakable Cu filament. Using a parallel conducting filaments model, the resistivity of this additional filament is calculated to be ∼10⁻⁷-10⁻⁵ Ω m, indicating that this filament is likely generated by oxygen vacancies rather than metal atoms in the ZnO films.