The forming process in planar-type Cu2O-based resistive switching devices is investigated. It is found that two forming processes occur in series, and the existence of a Cu filament is directly confirmed using transmission electron microscopy after each forming process. The time evolution of the surface is observed by an optical microscope during these processes. The first process accompanies the oxidation of the Cu2O surface, and the filament is created ∼15 μm below the surface; the second process involves melting of the region between the electrodes with the creation of a new filament ∼1 μm below the surface.
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