Observation of filament formation process of Cu/HfO 2 /Pt ReRAM structure by hard x-ray photoelectron spectroscopy under bias operation

We have demonstrated resistance switching using polycrystalline HfO 2 film with a Cu top electrode for nonvolatile memory applications and revealed the Cu diffusion into the HfO 2 layer during the filament formation process. Resistive switching was clearly observed in the Cu/HfO 2 /Pt structure by performing a current–voltage measurement. The current step from a high-resistive state to a low-resistive state was of the order of 10 3 –10 4 Ω, which provided a sufficient on/off ratio for use as a switching device. The filament formation process was investigated by employing hard x-ray photoelectron spectroscopy under bias operation. The application of a bias to the structure reduced the Cu 2 O state at the interface and the intensity ratio of Cu 2 p 3/2 /Hf 3 d 5/2 , providing evidence of Cu 2 O reduction and Cu diffusion into the HfO 2 layer. These results also provide evidence that the resistance switching of the Cu/HfO 2 /Pt structure originates in a solid electrolyte (nanoionics model) containing Cu ions.