The switching process of the conductive filament formed in Ni/HfO x /Pt resistive random access memory (ReRAM) devices were studied. We evaluated the oxide thickness dependence and temperature dependence of voltage for the Forming, Set and Reset operations for HfO x layers whose thickness are between 3.3 and 6.5 nm. The resistance of conductive filaments showed typical metallic behavior, which suggests Ni filament formation in the HfO x layer. There is a clear dependence of switching voltages for the Set and Reset processes on oxide thickness, which implies that the formation and rupture of conductive filaments occur in the entire thickness range of the HfO x layer. This finding differs from that of a previous study by Yang, which suggests the existence of a constant-thickness switching region. It is suggested that the thickness of the switching region in HfO x may be larger than 6.5 nm.

We investigated the resistive switching characteristics and temperature dependence of resistance of a Ni/NiO x /Pt resistive switching memory. The device was operated in a unipolar operation mode. From the temperature dependence of resistance, it is suggested that the conduction mechanism of the low-resistance state is metallic. Moreover, a deformed site that may correspond to the conductive filament appeared after the forming process. Cross-sectional transmission electron microscopy with energy dispersive X-ray analysis showed that the Ni atomic content at the NiO x of the deformed area was larger than those of other areas. It is strongly suggested that the conductive filament formed in the NiO x layer is composed of Ni atoms.

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