In this paper, we show the coexistence of the bipolar and unipolar resistive-switching modes in NiO cells realized using an optimized oxidation process of a Ni blanket layer used as the bottom electrode. The two switching modes can be activated independent of the cell switching history provided the appropriate programming conditions are applied. The bipolar and unipolar switching modes are discussed as driven by electrochemical- and thermal-based mechanisms, respectively. The switching versatility between these two modes is demonstrated both for large oxidized Ni films and for Ni films oxidized at the bottom of small dimension contact holes. The perspective of selecting the desired switching mode in a scaled device made in a small diameter single hole is highly attractive because the specific advantages of the two modes broaden the application scope of the cell and enable larger flexibility in terms of memory architecture.
Resistive switching controlled by external voltage has been reported in many Metal/Resistive oxide/Metal (MRM) structures in which the resistive oxide was simple transition metal oxide thin films such as NiO or TiO 2 deposited by reactive sputtering. In this paper, we have explored the possibility to form NiO-based MRM structures from the partial oxidation of a blanket Ni metallic film using a Rapid Thermal Annealing route, the remaining Ni layer being used as bottom electrode. X-ray diffraction was used to apprehend the Ni oxidation kinetics while transmission electron microscopy enabled investigating local microstructure and film interfaces. These analyses have especially emphasized the predominant role of the asdeposited Ni metallic film microstructure (size and orientation of crystallites) on (i) oxidation kinetics, (ii) NiO film microstructural characteristics (crystallite size, texture and interface roughness) and (iii) subsequent electrical behavior. On this latter point, the as-grown NiO films were initially in the low resistance ON state without the electro-forming step usually required for sputtered films. Above the threshold voltage varying from 2 to 5 V depending on oxidation conditions, the Pt/NiO/Ni MRM structures irreversibly switched into the high resistance OFF state. This irreversibility is thought to originate in the microstructure of the NiO films that would cause the difficulty to reform conductive paths.
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