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Effect of Cation Substitution on Bipolar Resistance Switching Behavior in Epitaxially Grown NiO Films

Epitaxial NiO films (epi-NiO) were successfully doped with Al by performing sequential deposition of NiO and AlOx by pulsed laser deposition. Surface morphology and crystallinity were confirmed by atomic force microscope images and X-ray diffraction, indicating that the samples were grown epitaxially. Resistance switching (RS) phenomena of Al-doped NiO films were investigated using epitaxial SrRuO3 (SRO) and CaRuO3 (CRO) as the bottom (BE) and top electrode (TE), respectively. When the epi-NiO/SRO interface was modified by inserting either a 1-nm-thick epitaxial AlOx layer or a 5-nm-thick epitaxial 10% Al-doped NiO (Al0.1Ni0.9Ox) layer, the RS of epi-NiO remained unchanged. However, when the CRO/epi-NiO interface was modified by inserting a 1-nm-thick epitaxial AlOx layer or a 5-nm-thick Al0.1Ni0.9Ox layer, switching behavior was significantly changed; in the case of the 1-nm-thick epitaxial AlOx layer, the switching behavior completely disappeared, while in the case of 5-nm-thick Al0.1Ni0.9Ox layer, number of cells which exhibited RS was drastically decreased. The switching phenomena may be suppressed due to the presence of Al cations whose free energy for oxidation is much lower than that of Ni. Our experimental results suggest that oxygen ions migration at the TE/epi-NiO interface may play a critical role in bipolar RS of epi-NiO. The relation between RS and free energy for oxidation is discussed.