Highly stable bipolar resistive switching behaviors of TiN/ZnO/Pt devices were demonstrated for the first time. The excellent memory characteristics including fast switching speed (<20 ns for set and <60 ns for reset), long retention (in the order of 10 5 s) and non-electroforming process were demonstrated. The bipolar switching behaviors can be explained by formation and rupture of the filamentary conductive path consisting of oxygen vacancies. The excellent bipolar switching behavior can be attributed to the significant amount of oxygen vacancies in ZnO film and the effect of TiN layer serving as an oxygen reservoir.

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The bipolar and unipolar resistive switching (BRS and URS) modes are first observed to coexist in the Au/SrTiO3/Pt cell fabricated by pulsed laser deposition. These two switching modes can be activated separately depending on the different current compliance (CC) during the electroforming process: with a lower CC (1 mA) the asymmetric BRS behaviour is measured in the voltage range −1.2 to +1 V, while the URS behaviour is observed with a higher CC (10 mA). On the basis of current–voltage characteristics, the switching mechanisms for the BRS and URS modes are considered as a change in Schottky-like barrier height and/or width at the Au/SrTiO3 interface and the formation and disruption of conduction filaments, respectively. The conversion between BRS and URS is reversible. Because each switching mode has a specific advantage, selecting the desired switching mode can broaden the application scope of the cell and enable large flexibility in terms of memory architecture.

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Two types of bipolar resistance switching with eightwise and counter eightwise polarities are observed to coexist in Au/SrTiO3/Ti memory cells. These two types of switching can be induced by different defect distributions which are activated by controlling the electric process. The analyses of I-V and C-V data reveal that the resistance switching with eightwise polarity originates from the change of Schottky barrier at the Au/SrTiO3 interface caused by trapping/detrapping effects at interface defect states, while the switching with counter eightwise polarity is caused by oxygen-vacancy migration.