Observation of two resistance switching modes in TiO2 memristive devices electroformed at low current

We report the observation of two resistance switching modes in certain 50 nm x 50 nm crossbar TiO2 memristive devices that have been electroformed with a low-current process. The two switching modes showed opposite switching polarities. The intermediate state was shared by both modes (the ON state of the high-resistance mode or the OFF state of the low-resistance mode) and exhibited a relaxation to a more resistive state, including an initial transient decay. The activation energies of such a decay and ON-switching to the intermediate state were determined to be 50-210 meV and 1.1 eV, respectively. Although they are attributed to the coexistence of charge trapping and ionic motion, the ionic motion dominates in both switching modes. Our results indicate that the two switching modes in our system correspond to different switching layers adjacent to the interfaces at the top and bottom electrodes.