According to their study, the electronic RS is induced by the non-uniform distribution of traps in the oxide and the asymmetric potential barrier at each end of the metal/TiO2 interfaces. When the traps were distributed non-uniformly, particularly when a much higher trap density was formed near one metal-insulator interface in a metal-insulator-metal structure, the barrier for trapping and detrapping becomes asymmetric with respect to bias polarity. Such asymmetric trap distributions invoke bias-polarity dependent hysteretic current-voltage (I-V) curves, which can be regarded as the advent of resistive switching phenomena.

Because such asymmetric I-V curves most likely have an electronic origin, the team expected that the reliability and endurance characteristics of the sample might be superior to other ion-migration related switching cases. However, this assumption proved to be wrong. This was ascribed to the highly asymmetric shape of the I-V curve for the single electronic RS cell; a sudden increase of current for the set switching, but a more gradual decrease of current for the reset switching.

Astonishing improvement

Therefore, the scientists attempted to make the I-V curve of the RS cell more symmetric while maintaining its hysteretic switching characteristics. Their idea was to anti-serially connect two RS cells, where the two RS cells may have similar or different trap densities, as can be seen in the schematic band structure shown above.

Depending on the detailed process conditions, such as the trap distribution and switching voltages, the anti-serially connected RS cell showed an astonishing improvement in endurance from several hundred cycles up to several tens of thousands of cycles. Subsequent work will focus on realizing the same improvement in a single device.

Additional information can be found in the journal Nanotechnology.