Unipolar resistive switching (URS) has been found to occur mainly because of the formation and rupture of local conduction paths (so-called conducting filaments) within the resistive switching film and works irrespective of the bias polarity. On the other hand, in bipolar resistive switching (BRS), the electrochemical potential barrier configuration of the material is altered as charged defects migrate in alternate directions according to electrical bias polarity.
In the study, the group from the dielectric thin-film laboratory at SNU has found a way to involve both the top and bottom metal/oxide interfaces in the switching of a single device, and in turn achieve tri-stable memristive switching. The team has taken notice of the fact that, even excluding complicated device fabrications, conventional BRS-like switching may occur in a single TiO2 layer only if a certain series of operating steps is adopted.
An electroforming step followed by unipolar reset switching led to the BRS-like switching mode. So it was postulated that the switching occurs at the location where the conductive filament, which has been initially formed during the previous electroforming step and known to be composed of Magnéli phase for TiO2, was ruptured by the subsequent unipolar reset process.
The migration of oxygen vacancies restricted at the ruptured filament region and the resulting modulation of the Schottky-like interfaces mainly accounts for the switching mechanism. Since the movable oxygen vacancies originate from the re-oxidation of Magnéli filament during unipolar reset, the amount of the available vacancies is limited so that the electrochemical barrier at both interfaces can be readily adjusted.
The group has also succeeded in making the device work in a cross-bar array format with electrode areas of 2 × 2, 4 × 4, 8 × 8 and 10 × 10 nm2.
More information is available in the journal Nanotechnology.