May 16, 2011

The switching location of a bipolar memristor: chemical, thermal and structural mapping

Three-dimensional steady-state heat transfer simulation. (a) Temperature map for an x–y slice taken through the middle of a 2 nm high and 100 nm wide cylindrical uniform heat source located directly on top of the bottom electrode (i.e. 1 nm above the surface of the bottom electrode, illustrated in right inset). Left inset shows a top view of the full device including membrane window. (b) Temperature contours for T = 760 K) along x–y slices for three different positions of heat source: directly on top of the bottom electrode (red, and the same as in panel a); exactly between top and bottom electrode (green) and directly below top electrode (blue). The x–y position for the cylindrical heat source is the same for all cases and shown with a black circle in both panels. (c) and (d) Similar temperature contours along an x–z slice (c) and y–z slice (d) going through the center of the conductive channel for three different positions of heat source. The heat source position in each case is indicated by the colored rectangle within the junction.

Nanotechnology journal - The switching location of a bipolar memristor: chemical, thermal and structural mapping (7 pages)

BBC News coverage - HP researchers have shown for the first time where the current switching process happens in memristors, and how heat affects memristors. Memristors have the potential to be a strong competitor to flash memory in about five years.

Memristors are memory resistors promising a rapid integration into future memory technologies. However, progress is still critically limited by a lack of understanding of the physical processes occurring at the nanoscale. Here we correlate device electrical characteristics with local atomic structure, chemistry and temperature. We resolved a single conducting channel that is made up of a reduced phase of the as-deposited titanium oxide. Moreover, we observed sufficient Joule heating to induce a crystallization of the oxide surrounding the channel, with a peculiar pattern that finite element simulations correlated with the existence of a hot spot close to the bottom electrode, thus identifying the switching location. This work reports direct observations in all three dimensions of the internal structure of titanium oxide memristors.

After electroforming and one switching cycle of a metal/TiO2/ metal memristor, we directly observed a ∼100 nm conducting channel that we identified to be oxygen-deficient (vacancyrich) relative to the deposited material. No structural order was found in this channel suggesting that composition (Ti to O ratio) is more crucial than long-range order in enabling resistance switching in titanium oxide. We observed a heat-induced transformation of amorphous TiO2 into an anatase phase around the channel, confirming that the channel was the conductor of current in the device. By comparing the semicircular shape and orientation of the observed anatase in the device with finite element simulations of the temperature profile caused by Joule heating, we deduced that the heating source was adjacent to the bottom electrode during switching. This also provided evidence that heating of the insulating gap between the tip of the conductive channel and the bottom electrode assisted the kinetics of the switching process.

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