Revealed: the material properties of memristors
Research associate John
Representing a fourth basic passive circuit element, memristors have the ability to ‘remember’ the total electrical charge that passes through them. As a result, they could potentially underpin a new generation of high density, non-volatile memory chips and logic circuits that mimic biological synapses.
Memristors were recognized only in theory until 2006, when HP Labs researchers first performed experiments to intentionally demonstrate their existence. While the electrical properties of these devices are now fairly well understood, very little has been known about how they actually undergo reversible changes in resistance.
"It's been a real challenge to non-destructively study, at the nanoscale, the material changes that the devices undergo during operation," reports John Paul Strachan of HP’s nanoElectronic Research Group, and lead author of the paper.
While the memristors that they investigated were relatively simple films of titanium dioxide sandwiched between layers of metal, the HP/UC Santa Barbara team sought to map out the chemistry and structure of the minute conductive channel which is responsible for the switching in the device.
Electrical charge flowing through a memristor changes the resistance state of the device, but actually observing the corresponding material changes has been a challenge. Highly focused x-rays were used to probe the memristor non-destructively and a ~100 nm region with concentrated oxygen vacancies (right, shown in blue) where the memristive switching occurs was discovered. Surrounding this region a newly developed structural phase (red) was also found, which acted like a thermometer telling researchers where and how hot it became.
The researchers used highly focused x-rays to localize the exact, one hundred nanometer channel where the resistance switching of memristors occurs. That gave them a detailed insight into the chemistry and structure changes that happen when the device is operating.
Additionally, says Strachan, "we now have a direct picture for the thermal profile which is highly localized around this channel during electrical operation, and is likely to play a large role in accelerating the physics driving the memristive behavior."
A better understanding of the physical processes that occur within memristors at the nanoscale is essential if memristors are to realize their potential as the basis for innovations in computer memory and logic, the researchers believe. Memristor-based devices could one day, for example, act like synapses inside computer circuits, mimicking the behavior of neurons in the human brain.
The team’s paper, The switching location of a bipolar memristor: Chemical, thermal, and structural mapping, appears as part of a special issue of Nanotechnology devoted to new research into non-volatile memory based on nanostructures.