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Scientists Report Smallest Phase-Change Material To Date

Materials: Nanowires of germanium telluride could lead to new types of ultra-compact digital memory devices

by Kate Greene
September 17, 2013

Inner Tubes
Credit: Nano Lett.
A model (left) depicts a carbon nanotube (gray) filled with germanium telluride, a phase-changing material. A micrograph (right) shows the actual tubes and the placement of the Ge (purple) and Te (green) atoms.
Schematic and micrograph of carbon nanotubes filled with germanium telluride.
Credit: Nano Lett.
A model (left) depicts a carbon nanotube (gray) filled with germanium telluride, a phase-changing material. A micrograph (right) shows the actual tubes and the placement of the Ge (purple) and Te (green) atoms.

To build fast smart phones and tablets that can store troves of data, some engineers have tinkered with memory devices that rely on phase-changing materials. With a little pulse of heat, these materials change quickly between crystalline and messy amorphous states, which serve as 1s and 0s in memory cells.

As with any memory technology, engineers want to make phase-change memory cells as small as possible to maximize the storage capacity of light-weight electronics. With that goal in mind, researchers now have developed a new synthesis technique to create the smallest working phase-change material yet: one-dimensional germanium-telluride nanowires with diameters smaller than 2 nm. (Nano Lett. 2013, DOI: 10.1021/nl4010354).

“None of the phase-change materials have been studied at this scale before,” says Cristina E. Giusca, a research scientist at the National Physical Laboratory, in the U.K, and lead author on the paper. The work could lay the foundation for completely novel architectures for memory devices, she says.

The most common phase-change memory devices are made of thin films. But below thicknesses of about 2 nm, these thin films no longer display the crystalline state needed to operate in memory cells. Previously, other researchers had developed one-dimensional phase-change nanowires, but the smallest were about 20 nm in diameter.

To make a thinner wire, Giusca and her team used carbon nanotubes, with diameters of at least 1.4 nm, as a template. They filled the inner cores of the nanotubes with molten GeTe using capillary action.

Characterizing the material at such a scale was challenging and pushed the limits of available analytical techniques, Giusca says. Her team used techniques including scanning transmission electron microscopy and photoelectron spectroscopy to determine if GeTe had been incorporated into the nanotubes and to analyze the material’s structural and electronic properties.

At this scale, the phase of the GeTe nanowires depended on the size of the nanotube. Nanotubes with 1.4-nm diameters created amorphous GeTe nanowires, but tubes with diameters of 1.3 nm or smaller produced crystalline structures. Wires in the amorphous state converted from the crystalline state and back again when heated by the electron beam of the scanning transmission electron microscope. Although the researchers could demonstrate transient phase changes at a scale below 2 nm for the first time, they couldn’t determine a phase transition temperature for the each structure.

“The scale of the memory element is impressively small,” says Yeon Sik Jung, a materials scientist at the Korea Advanced Institute of Science and Technology. “And the use of nanotubes was an ingenious method to prepare such tiny structures.” But he says it will take more innovation to incorporate the GeTe nanowires into a practical memory device, including finding a simple way to trigger the phase change.

Giusca agrees and says her team is currently working toward integrating individual structures into simple devices. This current work is just a first step, she says, a demonstration of the nanowire’s behavior and the underlying science in its simplest case.


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