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Kyoto University maps nanoscale changes in memory material

A team of scientists claims that they have created the world's most powerful electromagnetic pulses in the terahertz range to control in fine detail how a data-storage material switches physical form.

The Kyoto University team believes that the discovery could help find a way to scale down memory devices, eventually revolutionising how computers handle information.

Compact discs might be falling out of fashion, but they may have inspired the next generation of computer nanotechnology, the scientists explain. A glass layer in CDs consists of a phase-change material that can be encoded with information when light pulses cause crystals in small regions of the layer to either grow or melt.

Phase-change materials triggered by electrical impulses – rather than light – would offer new memory technologies with more stable and faster operation than that possible in many current types of memory devices. In addition, downscaling memory sites in phase-change materials could increase memory density. But this remains challenging because of the difficulty of controlling the crystal growth (crystallisation) and melting (amorphisation) processes.

The team observed nanometer-scale growth of individual crystals in a phase-change material composed of germanium, antimony and tellurium (or GST) after applying high-powered THz pulses as a trigger.

"One reason crystallisation and amorphisation of GST under an electric field are difficult to control is the heat diffusion effects in the micrometer scale associated with electrical inputs, which also contribute to the crystallisation," explains group leader Hideki Hirori. "Fortunately, THz technologies have matured to the point where we can use short pulses to generate strong electric fields while suppressing heating effects."

Hirori and his coworkers developed a THz pulse generator that delivered ultra-short and highly intense THz pulses across a pair of gold antennas. These pulses created an electric field in the GST sample comparable to that of an electrically switched device. Importantly, this approach greatly reduced the heat diffusion because of the extremely short duration of THz pulses – around 1 picosecond – enabling fine control over the rate and direction of GST crystallisation. A region of crystallisation grew in a straight line between the gold antennas in the direction of the field, at a few nanometers per pulse.

When the team tracked stepwise changes in crystallisation while increasing the number of THz pulses, they were surprised to find that after a certain point, crystal conductivity rapidly sped up instead of rising in line with the increase in THz strength. The researchers hypothesise that electrons jumping between states in the crystal added an unexpected source of heat to the system, boosting crystallisation.

Hirori explains: "Our experiment reveals how nanoscale and direction-controlled growth of crystals in GST can be achieved. We also identified a phenomenon which should assist in the design of new devices and ultimately realise the fast and stable digital information handling potential that this material promises."

Bethan Grylls

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