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Phase-change memory becomes more portable

11 Mar 2011 Belle Dumé
On and off: PCM cells in carbon nanotubes

Phase-change materials are already used to store data on rewritable discs, but their relatively high power requirements make them impractical for use in mobile phones and other portable devices. Now, researchers in the US have found a way to decrease the volume of phase-change material in the memory bit, cutting power requirements 100-fold compared with the best devices on the market today.

Phase-change materials are the active material in rewritable DVDs and are usually made of chalcogenides like germanium antimony telluride – GST for short. Using voltage pulses to produce heat, the materials are switched between an amorphous state (“off”) and crystalline state (“on”). The amorphous state has a very high resistance and the crystalline state a very low resistance.

Faster than Flash

These states endure once the power is turned off, so the materials are ideal for making nonvolatile memory similar to Flash or hard drives. What is more, the phases can be switched in just a matter of nanoseconds, which is much faster than Flash. However, the snag is that relatively high power levels are usually required to switch between the amorphous and crystalline states in GST memory bits.

To get around this problem, Eric Pop and colleagues at University of Illinois Urbana-Champaign used carbon nanotubes to “house” nanometre-scale GST memory bits. They began by creating tiny gaps within the nanotubes using a method called electrical breakdown. This simple technique produces gaps that vary in size from 20 to 300 nm – usually in the middle of a nanotube. Next, the researchers filled the nanogap with a small amount of GST.

The devices are initially in the off state because the as-deposited GST bits are amorphous, with a high resistance of around 50 MΩ. When a voltage is applied across the nanotube (which effectively acts as a contact or interconnect), an electric field is created across the nanogap and switches the GST bit to the crystalline phase. The resistance of the crystalline phase is around 100 times lower, at roughly 0.5 MΩ.

‘Extremely low power dissipation’

The switching only occurs in the small amount of material contained within the nanogap. “This means extremely low power dissipation compared to state-of-the-art devices that use much larger metallic wires to contact the phase-change material,” explained Pop.

The results are very important, say the researchers, because phase-change materials are the most promising technology for replacing Flash memory in laptops, cellphones and many other portable applications. “A 100-fold power reduction could go very far in extending battery life and portability, and could also ultimately lead to many novel applications,” says the team.

Although the Illinois researchers have reduced power dissipation by two orders of magnitude, it is possible that it may have not yet reached the fundamental lower limit for such a technology. “We will now seek to further reduce the programming power (we think another factor of 10 is possible) and also improve the long-term reliability of the memory bits,” Pop told

The work was described in Sciencexpress doi:10.1126/science.1201938.


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