Researchers from the Electrical and Computer Engineering Department of the University of Illinois have developed a new low-power digital memory which uses much less power and is faster than other solutions currently available. The breakthrough could give future consumer devices like smartphones and laptops a much longer battery life, but might also benefit equipment used in telecommunications, science or by the military.

Industry has recently been exploring the use of phase-change materials (PCM) as an alternative to the kind of memory that stores bits as a charge. With PCM, each bit is stored in the resistance of the material itself and can be reversibly switched with short voltage pulses and localized Joule heating. Advantages to such contenders for universal non-volatile data storage include low voltage operation, fast access times and high endurance, but the cost is the relatively high programming current needed to couple Joule heat to finite bit volume.

The Illinois research group, led by professor Eric Pop, has managed to lower the power per bit to a fraction of that used for existing PCM solutions by using the smallest known electronic conductors instead of metal wires. The carbon nanotubes are 10,000 times smaller than a human hair and grown by chemical vapor deposition with iron catalyst particles on silicon dioxide/silicon (SiO2/Si) substrates. The resulting single-wall and small diameter multi-wall nanotubes were both found to be capable of switching the PCM bits.

The nanotubes span titanium/palladium metal contacts, and nanogaps were created, typically situated in the middle of the nanotube. A thin film of PCM was then deposited over the nanotubes, which filled the nanogaps and created self-aligned lateral PCM bits. A layer of SiO2 was placed on top of the PCM film without breaking vacuum to prolong switching lifetimes and avoid the degradation often suffered by metal wires. The device is also immune to accidental erasure from a passing scanner or magnet.

Bridging the nanogap

The device is initially in the off state until a voltage is applied to the nanotube that switches the PCM bit to an on state. Although the PCM film covers the whole structure, the switching only occurs at the nanogap. The current used to switch the state was found to be two orders of magnitude lower than that of conventional PCM in the test devices, requiring programming currents from 1 to 8 µA.

"The energy consumption is essentially scaled with the volume of the memory bit," said graduate student Feng Xiong, the first author of the paper. "By using nanoscale contacts, we are able to achieve much smaller power consumption."

While the display on consumer devices like smartphones and notebooks takes up a lot of the battery's power, more and more is being dedicated to memory. As the ultra-low-power digital memory uses 100 times less energy and is faster than the currently available solutions, it could offer immediate battery life benefits to such devices.

"Anytime you're running an app, or storing MP3s, or streaming videos, it's draining the battery," said Albert Liao, a graduate student and co-author. "The memory and the processor are working hard retrieving data. As people use their phones to place calls less and use them for computing more, improving the data storage and retrieval operations is important."

Future applications

Pop says that he envisions a point where a device could get its power needs from harvested thermal or mechanical energy or sourced purely from solar. Consumer devices won't be the only beneficiaries, however.

"We're not just talking about lightening our pockets or purses," Pop said. "This is also important for anything that has to operate on a battery, such as satellites, telecommunications equipment in remote locations, or any number of scientific and military applications."

Server farms or data centers could also benefit from lower energy costs by utilizing the solution. The researchers say that the low-power memory could even lead to previously elusive three-dimensional stacking of chips.

However, before that happen, the solution needs to be upscaled. The group has so far created and tested a few hundred bits and is currently working towards doubling up each PCM bit through programming to achieve multibit memory. Pop also believes that the groundbreaking energy efficiency already achieved could be further improved by a factor of ten.

The research paper entitled Low-Power Switching of Phase-Change Materials with Carbon Nanotube Electrodes has now been published in Science Magazine.

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