Nanoscale engineering to spawn 'universal memory'
Weijie Wang and her fellow researchers have shown that nanometer-scale engineering of so-called phase-change materials could lead to the so-called "universal memory". The atoms in phase-change materials, such as Ge2Sb2Te5 (GST), can arrange themselves into one of two configurations. These two "phases" act as the ones and zeroes in digital information, and a pulse of electricity can change the material from one to the other. The ease with which GST changes phase, however, is both a blessing and a curse. On the plus side, it means that it can store data very quickly but, on the down side, it is prone to switching phase unexpectedly and thus losing the data.
Phase-change random-access memory (PCRAM) is one of the most promising approaches to universal memory. Previous research has shown that adding nitrogen to GST, creating NGST, makes a more stable material, but also slows the phase-change process.
Phase change in NGST. Faster when the grains are smaller and arranged into tinier cells, owing to an increase in the ratio of surface (blue) to internal (red) grains and the interface between grains.
Wang and her co-workers showed, however, that both high speed and high stability are possible simultaneously. They experimentally demonstrated that phase change in NGST became much faster by scaling down physically. "We developed a dual-scaling technique to reduce both the overall material volume and the size of the individual grains that make up NGST," she said.
When the researchers deposited small-grain NGST into the pores of a thin film of silicon dioxide, they found that phase change in 20nm -wide structures containing 5nm grains was as much as 17 times faster than devices created in 200nm pores. This increase in speed is because the mechanism that drives phase change is fundamentally different for smaller grains that are in smaller cells, owing to their higher surface-area-to-volume ratio.
"In principle, this method is applicable to all types of phase-change materials," stated Wang. "So, appropriate choice of device structure and phase-change material opens new opportunities for optimizing memory device performance."