Last week the University of California, San Diego announced that a group from its computer science department will be demonstrating a data storage system based on phase-change memory materials at the Device Automation Conference 2011 taking place this week in San Diego. According the press release, storage devices based on PCMMs are potentially thousands of times faster than conventional hard drive systems, and can be up to seven-times faster than current solid state drives, such as the flash memory currently used in laptops and consumer electronics like iPads. However, flash memory is still considered to be far too slow to meet the demands of high performance computing or quick, high-volume data sifting needed, for example, for analyzing data collected by environmental sensors or web searches conducted by Google.
In the press release, group leader Steven Swanson, professor of computer science and engineering and director of the Non-Volatile Systems Lab says, “As a society, we can gather all this data very, very quickly – much faster than we can analyze it with conventional, disk-based storage systems. Phase-change memory-based solid state storage devices will allow us to sift through all of this data, make sense of it, and extract useful information much faster. It has the potential to be revolutionary.”
PCMMss are binary or ternary chalocogenides that store data by switching between the amorphous and crystalline state by means of an applied electric current or optical pulse. The phase-change transition happens very quickly, hence the potential for high-speed, high-storage capacity devices. As with all earlier generation memory devices, there is a desire to scale down the size, which means the the storage density needs to be very high. Because each read–write event consumes a certain amount of power, addressing the power consumption issue is a major challenge to the development of the next generation of PCMMs.
Attention for nonvolatile storage devices has focused on chalcogenides in the Ge-Sb-Te ternary system, which are already being used for optical read-write applications such as DVDs and Blue Ray discs, and electronic memory systems, such as cell phones.
At present, PCMMs are synthesized by sputtering, however problems with void filling has been a barrier to continued downsizing of devices. At the recent GOMD meeting in Savannah, there were several presentations on the properties of PCMMs synthesized by electrodeposition and pulsed laser deposition Early results presented by Gang Chen’s group at Ohio University indicate that electrodeposition provides excellent void-filling capability.
Chen, an assistant professor in the department of physics and astronomy at OU, is also looking at approaches to reducing the power consumed to initiate the phase change. The phase transition temperature of tellurides ranges from 100-300 oC, depending on composition. In a conversation with me, Chen said that power consumption could be reduced by 30-50% if the phase transition temperature can be reduced to about 100 oC.
His group is looking at a “nanoscale confinement” approach, which is essentially a nanoscale composite device based on filling the pores of a mesoporous SiO2 matrix film with chalcogenide. By confining the amorphous chalcogenide in a nanoscale pore, surface energy is reduced and depresses the crystallization temperature without altering the composition of the material. His group reported on the successful electrodeposition of elemental Se and binary Sb-Te in SiO2 in several presentations at GOMD and in a poster, “Synthesis and characterization of Sb-Te and Ge-Te phase change memory materials,” by Chandrasiri Ihalawela (which captured first place, graduate student category).
The mesoporous SiO2 film is synthesized through a self-assembled sol-gel process and has a periodic, hexagonal arrangement of cylindrical pores in the 2-30 nm range. Electrodeposition of the chalcogenide within the film’s porosity creates a composite device with a honeycomb-like structure. Chen says the composite device has the potential to increase the data storage density while reducing the power consumption into target ranges.
A group at the University of Illinois, Urbana-Champaign is also working in a similar direction. In an article in the April 29 issue of Science (doi 10.1126/science.1201938), Xiong et al describe an approach to power reduction that uses carbon nanotube electrodes to trigger the phase change in a Ge-Sb-Te compound. They were able to achieve “best” results of under 1 μA to crystallize (set) and about 5 μA to amorphize (reset). This represents a reduction in switching current of two orders of magnitude over typical switching currents for other PCMM devices. The smallest amount of energy per bit of storage that they obtained was in the range of 100 femtojoules, however, because their research also showed that devices are highly scalable, they concluded that energy consumption of a fraction of a femtojoule per bit is achievable.