Xiong receives Gold Award from MRS

Consumers have come to expect the plummeting size and price of computer memory. Flash drives have become smaller and simultaneously gained capacity. Smartphones can store more data than some decade-old laptops. Digital cameras record thousands of images. 

To keep these size-and-price reductions coming, alumnus Feng Xiong (MSEE ’10 PhD ’14) has been exploring a promising next-generation technology known as phase-change memory. His research began during his time at ECE ILLINOIS, and recently, it earned him a Gold Graduate Student Award at the

Materials Research Society’s Spring Meeting and Exhibit, an international event that packs a convention hall in San Francisco with more than 5,000 attendees.

Phase-change materials themselves aren’t new, but their application to solid-state computer memory is. At present, phase-change materials are most widely used for optical data-storage devices like DVDs and CDs. When the laser is burning a disc, it is actually heating the phase-change material and, depending on the power and duration of the laser pulse, the molecular structure can be reversibly changed between an amorphous and crystalline state. These alternate structures reflect light at different wavelengths, and the pattern can then be interpreted as the zeros and ones used in binary data storage. 

These structures also have differing electrical resistivities, which means that the basic premise has potential utility within computers, smartphones, and other electronic devices. The trick is optimizing the materials for those applications. This is where Xiong’s research comes into play.  

“Flash memory is kind of reaching its scaling limit nowadays,” Xiong said. “That’s why people are trying to find a substitute for that, and phase-change memory is one of the most promising candidates.”

Scalability is one of Xiong’s primary research interests, proving that phase-change materials will be viable for multiple generations of smaller and sleeker devices.

“We want to see fundamentally how small we can make this material, down to nanometers, down to just a few atoms, and we want to see if there can still be this change in structure, this phase change,” Xiong said. “We were able to show that, even when the device dimension was down to just a few hundreds of cubic nanometers, we were still able to induce phase change.”

To scale the materials to this extreme, Xiong and his adviser, Adjunct Associate Professor Eric Pop (now a faculty member at Stanford), turned to carbon nanotubes, which function like nanoscopic wires, conducting current to each node in the phase change material, thus toggling between the zeros and ones. When their research was initially published in 2011, it was the cover article for Science. (Read additional coverage here.) They’ve continued to refine the scaling techniques in subsequent years.

The unique thing about phase-change memory is that the information is stored in the structure of the material rather than in an electrical charge. With Flash memory, for instance, there are billions of tiny transistors, either charged or uncharged, each representing one binary digit. This charge, however, can leak and degrade, especially after repeated read-write cycles or when exposed to radiation, a critical limitation for space equipment. Because phase-change memory is structure-based, these performance issues would be resolved. 

The other focus of Xiong’s research is the material itself. Last summer, he interned at IBM’s T.J. Watson Research Center, located north of New York City, where he worked with Dr. Simone Raoux and her research team, testing new materials for this application. With DVDs and CDs and other optical devices, a particular alloy of germanium, antimony, and tellurium, known as GST, is commonly used, but problematically, this material expands and shrinks during phase change by a factor of about 7 percent. This causes spatial limitations, of course, and repeated volume changes also reduce the lifetime of the device. 

“Imagine you have a memory device you can read and write, so you can cycle it between a binary zero and one phase, and then you have a volume expansion and shrinking,” Xiong said. “That’s not going to work very well.”

Working with the IBM researchers, an alloy of gallium, antimony, and trace amounts of other proprietary elements was selected. This material exhibits no volume change, and it also demonstrates a relatively high crystallization temperature. Memory devices composed of this material, therefore, could operate at higher temperatures, like the sweltering heat of data centers.

“Before it can be commercialized, there are still other tasks,” Xiong said. “I believe IBM has already been doing some device fabrication on that, in the sense that they want to make actual electrically programmable memory devices based on these materials. And then they’re going to test other electrically related performance, like how fast you can switch and the reliability.”

Once prototypes have been developed, Xiong, who is now a postdoctoral researcher at Stanford, will continue his collaboration with the IBM researchers, testing the scalability of their prototypes with the platform and techniques developed through the course of his graduate studies. 

“Hopefully, that’s going to happen later this year,” he said. “And hopefully, if that’s promising, they can start thinking of other ways to mass-produce.”

The Gold Graduate Student Award was granted based on a synoptic presentation Xiong gave at the Material Research Society’s meeting, which covered his research on phase-change materials and their scalability. Seven other graduate students were also awarded, selected from an international pool.  

“I was excited about it,” Feng said of the award. “And it was also a good opportunity for me to learn other people’s interesting work. ... It’s kind of a network-building experience. I got to meet some of the elite graduate students from around the world.”

In fact, after the awards ceremony, he and some of the other honorees gathered in a nearby restaurant to brainstorm and discuss their research in more depth. “We were just trying to see if there was a common area where we can kind of collaborate,” Feng said. “There are definitely more talks on that topic. ... So that’s a pretty good experience.”