By looking at the change in "real time," the Penn researchers could see what effect the electrical pulses were having at an atomic level of detail.
"The pulses create 'dislocations,' which are planes of atoms removed from the crystal pattern, disrupting the order locally on an atomic length scale," said Pavan Nukala, a co-author and member of the Agarwal group. "As we apply more and more pulses, the number of these dislocations start to increase."
"Eventually, the dislocations start to move down the nanowire in the direction of the current," Agarwal said. "At certain point, the number and density of dislocations becomes so huge that they jam in one spot
Like a traffic jam on a highway, the dislocations continue to pile up at that spot as more and more move down the length of the nanowire. At a critical point, the increasing disorder causes the material to amorphize the wire at the location of the jam.
The amorphous region, which always forms at the point of the jam and cuts through the entire cross section of the nanowire, is proof that this dislocation-based mechanism is fundamentally different from the melt-quench mechanism. With melting, the amorphous part should have spread along the surface of the material, rather than cut through its cross section
"Having the surface amorphize doesn't give us high resistance ratios because current can still travel through the crystalline interior," Agarwal said. "Cutting across the entire nanowire completely blocks the current, making for a much better memory devices.
"With surface melting, you can increase the resistance a few times at most, but our observation that the resistance increased by two or three orders of magnitude is another evidence of the new mechanism."
The PCM that researchers used in their study contained long tellurium-telluriumbonds that can easily slide apart, facilitating the planar dislocations that cause the material to amorphize.
The material, along with a better understanding of the mechanics of its phase change, will provide a starting point for picking the right qualities for future PCMs.
"If people think that melting is the only mechanism for phase change, then all the emphasis will be on making materials with low melting temperatures," Agarwal said. "But we've shown that we need to do something else, which is to also look for materials that can create dislocations easily
In addition to Agarwal, Li, Johnson and Nukala, the research was conducted by Sung-Wook Nam, Hee-Suk Chung, Yu Chieh Lo, Liang Qi, Ye Lu and Yeonwoong Jung, all of the Department of Materials Science and Engineering in Penn Engineering