Controlling nucleation in amorphous-to-crystal transition to break the speed limit of phase-change memory
Johns Hopkins University, USA
This talk describes an example of advancing materials performance from the nanoscale, the theme of research at CAMP-Nano. We report a success story (F. Rao et al., Science 2017) that take advantage of metastable alloys for nanoelectronics, setting an unprecedented operation speed for memory and switch applications. Specifically, we have designed a new phase-change alloy with drastically reduced crystal nucleation stochasticity to accomplish sub-nanosecond switching for cache-type phase-change random-access memory (PCRAM) technology.
Operation speed is currently a key challenge in PCRAM technology, especially for achieving sub-nanosecond high-speed cache-memory (such as SRAM). The limiting factor in the commercialized PCRAM products is the writing speed (~currently several tens of nanoseconds), which originates from the stochastic crystal nucleation during the crystallization of the amorphous Ge2Sb2Te5 glass. Here we demonstrate an alloying strategy that can speed up the crystallization kinetics by orders of magnitude in phase-change memory glass. The newly designed chalcogenide alloy enables a record-setting writing speed (as short as ~700 picoseconds) in a conventional PCRAM device, with no requirement for pre-programming or additional device design. This ultrafast crystallization stems from the reduced stochasticity of nucleation via geometrically matched and robust chemical bonds that stabilize crystal precursors in the amorphous state, which are found via ab initio simulations to exhibit long life-times. This discovery not only is a milestone that paves the way for the development of “universal memory” using PCRAM technology to boost the working efficiency of computing systems, but also highlights materials science principles in action, offering the insight to guide the alloy design from atomic (bonding configurations and sub-critical nuclei) scale.
Feng Rao, Keyuan Ding, Yuxing Zhou, Yonghui Zheng, Mengjiao Xia, Shilong Lv, Zhitang Song, Songlin Feng, Ider Ronneberger, Riccardo Mazzarello, Wei Zhang, and Evan Ma, Science 358 (6369), 1423 (2017).
E. Ma did his undergraduate work at Tsinghua University and graduate work at Tsinghua University and Caltech, followed by postdoc sojourns at MIT and University of Michigan. He is currently a professor in the Department of Materials Science and Engineering at Johns Hopkins University. Prof. Ma has published ~310 papers, with ~23,500 citations and h index=79 according to SCI (Web of Science), and ~32,500 citations and h index=90, according to Google Scholar. Dr. Ma has presented ~127 invited talks at international conferences (and another ~88 invited speeches at academic institutions). He is an elected Fellow of ASM, APS, and MRS. Dr. Ma has also been an adjunct professor (Qian Ren B) at Xi’an Jiaotong University since 2009. His current research interests include amorphous metals (metallic glasses), chalcogenide phase-change alloys for memory applications, strength/ductility and plasticity mechanisms of nanostructured metals, and in situ transmission electron microscopy of small-volume materials exposed to mechanical, thermal and environmental stimuli.