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Evan Ma1

1, Johns Hopkins University, Baltimore, Maryland, United States

This talk describes our recent success (F. Rao et al., Science 2017) in controlling the amorphous structure of chalcogenide Sc-Sb-Te glass to accelerate its crystallization, reaching an unprecedented operation speed for memory and switch applications. Specifically, we have designed a new phase-change memory alloy with drastically reduced crystal nucleation stochasticity from the parent amorphous phase. The ultrafast transition between the two metastable states accomplishes sub-nanosecond switching for cache-type phase-change random-access memory (PCRAM) technology. This is a milestone in memory materials, because 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 use alloying into the parent glass to speed up the crystallization kinetics by orders of magnitude. The newly designed chalcogenide Sc-Sb-Te 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, shortening the incubation time for crystallization. This discovery is an example of physical metallurgy principles in action, using atomic-scale insight into glass structures (bonding configurations and sub-critical nuclei) to control properties. For details, see F. Rao et al., Science 358 (6369), 1423 (2017).

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