Crystal growth of Ge 2 Sb 2 Te 5 at high temperatures
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Research Letter
Crystal growth of Ge2Sb2Te5 at high temperatures I. Ronneberger, Institute for Theoretical Solid State Physics, RWTH Aachen University, Aachen D-52074, Germany W. Zhang, Center for Advancing Materials Performance from the Nanoscale, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China R. Mazzarello, Institute for Theoretical Solid State Physics, RWTH Aachen University, Aachen D-52074, Germany; JARA-FIT and JARA-HPC, RWTH Aachen University, Aachen D-52074, Germany Address all correspondence to W. Zhang at [email protected], R. Mazzarello at [email protected] (Received 13 June 2018; accepted 5 July 2018)
Abstract Phase-change materials (PCMs) have important applications in optical and electronic storage devices. Ge2Sb2Te5 (GST) is a prototypical phase-change material (PCM) employed in state-of-the-art storage-class memories. In this work, we investigate crystallization of GST at temperatures 600–800 K by ab initio molecular dynamics. We consider large models containing 900 atoms, which enable us to investigate finitesize effects by comparison with smaller models. We use the metadynamics method to accelerate the formation of a large nucleus and then study the growth of the nucleus by unbiased simulations. The calculated crystal growth speed and its temperature-dependent behavior are in line with recent experimental work.
Introduction Phase-change materials (PCMs) are employed in storage devices (rewritable optical discs, non-volatile electronic memories), which exploit their ability to undergo fast and reversible transitions between the crystalline and amorphous state.[1–3] Long-term information storage is possible, thanks to the optical and electronic contrast between the two phases, as well as the high stability of the amorphous state near room temperature. Phase-change storage-class memories have recently entered the market[4,5]: these devices are significantly faster than flash memories, and thus, they fill the gap between DRAM and solidstate drives in terms of performance. The most important family of PCMs for memory applications belongs to the pseudobinary line connecting GeTe with Sb2Te3.[1–3,6–11] In particular, Ge2Sb2Te5 (GST) is currently used in electronic memories. Crystallization in PCMs is slower than the reverse amorphization process; hence, the former determines the maximum speed of the device. Recently, prototype phase-change cells containing Sc-alloyed Sb2Te3 (Sc0.2Sb2Te3) have shown subnanosecond crystallization.[6] Such switching speed is comparable with that of SRAM, and could lead to the development of a universal phase-change memory. Considerable experimental and theoretical work has been devoted to the study of the strong temperature dependence of the dynamics of PCMs.[12–18] This is a key property that ensures said fast crystallization at elevated temperatures and high stability of the amorphous phase at low temperatures. Such property has been linked to the high fragility of the supercooled liquid state,
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