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Microscopic origin of the fast crystallization ability of Ge–Sb–Te phase-change memory materials

Nature Materials volume 7, pages 399–405 (2008)Cite this article

Abstract

Ge–Sb–Te materials are used in optical DVDs and non-volatile electronic memories (phase-change random-access memory). In both, data storage is effected by fast, reversible phase changes between crystalline and amorphous states. Despite much experimental and theoretical effort to understand the phase-change mechanism, the detailed atomistic changes involved are still unknown. Here, we describe for the first time how the entire write/erase cycle for the Ge2Sb2Te5 composition can be reproduced using ab initio molecular-dynamics simulations. Deep insight is gained into the phase-change process; very high densities of connected square rings, characteristic of the metastable rocksalt structure, form during melt cooling and are also quenched into the amorphous phase. Their presence strongly facilitates the homogeneous crystal nucleation of Ge2Sb2Te5. As this simulation procedure is general, the microscopic insight provided on crystal nucleation should open up new ways to develop superior phase-change memory materials, for example, faster nucleation, different compositions, doping levels and so on.

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Figure 1: Model configurations showing crystallization of GST materials in simulations of slow cooling starting from the liquid, and of annealing the amorphous state obtained by rapid melt quenching.
Figure 2: Two representations of the high densities of crystal seeds in models of Ge2Sb2Te5 melts present at temperatures much above the melting point.
Figure 3: Different aspects of the structural evolution of a crystallizing melt of Ge2Sb2Te5 during a stepwise quench.
Figure 4: Structural characterization of the amorphous state of 225.
Figure 5: Structural evolution during a complete simulated phase-change cycle (liquid–amorphous–crystal) for 225.
Figure 6: Different evolution of the local structure around Ge atoms during slow and rapid quenches from the melt of Ge2Sb2Te5, leading to crystalline and amorphous products, respectively.

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Acknowledgements

Stimulating discussions with A. L. Greer, J. Hafner, G. Kresse, D. Cockayne, J.-Y. Raty, T. Bucko, S. Kugler, G. Csanyi, K. Borisenko and P. Jovari are gratefully acknowledged. J.H. is grateful for the award of a Marie-Curie Fellowship. All simulations were carried out using the Cambridge High-Performance Computer Facility.

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  1. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK

    J. Hegedüs & S. R. Elliott

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J.H. was responsible for carrying out the simulations and the analysis of the results. S.R.E. was responsible for the project planning and for writing much of the paper.

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Correspondence to S. R. Elliott.

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Hegedüs, J., Elliott, S. Microscopic origin of the fast crystallization ability of Ge–Sb–Te phase-change memory materials. Nature Mater 7, 399–405 (2008). https://doi.org/10.1038/nmat2157

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