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From local structure to nanosecond recrystallization dynamics in AgInSbTe phase-change materials

Abstract

Phase-change optical memories are based on the astonishingly rapid nanosecond-scale crystallization of nanosized amorphous ‘marks’ in a polycrystalline layer. Models of crystallization exist for the commercially used phase-change alloy Ge2Sb2Te5 (GST), but not for the equally important class of Sb–Te-based alloys. We have combined X-ray diffraction, extended X-ray absorption fine structure and hard X-ray photoelectron spectroscopy experiments with density functional simulations to determine the crystalline and amorphous structures of Ag3.5In3.8Sb75.0Te17.7 (AIST) and how they differ from GST. The structure of amorphous (a-) AIST shows a range of atomic ring sizes, whereas a-GST shows mainly small rings and cavities. The local environment of Sb in both forms of AIST is a distorted 3+3 octahedron. These structures suggest a bond-interchange model, where a sequence of small displacements of Sb atoms accompanied by interchanges of short and long bonds is the origin of the rapid crystallization of a-AIST. It differs profoundly from crystallization in a-GST.

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Figure 1: Phase diagram of PC materials and crystallization patterns.
Figure 2: HXRD data for AIST and GST, and atomic configurations of a-AIST and a-GST.
Figure 3: Valence-band spectra and bond-order distributions in c- and a-AIST.
Figure 4: PC mechanism of a-AIST.
Figure 5: Ring statistics and PC in a-AIST and a-GST.

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Acknowledgements

This work was supported by Core Research for Evolutional Science and Technology (CREST) ‘X-ray pinpoint structural measurement project—Development of the spatial- and time-resolved structural study for nano-materials and devices’ and by the Academy of Finland and the Japan Science and Technology Agency through the Strategic Japanese–Finnish Cooperative Program on ‘Functional materials’. The synchrotron radiation experiments were approved by the Japan Synchrotron Radiation Research Institute (proposal Nos 2007A1223, 2008A1409 and 2009A12386), and all calculations were carried out on the Jugene (IBM BlueGene/P) and Juropa (Xeon 5570) computers in the Forschungszentrum Jülich with grants from the John von Neumann Institute for Computing and the Forschungszentrum Jülich. We thank N. Yasuda and Y. Fukuyama for assistance in the density estimation measurement and H-P. Komsa for providing the initial 648-atom system coordinates for crystalline AIST.

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The experiments/calculations were carried out and analysed as follows: sample preparation, R.K., T.M., N.Y.; high-energy XRD and RMC, S.K., M.T.; EXAFS, T.H., S.K., T.M.; XPS, E.I., K.K., T.M.; DF-MD, J.A., R.O.J. The manuscript was planned by J.A., R.O.J., S.K., T.M. and N.Y. and written by R.O.J.

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Correspondence to Noboru Yamada.

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Matsunaga, T., Akola, J., Kohara, S. et al. From local structure to nanosecond recrystallization dynamics in AgInSbTe phase-change materials. Nature Mater 10, 129–134 (2011). https://doi.org/10.1038/nmat2931

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