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Computer Simulation of the Phase-change Cycle of GST-225

Published online by Cambridge University Press:  01 February 2011

Stephen Elliott  and
Jozsef Hegedus
Stephen Elliott
Affiliation:
[email protected], University of Cambridge, Chemistry, Lensfield Road, Cambridge, CB2 1EW, United Kingdom, 01223 336525, 01223 336362
Jozsef Hegedus
Affiliation:
[email protected], University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge, CB2 1EW, United Kingdom
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Abstract

We have simulated for the first time, by ab initio molecular dynamics, the complete phase-transformation cycle (liquid-crystal, liquid-amorphous-crystal) of the phase-change (PC) memory material Ge2Sb2Te5 (GST-225). We have observed that rapid cooling of the simulated melt leads to an amorphous product, whereas slow cooling results in the metastable rocksalt crystal. Furthermore, crystallization to the same structure is observed to occur on annealing the quenched amorphous model to temperatures below the melting temperature. The RDF of the energy-relaxed amorphous GST-225 structure agrees very well with experimental neutron-diffraction data, reproducing the shortening of the Ge-Te bond length relative to that in the rocksalt crystal structure observed experimentally.We have observed crystal-nucleation events in the simulated liquid that have been identified as the creation of connected near-regular square fourfold rings, the basic structural units of the rocksalt structure. These crystal nuclei are invariably found to be quenched into the amorphous state on rapid cooling of the simulated melt. This observation therefore explains why GST materials crystallize so readily and why homogeneous nucleation is so facile.

Keywords

amorphousphase transformationsimulation
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1072: Symposium G – Phase-Change Materials for Reconfigurable Electronics and Memory Applications , 2008 , 1072-G01-02
Copyright
Copyright © Materials Research Society 2008

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