Simulation of crystallization inGe2Sb2Te5: A memory effect in the canonical phase-change material

Abstract

Crystallization of amorphous Ge 2 Sb 2 Te 5 (GST) has been studied using four extensive (460 atoms, up to 4 ns) density functional/molecular dynamics simulations at 600 K. This phase change material is a rare system where crystallization can be simulated without adjustable parameters over the physical time scale, and the results could provide insight into order-disorder processes in general. Crystallization is accompanied by an increase in the number of ABAB squares (A: Ge, Sb; B: Te), percolation, and the occurrence of low-frequency localized vibration modes. A sample with a history of order crystallizes completely in 1.2 ns, but ordering in others was less complete, even after 4 ns. The amorphous starting structures without memory display phases (>1 ns) with subcritical nuclei (10-50 atoms) ranging from nearly cubical blocks to stringlike configurations of ABAB squares and AB bonds extending across the cell. Percolation initiates the rapid phase of crystallization and is coupled to the directional p-type bonding in metastable GST. Cavities play a crucial role, and the final ordered structure is distorted rock salt with a face-centered cubic sublattice containing predominantly Te atoms. We comment on earlier models based on smaller and much shorter simulations.

Densified network glasses and liquids with thermodynamically reversible and structurally adaptive behaviour

“…Crystallization can be hardly achieved on computer timescales (see however ref. 40) so that a direct connection between isostaticity and thermal stability cannot be made for the present densified silicates. However, experiments reveal that glass stability, quantified by the quantity DT ¼ T x À T g , is enhanced for such isostatic compositions.…”

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

“…Such tetrahedral bonding of Ge surrounded by Vcs has been conjectured some time ago also for the bulk of the GST-225 rocksalt structure by using electron microscopy and diffraction data in combination with DFT results [40]. However, this interpretation has not been confirmed in subsequent works using TEM [31,41,42], XRD [43,44], extended x-ray absorption fine structure (EXAFS) [45,46], and more refined DFT calculations, partially combined with molecular dynamics (MD) simulations [47,48]. Interestingly, a nuclear magnetic resonance (NMR) study provided some hints for the presence of tetrahedral Ge within a so-called nanocrystalline GST-225 phase, but not in its microcrystalline counterpart.…”

Collective excitations and viscosity in liquid Bi

“…30 Simulations with cells of this size are numerically very demanding, but the resulting structural data are more reliable than those obtained with smaller samples. In the case of crystallization of the amorphous phase change material Ge 2 Sb 2 Te 5 , for example, simulations using 460 atoms 31 gave qualitatively different but more convincing results than those with 180 atoms. 32 The starting structures were determined from reverse Monte Carlo (RMC) fits to x-ray diffraction data, 33 the time step was 3.025 fs, and the temperature was controlled by a Nosé-Hoover thermostat.…”