Ab Initio Computer Simulation of the Early Stages of Crystallization: Application to ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ Phase-Change Materials

By virtue of the ultrashort phase-transition time of phase-change memory materials, e.g., ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$, we successfully reproduce the early stages of crystallization in such a material using ab initio molecular-dynamics simulations. A stochastic distribution in the crystallization onset time is found, as generally assumed in classical nucleation theory. The critical crystal nucleus is estimated to comprise 5–10 $(\mathrm{Ge},\mathrm{Sb}{)}_4{\mathrm{Te}}_4$ cubes. Simulated growth rates of crystalline clusters in amorphous ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ are consistent with extrapolated experimental measurements. The formation of ordered planar structures in the amorphous phase plays a critical role in lowering the interfacial energy between crystalline clusters and the amorphous phase, which explains why Ge-Sb-Te materials exhibit ultrafast crystallization.

Figure 1

Atomic configurations (model 1) during the crystallization process in amorphous ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ and the evolution of structural units on annealing at 600 K. (a)–(d) Snapshots of model configurations at different stages of crystallization, periods I–IV (see text). The arrows in (f) indicate the times at which these snapshots were taken. (a) Formation of structural units during the incubation period I. A significant number of fourfold rings (silver) exist, but only a few planes (green) or cubes (red) are formed occasionally. (b) Development of ordered layer structures at the crystallization site. (c) A cube cluster and planes extending from the cluster interface. (d) Completely crystallized phase with a crystal-glass interface. (e) Mean-squared displacement (MSD) for each type of atom. (f)–(i) Evolution of structural units: (f) fourfold rings; (g) number of fourfold rings forming planes; (h) number of cubes; (i) number of atoms forming planes or cubes.

Figure 2

Time evolution of the effective radius ($R_{\mathrm{eff}}$) of the cube cluster for models 1–3. Dashed red lines are linear fits for the growth velocity. The vertical dotted lines indicate the periods I–IV, as in Fig. 1.

Figure 3

Chemical ordering during crystallization (model 1). (a) Proportion of wrong bonds on average and in fourfold rings. The large fluctuations in period I indicate that the discrete fourfold rings are relatively unstable compared to the fourfold rings comprising planes or cubes. (b) Model configuration showing a cube cluster with perfect chemical ordering. Ge (blue) and Sb (red) atoms are coordinated by Te (yellow) atoms in the cluster. Atoms in planes (green) lie at the interface between the cluster and the amorphous phase (purple).