Unique Melting Behavior in Phase-Change Materials for Rewritable Data Storage

${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST) is a technologically very important phase-change material for rewritable optical and electrical storage because it can be switched rapidly back and forth between amorphous and crystalline states for millions of cycles by appropriate pulsed heating. However, an understanding of this complicated phenomenon has not yet been achieved. Here, by ab initio molecular dynamics, we unravel the reversible phase transition process of GST. The melting of rocksalt-structured GST is unique in that it forms two-dimensional linear or tangled clusters while keeping order in the perpendicular direction. It is this specific character that results in the fast and reversible phase transition between amorphous and crystalline and hence rewritable data storage.

Figure 1

Radial distribution function of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ at temperatures from 600 to 2000 K.

Figure 2

Ion trajectories of atoms for ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ in the temperature range from 600 to 2000 K.

Figure 3

The mean square displacement averaged over atoms for ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. (a) 1500 K. (b) 2000 K, wherein the solid line, dashed line, and dash-dotted line represent the $[1\overline{1}0]$ ($R_x$), $[01\overline{1}]$ ($R_y$), and [111] ($R_z$) directions, respectively.

Figure 4

Structural evolutions at 600 K in the crystallization process for amorphous ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. (a) the amorphous structure obtained at 1500 K. (b) the amorphous structure obtained at 2000 K.