Initial Structure Memory of Pressure-Induced Changes in the Phase-Change Memory Alloy ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$

We demonstrate that while the metastable face-centered cubic (fcc) phase of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ becomes amorphous under hydrostatic compression at about 15 GPa, the stable trigonal phase remains crystalline. Upon higher compression, a body-centered cubic phase is obtained in both cases around 30 GPa. Upon decompression, the amorphous phase is retained for the starting fcc phase while the initial structure is recovered for the starting trigonal phase. We argue that the presence of vacancies and associated subsequent large atomic displacements lead to nanoscale phase separation and loss of initial structure memory in the fcc staring phase of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$.

Figure 1

Evolution of the x-ray diffraction patterns of the metastable fcc phase (left) and the stable trigonal phase (right) of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ upon compression. The corresponding pressures are marked next to each curve.

Figure 2

X-ray diffraction patterns of bcc phases of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ obtained starting with the fcc (left) and trigonal (right) phases.

Figure 3

Evolution of the x-ray diffraction patterns of the metastable fcc phase (left) and the stable trigonal phase (right) of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ upon decompression. The corresponding pressures are marked next to each curve.

Figure 4

Pressure dependence of the normalized volume per (Te) atom (assuming ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ stoichiometry throughout the complete cycle) upon compression and decompression for the metastable fcc and stable trigonal phases of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. The error bars are smaller than the size of the symbols used. The triangles correspond to the starting fcc phase and the circles correspond to the starting trigonal phase.

Figure 5

The bcc structure (top) and the corresponding diffraction pattern (bottom) for ${\mathrm{GeSb}}_2{\mathrm{Te}}_4$ “compressed” to 46 GPa using DFT simulations (Ge—red, Sb—green, Te—yellow). The insert shows a fragment of the obtained structure with clear bcc arrangement of atoms.