Crystallization of Ge₂Sb₂Te₅ under high hydrostatic pressures: Differences in nanoscale atomic ordering in as-deposited and pressure-induced amorphous phases

The crystallization process acts as a bottleneck to the development of phase-change memory devices. Here, we compare the crystallization of as-deposited and pressure-induced amorphous phases of Ge₂Sb₂Te₅ under hydrostatic pressures up to 8.5 GPa. The as-deposited phase fully converts to a stressed metastable cubic phase (at ca. 135 °C) at pressures below 3 GPa and remains cubic up to the maximum temperature used (240 °C). At higher pressures, the as-deposited phase partially crystallizes directly into the stable hexagonal phase at a significantly lower temperature (110 °C), however a significant volume fraction of the amorphous phase remains even for temperatures as high as 240 °C. The intensities of the Bragg diffraction peaks dramatically decrease with increasing pressure, further underscoring the suppression of crystal growth. In stark contrast, the pressure-induced amorphous phase- due to memory effects originating from the crystalline phase - crystallizes at ambient conditions at a lower temperature than its as-deposited counterpart. Furthermore, the pressure-induced amorphous phase also fully transforms directly into the hexagonal modification at pressures up to ca. 5 GPa. At higher pressure (8.5 GPa), an orthorhombic phase is formed. Different from the as-deposited phase, the crystallization temperature of pressure-induced amorphous Ge₂Sb₂Te₅ increases with pressure. The results reported here demonstrate that differences in nanoscale ordering in as-deposited (statistically ordered) and pressure-induced (chemically ordered) amorphous phases dramatically influence crystallization and will serve as a guideline for insightful development of phase-change devices.