Phase change behavior and critical size of Ge2Sb2Te5 nanowires and nanotubes

The phase change behavior of Ge2Sb2Te5 (GST) nanowires and nanotubes was investigated by using thermodynamic calculations. It is revealed that the melting and crystallization temperatures for both nanowires and nanotubes decrease as the diameter of the nanostructures is reduced. There exists a critical diameter for both nanowires and nanotubes, below which the crystallization could not take place. It determines the ultimate scaling limit of nanowires or nanostructures phase-change memory. The critical diameter for nanowires depends on the difference between the surface energy of solid and liquid phases, the bulk melting and crystallization temperatures, as well as the melting and crystallization entropy. The critical diameter of the nanotubes is larger than that of the nanowires. And it is also dependent on the ratio of outer diameter over inner diameter of the tubes. The lower ratio of outer diameter over inner diameter gives rise to the larger critical diameter of the nanotubes.