Absence of Partial Amorphization in GeSbTe Chalcogenide Superlattices

Abstract

Phase‐change materials (PCMs) are widely used for optical data storage due to their fast and reversible transitions between a crystalline and an amorphous phase that exhibit reflectivity contrast. In the last decade, PCMs have been found to be promising candidates for the development of nonvolatile electronic memories, as well. In this context, superlattices of thin layers of GeTe and Sb2Te3 show an unprecedented performance gain in terms of switching speed and power consumption with respect to bulk GeSbTe compounds. Models of crystalline–crystalline transitions, proposed to explain the improved properties, however, are challenged by recent experiments in which GeTe–Sb2Te3 superlattices are observed to reconfigure toward a van der Waals heterostructure of rhombohedral GeSbTe and Sb2Te3. Herein, ab initio molecular dynamics simulations are used to explore an alternative switching mechanism that comprises amorphous–crystalline transitions of ultrathin GeSbTe layers between crystalline Sb2Te3. Despite some positive results obtained by tailoring the quenching protocol, overall the extensive simulations do not yield clear evidence for this mechanism. Therefore, they suggest that the switching process probably involves a transition between two crystalline states.

Ab Initio Study of Novel Phase‐Change Heterostructures

“…In this respect, these superlattice models behave differently from the GST/Sb 2 Te 3 models forming the so called interfacial PCMs, [24,51] in which the Sb 2 Te 3 layers are not sufficiently robust (due to their low melting temperature) to serve as crystalline spacers between the actively switching GST layers. [52] In our simulations, the use of a single TiTe 2 trilayer was dictated by computational convenience. Using thicker CM layers may further improve the robustness of the PCH.…”

In‐Plane Twinning Defects in Hexagonal GeSb2Te4

“…[39,40] However, quasi-2D amorphization in the superlattice is unlikely due to the close melting temperature of GeTe and Sb 2 Te 3 (≈100 °C), which was corroborated by ab initio molecular dynamics simulations. [41] Partial amorphization was instead achieved in TiTe 2 /Sb 2 Te 3 phase-change heterostructures, [42][43][44][45] because of the large difference in melting temperature (>500 °C) between the two constituting alloys. Some models ascribe the switching in GST/Sb 2 Te 3 iPCMs to the presence of extended defects, [46] namely, swapped bilayers connecting the edges of adjacent atomic stacks, which are consistently found in GST samples grown by deposition and epitaxy methods.…”