Direct atomic identification of cation migration induced gradual cubic-to-hexagonal phase transition in Ge2Sb2Te5

Abstract

GeTe-Sb 2 Te 3 pseudobinary system, especially Ge 2 Sb 2 Te 5 alloy, is the most desirable material to be commercialized in phase change random access memory. Directly resolving the local atomic arrangement of Ge 2 Sb 2 Te 5 during intermediate steps is an effective method to understand its transition mechanism from face-centered-cubic to hexagonal phases. In this study, we provide insights into the atomic arrangement variation during face-centered-cubic to hexagonal transition process in Ge 2 Sb 2 Te 5 alloy by using advanced atomic resolution energy dispersive X-ray spectroscopy. Induced by thermal annealing, randomly distributed germanium and antimony atoms would migrate to the specific (111) layer in different behaviors, and antimony atoms migrate earlier than germanium atoms during the phase transition process, gradually forming intermediate structures similar to hexagonal lattice. With the migration completed, the obtained stable hexagonal structure has a partially ordered stacking sequence described as below:-Te-Sb x /Ge y-Te-Ge x /Sb y-Te-Ge x /Sb y-Te-Sb x /Ge y-Te-(x > y), which is directly related to the migration process. The current visual fragments suggest a gradual transition mechanism, and guide the performance optimization of Ge 2 Sb 2 Te 5 alloy.

Phase change thin films for non-volatile memory applications

“…This leads to the rearrangement of Ge and Sb atoms and to gradual ordering of structural vacancies in the planes in the GST225 lattice. 53,55,[66][67][68] Therefore, different distributions of vacancies within the structure of GST225 result in different metastable GST225 structures, indicating polymorphism of metastable GST225 crystal structures. Finally, the most thermodynamically favourable GST225 phase contains a highly ordered distribution of vacancies in {111} planes ( Fig.…”

“…The Te-Te distances across the vacancy layers vary depending on the degree of vacancy ordering. 68 The reported values for polycrystalline samples range between 0.33 nm (partially ordered) and 0.304 nm (fully ordered). 52,[66][67][68][69] However, GST-based materials of epitaxial thin lms grown on a substrate show slightly larger values for the Te-Te distances (0.33-0.34 nm) as for relaxed polycrystalline thin lms, probably either due to residual strain in the epitaxial lms or due to thermodynamic stabilization effects towards the lowest energy structure.…”

“…68 The reported values for polycrystalline samples range between 0.33 nm (partially ordered) and 0.304 nm (fully ordered). 52,[66][67][68][69] However, GST-based materials of epitaxial thin lms grown on a substrate show slightly larger values for the Te-Te distances (0.33-0.34 nm) as for relaxed polycrystalline thin lms, probably either due to residual strain in the epitaxial lms or due to thermodynamic stabilization effects towards the lowest energy structure. 69 In all cases, the distances between Te planes across the vacancy layers are slightly larger compared to the Te-Te distances of 0.284 nm across the van der Waals gaps, representing similar vacancy layers within the stable GST225 phase (see the next subchapter).…”

“…3(c)-(e)). 68,69 The preferential ordering of Sb atoms is however found in the cation layers next to the vacancy layers, whereas Ge species tend to accumulate preferentially in the middle of GST building units (Fig. 3).…”

“…4(b)-(h)). 54,59,67,68,[77][78][79] The mixed Ge/Sb layers close to the vdW always are rich in Sb (e.g. Sb66Ge33 (ref.…”