Effect of vacancy ordering in metastable Ge-Sb-Te phase-change materials

Chalcogenide ternary compounds Ge₂Sb₂Te₅ (GST) are well-known phase-change materials that exhibit fast structural transition between amorphous and crystalline phases. Metastable (m-)GST, its crystalline phase, is a distorted NaCl structure with 4(b) sites occupied by 40% Ge, 40% Sb, and 20% vacancies. Atomic configuration of 4(b) sites, especially the distribution of vacancy, is known to be crucial not only for phase-change process but also to the electrical transition such as Anderson localization. Despite its significance, consensus on the 4(b)-site configuration is yet to be established. In this talk, we present the effect of vacancy ordering in m-GST on material properties. We calculated energetics of m-GST for various vacancy configurations using first-principles calculations and tight-binding method. For quantitative analysis of the relationship between vacancy distribution and energetic stability, we established a set of parameters that quantify the degree of vacancy disorder in m-GST to identify key structural features that govern the system’s energy and electrical conductivity. Correlation between annealing temperature and vacancy ordering of m-GST was also investigated using molecular dynamics simulations.