Computer simulation of amorphous-crystal phase transformations of GST-225 memory material

We have simulated, by ab initio molecular dynamics (MD), the entire phase-change (PC) cycle in Ge-Sb-Te (GST) materials. Rapid quenching from the melt results in formation of the amorphous state but, on slower cooling, the models crystallize into the metastable rocksalt crystalline phase. Thermal annealing of the quenched amorphous models also results in crystallization to the same polymorph. Significant numbers of connected, near-planar 4-membered (square) rings exist in the liquid state and are quenched into the amorphous solid. The presence of these crystal seeds is responsible for the rapid, homogeneously-nucleated crystallization behaviour characteristic of GST PC memory materials. Key words: Phase-change memory materials; Ge-Sb-Te; ab initio molecular dynamics 1