Phase change materials: Density functional / molecular dynamics simulations of Ag/In-doped Sb4Te alloy

Phase change memories are based on the ultrafast (nanosecond scale) crystallization of nanosized amorphous “marks” in a polycrystalline layer. Until recently, the amorphous atomic structure of phase change alloys, such as Ge₂Sb₂Te₅ (GST, used in DVD-RAM optical recording) and Ag_3.5In_3.8Sb_75.0Te_17.7 (AIST, used in DVD±RW), has remained elusive. We have performed extensive density functional / molecular dynamics (DF/MD) simulations on the amorphous and crystalline structures of several phase change materials. Recently, the combination with experiment (XRD, EXAFS, hard x-ray photoelectron spectroscopy) and DF/MD simulations has enabled us to determine the amorphous atomic structures of GST and AIST, and how they differ from each other. Amorphous AIST has a range of ring sizes, while GST has many small rings and cavities, and the local environment of Sb in both forms of AIST is a distorted 3+3 octahedron (division between short/long bonds). We propose a “bond interchange” model, which is a sequence of small displacements of Sb atoms accompanied by interchanges of short and long bonds, as the origin of the rapid (slow nucleation) crystallization of AIST. It differs profoundly from the nucleation-driven crystallization in GST.