Experimentally constrained density-functional calculations of the amorphous structure of the prototypical phase-change material Ge2 Sb2 Te5

Phase change materials involve the rapid and reversible transition between nanoscale amorphous (a-) and crystalline (c-) spots in a polycrystalline film and play major roles in the multimedia world, including nonvolatile computer memory. The materials of choice are alloys of Ge, Sb, and Te, e.g., Ge2 Sb2 Te5 (GST) in digital versatile disk-random access memory. There has been much speculation about the structure of a- GST, but no model has yet received general acceptance. Here we optimize the structure by combining the results of density-functional calculations with high-energy x-ray diffraction data and x-ray photoelectron spectroscopy (XPS) measurements of the electronic densities of states to determine the structure. The structure agrees very well with available experimental data, including the differences in the XPS data between the amorphous and crystalline structures.