Experimentally constrained density-functional calculations of the amorphous structure of the prototypical phase-change material ${\text{Ge}}_2{\text{Sb}}_2{\text{Te}}_5$

Phase change materials involve the rapid and reversible transition between nanoscale amorphous $\left(a\text{−}\right)$ and crystalline $\left(c\text{−}\right)$ 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., ${\text{Ge}}_2{\text{Sb}}_2{\text{Te}}_5$ (GST) in digital versatile disk–random access memory. There has been much speculation about the structure of $a\text{−}$ 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.

Figure 1

(Color online) (a) Structure factor $S\left(Q\right)$ in $a\text{−}$GST. Green (gray): XRD (Ref. 3); black: calculated for RMC-optimized geometry. Inset: data for the high-$Q$ values. (b) Total radial distribution functions (XRD weights).

Figure 2

(Color online) (upper) RMC structure of $a\text{−}$GST [red (gray): Ge; blue (dark gray): Sb; and yellow (white): Te]. The 37 cavities are shown in light blue (light gray). (lower) Three cubes in part of optimized structure: two with $AB$ alternation and one with Sb-Sb bonds. A tetrahedral Ge-centered structure is shown.

Figure 3

(Color online) Irreducible rings in $a\text{−}$ GST (a) Ref. 4; (b) RMC-optimized geometry. “$13+$:” rings with more than 12 atoms.

Figure 4

(Color online) (a) (upper) XPS valence-band spectrum of $a\text{−}$GST [red (gray), arbitrary units] (Ref. 10) and the calculated electronic DOS [blue (dark gray), electrons per eV]. (b) (lower) ${\Delta }_{\text{DOS}}$ (see text). Blue (black): DF calculations, red (dark gray): XPS. Units as in (a).