Building blocks of amorphous ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$

The structural changes during the phase change between the amorphous and crystalline phase in ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ have been the subject of intense study due to the importance of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ in phase change memory devices. In our study, the energetics of the transition between the crystalline and the amorphous phase of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ is explored using density functional theory (DFT) calculations, electron diffraction, and reverse Monte Carlo model refinement. No energy barrier was found between the crystalline and the previously suggested amorphous structure of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. Further DFT calculations have led to a different building block of the amorphous structure, which is shown to agree with the experimental reduced density function determined from electron diffraction experiments and previously reported extended x-ray absorption fine structure measurements.

Figure 1

(Color online) One layer of the crystalline phase of GST (a) before and (b) after relaxation. Ge atoms are shown in red (smallest), Te in yellow (largest), and Sb in blue. The arrangement of vacancies is as in Ref. 5. Angles and distances between atoms are given in the figures.

Figure 2

(Color online) Atomic configurations before and after the flip. One unit cell of the atomic arrangement before and after the flip for [(a) and (b)] case 1 and [(c) and (d)] case 2 is shown. The red (short dark) arrows point at the Ge atom undergoing the flip. The yellow (long light with border) and blue (long light) arrows point at the Te and Sb nearest neighbor atoms, respectively, after the flip.

Figure 3

(Color online) Energies for the transitions shown in Figs. 2a, 2b, 2c, 2d. The energy difference between a “perfect” NaCl lattice and a lattice with a Ge atom moving along the path from a sixfold to a fourfold Te coordination is shown. The blue (upper) line shows case (1) [Figs. 2a, 2b] and the red (lower) line case (2) [Figs. 2c, 2d]. The vertical dashed line shows the “equilibrium” position of the flipped atom [position of the flipped Ge atom in Figs. 2b, 2d].

Figure 4

(Color online) RDF and building blocks of the amorphous phase. (a) shows the RDF acquired experimentally from a GST thin film (black line) of the unrefined distorted ring model (dashed line) and of the model after reverse Monte Carlo refinement (dotted line). The positions of the Ge-Te, Sb-Te, and Te-Te nearest neighbor distances determined using EXAFS in Ref. 5 are shown (+), as well as the distances as they would be in a perfect NaCl-type crystal with $a=6.03\mathrm{\AA }$ (vertical, dashed lines). (b) shows the ring obtained after relaxing the atoms around a vacancy without any surrounding environment [Ge, red (smallest); Te, yellow (largest); Sb, blue] and (c) shows a layer of the distorted ring model before (strong colors) and after (weak colors) refinement.