Intrinsic complexity of the melt-quenched amorphous Ge${}_2$Sb${}_2$Te${}_5$ memory alloy

Through the use of first-principles Ge $K$-edge XANES simulations we demonstrate that the structure of melt-quenched amorphous Ge-Sb-Te is intrinsically complex and is a mixture of Ge(3):Te(3) and Ge(4):Te(2) configurations in comparable concentrations, in contrast to the as-deposited amorphous phase that is dominated by the Ge(4):Te(2) configurations. The reasons for Ge-Te polyvalency are discussed and it is argued that both configurations are compatible with the Mott 8–N rule and the definition of an ideal amorphous solid. The near-perfect Te-Te distance match between the two major configurations accounts for the high cyclability of phase-change materials. Stable compositions in the Ge-Sb-Te system are suggested.

Figure 1

Experimental $K$-edge XANES spectra (left panel, raw spectra; left panel, vertically scaled data) measured for all constituent elements. The largest change is observed at the Ge $K$ edge.

Figure 2

Ge $K$-edge XANES spectra simulated for each of the 20 Ge atoms in the model.

Figure 3

(a) Simulated (averaged) Ge $K$-edge XANES spectra for the three main configurations found in the computer-generated amorphous GST model, namely tetrahedral sites ($T_d$ sites), pyramids ($P_y$), and distorted octahedra ($D$-$O_h$ sites). The typical local configurations of the analyzed groups are depicted in the top right-hand corner of each plot; Ge atoms are drawn in green and the larger Te atoms are drawn in gold; longer bonds are shown as solid lines and shorter bonds are shown as dashed lines. (b) Comparison of the experimental spectra for as-deposited, MQ amorphous, and laser-recrystallized layers with the simulated XANES spectra of characteristic configurations. (“Average” refers to an averaged spectrum taken over all Ge sites.) The energy origin is set at the Ge $K$ edge.

Figure 4

Ge-centered bond-length distributions for $T_d$, $P_y$, and distorted $O_h$ sites for the used model.

Figure 5

Isosurface plots (gray regions) of the difference in electron charge density for the simulated amorphous model (Ref. 13) and isolated pseudoatoms. Significant bonding charge can be seen midway along the bonds of the tetrahedrally coordinated Ge atom (green) shown on the left, while for the fragment shown on the right, bonding charge isosurfaces are present only for bonds of length less than $~$3 Å. Te (Sb) atoms are indicated as gold (purple).

Figure 6

Raman spectra for as-deposited, laser-amorphized, and laser-crystallized GST. The spectra are vertically displaced for clarity.

Figure 7

Simulated $K$-edge XANES spectra for twofold and threefold coordinated Te atoms in the amorphous phase as well as for a Te atom in the crystalline surrounding.

Figure 8

Schematic presentation of a generic structure containing continuously cross-linked tetrahedral and pyramidal configurations. Ge atoms are shown in green, Sb atoms are shown in purple, and Te atoms are shown in gold. Thin solid lines represent conventional covalent bonds where each atom contributes one electron and solid lines associated with an open ellipse represent dative bonds where two lone-pair electrons from Te interact with empty orbitals of Ge. The near-perfect match in Te-Te interatomic distances for the two configurations minimizes the potential internal stresses.

Figure 9

Simulated Ge $L_2$-edge XANES spectra for $T_d$, $P_y$, and $D$-$O_h$ configurations.