Structural singularities in GexTe100−x films

Abstract

Structural and calorimetric investigation of Ge(x)Te(100-x) films over wide range of concentration 10 < x < 50 led to evidence two structural singularities at x ∼ 22 at. % and x ∼ 33-35 at. %. Analysis of bond distribution, bond variability, and glass thermal stability led to conclude to the origin of the first singularity being the flexible/rigid transition proposed in the framework of rigidity model and the origin of the second one being the disappearance of the undercooled region resulting in amorphous materials with statistical distributions of bonds. While the first singularity signs the onset of the Ge-Ge homopolar bonds, the second is related to compositions where enhanced Ge-Ge correlations at intermediate lengthscales (7.7 Å) are observed. These two threshold compositions correspond to recently reported resistance drift threshold compositions, an important support for models pointing the breaking of homopolar Ge-Ge bonds as the main phenomenon behind the ageing of phase change materials.

Phase-change materials and rigidity

“…While the conclusion is drawn from structures modeled using ab initio methods, a structural investigation of coevaporated or -sputtered GexTe100-x films (0 < x < 50) based upon x-ray diffraction and EXAFS datasets simultaneously fitted by the reverse Monte Carlo technique points to the same conclusion. 6 A structural singularity in a composition region comprising 22-25 at.% Ge was identified by analysis of (1) Some other experimental data exhibit a singularity consistent with this proposal for compositions around 22 at.% Ge. 7,8 For example, the thermal stability T (T = Tx -Tg, where Tx is the temperature of crystallization onset…”

Structural properties of Ge-S amorphous networks in relationship with rigidity transitions: Anab initiomolecular dynamics study

“…Next, we calculate from the trajectories the fraction of homopolar bonds in the Ge-S glasses and compare them with previous results on Ge-Se 62 and bond statistics obtained from Reverse Monte Carlo modelling of Ge-Te 78 . Figure 7 shows the evolution of Ge-Ge and chalcogen-chalcogen (S-S, Se-Se, Te-Te) bonds with increasing Ge content.…”

“…For all, a decrease of the chalcogen-chalcogen bonds is obtained, accompanied by a growth of Ge-Ge bonds, the results exhibiting clear differences when the different families of binary chalcogenides are being directly compared. At low Ge composition (x <20 %), all Ge-based chalcogenides including Ge-Te 78 behave similarly, and this arises from the fact that GeTe 4/2 motifs are dominated by a tetrahedral geometry 6 for x <20 % so that the same short range order is encountered for all. At larger Ge content however (x >20 %), the decrease of Te-Te bond population behaves differently, and deviates from the S-S and Se-Se curves, and this arises from the increasing presence of Ge defect octahedra which increases the possibility of homopolar bonding in the structure.…”

Correlating ultrafast calorimetry, viscosity, and structural measurements in liquid GeTe and Ge15Te85

“…We are not aware of any data for the temperature dependence of the S(Q) for GeTe glass. Therefore S(Q 1 ) of amorphous thin-film GeTe at room temperature reported by Piarristeguy et al [40] and its extrapolation to T g using the temperature coefficient of Ge 15 Te 85 glass are plotted in Fig. 4.…”

“…The data points of Ge 15 Te 85 are vertically displaced by −0.2 for clarity. The data on the high-temperature liquid GeTe are taken from Weber et al [18] on cooling ( = −5 K min −1 ) and the room-temperature point is from Piarristeguy et al [40] (as-deposited thin-film GeTe, d = 1 − 3 μm); extrapolated to T g . The data on glassy (measured on heating, = 20 K min −1 ) and supercooled (measured on cooling, = −20 K min −1 ) Ge 15 Te 85 are from Wei et al [39] from measurements of bulk samples.…”