Liquid GeTe Research Articles

Atomistic Study of the Configurational Entropy and the Fragility of Supercooled Liquid GeTe

AbstractPhase‐change materials (PCMs) are an important family of alloys employed in non‐volatile memories and neuromorphic devices. These devices exploit the ability of PCMs to undergo rapid transitions between amorphous and crystalline phases at moderately high temperature. At ambient conditions, both phases are stable. These properties imply a very strong temperature dependence of the crystallization kinetics, which has been attributed to the high fragility of the supercooled liquid phase. In this work, the fragility index of GeTe, an important PCM, is extracted from a computational exploration of the potential energy landscape of the system by molecular dynamics simulations. For this purpose, a neural‐network potential is used to describe the interatomic interactions, which enables the evaluation of the configurational entropy in the deeply supercooled regime. The viscosity and the relaxation times are also calculated in the same temperature range. Finally, the Adam‐Gibbs relation is used to extrapolate the data to low temperatures and estimate the glass transition temperature and the fragility index. The latter quantity turns out to be of the order of 135–140, which shows that liquid GeTe is a highly fragile system.

Longitudinal acoustic and higher-energy excitations in the liquid phase-change materialGe2Sb2Te5

The dynamic structure factor $S(Q,E)$ of liquid ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ has been obtained by inelastic x-ray scattering, where $Q$ and $E$ are momentum and energy transfer, respectively. The dispersion curve of the longitudinal acoustic excitation energy exhibits flat-topped $Q$ dependence similarly to that in liquid GeTe and liquid Bi, where the local structure is modulated by Peierls distortion. The flat-topped energy in liquid ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ is split into low and high energy parts arising from Sb-Te and Ge-Te correlations, respectively. Furthermore, the high energy part depends on $Q$ like an optical mode with decreasing $Q$ to zero, and it approaches the vibrational energy of fourfold coordinated Ge atoms with octahedral configurations. The result indicates that a majority of Ge in the liquid stays in octahedral order as predicted by first-principles molecular dynamics simulations.

Viscosity, enthalpy relaxation and liquid-liquid transition of the eutectic liquid Ge15Te85

The viscosity and enthalpy relaxation in the Ge15Te85 eutectic liquid are investigated using a combination of parallel plate rheometry and conventional and modulated differential scanning calorimetry (DSC). The results of these measurements provide evidence for similar temperature dependence of enthalpy and shear relaxation timescales in this liquid. However, the enthalpy relaxation is found to be more than an order of magnitude slower than the shear relaxation process. The temperature dependence of the viscosity of the Ge15Te85 liquid in the supercooled state and above its melting point can be described using a single fragility index, except over a narrow temperature region in the immediate vicinity of the melting point where the viscosity abruptly increases, signifying a liquid-liquid transition. This anomalous viscosity jump coincides with the onset of an exothermic event observed in the DSC cooling curve. Together, these results are consistent with a transition that involves incipient liquid-liquid immiscibility that can be bypassed via rapid supercooling.

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

Two distinct trends in the temperature dependence of viscosity, measured directly and inferred from calorimetry by analyzing crystallization kinetics, can be correlated with the temperature evolution of the height of the first peak of the x-ray total structure factor for liquid GeTe and ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$. The phase-change chalcogenide GeTe is a high-fragility liquid with the kinetic fragility value of 76, at the glass-transition temperature, being between those for liquid (Ag,In)-doped ${\mathrm{Sb}}_{2}\mathrm{Te}$ and ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. The viscosity of the high-temperature liquid shows Arrhenius kinetics on cooling to the melting point, and the structure factor conforms to the fragile liquid. For liquid ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$, the temperature evolution of the structure factor suggests a transition in the temperature range of about 100 K above the melting. The crystallization shows a wide range of Arrhenius kinetics in the supercooled liquid region. This finding combined with the dynamic viscosity measurements is interpreted by invoking a weak fragile-to-strong crossover on cooling the liquid ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$. The differences in structures and dynamics of liquid GeTe and ${\mathrm{Ge}}_{15}{\mathrm{Te}}_{85}$ appear closely correlated to their distinctly different crystallization mechanisms.

Peculiar atomic dynamics in liquid GeTe with asymmetrical bonding: Observation by inelastic x-ray scattering

Collective dynamics in liquid GeTe was investigated by inelastic x-ray scattering at $2\ensuremath{\le}Q\ensuremath{\le}31\phantom{\rule{4pt}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$. The dynamic structure factor shows clear inelastic excitations. The excitation energies at low $Q$ disperse with increasing $Q$, consistent with the behavior of a longitudinal-acoustic excitation. The dispersion curve has a flat-topped region around the pseudo-Brillouin-zone boundary, similar to what is observed in liquid Bi [Inui et al., Phys. Rev. B 92, 054206 (2015)]. The dynamic structure factor shows a low-frequency excitation, and its coupling with the longitudinal-acoustic mode plays an important role for a flat-topped dispersion. From these results, it is inferred that atomic dynamics in liquid GeTe is strongly affected by a Peierls distortion similar to liquid Bi. By comparing the momentum transfer dependence of the excitation energy and quasielastic linewidth to partial structure factors obtained by our own ab initio molecular dynamics simulation for liquid GeTe, the quasielastic and inelastic components were found to be correlated with Te-Te and Ge-(Ge,Te) partial structure factors, respectively.

Experimental and ab initio molecular dynamics study of the structure and physical properties of liquid GeTe

GeTe is a prototypical phase-change material employed in data storage devices. In this work, the atomic structure of liquid GeTe is studied by x-ray and neutron diffraction in the temperature range from 1197 to 998 K. The dynamic viscosity is measured from 1273 to 953 K, which is 55 K below the solidification point, using an oscillating-cup viscometer. The density of liquid GeTe between 1293 and 973 K is determined by the high-energy $\ensuremath{\gamma}$-ray attenuation method. The experiments are complemented with ab initio molecular dynamics (AIMD) simulations based on density functional theory (DFT). Compatibility of the AIMD-DFT models with the diffraction data is proven by simultaneous fitting of all data sets in the frame of the reverse Monte Carlo simulation technique. It is shown that octahedral order dominates in liquid GeTe, although tetrahedral structures are also present. The viscosity of the equilibrium and weakly undercooled liquid GeTe obeys the Arrhenius law with a small activation energy of the order of 0.3 eV, which is indicative of a highly fragile liquid. The calculated density of states and electronic wave functions point to the existence of a pseudogap and localized electron states within the gap in the equilibrium liquid near the melting point as well as in the undercooled liquid.

Unraveling the Crystallization Kinetics of Supercooled Liquid GeTe by Ultrafast Calorimetry

Crystallization kinetics of phase change materials (PCMs) at high temperatures is of key importance for the extreme speed of data writing and erasing. In this work, the crystallization behavior of one of the typical PCMs, GeTe, has been studied using ultrafast differential scanning calorimetry (DSC) at high heating rates up to 4 × 104 K s–1. A strong non-Arrhenius temperature-dependent viscosity has been observed. We considered two viscosity models for estimating the crystal growth kinetics coefficient (Ukin). The results showed that the MYEGA model was more suitable to describe the temperature-dependent viscosity and the crystal growth kinetics for supercooled liquid GeTe. The glass transition temperature (Tg) and fragility m were estimated to be 432.1 K and 130.7, respectively. The temperature-dependent crystal growth rates, which were extrapolated by the MYEGA model, were in line with the experimental results that were measured by in situ transmission electron microscopy at a given temperature. The cry...

Structural evolution on medium-range-order during the fragile-strong transition in Ge15Te85

Using synchrotron X-ray scattering, we investigate liquid Ge15Te85 spanning a wide temperature range from near Tg to the melt, and demonstrate that the density anomaly and fragile-strong transition are not only related to short-range-order (SRO) structural change (e.g. Peierls-like distortion), but also accompanied by a remarkable development of medium-range-order (MRO). The latter manifests as an emerging pre-peak in total structure factor S(Q) and atomic pair correlations on the length scale of ∼8 Å in the real space G(r) function. The results highlight the role of medium-range structural ordering in the evolution of the configurational entropy which, according to the Adam-Gibbs theory, can be linked to the fragile-strong transition (FS-transition). Based on the relation between structure and liquid dynamics, the FS-transitions at high pressures are examined in terms of experimental data and the Ehrenfest relation. This work identifies the length scale for the atomic correlations in MRO structural evolutions and presents a structural approach to exploring liquid dynamics, which may be useful for investigating relevant phase-change alloys.

Pressure dependence of the static structure of liquid GeTe based on ab initio molecular dynamics simulations

We have investigated the pressure dependence of the static structure of liquid GeTe based on ab initio molecular dynamics simulations. The pressure range is between ambient pressure and 250 GPa, and their temperatures between 1000 K and 4000 K, which keep the liquid state. In this study, we found two transition stages caused by the compression. At the first stage, below 12 GPa, atomic distances elongate and Peierls-type distortion is dissolved with increasing pressure. At the second stage, above 12 GPa, atomic distances shorten and the electronic states shows metallic.

A structural study of liquid Te-rich Si-Te alloys

X-ray diffraction measurements using synchrotron radiation on liquid SixTe1−x (x≥0.2) were carried out at several temperatures. At constant temperature, the first peak, which is composed of Si-Te and Te-Te correlations, of the pair distribution function gradually shifts to the shorter distances with increasing Si content. These shifts indicate the substitution of Si-Te bond for the Te-Te bonds in the first peak. The total coordination number shows little temperature variation and remains about two in the temperature interval of a few hundreds of degrees. The composition and temperature behaviour of the Te-rich side of Si-Te liquid alloys has already been investigated, among which thermodynamic behaviour is a quite contrast to that of the Ge-Te alloys in the overlapping composition-temperature range of the present investigation. We clarify the evolution of the local structure of the liquid Si-Te alloys in comparison with the liquid Ge-Te alloys.

Communication: Van der Waals corrections for an improved structural description of telluride based materials

Density functional theory (DFT), using the generalized gradient approximation, fails to reproduce the structure of liquid tellurides, which manifests by an overestimation of the interatomic bond distances. Here, we take into account dispersion forces in a semi-empirical way and apply such DFT simulations to liquid Ge(15)Te(85). Substantial improvement of the simulated structure factor and pair distribution function is found, together with a change in the diffusion constant. A detailed analysis shows that such dispersion forces strongly affect the local geometry and first coordination shell of the atoms, whereas angular distributions remain unchanged.

Pressure-Induced Structural Changes in Liquid Ge33Te67 and Liquid Ge15Te85

We have performed energy-dispersive X-ray diffraction measurements for liquid Ge 33 Te 67 up to 9 GPa and observed a pressure dependence of the structure for the first time. We confirmed that the structure is different from the Ge(Te 1/2 ) 4 tetrahedral network in any pressure at the temperature higher than 740 °C. We observed a small expansion in the bond length with increasing pressure up to 2.4 GPa. We also investigated the pressure dependence of the structure of liquid Ge 15 Te 85 at constant temperature at around 730 °C. The pressure dependence was found to be similar to that of liquid Ge 33 Te 67 , i.e., we confirmed that the structure is different from the tetrahedral network in this high temperature. Although the structure at 730 °C at ambient pressure is regarded as a “densely packed” structure in comparison to the lower-temperature structure, we clarified that the structure is still highly compressible.

Pressure-induced structural change in liquid GeTe and liquid GeSe

X-ray diffraction has been measured up to 19 GPa for liquid GeTe and up to 15 GPa for liquid GeSe using synchrotron radiations. Static structure factor S(Q) and pair distribution function g(r) was obtained. Both S(Q) of liquid GeTe and liquid GeSe change their shape with increasing pressure, indicating anisotropic contraction of the local structure. A sharp change in S(Q) and g(r) was observed in liquid GeTe between 1.8 GPa and 4.0 GPa. Pressure dependences of S(Q) and g(r) of liquid GeSe change around 6 GPa. In liquid GeSe a small pre-peak was observed around 1.2 Å-1 up to 1.7 GPa which suggests the presence of the medium range order of covalent bond network. With increasing pressure the Peierls-type distortion becomes small and the bond angle decreases; the local structure becomes a rhombohedral structure. These pressure dependences of the structure of liquid GeTe and liquid GeSe are different from those of the crystals. These results are compared with those of other liquids and discussed in relation to the bonds between atoms in the liquids.

液体ヒ素および ＩＶ−ＶＩ 化合物の圧力誘起構造変化

In this article, we review some recent advances in the studies of the pressure-induced structural changes in liquid arsenic and liquid IV-VI compounds. For liquid As, the A7-like local structure has been experimentally observed and attributed to a “Peierls distortion in the liquid state”. Most recently the suppression of the distortion has been observed at high pressures. We also observed the similar pressure dependence in liquid GeS, GeSe, and GeTe, where their structures are distorted at low pressures. We describe the pressure-induced suppression of Peierls-type distortion in these liquids, and show the differences between liquid and crystalline states.

Static and dynamic structures of liquid GeTe mixtures

Neutron diffraction measurements of the liquid Ge–Te system and quasi- and inelastic neutron scattering experiments of the eutectic liquid Ge 15Te 85 were performed in the temperature range from 400 to 560 °C. The large temperature dependence of the static structure for liquid Ge 15Te 85 is reconfirmed; however it is found that the compositional dependence obtained at around 80 °C above the liquidus curve is rather small. The dynamic structure factors of liquid Ge 15Te 85 in the Q-region from 0.2 to 2.6 Å −1 show a normal temperature dependence. From the vibrational density of states, it is found that the Te–Te covalent bond persists in liquid Ge 15Te 85; however a bending mode was not assigned clearly.

Structural studies on Te-rich Ge–Te melts

The structure of liquid Ge15Te85, Ge25Te75 and Ge33Te67 alloys has been studied by neutron and x-ray diffraction. Datasets obtained bythe two techniques have been modelled simultaneously with the reverse MonteCarlo simulation method. As a result, the first coordination numbers andnearest neighbour distances for the studied alloys were estimated. On the basisof the experimental data, Ge–Ge bonding appears to be present in liquidGe25Te75 and Ge33Te67. It has been found that temperature dependences of thephysico-chemical properties correlate with the structural changes in liquidGe15Te85.

Simulating Liquid GeTe

AbstractOne of the most difficult problems in condensed matter physics is to describe the microscopic liquid state. Owing to the dynamical nature of the liquid state, it is not possible to discuss specific microscopic structures, only ensemble averages can be specified. Such averages can be performed via molecular dynamics simulations. A problematic issue in performing such simulations is computing accurate interatomic forces. Although classical many-body potentials can be use for simulations of covalent materials, one must effectively map quantum phenomena such as hybridization onto such potentials. This mapping is complex and lacking a well-defined prescription. This step can be avoided by employing quantum forces from the pseudopotential-density functional method. Using molecular dynamics with quantum forces, we examine the local atomic order as well as some dynamic and electronic properties of the semiconducting liquid GeTe. Near the melting temperature, the Peierls distortion responsible for the lower temperature crystal phase of GeTe is shown to manifest itself within the liquid structure. At higher temperatures in the liquid, increasing disorder leads to an eventual semiconductor-metal transition. The calculated kinematic viscosity of the liquid is found to agree with the experimental value and is shown to arise from the small diffusion coefficient of the Te atoms. Using an ensemble average, we predict the dc conductivity of the melt to be consistent with recent measurements.

Temperature-Induced Density Anomaly in Te-Rich Liquid Germanium Tellurides:pversussp3Bonding?

The density anomaly of liquid Ge(0.15)Te(0.85) measured between 633 and 733 K is investigated with ab initio molecular dynamics calculations at four temperatures and at the corresponding experimental densities. For box sizes ranging from 56 to 112 atoms, an 8 k-points sampling of the Brillouin zone is necessary to obtain reliable results. Contrary to other Ge chalcogenides, no sp(3) hybridization of the Ge bonding is observed. As a consequence, the negative thermal expansion of the liquid is not related to a tetrahedral bonding as in the case of water or silica. We show that it results from the symmetry recovery of the local environment of Ge atoms that is distorted at low temperature by a Peierls-like mechanism acting in the liquid state in the same way as in the parent solid phases.

Thermodynamics of Structural Changes in Liquid Ge–Te Alloys Around the Eutectic Composition: Specific Heat Measurements and Thermodynamic Stability

We have obtained the specific heat, C p , of liquid Ge–Te alloys around the eutectic composition (15 at.% Ge) and alloys up to 50 at.% Ge by combining the data obtained by modulation and adiabatic calorimetries. The results cover a wide temperature range from deeply undercooled state to 800°C. A very pronounced peak of C p was observed for Te and alloys up to 20 at.% Ge. The peak position of C p takes a maximum around the eutectic composition where the peak width becomes narrowest. Using a new thermodynamic relation, we clarify a correlation between the thermodynamic stability and structural changes in the liquid Ge–Te system.

Thermodynamics of the structural changes in the liquid Ge–Te system around the Te-rich eutectic composition

To clarify the thermodynamics of the structural changes in a liquid Ge–Te alloy around the Te-rich eutectic composition (15 at.% Ge), detailed measurements of the molar volume, sound velocity and specific heat measurements were performed. The crossover temperature manifested by a sharp extremum in those thermodynamic quantities takes a maximum around the eutectic composition where the peak width becomes most narrow. The structural changes are discussed in connection with the thermodynamic fluctuations.