Littérature scientifique sur le sujet « Phase change materials, phase change memories, first principles simulations, molecular dynamics »

We review our results on large-scale atomistic simulations of the phase change compound GeTe of interest for applications in nonvolatile electronic memories. The simulations are based on an interatomic potential with an accuracy close to that of the density functional theory (DFT). The potential was generated by fitting a DFT database by means of an artificial neural network method. This methodological advance allowed us to perform molecular dynamics simulations with several thousand atoms for several ns that provided useful insights on several properties of interest for the operation of phase change memories, including the crystallization kinetics, the dynamics of the supercooled liquid, the structural relaxation in the glass and the properties of nanowires.

Unusual force constants originating from the local charge distribution in crystalline GeTe and Sb2Te3 are observed by using the first-principles calculations. The calculated stretching force constants of the second nearest-neighbor Sb-Te and Ge-Te bonds are 0.372 and −0.085 eV/Å2, respectively, which are much lower than 1.933 eV/Å2 of the first nearest-neighbor bonds although their lengths are only 0.17 Å and 0.33 Å longer as compared to the corresponding first nearest-neighbor bonds. Moreover, the bending force constants of the first and second nearest-neighbor Ge-Ge and Sb-Sb bonds exhibit large negative values. Our first-principles molecular dynamic simulations also reveal the possible amorphization of Sb2Te3 through local distortions of the bonds with weak and strong force constants, while the crystalline structure remains by the X-ray diffraction simulation. By identifying the low or negative force constants, these weak atomic interactions are found to be responsible for triggering the collapse of the long-range order. This finding can be utilized to guide the design of functional components and devices based on phase change materials with lower energy consumption.

Fifty years after its discovery, the ovonic threshold switching (OTS) phenomenon, a unique nonlinear conductivity behavior observed in some chalcogenide glasses, has been recently the source of a real technological breakthrough in the field of data storage memories. This breakthrough was achieved because of the successful 3D integration of so-called OTS selector devices with innovative phase-change memories, both based on chalcogenide materials. This paves the way for storage class memories as well as neuromorphic circuits. We elucidate the mechanism behind OTS switching by new state-of-the-art materials using electrical, optical, and x-ray absorption experiments, as well as ab initio molecular dynamics simulations. The model explaining the switching mechanism occurring in amorphous OTS materials under electric field involves the metastable formation of newly introduced metavalent bonds. This model opens the way for design of improved OTS materials and for future types of applications such as brain-inspired computing.

ABSTRACT: We reproduced mechanical behaviors, such as the reinforcement effect, hysteresis, and stress softening, of filled rubber under cyclic deformations using coarse-grained molecular dynamics simulations. We measured polymer density distribution in the nonload equilibrium state and conformational changes in polymer chains during deformation for dispersed and aggregated filler structures. We found that the polymer–filler attractive interactions increase the polymer density in the vicinity of fillers and decrease the polymer density in the other regions. The polymer bonds that connect polymer particles away from fillers are extended when the polymer density decreases. This alteration increases the modulus of the polymer phase, and the reinforcement effect appears. For aggregated filler structures, the polymer chains interacting with adjacent fillers act as a bridge between these fillers and increase the modulus, especially when the strain is low. To test the mechanisms of hysteresis and stress softening, we measured the changes in the polymer paths. A polymer path is the minimal path of polymer networks between two fillers; in other words, it is the “bridge” that connects two fillers. We found that the polymer paths increase in length, especially during primary loading, because of polymer adsorption/desorption on the filler surface to adjust the change of filler positions. It was also found that the influence of the filler structure diminishes in the first loading. During subsequent unloading, a long path does not become a short path again but will be folded even though the filler distance reduces. Hence, the change in the polymer paths in the second cycle is smaller than that in the first cycle because the polymer path is just unfolded. We confirmed the hysteresis and stress-softening result from these conformational changes. In this article, we also discuss the recovery mechanism for stress softening and the history dependence.

Molecular dynamics simulations of solids are often performed using anisotropic barostats that allow the shape and volume of the periodic cell to change during the simulation. Most existing schemes are based on a second-order differential equation that might lead to undesired oscillatory behaviors and should not be used in the equilibration phase. We recently introduced stochastic cell rescaling, a first-order stochastic barostat that can be used for both the equilibration and production phases. Only the isotropic and semi-isotropic variants have been formulated and implemented so far. In this paper, we develop and implement the equations of motion of the fully anisotropic variant and test them on Lennard-Jones solids, ice, gypsum, and gold. The algorithm has a single parameter that controls the relaxation time of the volume, results in the exponential decay of correlation functions, and can be effectively applied to a wide range of systems.

Molten salts constitute one kind of PCMs (Phase Change Materials) widely used in concentrating solar power facilities for heat storage and heat transfer. This paper aims to simulate nanofluid PCMs with molecular dynamics method. Concretely, the thermophysical properties of a nanofluid of KNO3 doped with SiO2 nanoparticle are investigated by equilibrium and nonequilibrium molecular dynamics simulations. For the first time, these properties of a nanofluid in the family of PCMs are calculated. The density, thermal expansion coefficient, specific heat capacity, thermal conductivity, and viscosity are characterized as functions of the SiO2 nanoparticle concentration. The effect of the SiO2 nanoparticle size on the nanofluid’s properties is also investigated. The simulation results present an enhancement of the thermophysical properties, especially for the specific heat capacity, in good agreement with the existing experimental results on a representative nanofluid PCM, and open prospects for the understanding of microscopic mechanism leading to such enhancements.

The potential of thermoelectric power to reduce energy waste and mitigate climate change has led to renewed interest in “phonon-glass electron-crystal” materials, of which the inorganic clathrates are an archetypal example. In this work we present a detailed first-principles modelling study of the structural dynamics and thermal transport in bulk diamond Si and five framework structures, including the reported Si Clathrate I and II structures and the recently-synthesised oC24 phase, with a view to understanding the relationship between the structure, lattice dynamics, energetic stability and thermal transport. We predict the IR and Raman spectra, including ab initio linewidths, and identify spectral signatures that could be used to confirm the presence of the different phases in material samples. Comparison of the energetics, including the contribution of the phonons to the finite-temperature Helmholtz free energy, shows that the framework structures are metastable, with the energy differences to bulk Si dominated by differences in the lattice energy. Thermal-conductivity calculations within the single-mode relaxation-time approximation show that the framework structures have significantly lower κlatt than bulk Si, which we attribute quantitatively to differences in the phonon group velocities and lifetimes. The lifetimes vary considerably between systems, which can be largely accounted for by differences in the three-phonon interaction strengths. Notably, we predict a very low κlatt for the Clathrate-II structure, in line with previous experiments but contrary to other recent modelling studies, which motivates further exploration of this system.

AbstractThe Ge2Sb2Te5 alloy has served as the core material in phase-change memories with high switching speed and persistent storage capability at room temperature. However widely used, this composition is not suitable for embedded memories—for example, for automotive applications, which require very high working temperatures above 300 °C. Ge–Sb–Te alloys with higher Ge content, most prominently Ge2Sb1Te2 (‘212’), have been studied as suitable alternatives, but their atomic structures and structure–property relationships have remained widely unexplored. Here, we report comprehensive first-principles simulations that give insight into those emerging materials, located on the compositional tie-line between Ge2Sb1Te2 and elemental Ge, allowing for a direct comparison with the established Ge2Sb2Te5 material. Electronic-structure computations and smooth overlap of atomic positions (SOAP) similarity analyses explain the role of excess Ge content in the amorphous phases. Together with energetic analyses, a compositional threshold is identified for the viability of a homogeneous amorphous phase (‘zero bit’), which is required for memory applications. Based on the acquired knowledge at the atomic scale, we provide a materials design strategy for high-performance embedded phase-change memories with balanced speed and stability, as well as potentially good cycling capability.

ABSTRACTThe amorphous structure of InxSey has been studied by a first principles tight-binding molecular dynamics technique using the program package PLATO (Package for Linear-combination of Atomic Type Orbitals). The three-dimensional amorphous structures with different densities were prepared by quick quenching from the liquid phase. The characteristics of short-range order such as coordination numbers, radial and bond angle distributions, and ring statistics have been analyzed.

I materiali a cambiamento di fase sono calcogenuri a base di tellurio di notevole interesse tecnologico per la realizzazione di memorie ottiche (DVD) e di memorie elettroniche non volatili di nuova concezione, le memorie a cambiamento di fase o PCM. Questi dispositivi si basano su una veloce (50 ns) e reversibile transizione di fase amorfo-cristallo indotta per riscaldamento. Le due fasi corrispondono ai due stati di memoria che possono essere distinti grazie alla grande differenza tra le proprietà ottiche ed elettroniche dell'amorfo e quelle del cristallo. Nonostante il Ge2Sb2Te5 (GST) sia il materiale attualmente usato nelle PCM, si stanno studiando nuovi materiali con una temperatura di cristallizzazione più alta per aumentare la stabilità termica delle PCM. A questo proposito in questa tesi sono state studiate, attraverso simulazioni di dinamica molecolare ab-initio, diverse leghe ad alta temperatura di cristallizzazione con composizione In3Sb1Te2, In13Sb11Te3 e Ga4Sb6Te3. Queste leghe sono state studiate sperimentalmente e proposte come sostituti del GST, ma le proprietà strutturali e l'origine microscopica dell'elevata temperatura di cristallizzazione della fase amorfa di questi composti non è ancora del tutto chiara. Sono stati, quindi, generati modelli di qualche centinaio di atomi della fase amorfa raffreddando dal liquido in centinaia di ps allo scopo di trovare una relazione tra la struttura dell'amorfo e l'alta temperatura di cristallizzazione di queste leghe. La topologia di legame dei modelli amorfi risulta principalmente tetraedrica, molto diversa dalla geometria della fase cristallina che presenta invece intorni ottaedrici. La presenza di strutture tetraedriche nell'amorfo, assenti invece nella fase cristallina, può quindi costituire un ostacolo alla cristallizzazione con l'effetto di innalzare la temperatura di cristallizzazione rispetto al GST che presenta una geometria di legame prevalentemente ottaedrica sia nell'amorfo che nel cristallo. Nella seconda parte di questo lavoro è stato affrontato il problema del drift, che consiste in un aumento della resistenza elettrica della fase amorfa con il tempo. Questo fenomeno rappresenta un problema nelle celle PCM in quanto modifica le caratteristiche elettriche del dispositivo; tuttavia, manca ancora una spiegazione completa del meccanismo microscopico alla base di questo processo. Il drift sembra però legato al fenomeno del rilassamento strutturale che si verifica nei semiconduttori amorfi e che modifica nel tempo gli stati di difetto in prossimità degli edge delle bande di valenza e di conduzione, da cui dipende la conduzione nella fase amorfa. Per studiare il fenomeno del drift sono stati generati modelli di grandi dimensioni (circa duemila atomi) di GeTe amorfo raffreddando dal liquido in 100 ps attraverso simulazioni di dinamica molecolare classica con un potenziale Neural-Network. Una volta rilassati ab-initio, i modelli presentano diversi stati nel gap localizzati su catene di atomi di Ge. Dopo aver riscaldato i modelli a 500 K in modo da accelerare il processo di drift, si osserva una riduzione del numero di catene di Ge e di legami omopolari Ge-Ge con un conseguente allargamento del gap e riduzione dell'ampiezza delle code di Urbach che possono giustificare un aumento della resistenza. Si propone quindi che il drift sia dovuto al rilassamento strutturale della fase amorfa che porta alla riduzione delle catene di legami omopolari di Ge.
Phase change materials based on chalcogenide alloys are of great technological importance because of their use in optical data storage devices (DVDs) and electronic non-volatile memories of new concept, the Phase Change Memory cell (PCM). These applications rely on a fast (50 ns) and reversible change between the crystalline and the amorphous phases upon heating. The two phases correspond to the two states of the memory that can be discriminated thanks to a large difference in their optical and electronic properties. Although Ge2Sb2Te5 (GST) is the compound presently used as active layer in PCMs, alternative materials with a higher crystallization temperature are under scrutiny in order to increase the thermal stability of the PCM devices. In this respect, we analysed, by means of ab-initio molecular dynamics simulations, different high crystallization temperature alloys with composition In3Sb1Te2, In13Sb11Te3 and Ga4Sb6Te3, which have been experimentally proposed as substitute of GST. However, the structural properties and the microscopical reason of the high thermal stability of the amorphous phases of these compounds is still unclear. We, thus, generated models of the amorphous phase of few hundreds of atoms by quenching from the melt in few hundreds of ps aiming at finding out a relation between the structural properties of the amorphous phase and the high crystallization temperature of these alloys. The topology of our amorphous models turned out to be mostly tetrahedral which differs from the octahedral-like geometry of the crystalline phases. The presence of tetrahedral structures in the amorphous which are absent in the crystalline phase, probably hinders the crystallization process resulting in a higher crystallization temperature with respect to GST which display a mostly octahedral-like structures in both amorphous and the crystalline phase. In the second part of this work we addressed the issue of the resistance drift phenomenon, which consists of an increase of the electrical resistance of the amorphous phase with time. This effect is detrimental in PCMs since it changes the electrical characteristics of the devices. This process is believed to be due to an aging of the amorphous phase which modifies during time the defect states in the proximity of the valence and conduction band edges which control the electrical conductivity. The microscopic origin of the structural relaxations leading to the drift is still unknown. To address this problem, we generated large models (about two thousand atoms) of amorphous GeTe by quenching from the melt in 100 ps with classical molecular dynamics simulations by using a neural-network potential. Once relaxed by first principles, the models showed the presence of several in-gap states localized on chains of Ge atoms. After an annealing at 500 K, performed to accelerate the drift process, Ge chains and homopolar Ge-Ge bonds reduce in number resulting in a band gap widening and a reduction of the Urbach tails at the band edges which can account for the increase of the resistance. We thus propose that the resistance drift originates from structural relaxations leading to the removal of Ge chains.