Crystallization Of Phase Change Materials Research Articles

Programming multilevel crystallization states in phase-change-material thin films

We propose and demonstrate a simple method to accurately monitor and program arbitrary states of partial crystallization in phase-change materials (PCMs). The method relies both on the optical absorption in PCMs as well as on the physics of crystallization kinetics. Instead of raising temperature incrementally to increase the fraction of crystallized material, we leverage the time evolution of crystallization at constant temperatures and couple this to a real-time optical monitoring to precisely control the change of phase. We experimentally demonstrate this scheme by encoding a dozen of distinct states of crystallization in two different PCMs: GST and Sb2S3. We further exploit this ’time-crystallization’ for the in-situ analysis of phase change mechanisms and demonstrate that the physics of crystallization in Sb2S3 is fully described by the so-called Johnson-Mehl-Avrami-Kolmogorov formalism. The presented method not only paves the way towards real-time and model-free programming of non-volatile reconfigurable photonic integrated devices, but also provides crucial insights into the physics of crystallization in PCMs.

Highly tunable β-relaxation enables the tailoring of crystallization in phase-change materials

In glasses, secondary (β-) relaxations are the predominant source of atomic dynamics. Recently, they have been discovered in covalently bonded glasses, i.e., amorphous phase-change materials (PCMs). However, it is unclear what the mechanism of β-relaxations is in covalent systems and how they are related to crystallization behaviors of PCMs that are crucial properties for non-volatile memories and neuromorphic applications. Here we show direct evidence that crystallization is strongly linked to β-relaxations. We find that the β-relaxation in Ge15Sb85 possesses a high tunability, which enables a manipulation of crystallization kinetics by an order of magnitude. In-situ synchrotron X-ray scattering, dielectric functions, and ab-initio calculations indicate that the weakened β-relaxation intensity stems from a local reinforcement of Peierls-like distortions, which increases the rigidity of the bonding network and decreases the dynamic heterogeneity. Our findings offer a conceptually new approach to tuning the crystallization of PCMs based on manipulating the β-relaxations.

Monolithic TCAD simulation of phase-change memory (PCM/PRAM) + Ovonic Threshold Switch (OTS) selector device

Owing to the increasing interest in the commercialization of phase-change memory (PCM) devices, a number of TCAD models have been developed for their simulation. These models formulate the melting, amorphization and crystallization of phase-change materials as well as their extreme conductivity dependence on both electric field and temperature into a set of self-consistently-solved thermoelectric and phase-field partial-differential equations. However, demonstrations of the ability of such models to match actual experimental results are rare. In addition, such PCM devices also require a so-called selector device – such as an Ovonic Threshold Switching (OTS) device – in series for proper memory operation. However, monolithic simulation of both the PCM and OTS selector device in a single simulation is largely absent from the literature, despite its potential value for material- and design-space explorations. It is the goal of this work to first characterize a PCM device in isolation against experimental data, then to demonstrate the qualitative behavior of a simulated OTS device in isolation and finally to perform a single monolithic simulation of the PCM + OTS device within the confines of a commercially available TCAD solver: GTS Framework.

Low‐temperature thermal energy storage with polymer‐derived ceramic aerogels

AbstractThermal energy storage (TES) with phase change materials (PCMs) presents some advantages when shape‐stabilization is performed with ceramic aerogels. These low‐density and ultra‐porous materials guarantee high energy density and can be easily regenerated through simple pyrolysis while accounting for moderate mechanical properties. However, the small pore size that typically characterizes these sorbents can hinder the crystallization of PCMs, slightly reducing the energy density of the stabilized compound. In this work, we present the use of polymer‐derived mesoporous SiC and SiOC aerogels for the stabilization of polyethylene glycol and a fatty alcohol (PureTemp 23), having a melting temperature of 17 and 23°C, respectively. Their TES performances point out maximum thermal efficiency values of around 80%. These performances are discussed accounting for the results of thermogravimetric analysis, differential scanning calorimetry, and leaking tests.

Oxidation Process in Ge-Rich GeSbTe Alloy for Phase-Change Memory: Mechanism, Kinetic and N-Doping Influence

Phase-change materials (PCMs) based on GeSbTe alloys are well known and widely used in optical storage devices. The recent progress in scaling and material engineering made PCMs also promising candidates for high-speed non-volatile memory applications [1]. Indeed, Phase-Change Memory (or PCM) devices feature high writing and reading speed and compatibility with high working and storage temperatures.Ge-rich GeSbTe is a promising material for embedded applications requiring high data stability at high temperature, thanks to its higher crystallization temperature wrt standard PCMs such as Ge2Sb2Te5 [2-3]. Moreover, N doping in Ge-rich GeSbTe was also introduced in order to stabilize even more the material at high temperature, and improve its programming performances [4-5]. However, fabrication steps like etching or stripping that imply an exposure of the PCMs to air, could affect the material properties and the final device performance [6]. It is known that Ge is particularly sensitive to oxidation, and this makes even more important to well understand and study such phenomena in Ge-rich GeSbTe. Recent observations already reported the appearing of segregation mechanisms due to surface oxidation in Ge2Sb2Te5 and Ge-rich GeSbTe [7]. Such material degradation could significantly affect the final device performance and the electrical parameters variability. Therefore, the understanding of the physics of these mechanisms become fundamental for improving the fabrication steps for future further improvements of the device (e.g. new barriers, new etching strategies, etc.).This study focuses on the oxidation mechanism and on its kinetic in Ge-rich GeSbTe, and how it is influenced by N doping. We analyzed the oxidation in amorphous and crystalline samples, at room temperature and after annealing of increasing duration at high temperature in order to accelerate oxidation process. Using surface characterization by pARXPS (parallel angular resolved XPS), we monitor with precision the evolution in time of the oxide thickness (figure 1) and of the composition of different N-doped Ge-rich GeSbTe samples (figure 2). We report the evolution of our samples along more than one year, showing a double steps oxidation mechanism, with a kinetic strongly dependent on N content. Oxidation speed and thickness are then correlated with N content, leading to an analytical model that is in agreement with all the data acquired (figure 3), and that allows to describe the ongoing oxidation and segregation phenomena. TEM images completed by EDX analyses (figure 4) are here shown to support our findings.In this work, we present the study of the oxidation mechanism and kinetic in N-doped Ge-rich GeSbTe materials by pARXPS, TEM and EDX analyses. We propose an analytical model for the oxidation kinetic based on our data, highlighting the strong impact of N doping concentration on the parameters of our model. [1] G. Navarro et al., “Phase-Change Memory: Performance, Roles and Challenges,” in 2018 IEEE International Memory Workshop (IMW), Kyoto, 2018, pp. 1–4, doi: 10.1109/IMW.2018.8388845. [2] P. Zuliani et al., "Overcoming Temperature Limitations in Phase Change Memories With Optimized GexSbyTez" in IEEE Transactions on Electron Devices, vol. 60, no. 12, pp. 4020-4026, Dec. 2013. [3] J. Kluge et al., “High Operating Temperature Reliability of Optimized Ge-Rich GST Wall PCM Devices,” 2016, pp. 1–4, doi: 10.1109/IMW.2016.7495273 [4] G. Navarro et al., “Trade-off between SET and data retention performance thanks to innovative materials for phase-change memory,” 2013, pp. 21.5.1-21.5.4, doi: 10.1109/IEDM.2013.6724678. [5] A. Kiouseloglou et al., “A Novel Programming Technique to Boost Low-Resistance State Performance in Ge-Rich GST Phase Change Memory,” IEEE Transactions on Electron Devices, vol. 61, no. 5, pp. 1246–1254, May 2014, doi: 10.1109/TED.2014.2310497. [6] P. Noé, C. Sabbione, N. Bernier, N. Castellani, F. Fillot, and F. Hippert, “Impact of interfaces on scenario of crystallization of phase change materials,” Acta Materialia, vol. 110, pp. 142–148, May 2016, doi: 10.1016/j.actamat.2016.03.022. [7] M. Agati, C. Gay, D. Benoit, and A. Claverie, “Effects of surface oxidation on the crystallization characteristics of Ge-rich Ge-Sb-Te alloys thin films,” Applied Surface Science, vol. 518, p. 146227, Jul. 2020, doi: 10.1016/j.apsusc.2020.146227. Figure 1

In-situ observation of Ge2Sb2Te5 crystallization at the passivated interface

Abstract The unique properties of phase change materials (PCMs) in the amorphous and crystalline phase, and the feasibility of manipulating the phase transition process, have enabled the application of PCMs in optics and electronics systems, for data storage and neuromorphic computing. As the semiconductor device employing PCMs scales down with the complementary metal oxide semiconductor (CMOS) technology node, the impact of interface on the crystallization of PCMs becomes increasingly significant, that is of great importance for the system performance, e.g. phase change memory (PCM). In this work, crystallization of Ge2Sb2Te5 (GST) on passivated SiO2 surface is triggered by thermal pulses, in-situ observed in transmission electron microscope. A distinguishing heterogenous nucleation dominated crystallization behavior of GST has been verified at the GST/SiO2 interface. Meanwhile, in the in-situ prepared reactive GST/SiO2 interface, the interfacial covalent interaction between atoms in GST and SiO2 is believed to hamper the GST nucleation at the interface. Interfaces consist of the same materials but with different reactivity will lead to distinct crystallization behaviors of PCMs. By forming passive interface between PCMs and surrounding dielectric materials, promotion in nucleation and enhancement in operation speed can be expected in PCM with decreasing cell size.

Designing crystallization in phase-change materials for universal memory and neuro-inspired computing

The global demand for data storage and processing has increased exponentially in recent decades. To respond to this demand, research efforts have been devoted to the development of non-volatile memory and neuro-inspired computing technologies. Chalcogenide phase-change materials (PCMs) are leading candidates for such applications, and they have become technologically mature with recently released competitive products. In this Review, we focus on the mechanisms of the crystallization dynamics of PCMs by discussing structural and kinetic experiments, as well as ab initio atomistic modelling and materials design. Based on the knowledge at the atomistic level, we depict routes to improve the parameters of phase-change devices for universal memory. Moreover, we discuss the role of crystallization in enabling neuro-inspired computing using PCMs. Finally, we present an outlook for future opportunities of PCMs, including all-photonic memories and processors, flexible displays with nanopixel resolution and nanoscale switches and controllers. Chalcogenide phase-change materials (PCMs) are leading candidates for non-volatile memory and neuro-inspired computing devices. This Review focuses on the crystallization mechanisms of PCMs as well as the design principles to achieve PCMs with high switching speeds and good data retention, which will aid the development of PCM-based universal memory and neuro-inspired devices.

Relative activation energy for laser-induced crystallization of phase change materials

The relative activation energy () derived from a pulsed laser-induced crystallization of phase change materials (PCMs) was suggested as a useful quantitative parameter to represent the practical switching speed, energy consumption, and storage reliability of phase change random access memory (PRAM). We employed the time-for-constant-fraction technique using the reflectivity change of PCMs and the irradiated laser power/pulse width. Using the suggested method, the values for the Ge2Sb2Te5 (GST), Bi-doped GST (5.9 at%), and Sn-doped GST (17.7 at%) films were determined to be 6.34 × 10−9, 4.56 × 10−9, and 3.77 × 10−9 J, respectively, which reflected their expected device performances of PRAM.

Evaluation of paraffin infiltrated in various porous silica matrices as shape-stabilized phase change materials for thermal energy storage

Phase change behaviors of organic phase change materials (PCMs) confined in porous silica matrices, which are determined by the pore characteristics as well as interfacial interactions between PCM molecules and supporting solid, are crucial to PCMs for thermal energy storage performance. In this paper, a series of shape-stabilized phase change materials (ss-PCMs) were engineered with three silica matrices as support and paraffin as PCMs through solution impregnation. These porous silica matrices serve as an ideal skeleton for shape stabilization of melted paraffin PCMs and yield desirable thermal properties of paraffin confined in silica pores. The textural and chemical properties, crystallization, interfacial interactions, and thermal properties and stability are investigated using various techniques including nitrogen adsorption-desorption isotherms, Fourier transformation infrared (FT-IR) spectroscopy, small- and wide-angle X-ray diffraction (XRD), and scanning electron microscopy, as well as differential scanning calorimetry (DSC) analysis. The effect of the pore structure on crystallization of PCMs was studied and a novel phase change was unveiled. The phenomena of melting point depression and confinement effects are in line with theoretical thermodynamics, and the physical state of paraffin at the interfacial region in mesopore channel is analyzed considering pore geometric factors, revealing a non-melting interface layer in MCM-41. This evaluation of various silica matrices may provide important and general implications for the fundamental understanding of porous silica ss-PCMs performance with cost-effective and readily available raw materials.

Supercooling Suppression and Thermal Conductivity Enhancement of Na2HPO4·12H2O/Expanded Vermiculite Form-Stable Composite Phase Change Materials with Alumina for Heat Storage

In the heat storage applications, Na2HPO4·12H2O phase change materials (PCM) show significant defects including form instability, high supercooling degree and low thermal conductivity. Aiming at these drawbacks, the Na2HPO4·12H2O-alumina/expanded vermiculite (EVM) form-stable composite phase change materials (NEA fs-CPCMs) with supercooling suppression and heat transfer enhancement were prepared. The favorable wettability of NEA fs-CPCMs was beneficial to their form-stabilization (encapsulation mass fractions above 59.7 wt %). The supercooling degree of NE5.3 was reduced to 1.4 °C after adding 5.3 wt % alumina as the nucleating agent (decreased by 90%). The surface effects of EVM and alumina and high wettability of PCM-alumina were responsible for the supercooling suppression of PCM. Large specific surface area of EVM could provide nucleation sites for crystallization of PCM. The surface electronegativity of alumina increased the affinity ability and diffusion rate of ions, thus effectively increasing the...

Electron-beam-irradiation-induced crystallization of amorphous solid phase change materials

The electron-beam-irradiation-induced crystallization of phase change materials in a nano sized area was studied by in situ transmission electron microscopy and selected area electron diffraction. Amorphous phase change materials changed to a polycrystalline state after being irradiated with a 200 kV electron beam for a long time. The results indicate that the crystallization temperature strongly depends on the difference in the heteronuclear bond enthalpy of the phase change materials. The selected area electron diffraction patterns reveal that Ge2Sb2Te5 is a nucleation-dominated material, when Si2Sb2Te3 and Ti0.5Sb2Te3 are growth-dominated materials.

Impact of interfaces on scenario of crystallization of phase change materials

Chalcogenide phase change materials (PCMs), such as Ge-Sb-Te alloys, have outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and, more recently, in Phase Change Random Access Memories. The latter are the most promising candidate to replace the current FLASH memories. The main feature of PCMs are fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties. Controlling their crystallization, however, is a challenge. In the present work, a new picture of the prototypical GeTe and Ge2Sb2Te5 thin film crystallization is provided from optical reflectivity, resistivity and Scanning Transmission Electron Microscopy studies of 100 nm thick films. The crystallization temperature Tx significantly varies as a function of surface oxidation. Whereas Ge2Sb2Te5 has been, and is still, the subject of many publications, the present results reveal that its crystallization mechanism was not fully understood. Heterogeneous crystallization of Ge2Sb2Te5 at the upper film surface, reported so far, results from a surface alteration. Adequately protected Ge2Sb2Te5 and GeTe crystallize through nucleation inside the amorphous phase. Preventing surface alteration due to oxidation as demonstrated in the present study, through suitable capping layer deposition methods, yields a spectacular increase in Tx above the values that have been reported in the literature. The present study demonstrates that interface engineering allows one to select the crystallization mechanism, and hence control the stability of the amorphous phase in PCMs.

Crystallization Kinetics of Supercooled Liquid Ge–Sb Based on Ultrafast Calorimetry

The crystallization kinetics of phase-change materials (PCMs) entails a crucial aspect of phase-change memory technology, and their study is also of interest to advance the understanding of crystallization in general. Research on crystallization of PCMs remains challenging because of the short (nanosecond) time and small (nanometer) length scales involved. Ultrafast differential scanning calorimetry (DSC) offers a powerful tool to study crystallization via ultrahigh heating rates. Here, we used this tool to study the crystallization kinetics of growth-dominant Ge7Sb93. Two models describing the viscosity of the undercooled liquid were used to interpret the data and were subsequently crosschecked by independent growth-rate data. With both models the data in Kissinger plots could be fitted well, but one of the models resulted in a large discrepancy with the independent data. These results demonstrate that great care is needed when deriving crystal-growth rates from ultrafast DSC measurements because orders ...

Microscopic Mechanism of Doping-Induced Kinetically Constrained Crystallization in Phase-Change Materials.

A comprehensive microscopic mechanism of doping-induced kinetically constrained crystallization in phase-change materials is provided by investigating structural and dynamical dopant characteristics via ab initio molecular dynamics simulations. The information gained from this study may provide a basis for a fast screening of dopant species for electronic memory devices, or for understanding the general physics involved in the crystallization of doped glasses.

The Effect of Additives on the Crystallization PCM CaCl<sub>2</sub>.6H<sub>2</sub>O for Stabilizing the Air Temperature for Energy Conservation

Phase Change Material (PCM) is a material that uses the phase transition properties as latent heat storage for certain application. Salt hydrate CaCl2.6H2O (melting temperature, Tm = 29°C, and melting entalphy, DH = 190 kJ/kg) is a common inorganic PCM that can be used for regulating the air temperature of the room, in order to reduce the electrical energy consumption for energy conservation. General problems of inorganic PCM, in particular during the crystallization or latent heat release, are the subcooling and phase separation effects. It is worth mentioning that understanding the crystallization of PCM is important to keep its performance for application. In this research we choose two types additive materials as nucleator to CaCl2.6H2O to overcome those two negative effects, namely Ba2CO3 and K2CO3. We also study the amount of additives needed for effective reduction of subcooling and its phase stability by performing the cycling process. The average temperature and maximum temperature of the subcooling will be determined from temperature vs time recording.

Dynamical heterogeneity in the supercooled liquid state of the phase change material GeTe.

A contending technology for nonvolatile memories of the next generation is based on a remarkable property of chalcogenide alloys known as phase change materials, namely their ability to undergo a fast and reversible transition between the amorphous and crystalline phases upon heating. The fast crystallization has been ascribed to the persistence of a high atomic mobility in the supercooled liquid phase, down to temperatures close to the glass transition. In this work we unravel the atomistic, structural origin of this feature in the supercooled liquid state of GeTe, a prototypical phase change compound, by means of molecular dynamic simulations. To this end, we employed an interatomic potential based on a neural network framework, which allows simulating thousands of atoms for tens of ns by keeping an accuracy close to that of the underlying first-principles framework. Our findings demonstrate that the high atomic mobility is related to the presence of clusters of slow and fast moving atoms. The latter contain a large fraction of chains of homopolar Ge-Ge bonds, which at low temperatures have a tendency to move by discontinuous cage-jump rearrangements. This structural fingerprint of dynamical heterogeneity provides an explanation of the breakdown of the Stokes-Einstein relation in GeTe, which is the ultimate origin of the fast crystallization of phase change materials exploited in the devices.

Irreversible reactions studied with nanosecond transmission electron microscopy movies: Laser crystallization of phase change materials

We use multi-frame, nanosecond-scale photo-emission transmission electron microscopy to create movies of irreversible reactions that occur too rapidly to capture with conventional microscopy. The technique is applied to the crystallization of phase change materials used for optical and resistive memory. For those applications, laser- or current-induced crystallization is orders of magnitude too fast to capture with other imaging techniques. We recorded movies of laser-induced crystallization and measured crystal growth rates at temperatures close to where the maximum growth rate occurs. This paves the way for studying crystallization kinetics of phase change materials over the whole range of technologically relevant temperatures.

Crystallization of PCMs inside an emulsion: Supercooling phenomenon

Heat transfer characteristics during crystallization and melting of the phase change material (PCM) dispersed inside an emulsion are investigated theoretically and experimentally. The dispersed PCMs are hexadecane, octadecane and water. The crystallization and melting processes are not symmetrical because of the supercooling phenomenon induced by the smallness of the emulsion droplet sizes. Nucleation laws are used to simulate the supercooling phenomenon. Energy analysis is carried out to understand the behavior of the emulsion using two paraffins (octadecane and hexadecane). The effects of different parameters such as the mass fraction of PCM and the stabilization temperature on thermal behavior of emulsion are also analyzed.

Electric field induced crystallization in phase-change materials for memory applications

Emerging electrical memory technologies based on phase-change materials capitalize on a fast amorphous-to-crystalline transition. Recent evidence from measurements of relaxation oscillations and switching statistics in phase-change memory devices indicates the possibility that electric field induced crystal nucleation plays a dominant role in defining the characteristic electrical switching behavior. Here we present a detailed kinetics study of crystallization in the presence of an electric field for the phase-change material Ge2Sb2Te5. We derive quantitative crystallization maps to show the effects of both temperature and electric field on crystallization and we identify field ranges and parameter values where the electric field effects might play a significant role.

Thermal and elastic properties of Ge-Sb-Te based phase-change materials

ABSTRACTPhase-change materials undergo a change in bonding mechanism upon crystallization, which leads to pronounced modifications of the optical properties and is accompanied by an increase in average bond lengths as seen by extended x-ray absorption fine structure (EXAFS), neutron and x-ray diffraction. The reversible transition between a crystalline and an amorphous phase and its related property contrast are already employed in non-volatile data storage devices, such as rewritable optical discs and electronic memories. The crystalline phase of the prototypical material GeSb2Te4 is characterized by resonant bonding and pronounced disorder, which help to understand their optical and electrical properties, respectively. A change in bonding, however, should also affect the thermal properties, which will be addressed in this study. Based on EXAFS data analyses it will be shown that the thermal and static atomic displacements are larger in the meta-stable crystalline state. This indicates that the bonds become softer in the crystalline phase. At the same time, the bulk modulus increases upon crystallization. These observations are confirmed by the measured densities of phonon states (DPS), which reveal a vibrational softening of the optical modes upon crystallization. This demonstrates that the change of bonding upon crystallization in phase-change materials also has a profound impact on the lattice dynamics and the resulting thermal properties.