Controlling crystallization kinetics is key to overcome the temperature–time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 ...years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3 Tm to compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge2Sb2Te5 with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperature Tg. Moreover, highly unusual for glass‐forming systems, crystallization at conventional heating rates is observed more than 50 °C below Tg, where the atomic mobility should be vanishingly small.
Combining kinetic, thermodynamic, and microscopic measurements of crystallization kinetics in the classic phase‐change material Ge2Sb2Te5, it is demonstrated that Ge2Sb2Te5 crystallizes from the glassy phase at heating rates up to 10 000 K s−1 and from the undercooled liquid at higher rates. Due to the concurrence of emerging glass transition and crystallization, the activation energy of crystallization drops by more than fourfold.
Chalcogenide phase‐change materials combine a remarkable set of properties that makes them promising candidates for future non‐volatile memory applications. Binary data storage exploits the high ...contrast in electrical and optical properties between the covalent amorphous and metavalent crystalline phase. Here the authors perform an analysis of the liquid phase kinetics of the phase‐change material Ge3Sb6Te5, which is the key to ultrafast switching speeds. By employing four experimental techniques, the viscosity is measured over sixteen orders of magnitude despite its propensity for fast crystallization. These measurements reveal that the liquid undergoes a transition in viscosity–temperature dependence associated with a liquid–liquid phase transition. The system exhibits a shallow viscosity change with temperature near the glass transition which stabilizes the memory cells in the amorphous state and which limits the severity of relaxation processes. Meanwhile, when heated during the writing process, the fragility increases to more than double, causing the viscosity to drop rapidly enabling a nanosecond crystallization speed. This change in viscosity–temperature dependence is highly unusual among glass forming liquids and is reminiscent of the behavior of water. This viscosity transition is also key to the technological success of phase‐change materials for computer memory applications.
Phase‐change materials (PCMs) show a large property contrast that is exploited in memory storage. Fragile‐to‐strong transitions (FST) aid the application of PCMs, as ultrafast crystallization and amorphous phase stability are simultaneously realized. Here, for the PCM Ge3Sb6Te5 the viscosity is measured over sixteen orders of magnitude, clearly demonstrating an FST.
Abstract
Controlling a state of material between its crystalline and glassy phase has fostered many real-world applications. Nevertheless, design rules for crystallization and vitrification kinetics ...still lack predictive power. Here, we identify stoichiometry trends for these processes in phase change materials, i.e. along the GeTe-GeSe, GeTe-SnTe, and GeTe-Sb
2
Te
3
pseudo-binary lines employing a pump-probe laser setup and calorimetry. We discover a clear stoichiometry dependence of crystallization speed along a line connecting regions characterized by two fundamental bonding types, metallic and covalent bonding. Increasing covalency slows down crystallization by six orders of magnitude and promotes vitrification. The stoichiometry dependence is correlated with material properties, such as the optical properties of the crystalline phase and a bond indicator, the number of electrons shared between adjacent atoms. A quantum-chemical map explains these trends and provides a blueprint to design crystallization kinetics.
The calorimetric features that have been broadly used to assign a glass transition temperature Tg of 136 K to amorphous water are qualitatively reproduced with a phase change material. Annealing ...treatments and ultrafast calorimetry measurements indicate that this feature is only a shadow-Tg and that the real Tg lies at higher temperature above the glass transition. A Kissinger analysis of the crystallization kinetics confirms that crystallization occurs below Tg from the glassy state at conventional heating rates. These results strongly suggest that the amorphous water endotherm at 136 K is indeed a shadow-Tg and that the real Tg lies at higher temperature as predicted from structural relaxation considerations.
Engineering phase change materials (PCM) to realize superior data storage devices requires a detailed understanding of crystallization kinetics and its temperature dependence. The temperature ...dependence of crystallization differs distinctly between crystallizing from the glassy phase and the undercooled liquid (UCL). Hence, knowing the phase from which crystallization occurs is necessary for predicting the switching ability. Here, we measure the glassy dynamics and crystallization kinetics using calorimetry for heating rates spanning over six orders of magnitude. Our results show that the prominent PCM (Ag,In)-doped Sb2Te (AIST) exhibits a change from crystallizing from the glassy phase to crystallizing from the UCL at a critical heating rate of 5000 K/s. Above the glass transition, the activation energy of crystallization changes drastically enabling rapid crystallization at elevated temperatures.
Calorimetric data on the excess heat capacity Cpexc and the crystallization peak temperature Tp is combined to identify the amorphous state the phase change material AIST crystallizes from, i.e. the glassy phase or the undercooled liquid. When glass transition and crystallization interfere, the Kissinger activation energy Ek drops by more than threefold, enabling rapid crystallization. The glass transition temperature Tg is estimated to be about 182.5 °C. Hence, at conventional heating rates, AIST unconventionally crystallizes ~ 27.5 °C below the actual glass transition. Display omitted
Abstract
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 Ge
15
Sb
85
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.
Like many phase‐change materials, GeTe crystallizes upon heating at a conventional rate before the calorimetric glass transition is reached. This has so far prevented an unambiguous determination of ...its glass transition temperature Tg. Herein, a new approach is realized to estimate the glass transition temperature Tg for GeTe through progressive crystallization of Ge15Te85. Selective crystallization of pure tellurium during sub‐Tg annealing leads to a gradual change in the composition of the amorphous surrounding toward that of GeTe. This gives rise to a new endotherm whose onset temperature gradually approaches the Tg of GeTe.
When the eutectic composition Ge15Te85 crystallizes, it transforms into tellurium and GeTe during two distinct crystallization events. As tellurium crystallizes first, the composition of the remaining amorphous material changes toward that of GeTe. During that shift in composition, an endotherm of a glass transition develops, whose onset approaches the glass transition temperature Tg of GeTe.
Glasses frequently reveal structural relaxation that leads to changes in their physical properties including enthalpy, specific volume, and resistivity. Analyzing the short‐range order (SRO) obtained ...from electron diffraction by transmission electron microscopy (TEM) in combination with Reverse‐Monte‐Carlo (RMC) simulations is shown to provide information on the atomic arrangement. The technique elaborated here features several benefits including reliability, accessibility, and allows for obtaining detailed structural data quickly. This is demonstrated with a detailed view of the structural changes in the as‐deposited amorphous phase change material (PCM) GeTe. The data show a significant increase in the average bond angle upon thermal treatment. At the same time the fraction of tetrahedrally coordinated Ge atoms decreases due to an increase in octahedrally distorted and pyramidal motifs. This finding provides further evidence for the atomic processes that govern structural relaxation in amorphous GeTe and other PCMs. A thorough literature review finally unveils possible origins of the large discrepancies reported on the structure of amorphous GeTe.
Structure function ϕ(q) and Pair Distribution Function G(r) of the technologically relevant phase‐change‐material GeTe are obtained by TEM diffraction. Reverse‐Monte‐Carlo simulations based on this structural data reveal atomic rearrangement processes, shedding light on structural relaxation that leads to resistance drift in GeTe. The here elaborated method is capable of reliably reproducing structural models obtained from large scale synchrotron‐facilities as shown for several materials like amorphous silica.
Reducing the enthalpy of as-deposited amorphous films is desirable as it improves their kinetic stability and enhances the reliability of resulting devices. Here we demonstrate that Ge15Te85 glass ...films of lower enthalpy are produced by increasing the voltage during magnetron sputter deposition. An increase of ~100 V leads to a drop in effective cooling rate of almost three orders of magnitude, thereby yielding markedly lower enthalpy glasses. The sputtering voltage therefore constitutes a novel parameter for tuning the fictive temperature of glass films, which could help to obtain ultra-stable glasses in combination with substrate temperature control.
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