Growth-dominated Crystallization Research Articles

Understanding the microstructure evolution of carbon-doped Sb2Te3 phase change material for high thermal stability memory application

The Sb2Te3 phase change material shows a growth-dominated crystallization mechanism with fast phase transition but poor thermal stability of the amorphous state. This work investigated the effects of carbon doping on the thermal stability, microstructure, and electrical properties of the Sb2Te3 material. The 10-year data retention temperature of the material increased to ∼147.3 °C and the size of the grains was limited to ∼10 nm by carbon doping. The formation of the C cluster upon crystallization was found at the grain boundaries, which was accelerated as the temperature increased due to the break of the Sb–C bonds. The memory device based on the carbon-doped Sb2Te3 material exhibited a switching speed of 15 ns and an endurance of ∼105 cycles with a resistance ratio of more than two orders of magnitude. This work suggests that the carbon-doped Sb2Te3 material is a promising candidate for memory applications that require high thermal stability, fast speed, and high endurance.

Tuning the Crystallization Mechanism by Composition Vacancy in Phase Change Materials.

Interface-influenced crystallization is crucial to understanding the nucleation- and growth-dominated crystallization mechanisms in phase-change materials (PCMs), but little is known. Here, we find that composition vacancy can reduce the interface energy by decreasing the coordinate number (CN) at the interface. Compared to growth-dominated GeTe, nucleation-dominated Ge2Sb2Te5 (GST) exhibits composition vacancies in the (111) interface to saturate or stabilize the Te-terminated plane. Together, the experimental and computational results provide evidence that GST prefers (111) with reduced CN. Furthermore, the (8 - n) bonding rule, rather than CN6, in the nuclei of both GeTe and GST results in lower interface energy, allowing crystallization to be observed at the simulation time in general PCMs. In comparison to GeTe, the reduced CN in the GST nuclei further decreases the interface energy, promoting faster nucleation. Our findings provide an approach to designing ultrafast phase-change memory through vacancy-stabilized interfaces.

Controllable single phase enables superior thermal stability in Sb-Sn-S films

Sb-Sn-S thin films were prepared by magnetron sputtering, and the effects of the SnS content on the macroscopic performance and microstructure of Sb were investigated. Results show that Sb55(SnS)45 exhibited the best performance, with a crystallization temperature of 210 °C, solving the issue of spontaneous Sb crystallization. However, the thermal stability of Sb37(SnS)63 film could no longer be significantly improved. The investigated crystallization mode revealed that Sb55(SnS)45 exhibited a growth-dominated crystallization behavior, maintaining a high crystallization rate while substantially improving the thermal stability. In terms of the microstructure, high-energy Sb–S and Sb–Sn bonds were formed, breaking the originally unstable Sb–Sb bonds. In the case of Sb37(SnS)63, the internal Sb–S and Sb–Sn bonding environment reached a saturation state, and the excessive introduction of SnS resulted in the stacking arrangement of the Sb–Sn–S amorphous alloy, leading to lattice distortion. However, an appropriate amount of Sb–Sn–S amorphous alloy polymerized around the SbSn grains, limiting the grain growth and the element segregation, thereby enhancing the long-term stability of the phase-change thin film.

Development of Sb2Se3 alloys by Ti-doping with ultralow resistance drift and improved microstructure for nonvolatile memory applications

The Sb2Se3 and Ti-doped Sb2Se3 phase change thin films were prepared by magnetron sputtering. The relationship between resistance drift and crystallization behavior of Sb2Se3 and Ti-doped Sb2Se3 thin films were thoroughly investigated. The results revealed that when Ti-doping concentration reaches 5.4 at. %, Ti5.4(Sb2Se3)95.4 thin film exhibited a high thermal stability with crystallization temperature of 225 °C and 10-year data retention temperature of 129.5 °C. This benefits to lower resistance drift coefficient from 0.067 for Sb2Se3 to 0.002 for Ti5.4(Sb2Se3)95.4. Further microstructural analysis revealed the suppression of large grain growth in Ti-doped Sb2Se3 thin films, while the formation of Ti–Sb and Ti–Se bonds being responsible for enhanced stability of the amorphous Ti-doped thin films. Moreover, the Ti doping promoted one-dimensional growth-dominated crystallization mechanism of the studied alloys, leading to the reduced nucleation index compared to Sb2Se3. The present study sheds valuable light on the effectively reducing nucleation randomness in chalcogenide-based phase-change materials.

Elemental Redistribution During the Crystallization of Ge–Cu–Te Thin Films for Phase-Change Memory

Herein, a GeCu2Te2 alloy is proposed as a phase-change material for application in nonvolatile phase-change random access memory (PRAM). The crystallization kinetics and microchemical changes during phase transformation are investigated, and their correlation with the electrical behaviors of the GeCu2Te2 thin films are examined. The key findings are as follows: (ⅰ) the GeCu2Te2 alloy shows a higher crystallization temperature (∼185 °C) than the classic Ge2Sb2Te5 (GST) thin films, thus demonstrating superior thermal stability; (ⅱ) the crystallization kinetics demonstrate a decreasing in the Avrami exponent n from 4, which is related to the growth-dominated crystallization process evidenced by the micromorphology; (ⅲ) a massive redistribution of the chemical elements along the depth of the thin films during crystallization is considered to be driven by selective surface oxidation at amorphous state, and stress buildup during crystallization. In addition, the crystallization-induced stress is determined as ∼168 MPa by utilizing the wafer curvature and X-ray diffraction methods for the GeCu2Te2 thin films. Finally, the lower threshold switching voltage ∼1.72 V for amorphous GeCu2Te2 thin films is beneficial for reducing the SET operating power consumption. The authors believe that these results are valuable for the optimal phase change material design.

Uncovering the physical properties, structural characteristics, and electronic application of superlattice-like Ti/Sb phase-change thin films

Superlattice-like (SLL) Ti/Sb thin films were proposed and investigated from the viewpoint of physical properties, structural characteristics, and electronic application. Magnetron sputtering was employed to deposit the SLL Ti/Sb thin films with different thickness ratios. In-situ resistance–temperature measurement indicates that the crystallization temperature, crystallization-activation energy, and data-retention capacity increase significantly and the resistance drift index reduces with an increment in thickness ratio of the Ti to Sb layer, meaning higher amorphous thermal stability and reliability of SLL Ti/Sb thin films. X-ray diffraction and Raman spectra reveal that the inserted Ti layer can inhibit grain growth and refine the grain size, causing remarkable improvement of thermal stability and crystalline resistance. Analyses of x-ray reflectivity and atomic force microscopy demonstrate that the thickness fluctuation of SLL Ti/Sb thin films becomes smaller and the surface topography becomes smoother, respectively. The Avrami exponent of the SLL (Ti3Sb7)5 thin film reflects the growth-dominated crystallization mechanism, implying a rapid phase transition speed. Phase-change memory cells based on the SLL (Ti3Sb7)5 thin film can realize a reversible SET/RESET operation under an electrical pulse with a width of 100 ns. The RESET power consumption was estimated to be much lower than that of traditional Ge2Sb2Te5 material. The above results strongly prove that the suitable SLL structure of Ti/Sb thin films have tremendous potential in the area of high-temperature and low-power electronic storage.

Production of Sucrose Crystals of Uni-Modal Size Distribution by Seeded Batch Cooling Crystallization

From sucrose aqueous solution of high viscosity, sucrose was crystallized by seeded cooling in a batch crystallizer. At low seed loadings, the product crystal size distribution (CSD) was wide-spread because of enormous secondary and primary nucleation. However, at high seed loadings, it became uni-modal, where the crystallization was dominated by seed growth with practically no secondary nucleation. Enough seeding was thus effective in suppressing nucleation even during batch crystallization with high viscosity solution. The volume mean size of the product crystals obtained at high seed loadings agreed with that calculated by a simple mass balance assuming growth-dominated crystallization with no change in the number of crystals.

Photoexcitation Induced Ultrafast Nonthermal Amorphization in Sb2Te3

Phase-change materials are of great interest for low-power high-throughput storage devices in next-generation neuromorphic computing technologies. Their operation is based on the contrasting properties of their amorphous and crystalline phases, which can be switched on the nanosecond time scale. Among the archetypal phase change materials based on Ge-Sb-Te alloys, Sb2Te3 displays a fast and energy-efficient crystallization-amorphization cycle due to its growth-dominated crystallization and low melting point. This growth-dominated crystallization contrasts with the nucleation-dominated crystallization of Ge2Sb2Te5. Here, we show that the energy required for and the time associated with the amorphization process can be further reduced by using a photoexcitation-based nonthermal path. We employ nonadiabatic quantum molecular dynamics simulations to investigate the time evolution of Sb2Te3 with 2.6, 5.2, 7.5, 10.3, and 12.5% photoexcited valence electron-hole carriers. Results reveal that the degree of amorphization increases with excitation, saturating at 10.3% excitation. The rapid amorphization originates from an instantaneous charge transfer from Te-p orbitals to Sb-p orbitals upon photoexcitation. Subsequent evolution of the excited state, within the picosecond time scale, indicates an Sb-Te bonding to antibonding transition. Concurrently, Sb-Sb and Te-Te antibonding decreases, leading to formation of wrong bonds. For photoexcitation of 7.5% valence electrons or larger, the electronic changes destabilize the crystal structure, leading to large atomic diffusion and irreversible loss of long-range order. These results highlight an ultrafast energy-efficient amorphization pathway that could be used to enhance the performance of phase change material-based optoelectronic devices.

Heterogeneous Nucleation and Self-Nucleation of Isotactic Polypropylene Microdroplets in Immiscible Blends: From Nucleation to Growth-Dominated Crystallization

Nucleating agents (NAs) are widely used for speeding up processing and tuning the final optical and mechanical properties. Despite their industrial importance, the search for highly efficient NAs i...

Review and Modeling of Crystal Growth of Atropisomers from Solutions

In this paper, theories on anisotropic crystal growth and crystallization of atropisomers are reviewed and a model for anisotropic crystal growth from solution containing slow inter-converting conformers is presented. The model applies to systems with growth-dominated crystallization from solutions and assumes that only one conformation participates in the solute integration step and is present in the crystal lattice. Other conformers, defined as the wrong conformers, must convert to the right conformer before they can assemble to the crystal lattice. The model presents a simple implicit method for evaluating the growth inhibition effect by the wrong conformers. The crystal growth model applies to anisotropic growth in two main directions, namely a slow-growing face and a fast-growing face and requires the knowledge of solute crystal face integration coefficients in both directions. A parameter estimation algorithm was derived to extract those coefficients from data about temporal concentration and crystal size during crystallization and was designed to have a short run time, while providing a high-resolution estimation. The model predicts a size-dependent growth rate and simulations indicated that for a given seed size and solvent system and for an isothermal anti-solvent addition crystallization, the seed loading and the supersaturation at seeding are the main factors impacting the final aspect ratio. The model predicts a decrease of the growth inhibition effect by the wrong conformer with increasing temperature, likely due to faster equilibration between conformers and/or a decrease of the population of the wrong conformer, if of low energy, at elevated temperatures. Finally, the model predicts that solute surface integration becomes the rate-limiting mechanism for high solute integration activation energies, resulting in no impact of the WC on the overall crystal growth process.

Dependence of transition behaviors on structure of Sb100−xErx films for broadband nonvolatile optical memory

The crystallization temperature (Tc) and 10-year data-retention temperature enhance from 176 °C to 217 °C and from 61.5 °C to 120.6 °C, respectively, when the Er concentration increases from 16 at. % to 28 at. % for Sb100−xErx films. The improvement in the thermal stability of the Sb100−xErx results from Er doping induced the suppression of the A1g mode from Sb-Sb bonds. The fast crystallization of the Sb100−xErx film is ascribed to the growth-dominated crystallization mechanism which was confirmed by the in situ microstructure observation. A large optical contrast of Sb100−xErx such as high ON/OFF ratios of both the refractive index (n) and the extinction coefficient (k) between the amorphous and crystalline states results from the formation of resonant bonding in crystalline states. Sb100−xErx demonstrated the repeatable and reversible phase change between two states induced by optical pulses, suggesting a potential candidate for optical storage.

Effect of copper doping on the crystallization behavior of TiSbTe for fast-speed phase change memory

In this study, Ti-Sb-Te (TST) alloy doped with Cu has been proposed to enhance the phase change and electrical properties of TST. The formation of Cu-Te bond was suggest to be responsible for the improved thermal stability of the Cu0.28(Ti0.09Sb0.38Te0.53)0.72 (Cu0.28TST) film. Cu0.28TST-based phase change memory cell can be triggered by a 5 ns electric pulse. Such fast speed was ascribed to its growth-dominated crystallization mechanism and the precipitation of Sb in the crystallization process. Furthermore, an endurance cycles up to 105 could be achieved in the memory cell.

Crystallization Properties of Mg35Sb65/Sb Nanocomposite Multilayer Films for Phase Change Memory Application

The phase transition properties of Mg35Sb65/Sb multilayer thin films were studied, including the crystallization mechanism, surface morphology, atomic bonding mode and adhesion strength. With the increase of the thickness of Mg35Sb65 interlayers, Mg35Sb65/Sb thin film had better thermal stability and lower electrical conductivity. The growth-dominated crystallization mechanism made Mg35Sb65/Sb have ultra-fast phase change speed. The subtle change on surface morphology was ascribed to the grain growth and interface stress during crystallization. The scratch test revealed the adhesion strength contrast before and after crystallization. This work showed that Mg35Sb65/Sb multilayer film was a potential material with fast speed and good thermal stability for phase change memory application.

Phase-change properties of GeSbTe thin films deposited by plasma-enchanced atomic layer depositon.

Phase-change access memory (PCM) appears to be the strongest candidate for next-generation high-density nonvolatile memory. The fabrication of ultrahigh-density PCM depends heavily on the thin-film growth technique for the phase-changing chalcogenide material. In this study, Ge2Sb2Te5 (GST) and GeSb8Te thin films were deposited by plasma-enhanced atomic layer deposition (ALD) method using Ge [(CH3)2 N]4, Sb [(CH3)2 N]3, Te(C4H9)2 as precursors and plasma-activated H2 gas as reducing agent of the metallorganic precursors. Compared with GST-based device, GeSb8Te-based device exhibits a faster switching speed and reduced reset voltage, which is attributed to the growth-dominated crystallization mechanism of the Sb-rich GeSb8Te films. These results show that ALD is an attractive method for preparation of phase-change materials.

Spherulitic crystallization mechanism of a Zr-based bulk metallic glass during laser processing

Laser deposition of a Zr–Cu–Ni–Al–Nb metallic glass has been studied in an effort to understand and evaluate the challenges of fabricating metallic glass components via additive manufacturing techniques. The parent amorphous alloy crystallizes into micro-scale spherulites at heating rates up to 104 K/s during laser processing. Detailed microstructural and compositional examinations of the spherulites reveals that rapid heating suppresses phase separation and nucleation at the initial stage of crystallization, resulting in a growth-dominated crystallization behavior. The activation energy of spherulitic crystallization is estimated to be 124 kJ/mol, significantly lower than that of multiphase nanocrystallization reported elsewhere. The low activation energy of spherulitic crystallization is consistent with observations of the short-range redistribution of constituent elements at the amorphous–crystalline interfaces during growth of the spherulites.

Pre-eruptive Magmatic Conditions at Augustine Volcano, Alaska, 2006: Evidence from Amphibole Geochemistry and Textures

Variations in the geochemistry and texture of amphibole phenocrysts erupted from Augustine Volcano in 2006 provide new insights into preand syn-eruptive magma storage and mixing. Amphiboles are rare but present in all magma compositions (lowto high-silica andesites) from the 3 month long eruption. Unzoned magnesiohornblende in the highand low-silica andesites exhibit limited compositional variability, relatively high SiO2 (up to 49·7 wt %), and relatively lowAl2O3 (511·1wt %). Intermediate-silica andesites and quenched mafic enclaves contain amphiboles that vary in composition (e.g. SiO2 40·8^48·9 wt %, Al2O3 6·52^15·2 wt %) and classification (magnesiohornblende^magnesiohastingsite^ tschermakite). Compositional variation in amphibole is primarily controlled by temperature-dependent substitutions. Both highand low-silica andesites represent remnant magmas that were stored in the shallow crust at 4^8 km depth, remaining distinct owing to a complex subsurface plumbing system. Intermediate-silica andesites and quenched mafic inclusions represent pre-eruptive hybrids of resident highand low-silica andesite magmas and an intruding basalt. Amphiboles in explosive phase high-silica andesites are largely euhedral and unreacted, consistent with the high magma flux rates from depth during this phase (up to 13 800 m s). Phenocrysts from the other lithologies have reaction rims that range from 1 to41000 m in thickness. Reaction rim microlite sizes correlate with reaction rim thicknesses. Reaction rims550 m thick contain microlites 1^10 m in length whereas reaction rims480 m thick contain microlites 10^100 m in length. Differentiating between heatingand decompression-induced amphibole reaction rim formation is problematic because of a lack of experimental constraints.We attempt a new approach to assessing reaction rim formation, based on a kinetic theory of crystal nucleation and growth, in which the differences in reaction rim textures represent different degrees of amphibole disequilibrium. Large crystals and low number densities suggest relatively lower levels of disequilibrium resulting in growth-dominated crystallization. Smaller crystals and larger number densities are indicative of higher nucleation rates and a high driving force.

Crystallization behavior of Si-added amorphous Ga19Sb81 films for phase-change memory

Crystallization behavior of Si-added amorphous Ga19Sb81 films was studied by measuring electrical resistance (R) versus temperature (T) at various heating rates. The crystallization temperatures obtained from the R–T curves increase from 228 to 284°C measured at the heating rate 10°C/min with increasing Si content to 14at.%. All films with full-crystallization show phases of Sb and GaSb. At 9at.% Si the activation energy of crystallization and the rate factor reach the maxima of 9.1eV and 6.8×1084min−1, respectively. The kinetic exponent (n) of the film with 9at.% Si is below 1.5 which indicates growth-dominated crystallization, while that of other films falls in 1.5≤n≤2.5 which denotes decreasing nucleation rate during nuclei growth. Time of 90% crystallization slightly decreases owing to Si addition as estimated using JMA theory. The temperature corresponding to 10-year failure-time obtained through isothermal Arrhenius plot effectively increases from 156 to 217°C as Si content is from 0 to 14at.%. The melting temperature slightly reduces from 591 to 581°C with Si addition. The reset voltage of test cells based on Si4(Ga19Sb81)96 is reduced by 0.5V to ~2.75V and can be set–reset switched within pulse-widths of 40–200ns.

The Evolution of the Peach Spring Giant Magma Body: Evidence from Accessory Mineral Textures and Compositions, Bulk Pumice and Glass Geochemistry, and Rhyolite-MELTS Modeling

The Miocene Peach Spring Tuff is a giant (� 640 km 3 dense rock equivalent) pyroclastic deposit that is extensively exposed in the southwestern USA. Evidence from geochemical and textural analyses of bulk-rocks, glasses, and accessory minerals (zircon, titanite, allanite, chevkinite, magnetite) from outflow and intra-caldera pumice clasts and fiamme, in combination with field observations and rhyolite-MELTS modeling, suggests that the Peach Spring magma body was compositionally and thermally zoned with a basal cumulate, and that it crystallized over millennial timescales before being remobilized by mafic input prior to erupting. Crystal contents, bulk compositions, spatial distributions, and temperatures (recorded by Ti in zircon and Zr in titanite) of pumice clasts and fiamme vary systematically: distal outflow high-silica rhyolites are crystal-poor and document lower temperatures; intra-caldera trachytes are crystal-rich and record higher temperatures. These variations indicate that the Peach Spring magma body was zoned. We interpret the outflow high-silica rhyolites to represent the first portion of the magma body to erupt. Zircon and titanite display core-to-edge reductions in rare earth element (REE) concentration and temperature, suggestive of relatively uninterrupted crystallization as the magma body cooled; crystallization temperature intervals from rhyoliteMELTS are consistent with those recorded by zircon and titanite. Exponential size distributions for accessory minerals and phenocryst textures are consistent with geochemical evidence for a simple cooling and crystallization history. Intra-caldera trachytes and outflow low-silica rhyolites represent the later portion of the magma body to erupt.This magma experienced a late-stage heating event potentially associated with the onset of the eruption.The edges of titanite crystals are enriched in REE and Zr, and zircon edges are enriched in Ti, suggesting higher temperatures during edge crystallization (at least 9008C). Concave-down crystal size distributions and resorption features on phenocrysts are additional signs of heating. Rare trachyandesite enclaves and the presence of mafic to intermediate lavas immediately underlying the Peach Spring Tuff suggest that a mafic magma input may have been the cause of the heating. Evidence further suggests that the intra-caldera trachytes may represent a remobilized cumulate at the base of the magma body, which retained some melt prior to rejuvenation. Bulk pumice and fiamme compositions are very rich in feldspar and accessory mineral phenocrysts, indicative of accumulation of these minerals; high crystal contents (� 35%) and evidence of heating and resorption imply that this magma was even more crystal-rich prior to the heating event. Rhyolite-MELTS simulations suggest that the trachyte magma had roughly 1wt % water, which cannot be totally accounted for by hydrous phases, thus requiring some amount of melt within the cumulate. Kinked magnetite size distributions are interpreted to represent a change from growth-dominated crystallization (larger crystals, shallow slopes) to nucleation-dominated (small crystals, steep slopes) owing to the onset of eruptive decompression. Timescales of magnetite crystallization calculated from these slopes indicate that the Peach Spring magma body crystallized over millennial timescales, and that eruptive decompression began 10 � 1 ^10 2 years prior to eruption.

How Gold Nanoparticles Influence Crystallization of Polyethylene in Rigid Cylindrical Nanopores

Even high amounts of gold nanoparticles (AuNPs) only moderately influence crystallization of bulk polyethylene (PE). However, under the rigid two-dimensional confinement of aligned cylindrical nanopores in anodic aluminum oxide (AAO) the presence of Au turns nucleation-dominated crystallization of PE at high supercooling into growth-dominated crystallization at lower supercooling. Transmission electron microscopy investigations revealed formation of larger Au crystals from AuNPs by Ostwald ripening. These larger Au crystals apparently acted as heterogeneous nucleation sites initiating PE crystallization in AAO nanopores. Thus, PE/Au composites in AAO exhibited significantly higher crystallization and melting onset temperatures as well as significantly weaker dependence of crystallization half-times on crystallization temperatures. X-ray texture analysis revealed for pure PE in AAO the existence of two copopulations of crystals with different orientations (indicative of nucleation-dominated crystal growth)...

Experimental Evaluation of the Targeted Direct Design of Temperature Trajectories for Growth-Dominated Crystallization Processes Using an Analytical Crystal Size Distribution Estimator

The paper presents an experimental validation of a novel methodology for the systematic design of the set point operating curves for supersaturation-controlled, seeded crystallization processes, which produces a desired target crystal size distribution (CSD). The direct design approach is based on the idea of operating the system within the metastable zone (MSZ) bounded by the nucleation and the solubility curves. The proposed approach is based on an analytical CSD estimator, obtained by the analytical solution of the population balance equation for supersaturation-controlled growth-dominated processes. Based on the analytical estimator a design parameter for supersaturation-controlled processes is defined as a function of the supersaturation, time, and growth kinetics. Using the design parameter and the analytical CSD estimator, the temperature profiles in the time domain are determined to obtain a target distribution with a desired shape, while maintaining the constant supersaturation. The resulting tem...