Fragile-to-Strong Crossover in Supercooled Liquid Ag-In-Sb-Te Studied by Ultrafast Calorimetry

Phase-change random-access memory relies on the reversible crystalline-glassy phase change in chalcogenide thin films. In this application, the speed of crystallization is critical for device performance: there is a need to combine ultrafast crystallization for switching at high temperature with high resistance to crystallization for non-volatile data retention near to room temperature. In phase-change media such as nucleation-dominated Ge2Sb2Te5, these conflicting requirements are met through the highly “fragile” nature of the temperature dependence of the viscosity of the supercooled liquid. The present study explores, using ultrafast-heating calorimetry, the equivalent temperature dependence for the growth-dominated medium Ag-In-Sb-Te. The crystallization shows (unexpectedly) Arrhenius temperature dependence over a wide intermediate temperature range. Here it is shown that this is evidence for a fragile-to-strong crossover on cooling the liquid. Such a crossover has many consequences for the interpretation and control of phase-change kinetics in chalcogenide media, helping to understand the distinction between nucleation- and growth-dominated crystallization, and offering a route to designing improved device performance.J.O., D.W.H. and A.L.G. acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC, UK), D.W.H. in part through the EPSRC Centre for Innovative Manufacturing in Photonics. J.O. and A.L.G. acknowledge support from the World Premier International Research Center Initiative (WPI), MEXT, Japan. C. A. Angell, L. Battezzati, G. Dalla Fontana and M. Salinga are thanked for helpful discussions.This is the author accepted manuscript. The final version is available from Wiley at http://onlinelibrary.wiley.com/doi/10.1002/adfm.201501607/abstract

Fast crystal growth in glass-forming liquids

In liquids of high glass-forming ability, in which crystal growth rates are low, the rates can be measured over the full range of supercooling from the liquidus temperature down to the glass transition. For systems of low glass-forming ability, growth rates are readily measured at small supercooling and at very large supercooling around the glass-transition temperature, but it is difficult to acquire data over the full range of intermediate supercooling, especially at the maximum in growth rate. Data at intermediate supercoolings are however of considerable interest for understanding glass formation in such systems as pure metals and chalcogenides for phase-change data storage. We will review the methods emerging for making such measurements, and will note that the fragility of the liquid (including possible crossover from ‘fragile’ to ‘strong’ liquid behaviour on cooling) is an important part of understanding fast crystal growth. We also note that there are deficiencies in existing theories of fast crystal growth.oa-900

Rejuvenation through plastic deformation of a La-based metallic glass measured by fast-scanning calorimetry

We explore the glassy states achievable after a metallic glass is formed on liquid quenching. Samples of La55Al25Ni20 (at.%) metallic glass (rod and ribbon) are studied. The extent of structural relaxation at room temperature is characterized for this low-glass-transition temperature glass. Plastic deformation (uniaxial compression) rejuvenates the glass to states of higher enthalpy characteristic of glass formation at high cooling rate. Deformation increases the heterogeneity of the glass, widening the spectrum of relaxation times. The extent of rejuvenation in samples of low aspect ratio is compared with that under conditions of high constraint in notched samples. The deformation-induced rejuvenation is particularly susceptible to reduction on subsequent ageing. Fast-scanning calorimetry is useful in characterizing the dynamics of structural relaxation. The shadow glass transition is more evident on fast heating, and is observed in this glass for the first time. A new excess exothermic effect is observed before the glass transition

Reversible migration of silver on memorized pathways in Ag-Ge<inf>40</inf>S<inf>60</inf> films

Reversible and reproducible formation and dissolution of silver conductive filaments are studied in Ag-photodoped thin-film Ge40S60 subjected to electric fields. A tip-planar geometry is employed, where a conductive-atomic-force microscopy tip is the tip electrode and a silver patch is the planar electrode. We highlight an inherent “memory” effect in the amorphous chalcogenide solid-state electrolyte, in which particular silver-ion migration pathways are preserved “memorized” during writing and erasing cycles. The “memorized” pathways reflect structural changes in the photodoped chalcogenide film. Structural changes due to silver photodoping, and electrically-induced structural changes arising from silver migration, are elucidated using Raman spectroscopy. Conductive filament formation, dissolution, and electron (reduction) efficiency in a lateral device geometry are related to operation of the nano-ionic Programmable Metallization Cell memory and to newly emerging chalcogenide-based lateral geometry MEMS technologies. The methods in this work can also be used for qualitative multi-parameter sampling of metal/amorphous-chalcogenide combinations, characterizing the growth/dissolution rates, retention and endurance of fractal conductive filaments, with the aim of optimizing devices.This work was supported by World Premier International Research Center Initiative (WPI), MEXT, Japan. J.O. and A.L.G. acknowledge support from the Engineering and Physical Sciences Research Council (EPSRC, UK).This is the final version of the article. It first appeared from AIP via http://dx.doi.org/10.1063/1.492700

Ultra-fast calorimetry study of Ge₂Sb₂Te₅ crystallization between dielectric layers

Phase changes in chalcogenides such as Ge2Sb2Te5 can be exploited in non-volatile random-access memory, with fast crystallization crucial for device operation. Ultra-fast differential scanning calorimetry, heating at rates up to 40,000K s-1, has been used to study the crystallization of amorphous Ge2Sb2Te5 with and without sandwich layers of ZnS-SiO2. At heating rates up to 1000K s-1, the sandwich layers retard crystallization, an effect attributed to crystallization-induced stress. At greater heating rates (&gt;or = 5000K s-1), and consequently higher crystallization temperatures, the stress is relaxed, and sandwich layers catalyze crystallization. Implications for memory-device performance are discussed

Classical-nucleation-theory analysis of priming in chalcogenide phase-change memory

The chalcogenide Ge2Sb2Te5 (GST) is of interest for use in phase-change memory. Crystallization is the rate-limiting step for memory operation, and can be accelerated by the prior application of a “priming” heating pulse. There is characteristic fading of the priming effect if there is a time interval between the priming pulse and the main heating pulse to achieve crystallization. We apply classical nucleation theory to interpret these effects, based on a fitting of nucleation kinetics (steady-state and transient) over the full temperature range of the supercooled liquid. The input data come from both physical experiments and atomistic simulations. Prior studies of conventional glass-formers such as lithium disilicate preclude any possibility of fading; the present study shows, however, that fading can be expected with the particular thermodynamic parameters relevant for GST and, possibly, other phase-change chalcogenides. We also use the nucleation analysis to highlight the distinction between GST and the other archetypical chalcogenide system (Ag,In)-doped Sb2Te. Classical nucleation theory appears to be applicable to phase-change chalcogenides, and to predict performance consistent with that of actual memory cells. Nucleation modeling may therefore be useful in optimizing materials selection and performance in device applications.We acknowledge financial support by the World Premier International Research Center Initiative (WPI), MEXT, Japan, and from the European Research Council under the European Union's Horizon 2020 research and innovation program (grant ERC-2015-AdG-695487: ExtendGlass)

Search for highly-ionizing particles in pp collisions at the LHC's Run-1 using the prototype MoEDAL detector

A search for highly electrically charged objects (HECOs) and magnetic monopoles is presented using 2.2 fb(-1) of p - p collision data taken at a centre of mass energy (E-CM) of 8 TeV by the MoEDAL detector during LHC's Run-1. The data were collected using MoEDAL's prototype Nuclear Track Detectord array and the Trapping Detector array. The results are interpreted in terms of Drell-Yan pair production of stable HECO and monopole pairs with three spin hypotheses (0, 1/2 and 1). The search provides constraints on the direct production of magnetic monopoles carrying one to four Dirac magnetic charges and with mass limits ranging from 590 GeV/c(2) to 1 TeV/c(2). Additionally, mass limits are placed on HECOs with charge in the range 10e to 180e, where e is the charge of an electron, for masses between 30 GeV/c(2) and 1 TeV/c(2).Peer reviewe

Preferred location for conducting filament formation in thin-film nano-ionic electrolyte: study of microstructure by atom-probe tomography

© 2017, The Author(s). Atom-probe tomography of Ag-photodoped amorphous thin-film Ge 40 S 60 , the material of interest in nano-ionic memory and lateral geometry MEMS technologies, reveals regions with two distinct compositions on a nanometer length-scale. One type of region is Ag-rich and of a size typically extending beyond the measured sample volume of ~40 × 40 × 80 nm 3 . These type-I regions contain aligned nanocolumns, ~5 nm wide, that are the likely location for reversible diffusion of Ag + ions and associated growth/dissolution of conducting filaments. The nanocolumns become relatively Ag-rich during the photodoping, and the pattern of Ag enrichment originates from the columnar-porous structure of the as-deposited film that is to some extent preserved in the electrolyte after photodoping. Type-II regions have lower Ag content, are typically 10–20 nm across, and appear to conform to the usual description of the photoreaction products of the optically-induced dissolution and diffusion of silver in a thin-film chalcogenide. The microstructure, with two types of region and aligned nanocolumns, is present in the electrolyte after photodoping without any applied bias, and is important for understanding switching mechanisms, and writing and erasing cycles, in programmable-metallization-cell memory

Fast-heating-induced formation of metallic-glass/crystal composites with enhanced plasticity

Bulk metallic glasses are known to have poor plasticity which limits their application as structural materials. Due to the lack of atomic periodicity in metallic glasses, their mechanical properties cannot be controlled the same way as in crystalline materials. Fast-heating-induced, a heating rate of 10 K s−1 and higher, crystallization of ductile nanocrystalline phase(s) leads to enhanced plasticity of metallic-glass/crystal composites. Here, an overview of controlling the microstructure on fast heating, the suggested crystallization mechanism of metastable phases and the principle of enhanced plasticity of the composites is presented and discussed, with a special focus on Cu–Zr-based metallic glasses

High resolution pixel detectors for e+e- linear colliders

The physics goals at the future e+e- linear collider require high performance vertexing and impact parameter resolution. Two possible technologies for the vertex detector of an experimental apparatus are outlined in the paper: an evolution of the Hybrid Pixel Sensors already used in high energy physics experiments and a new detector concept based on the monolithic CMOS sensors.Comment: 8 pages, to appear on the Proceedings of the International Workshop on Linear Colliders LCWS99, Sitges (Spain), April 28 - May 5, 199