Amorphous Phase Change Materials Research Articles

Neutron Radiation-induced Damage to the Local Structure and Optical Properties of Amorphous Phase-Change Materials

Neutron Radiation-induced Damage to the Local Structure and Optical Properties of Amorphous Phase-Change Materials

Medium-range structural order in amorphous Ge2Sb2Te5 phase change material

The crystallization rate is an important factor for the application of phase change materials as memory devices, which, thus, motivates a strong interest in the underlying crystallization processes. We analyzed the crystallization kinetics of as-deposited amorphous Ge2Sb2Te5 by differential scanning calorimetry, revealing that the thermal treatment below the crystallization onset temperature effectively enhances the crystallization rate compared to the as-deposited samples. Variable resolution fluctuation electron microscopy was carried out to characterize the corresponding amorphous structure of the as-deposited films and the differently annealed states regarding their medium-range order (MRO). As a result, relatively large MRO correlation lengths in the range of 3–4 nm were observed, whereas the MRO volume fraction doubled after longer annealing treatments, demonstrating the importance of MRO and MRO tuning for optimizing the properties of amorphous phase change materials. The increased volume fraction of the MRO seems to also favor an increased nanoindentation hardness.

Evolution of Short-Range Order of Amorphous GeTe Upon Structural Relaxation Obtained by TEM Diffractometry and RMC Methods.

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 amorphousGeTe.

Tellurium self-diffusion in amorphous Ge2Sb2Te5 phase change materials

Amorphous Ge2Sb2Te5 phase change material is considered as a prototype material for non-volatile memories due to the reversibility of the amorphous-to-crystalline transition on a nanosecond timescale. In this context, the kinetics of atomic self-diffusion has important bearings for the crystallization process and the switching behavior of phase change materials. It is thus important for applications as well as for the general understanding of the rapid and reversible switching behavior to understand atomic self-diffusion, especially in the amorphous phase. However, up to now, reliable data on the kinetics of atomic self-diffusion in amorphous phase change materials is almost non-existent. For this reason, Te tracer diffusion was measured using secondary ion mass spectroscopy and applying the highly enriched natural 122Te isotope. For the first time, Te self-diffusion coefficients in amorphous Ge2Sb2Te5 are experimentally measured and the activation energy of Te self-diffusion was determined as (1.43±0.08) eV.

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.

Phase-Change Materials, 1/f Noise, and Phase Synchrony

In this article, we study 1 / f γ , γ ≈ 1 electrical noise in amorphous phase-change materials. Given the relevance of noise in recent applications, it is necessary to gain a deeper perspective on its nature in phase-change semiconductors, a promising class of materials. Electron conduction is envisaged in terms of an envelope function and a field-dependent Bloch wave function; the electron transport across the structure is modeled as driven phase oscillators under a weak field and obeys a Kuramoto-type equation. Its solutions naturally divide into a phase-synchronized group and phase-desynchronized oscillators. The former is comprised by long-lived pairs or aggregates and are responsible for 1 / f , γ = 1 noise. We identify the dividing frequency between γ = 1 noise and γ ≠ 1 noise. The phase-desynchronized carriers generate γ ≠ 1 noise and are single carriers, not aggregates, and are short-lived. We apply our analysis to recent experiments.

Resistance Drift Convergence and Inversion in Amorphous Phase Change Materials

AbstractPhase change materials (PCMs) are key to the development of artificial intelligence technologies such as high‐density memories and neuromorphic computing, thanks to their ability for multi‐level data storage through stepwise resistive encoding. Individual resistance levels are realized by adjusting the crystalline and amorphous volume fraction of the memory cell. However, the amorphous phase exhibits a drift in resistance over time that has so far hindered the commercial implementation of multi‐level storage schemes. In this study, the underlying physical process of resistance drift with the goal of modeling is elucidated that will help minimize and potentially overcome drift in PCM memory devices. Clear evidence is provided that the resistance drift is dominated by glass dynamics. Resistivity convergence and drift inversion for the amorphous chalcogenide Ge15Te85 and the PCM Ge3Sb6Te5 are experimentally demonstrated and these changes are successfully predicted with a glass dynamics model. This new insight into the resistance drift process provides tools for the development of advanced PCM devices.

Electric-field-induced annihilation of localized gap defect states in amorphous phase-change memory materials

Structural relaxation of amorphous phase-change-memory materials has been attributed to defect-state annihilation from the band gap, leading to a time-dependent drift in the electrical resistance, which hinders the development of multi-level memory devices with increased data-storage density. In this computational study, homogeneous electric fields have been applied, by utilizing a Berry-phase approach with hybrid-density-functional-theory simulations, to ascertain their effect on the atomic and electronic structures associated with the mid-gap states in models of the prototypical glassy phase-change material, Ge2Sb2Te5. Above a threshold value, electric fields remove spatially localized defects from the band gap and transform them into delocalized conduction-band-edge electronic states. A lowering of the nearest-neighbor coordination of Ge atoms in the local environment of the defect-host motif is observed, accompanied by a breaking of 4-fold rings. This engineered structural relaxation, through electric-field tuning of electronic and geometric properties in the amorphous phase, paves the way to the design of optimized glasses.

Low resistance-drift characteristics in Cr2Ge2Te6-based phase change memory devices with a high-resistance crystalline phase

Resistance drift affects the reliability of data reading in phase change material (PCM)-based memory devices. In general, resistance drift occurs in high-resistance amorphous state in the PCM-based memory devices. The conduction mechanism such as Poole-Frenkel conduction (PF) in amorphous PCMs causes severe resistance drift in the device. Cr2Ge2Te6 (CrGT) exhibiting a phase transition between low-resistance amorphous and high-resistance crystalline phases demonstrates band conduction in the amorphous phase and mixed conduction with band conduction and nearest-neighbor hopping conduction in the crystalline phase. Since the conduction mechanisms of both phases in the CrGT are not governed by PF, the resistance-drift characteristics are expected to be different from that in conventional PCMs. In this study, the resistance-drift characteristics of the CrGT-based devices were investigated. The fabricated CrGT-based devices exhibited a cyclic endurance of ~7 × 105 times with a clear difference in resistance between the high- and low-resistance states. The drift coefficient, v, at 40 °C was evaluated for the CrGT- and Ge-Sb-Te (GST)-based devices; the value of v for the CrGT-based device was smaller than that for the GST-based device in both high- and low-resistance states. The current–voltage analysis revealed that mixed-conduction mechanism plays an important role to suppress the resistance drift in the high-resistance state of the CrGT-based devices. These findings provide new insights to realize a low resistance-drift PCM-based memory device.

Direct evidence for structural transformation and higher thermal stability of amorphous insbte phase change material

The search for novel phase-change materials with enhanced electrical and thermal properties are utmost importance for the development of reliable next-generation high-speed, non-volatile random access memory (NVRAM). In this paper, we investigate local structural change and crystallization kinetics of ternary InSbTe thin films using temperature-dependent resistivity measurement and in situ X-ray diffraction by synchrotron radiation source. Our experimental results reveal that crystallization of InSbTe begins with the formation of binary phases (InSb and InTe). Furthermore, cubic In3SbTe2 phase is emerged at 300 °C and remains stable up to 410 °C. The calculated values of texture coefficient indicate an increased orientation of In3SbTe2 phase upon increasing the temperature. Also, higher activation energy of 5.2–5.6 eV for crystallization is observed using Kissinger's method. These enhanced electrical, thermal and structural properties of InSbTe thin films would be suitable for high-speed NVRAMs as well as multi-bit data storage applications.

In situ TEM revealing pretreatment and interface effects in Ge2Sb2Te5

Random access memory devices that rely on phase changes are primarily limited by the speed of crystallization. However, imaging structural dynamics in working devices with nanoscale resolution remains elusive. Using an ultrafast liquid-quenching system in transmission electron microscope, we obtain a melt-quenched amorphous phase-change material Ge2Sb2Te5 nanosheet. We find that the applied incubation field (pretreatment) leads to nonuniform nucleus formation and growth in the amorphous Ge2Sb2Te5, accompanied by a slight change in electric properties. Moreover, the phase-change mechanism changes from the material inherent crystallization mechanism (nucleation-dominated) to the heterocrystallization mechanism, indicating the incubation period of nucleation might be bypassed.

Chemical Bonding in Chalcogenides: The Concept of Multicenter Hyperbonding.

The precise nature of chemical-bonding interactions in amorphous, and crystalline, chalcogenides is still unclear due to the complexity arising from the delocalization of bonding, and nonbonding, electrons. Although an increasing degree of electron delocalization for elements down a column of the periodic table is widely recognized, its influence on chemical-bonding interactions, and on consequent material properties, of chalcogenides has not previously been comprehensively understood from an atomistic point of view. Here, a chemical-bonding framework is provided for understanding the behavior of chalcogenides (and, in principle, other lone-pair materials) by studying prototypical telluride nonvolatile-memory, "phase-change" materials (PCMs), and related chalcogenide compounds, via density-functional-theory molecular-dynamics (DFT-MD) simulations. Identification of the presence of previously unconsidered multicenter "hyperbonding" (lone-pair-antibonding-orbital) interactions elucidates not only the origin of various material properties, and their contrast in magnitude between amorphous and crystalline phases, but also the very similar chemical-bonding nature between crystalline PCMs and one of the bonding subgroups (with the same bond length) found in amorphous PCMs, in marked contrast to existing viewpoints. The structure-property relationship established from this new bonding-interaction perspective will help in designing improved chalcogenide materials for diverse applications, based on a fundamental chemical-bonding point of view.

A unified mid-gap defect model for amorphous GeTe phase change material

Understanding the nature of the mid-gap defects in amorphous chalcogenide alloy-based phase change materials is crucial to guide the development of multi-level memories, selectors, and neuromorphic devices. Currently, however, the knowledge of the mid-gap defects is fragmented and the known structural and electrical characteristics of the defects cannot be unified in a single atomic model. In this work, a Ge–Ge chain defect model is built by artificial bond rearrangement in an amorphous GeTe network generated by atomic distortion, revealing a possible valence alternation mechanism in its formation. Atoms in the Ge–Ge chain are found to have a crystalline-like environment. The chain is found to have a negative-U property and to introduce mid-gap states. This model unifies the main known properties of the mid-gap defects in amorphous chalcogenide alloy-based phase change materials.

Uncovering β-relaxations in amorphous phase-change materials.

Relaxation processes are decisive for many physical properties of amorphous materials. For amorphous phase-change materials (PCMs) used in nonvolatile memories, relaxation processes are, however, difficult to characterize because of the lack of bulk samples. Here, instead of bulk samples, we use powder mechanical spectroscopy for powder samples to detect the prominent excess wings-a characteristic feature of β-relaxations-in a series of amorphous PCMs at temperatures below glass transitions. By contrast, β-relaxations are vanishingly small in amorphous chalcogenides of similar composition, which lack the characteristic features of PCMs. This conclusion is corroborated upon crossing the border from PCMs to non-PCMs, where β-relaxations drop substantially. Such a distinction implies that amorphous PCMs belong to a special kind of covalent glasses whose locally fast atomic motions are preserved even below the glass transitions. These findings suggest a correlation between β-relaxation and crystallization kinetics of PCMs, which have technological implications for phase-change memory functionalities.

Electron beam irradiation induced crystallization behavior of amorphous Ge2Sb2Te5 chalcogenide material

The crystallization of amorphous Ge2Sb2Te5 phase change material induced by electron beam irradiation was investigated by in-situ transmission electron microscopy (TEM). Amorphous matrix transformed into a partially crystalline state after being irradiated with a 200-keV electron beam for a long time. Real-time observation revealed that the crystallization of amorphous Ge2Sb2Te5 film occurs through a nucleation and growth mechanism under electron beam irradiation in TEM. While uncertainty from the 2D projection remains, the nuclei have been observed to grow preferentially along the < 100> direction.

Localised states and their capture characteristics in amorphous phase-change materials

As phase-change materials are poised to play a key role in next-generation computing systems, improving the current understanding of electrical transport in their amorphous phase can further strengthen their technological competitiveness. Even though the interaction of charge carriers with disorder-induced localised states largely affect the field-dependent conductivity, a clear link between electrical transport and specific features of the electronic density of states (DOS) could not be established yet due to a lack of knowledge of the capture characteristics of trap states. Here, we address this knowledge gap and employ modulated photocurrent spectroscopy (MPC) to investigate localised states in the frequently studied amorphous phase of Ge2Sb2Te5. Additionally, we present results on the DOS in the bandgap of amorphous AgIn-doped Sb2Te, which has not been subject to high-resolution DOS spectroscopy before. We find experimental evidence for clearly non-constant capture coefficients among a continuous spectrum of localised states in both studied materials. According to this observation especially in AgIn-doped Sb2Te, where no pronounced defect can be detected as main channel for carrier emission, we point out the necessity of modifying the current Poole-Frenkel-based transport modelling.

Polarity-dependent resistance switching in crystalline Ge1Sb4Te7 film

Phase-change memory (PCM) utilizes the fast reversible phase transition between crystalline and amorphous chalcogenide phase-change materials to achieve the data storage. The rather large density difference between crystalline and amorphous phases will induce device failure with repeated switching. Further, the melting-quenching process induced amorphous-crystalline phase-change needs high energy consumption. In this work, reversible resistance switching is observed in crystalline Ge1Sb4Te7 ribbons driven by voltage polarity, without amorphous-crystalline phase-change. Consequently, the large density variation and the high energy consumption are avoided, which overcomes those two restrictions of PCM. Moreover, on the basis of ab initio calculations, the underlying mechanism is further analyzed and it is concluded that this switching is induced by the reversible swapping of antimony between its lattice site and the center of the Te-Te van der Waals bilayers.

Laser-induced modification of amorphous GST225 phase change materials

In this paper, we have studied the crystallization behavior of amorphous GST225 thin films upon irradiation with nanosecond laser pulses. Crystalline and melt-quenched amorphous regions were produced by exposure to laser single or multipulses, and were characterized by the optical microscopy and by the micro-Raman spectroscopy. Transition region between the amorphous and crystalline parts of the laser-modified area was investigated by atomic force microscopy. Using irradiation by single laser pulses with varying fluence, it was verified that crystallization was possible if the fluence is more than 90.4 mJ/cm2.

Laser-induced modification and formation of periodic surface structures (ripples) of amorphous GST225 phase change materials

In this paper, we have studied the crystallization behavior of amorphous Ge2Sb2Te5 thin films upon irradiation with femtosecond laser pulses. Crystalline and melt-quenched amorphous regions were produced by exposure to laser multipulses, and were characterized by the optical microscopy and by the micro-Raman spectroscopy. Using irradiation with single laser pulses of varying fluence it was verified that crystallization was not possible, requiring minimum ten pulses for triggering partial crystallization. We obtained laser-induced periodic surface structures in the form of near-subwavelength ripples of two types with different periods. It was found that the fraction of the crystalline phase is higher in the ridges of the ripples compared to the valleys. We used the model of heterogeneous crystallization for bodies with different curvatures of the boundary surface to explain the observed effect of the phase transformation in laser-induced periodic surface structures.

Ab initio computer simulations of non-equilibrium radiation-induced cascades in amorphous Ge2Sb2Te5

Ion irradiation corresponds to a process that involves the production of non-equilibrium cascades in the host material, and the atomistic modelling of such events in glasses is challenging. Here, non-equilibrium cascades in amorphous Ge2Sb2Te5 phase-change memory material have been investigated by means of first-principles molecular-dynamics simulations. A stochastic boundary-conditions approach is employed to treat the thermal nature of the cascades and drive the modelled system back to equilibrium in a natural way, while four different initial thermal-spike energies are considered. A comprehensive analysis of the cascade evolution is presented with respect to the kinetic profile and the dynamics of the cascade inside the glass structure. The modelling results show that the instantaneous maximum kinetic energy decays rapidly with time, and that the time-scale of the ballistic phase of the cascade inside the glass model is very short. The quality of the implemented approach is validated through a comparison of the calculated structure factor for the modelled glasses with experimental data from the literature. Analysis of the bonding for all the species in the glass structure highlights particular structural modifications in the connectivity of the amorphous network due to the simulated cascade.