Chalcogenide-based Phase-change Materials Research Articles

Electrically Reconfigurable Phase-Change Transmissive Metasurface.

Programmable and reconfigurable optics hold significant potential for transforming a broad spectrum of applications, spanning space explorations to biomedical imaging, gas sensing, and optical cloaking. The ability to adjust the optical properties of components like filters, lenses, and beam steering devices could result in dramatic reductions in size, weight, and power consumption in future optoelectronic devices. Among the potential candidates for reconfigurable optics, chalcogenide-based phase change materials (PCMs) offer great promise due to their non-volatile and analogue switching characteristics. Although PCM have found widespread use in electronic data storage, these memory devices are deeply sub-micron-sized. To incorporate phase change materials into free-space optical components, it is essential to scale them up to beyond several hundreds of microns while maintaining reliable switching characteristics. This study demonstrated a non-mechanical, non-volatile transmissive filter based on low-loss PCMs with a 200 µm×200 µm switching area. The device/metafilter can be consistently switched between low- and high-transmission states using electrical pulses with a switching contrast ratio of 5.5 dB. The device wasreversibly switched for 1250 cycles before accelerated degradation took place. The work represents an important step toward realizing free-space reconfigurable optics based onPCMs. This article is protected by copyright. All rights reserved.

Toward Accurate Thermal Modeling of Phase Change Material-Based Photonic Devices.

Reconfigurable or programmable photonic devices are rapidly growing and have become an integral part of many optical systems. The ability to selectively modulate electromagnetic waves through electrical stimuli is crucial in the advancement of a variety of applications from data communication and computing devices to environmental science and space explorations. Chalcogenide-based phase-change materials (PCMs) are one of the most promising material candidates for reconfigurable photonics due to their large optical contrast between their different solid-state structural phases. Although significant efforts have been devoted to accurate simulation of PCM-based devices, in this paper, three important aspects which have often evaded prior models yet having significant impacts on the thermal and phase transition behavior of these devices are highlighted: the enthalpy of fusion, the heat capacity change upon glass transition, as well as the thermal conductivity of liquid-phase PCMs. The important topic of switching energy scaling in PCM devices, which also helps explain why the three above-mentioned effects have long been overlooked in electronic PCM memories but only become important in photonics, is further investigated. These findings offer insight to facilitate accurate modeling of PCM-based photonic devices and can inform the development of more efficient reconfigurableoptics.

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.

Altered the structural, morphological and optical properties of SbSe thin films through swift heavy ion irradiation

Chalcogenide based phase change materials are gaining attention due to their ability to exhibit expeditious and reversible structural transition from amorphous to crystalline phase. This work included the effect of swift heavy silver (Ag9+) ion-irradiation (120 MeV), at various fluences (5E11, 1E12, 5E12 and 1E13 ions/cm2) on the structural, optical and morphological properties of pristine and annealed (250 ° C) SbSe thin films. The pristine films undergo a structural transition from amorphous to crystalline upon annealing and from crystalline to amorphous upon irradiation of annealed films. Structural transition caused by annealing and ion-irradiation resulted in a drastic change in morphology and optical properties. The annealed films exhibited less transmission than the pristine and irradiated films, which increased with increase in ion-irradiation fluences because of phase transition. After irradiation, the optical band decreased for pristine thin films, because the forbidden gap defect concentration has increased, but increased after irradiating the annealed thin films that may be due to annealing out of dense localized defect states. The significant optical contrast upon phase transition in near infrared region can be utilized for different optoelectronic applications.

Recent advances in doped Ge2Sb2Te5 thin film based phase change memories

Nowadays, chalcogenide-based phase change materials (PCMs) are proving themselves superior in the category of memory devices due to a combination of their unique set of properties.

Phase change in GeTe/Sb2Te3 superlattices: Formation of the vacancy-ordered metastable cubic structure via Ge migration

Interfacial phase-change memory (iPCM), comprising alternating layers of two chalcogenide-based phase-change materials—Sb2Te3 (ST) and GeTe (GT)—has demonstrated outstanding performance in resistive memories. However, its comprehensive understanding is controversial. Herein, the phase-change characteristic of iPCM is identified using atomic scale imaging, X-ray diffraction, and chemical analysis with first-principles density functional theory (DFT) calculations. By inducing laser pulsing, the ST/GT superlattice structure in the low-resistance state tends to reversibly convert into the modified metastable face-centered cubic (fcc) GeSbTe structure in the high-resistance state. This transition is driven by Ge atom rearrangement to pre-existing vacancy layers and ordered vacancy-layer formation. DFT atomistic modeling shows that the resistance difference of 102 orders between low- and high-resistance states is a direct consequence of the intercalation of Ge atoms into the vacancy layer. These results provide insights into iPCM phase-change mechanisms and phase-change random access memory design with low energy and high speed.

Understanding the low resistivity of the amorphous phase of Cr2Ge2Te6 phase-change material: Experimental evidence for the key role of Cr clusters

Different from the prototypical elemental semiconductors such as Si and Ge, chalcogenide-based phase-change materials (PCMs) generally show very high resistivity contrast between the amorphous and crystalline phases. In contrast to conventional PCMs, such as Ge-Sb-Te alloys, where the amorphous phase possesses higher resistivity, ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CrGT) exhibits the opposite dependence. Namely, the amorphous phase is characterized by a lower resistivity than the crystalline phase. Although density functional theory calculations suggest that Cr clusters are responsible for the low resistivity of amorphous CrGT, the effects of composition on the electrical properties have yet to be investigated. In this work, the dependence of the electrical properties on Cr content and the role of the Cr clusters were investigated experimentally using Hall effect, hard x-ray photoelectron spectroscopy (HAXPES), and optical property measurements. The electrical properties were found to be dependent on the Cr content. From a HAXPES core-level spectra analysis, it was found that the increased carrier density correlated with the extent of Cr clusters, indicating that the hole carriers present likely originated from Cr clusters. The increased concentration of Cr clusters was also found to lead to a shift of the valence band edge toward the Fermi level as well as to a decrease in the optical band gap. It has been suggested that the Cr clusters may induce the formation of new energy states close to the valence band edge. These results indicate that the Cr clusters play an essential role in determining the electrical properties of amorphous CrGT, and that tuning the film composition is an effective way to optimize device properties for nonvolatile memory applications.

Evolution of the local structure surrounding nitrogen atoms upon the amorphous to crystalline phase transition in nitrogen-doped Cr2Ge2Te6 phase-change material

Phase-change memory based upon resistive switching due to phase transitions between the amorphous and crystalline phases of chalcogenide-based phase-change materials (PCMs) has been widely studied for the next generation non-volatile memory (NVM). Unlike traditional PCMs such as Ge2Sb2Te5, nitrogen doped Cr2Ge2Te6 (NCrGT) was reported to not exhibit a bulk resistivity change upon the amorphous to crystalline phase transition, instead the contact resistivity contrast in the presence of metal electrodes was found to play an essential role. In the current study, a distinct difference in the local structure around N atoms in amorphous and crystalline NCrGT films was found using X-ray absorption near-edge spectroscopy (XANES) and hard X-ray photoelectron spectroscopy (HAXPES). CrN and GeN bonds and N2 molecular vibrational modes were found to be present in an amorphous NCrGT film, while N atoms were found to be present at Te substitutional positions in the crystalline phase. A previous study revealed that Cr nanoclusters are present in the undoped amorphous phase, whose concentration was found to decrease as crystallization progressed leading to an increase in the resistivity. In the present study, it was determined that N doping inhibits Cr nanocluster formation in the amorphous NCrGT phase and leads to a constant Cr nanocluster volume fraction even after crystallization, ensuring negligible resistance contrast between the amorphous and crystalline phases. These results will prove useful in understanding the role of dopant elements in the promising PCM CrGT.

Conversion of p\u2013n conduction type by spinodal decomposition in Zn-Sb-Bi phase-change alloys

Phase-change films with multiple resistance levels are promising for increasing the storage density in phase-change memory technology. Diffusion-dominated Zn2Sb3 films undergo transitions across three states, from high through intermediate to low resistance, upon annealing. The properties of the Zn2Sb3 material can be further optimized by doping with Bi. Based on scanning transmission electron microscopy combined with electrical transport measurements, at a particular Bi concentration, the conduction of Zn-Sb-Bi compounds changes from p- to n-type, originating from spinodal decomposition. Simultaneously, the change in the temperature coefficient of resistivity shows a metal-to-insulator transition. Further analysis of microstructure characteristics reveals that the distribution of the Bi-Sb phase may be the origin of the driving force for the p–n conduction and metal-to-insulator transitions and therefore may provide us with another way to improve multilevel data storage. Moreover, the Bi doping promotes the thermoelectric properties of the studied alloys, leading to higher values of the power factor compared to known reported structures. The present study sheds valuable light on the spinodal decomposition process caused by Bi doping, which can also occur in a wide variety of chalcogenide-based phase-change materials. In addition, the study provides a new strategy for realizing novel p–n heterostructures for multilevel data storage and thermoelectric applications.

Metasurfaces Enabled by Locally Tailoring Disorder in Phase-Change Materials

Active optical metasurfaces with dynamic switchable, tunable, and reconfigurable optical functionalities are an emerging field in photonics and optoelectronics. Especially, chalcogenide-based phase-change materials, such as Ge2Sb2Te5 (GST), can be fast and repeatedly switched by external stimuli between crystalline and amorphous states, typically accompanied by a tremendous difference of the electronic and photonic properties. Here, we demonstrate that focused ion beam-induced disorder in highly confined regions can transform phase-change materials in active optical metasurfaces by locally adjusting the phase. A careful control of the amount of disorder can locally tailor the effective refractive index in GST films on the nanometer scale, which is highly promising for multilevel switching applications. In contrast to direct laser writing, focused ion beam irradiation enables the fabrication of subwavelength-scaled, planar, nonvolatile, and reconfigurable optical metasurfaces, with pattern sizes clearly be...

Resolving Crystallization Kinetics of GeTe Phase-Change Nanoparticles by Ultrafast Calorimetry.

Chalcogenide-based phase change materials (PCMs) are promising candidates for the active element in novel electrical nonvolatile memories and have been applied successfully in rewritable optical disks. Nanostructured PCMs are considered as the next generation building blocks for their low power consumption, high storage density, and fast switching speed. Yet their crystallization kinetics at high temperature, the rate-limiting property upon switching, faces great challenges due to the short time and length scales involved. Here we present a facile method to synthesize highly controlled, ligand-free GeTe nanoparticles, an important PCM, with an average diameter under 10 nm. Subsequent crystallization by slow and ultrafast rates allows unravelling of the crystallization kinetics, demonstrating the breakdown of Arrhenius behavior for the crystallization rate and a fragile-to-strong transition in the viscosity as well as the overall crystal growth rate for the as-deposited GeTe nanoparticles. The obtained results pave the way for further development of phase-change memory based on GeTe with sub-lithographic sizes.

Local structure of amorphous Ag5In5Sb60Te30 and In3SbTe2 phase change materials revealed by X-ray photoelectron and Raman spectroscopic studies

Reversible switching between highly resistive (binary “0”) amorphous phase and low resistive (binary “1”) crystalline phase of chalcogenide-based Phase Change Materials is accredited for the development of next generation high-speed, non-volatile, data storage applications. The doped Sb-Te based materials have shown enhanced electrical/optical properties, compared to Ge-Sb-Te family for high-speed memory devices. We report here the local atomic structure of as-deposited amorphous Ag5In5Sb60Te30 (AIST) and In3SbTe2 (IST) phase change materials using X-ray photoelectron and Raman spectroscopic studies. Although AIST and IST materials show identical crystallization behavior, they differ distinctly in their crystallization temperatures. Our experimental results demonstrate that the local environment of In remains identical in the amorphous phase of both AIST and IST material, irrespective of its atomic fraction. In bonds with Sb (∼44%) and Te (∼56%), thereby forming the primary matrix in IST with a very few Sb-Te bonds. Sb2Te constructs the base matrix for AIST (∼63%) along with few Sb-Sb bonds. Furthermore, an interesting assimilation of the role of small-scale dopants such as Ag and In in AIST, reveals rare bonds between themselves, while showing selective substitution in the vicinity of Sb and Te. This results in increased electronegativity difference, and consequently, the bond strength is recognized as the factor rendering stability in amorphous AIST.

Redefining the Speed Limit of Phase Change Memory Revealed by Time-resolved Steep Threshold-Switching Dynamics of AgInSbTe Devices.

Although phase-change memory (PCM) offers promising features for a ‘universal memory’ owing to high-speed and non-volatility, achieving fast electrical switching remains a key challenge. In this work, a correlation between the rate of applied voltage and the dynamics of threshold-switching is investigated at picosecond-timescale. A distinct characteristic feature of enabling a rapid threshold-switching at a critical voltage known as the threshold voltage as validated by an instantaneous response of steep current rise from an amorphous off to on state is achieved within 250 picoseconds and this is followed by a slower current rise leading to crystallization. Also, we demonstrate that the extraordinary nature of threshold-switching dynamics in AgInSbTe cells is independent to the rate of applied voltage unlike other chalcogenide-based phase change materials exhibiting the voltage dependent transient switching characteristics. Furthermore, numerical solutions of time-dependent conduction process validate the experimental results, which reveal the electronic nature of threshold-switching. These findings of steep threshold-switching of ‘sub-50 ps delay time’, opens up a new way for achieving high-speed non-volatile memory for mainstream computing.

Stochastic phase-change neurons.

Artificial neuromorphic systems based on populations of spiking neurons are an indispensable tool in understanding the human brain and in constructing neuromimetic computational systems. To reach areal and power efficiencies comparable to those seen in biological systems, electroionics-based and phase-change-based memristive devices have been explored as nanoscale counterparts of synapses. However, progress on scalable realizations of neurons has so far been limited. Here, we show that chalcogenide-based phase-change materials can be used to create an artificial neuron in which the membrane potential is represented by the phase configuration of the nanoscale phase-change device. By exploiting the physics of reversible amorphous-to-crystal phase transitions, we show that the temporal integration of postsynaptic potentials can be achieved on a nanosecond timescale. Moreover, we show that this is inherently stochastic because of the melt-quench-induced reconfiguration of the atomic structure occurring when the neuron is reset. We demonstrate the use of these phase-change neurons, and their populations, in the detection of temporal correlations in parallel data streams and in sub-Nyquist representation of high-bandwidth signals.

High-field electrical transport in amorphous phase-change materials

Electrical transport in chalcogenide-based phase change materials is an active area of research owing to the prominent role played by these materials in the field of information technology. Here, we present transport measurements (IV curves) obtained on line-cells of as-deposited amorphous phase change materials (Ge2Sb2Te5, GeTe, Ag4In3Sb66Te27) over a wide voltage and temperature range (300 K to 160 K). The well defined geometry of our devices enables a description of the transport behavior in terms of conductivity vs. electric field. At higher temperatures (300 K ≥ T ≥ 220 K) and low to intermediate fields (F < 20 V/μm), the data can be described within the framework of a previously developed model, which is based on multiple trapping transport together with 3D Poole-Frenkel emission from a two-center Coulomb potential. Based on this model, we observe a temperature dependence of the inter-trap distance, which we can relate to a temperature dependence in the occupation of the defect creating the Coulomb potential governing Poole-Frenkel emission. At higher fields and lower temperatures, the dependency of the IV curve on the electric field can be described by ln(I/I0) = (F/Fc)2. By combining this contribution with that of the Poole-Frenkel emission, we can show that the slope at high fields, Fc, is independent of temperature. We argue that models based on direct tunneling or thermally assisted tunneling from a single defect into the valence band cannot explain the observed behavior quantitatively.

Subthreshold electrical transport in amorphous phase-change materials

Chalcogenide-based phase-change materials play a prominent role in information technology. In spite of decades of research, the details of electrical transport in these materials are still debated. In this article, we present a unified model based on multiple-trapping transport together with 3D Poole–Frenkel emission from a two-center Coulomb potential. With this model, we are able to explain electrical transport both in as-deposited phase-change material thin films, similar to experimental conditions in early work dating back to the 1970s, and in melt-quenched phase-change materials in nanometer-scale phase-change memory devices typically used in recent studies. Experimental measurements on two widely different device platforms show remarkable agreement with the proposed mechanism over a wide range of temperatures and electric fields. In addition, the proposed model is able to seamlessly capture the temporal evolution of the transport properties of the melt-quenched phase upon structural relaxation.

Ferromagnetism modulation by phase change in Mn-doped GeTe chalcogenide magnetic materials

In this work, an effective method to modulate the ferromagnetic properties of Mn-doped GeTe chalcogenide-based phase change materials is presented. The microstructure of the phase change magnetic material Ge1−x Mn x Te thin films was studied. The X-ray diffraction results demonstrate that the as-deposited films are amorphous, and the crystalline films are formed after annealing at 350 °C for 10 min. Crystallographic structure investigation shows the existence of some secondary magnetic phases. The lattice parameters of Ge1−x Mn x Te (x = 0.04, 0.12 and 0.15) thin films are found to be slightly different with changes of Mn compositions. The structural analysis clearly indicates that all the films have a stable rhombohedral face-centered cubic polycrystalline structure. The magnetic properties of the amorphous and crystalline Ge0.96Mn0.04Te were investigated. The measurements of magnetization (M) as a function of the magnetic field (H) show that both amorphous and crystalline phases of Ge0.96Mn0.04Te thin film are ferromagnetic and there is drastic variation between amorphous and crystalline states. The temperature (T) dependence of magnetizations at zero field cooling (ZFC) and field cooling (FC) conditions of the crystalline Ge0.96Mn0.04Te thin film under different applied magnetic fields were performed. The measured data at 100 and 300 Oe applied magnetic fields show large bifurcations in the ZFC and FC curves while on the 5,000 Oe magnetic field there is no deviation.

An optoelectronic framework enabled by low-dimensional phase-change films.

The development of materials whose refractive index can be optically transformed as desired, such as chalcogenide-based phase-change materials, has revolutionized the media and data storage industries by providing inexpensive, high-speed, portable and reliable platforms able to store vast quantities of data. Phase-change materials switch between two solid states--amorphous and crystalline--in response to a stimulus, such as heat, with an associated change in the physical properties of the material, including optical absorption, electrical conductance and Young's modulus. The initial applications of these materials (particularly the germanium antimony tellurium alloy Ge2Sb2Te5) exploited the reversible change in their optical properties in rewritable optical data storage technologies. More recently, the change in their electrical conductivity has also been extensively studied in the development of non-volatile phase-change memories. Here we show that by combining the optical and electronic property modulation of such materials, display and data visualization applications that go beyond data storage can be created. Using extremely thin phase-change materials and transparent conductors, we demonstrate electrically induced stable colour changes in both reflective and semi-transparent modes. Further, we show how a pixelated approach can be used in displays on both rigid and flexible films. This optoelectronic framework using low-dimensional phase-change materials has many likely applications, such as ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses and artificial retina devices.

Nanosecond in situ transmission electron microscope studies of the reversible Ge2Sb2Te5 crystalline ⇔ amorphous phase transformation

Chalcogenide-based phase-change materials have wide use in optical recording media and are growing in importance for use in non-volatile electronic memory. For both applications, rapid switching between the amorphous and crystalline phases is necessary, and understanding the changes during rapidly driven phase transitions is of scientific and technological significance. Laser-induced crystallization and amorphization occur rapidly and changes in atomic structure, microstructure, and temperature are difficult to observe experimentally and determine computationally. We have used nanosecond-scale time-resolved diffraction with intense electron pulses to study Ge2Sb2Te5 during laser crystallization. Using a unique and unconventional specimen geometry, cycling between the amorphous and crystalline phases was achieved, enabling in situ transmission electron microscope (TEM) study of both microstructural and crystallographic changes caused by repeated switching. Finite element analysis was used to simulate interactions of the laser with the nano-structured specimens and to model the rapidly changing specimen temperature. Such time-resolved experimental methods combined with simulation of experimentally inaccessible physical characteristics will be fundamental to advancing the understanding of rapidly driven phase transformations.

Solution-Based Processing of the Phase-Change Material KSb5S8

A hydrazine-based process for solution-depositing phase-change materials (PCMs) is demonstrated, using KSb{sub 5}S{sub 8} (KSS) as an example. The process involves dissolving the elemental metals and chalcogen in hydrazine at room temperature and spin-coating the solution onto a substrate, followed by a short low-temperature (T {<=} 250 C) anneal. The spin-coated KSS films, which range in thickness from 10 to 90 nm, are examined using variable temperature X-ray diffraction, medium energy ion scattering (MEIS), Rutherford backscattering spectroscopy (RBS), and scanning electron microscopy (SEM). The spin-coated KSS films exhibit a reversible amorphous-crystalline transition with a relatively high crystallization temperature ({approx}280 C). Selected other chalcogenide-based PCMs are also expected to be suitable for thin-film deposition using this approach.