Chalcogenide Phase-change Materials Research Articles

A multi-level optical storage scheme via two-step picosecond laser irradiations: time/space modulations of microstructure and its optical property

Achieving multi-level properties of chalcogenide phase change materials is highly desirable for high-density data storage, photonic switches and neuromorphic computing etc. In this paper, we report a multi-level implement scheme via a two-step picosecond laser irradiation strategy on the basis of time/space modulations of microstructure and property of Ge2Sb2Te5 (GST) phase change thin films. Unlike the common grey-scale method by multiple laser irradiations at a fixed position, multi-levels of optical properties were obtained by two-step overlapping laser irradiations together with such varied parameters as the overlapping ratio, input energy and two-pulse duration. Variations of the microstructure and optical property were characterized by transmission electron microscope (TEM) and microarea reflectance testing. It showed that the overlapping ratio was an important factor to adjust microstructure patterns and optical properties. With the increase of the overlapping ratio, the optical reflectivity presented a trend of increasing first and then decreasing, mainly because of the effects of the size and distribution of grains from the second laser irradiations. Based on the scheme proposed, a 10-level optical reflectivity could be obtained. The fundamental research performed in this study is of importance to give guideline for high-density storage or big-data processing with phase change materials in the future.

Functional Coatings on High‐Performance Polymer Fibers for Smart Sensing

AbstractAbove a critical temperature, high‐performance fibers may lose their mechanical properties resulting in catastrophic events of damage when, e.g., used as load‐carrying ropes. Here, a method to functionalize polymer fibers with thermochromic optical coatings that enable signaling of damaging thermal history is introduced. These smart coatings are comprised of an index‐tunable anti‐reflection coating based on chalcogenide phase change materials (PCM). It is demonstrated that the insulator−metal phase transition of these materials can be aligned with the critical deterioration temperature of both polyethylene terephthalate (PET) monofilaments and liquid‐crystal polyester (LCP) yarns by composition tuning. The carefully designed optical system amplifies the change in optical properties of its constituents upon phase change. The thermal and mechanical degradation of these fibers can thus be monitored and displayed by eye.

Investigation of the crystallization behavior of laser‐irradiated EXTREME pattern by Raman spectroscopy

AbstractThis paper investigates the crystallization behavior of a Ge2Sb2Se4Te1 (GSST) chalcogenide phase change material film using Raman spectroscopy. The properties of as‐deposited, thermally treated, and laser‐written GSST films are investigated. A single mask exposure laser excitation process was developed using a spatial light modulator with an 800 nm femto‐second laser to generate a direct laser‐written EXTREME pattern. The correlation between the laser‐imprinted EXTREME pattern and the phase change transition from amorphous to crystalline state of the GSST film was characterized using Raman spectroscopy.

GeTe photoresist films for both positive and negative heat-mode nanolithography

Binary chalcogenide phase change material GeTe was proposed and investigated as a novel high-resolution inorganic photoresist for laser heat-mode lithography. GeTe possesses both positive and negative photoresist characteristics based on alkaline or acid developing solutions, which can improve the flexibility in fabricating micro/nano devices. Different relief gratings with feature sizes in the range of nanometer to micron were fabricated, and the minimum linewidths can be reduced to 137 nm (positive tone) and 98 nm (negative tone) respectively. The selective wet etching mechanism was discussed with local oxygen concentration mapping of nanopatterns. Moreover, the patterns were successfully transferred onto Si substrate by plasma etching.

Dynamically manipulating third-harmonic generation of phase change material with gap-plasmon resonators.

Here we theoretically demonstrate a dynamic third-harmonic generation (THG) device by incorporating chalcogenide phase change material (Ge2Sb2Te5) into ultra-small cavities of gap-plasmon resonators. Through coupling to plasmonic resonances, the THG efficiency of Ge2Sb2Te5 is increased up to five orders of magnitude due to the greatly enhanced electromagnetic fields within the ultra-small cavities. More interestingly, the THG signals of the proposed device can be dynamically switched on and off by tuning the phase states of Ge2Sb2Te5 between amorphous and crystalline. Our proposed dynamic THG nanostructures are beneficial to developing dynamic nonlinear nanophotonic devices.

Integrated phase-change photonic devices and systems

Abstract

Progressive amorphization of GeSbTe phase-change material under electron beam irradiation

Fast and reversible phase transitions in chalcogenide phase-change materials (PCMs), in particular, Ge-Sb-Te compounds, are not only of fundamental interests but also make PCMs based random access memory a leading candidate for nonvolatile memory and neuromorphic computing devices. To RESET the memory cell, crystalline Ge-Sb-Te has to undergo phase transitions first to a liquid state and then to an amorphous state, corresponding to an abrupt change in electrical resistance. In this work, we demonstrate a progressive amorphization process in GeSb2Te4 thin films under electron beam irradiation on a transmission electron microscope (TEM). Melting is shown to be completely absent by the in situ TEM experiments. The progressive amorphization process resembles closely the cumulative crystallization process that accompanies a continuous change in electrical resistance. Our work suggests that if displacement forces can be implemented properly, it should be possible to emulate symmetric neuronal dynamics by using PCMs.

Phase Change Materials: Fundamentals and Applications of Chalcogenide Phase‐Change Material Photonics (Adv. Theory Simul. 8/2019)

Chalcogenide phase-change materials (CPCMs) possess a radically changeable optical property as switching the state between amorphous and crystalline. This makes CPCMs attractive for reconfigurable photonic devices. In article number 1900094 by Tun Cao and Mengjia Cen, the fundamentals in chalcogenide-tuned photonics is introduced. Recent developments are reviewed in the field, before concentrating on metal-chalcogenide-metal meta-devices. The requirements and future outlooks in this rising field are discussed.

Multilevel reflectance switching of ultrathin phase-change films

Several design techniques for engineering the visible optical and near-infrared response of a thin film are explored. These designs require optically active and absorbing materials and should be easily grown on a large scale. Switchable chalcogenide phase-change material heterostructures with three active layers are grown here using pulsed laser deposition. Both Fabry–Perot and strong interference principles are explored to tune the reflectance. Robust multilevel switching is demonstrated for both principles using dynamic ellipsometry, and measured reflectance profiles agree well with simulations. We find, however, that switching the bottom layer of a three-layer device does not yield a significant change in reflectance, indicating a maximum in accessible levels. The pulsed laser deposition films grown show promise for optical display applications, with three shown reflectance levels.

Understanding the Crystallization Behavior of Surface-Oxidized GeTe Thin Films for Phase-Change Memory Application

The outstanding properties of chalcogenide phase-change materials (PCMs) led to their successful use in innovative resistive memory devices where the material is switched between its amorphous and ...

Rapid Assembly of Colloidal Crystals under Laser Illumination on a GeSbTe Substrate.

Optical techniques have been actively studied for manipulating nano- to microsized objects. However, long-range attraction and rapid transport of particles within thin quasi-two-dimensional systems are difficult because of the weak thermophoretic forces. Here, we introduce an experimental system that can rapidly generate quasi-two-dimensional colloidal crystals in deionized water, sandwiched between two hard plates. When a pulsed laser is irradiated on a chalcogenide phase-change material spattered on one side of the plates, the induced Marangoni-like flow causes a colloidal self-assembly in the order of tens of micrometers within the laser spot, with a transport velocity of a few tens of micrometers per second. This is due to the large thermal gradient induced by chalcogenide characteristicsof high laser absorption and low thermal conductivity, and a strong hydrodynamic slip flow at the hydrophobic chalcogenide interface. Moreover, the colloidal crystals exhibit various lattice structures, depending on the laser intensity and chamber distance. For a certain range of the chamber distance, the colloidal crystal phases can be alternated by tuning the laser intensity in real time. Our system forms and deforms quasi-two-dimensional colloidal crystals at an on-demand location on a GeSbTe substrate.

Near-infrared modulation by means of GeTe/SOI-based metamaterial.

Today, nanophotonics still lacks components for modulation that can be easily implementable in existing silicon-on-insulator (SOI) technology. Chalcogenide phase change materials (PCMs) are promising candidates for tuning in the near infrared: at the nanoscale, thin layers can provide enough contrast to control the optical response of a nanostructure. Moreover, all-dielectric metamaterials allow for resonant behavior without having ohmic losses in the telecom range. Here, a novel hybridization of a SOI-based metamaterial with PCM GeTe is experimentally investigated. A metamaterial based on Si nanorods, covered by a thin layer of GeTe, is designed and fabricated. Switching GeTe from amorphous to crystalline leads to a rather high resonance-governed reflection contrast at 1.55μm. Additional confocal Raman imaging is done to differentiate the crystallized zones of the metamaterials' unit cell. The findings are in good agreement with numerical analysis and show good perspectives of all-dielectric tunable near-infrared nanophotonics.

Direct observation of partial disorder and zipperlike transition in crystalline phase change materials

Chalcogenide phase change materials, such as $\mathrm{G}{\mathrm{e}}_{1}\mathrm{S}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ (GST), are of tremendous importance in emerging data storage technology, which takes advantage of rapid and reversible switching of GST between the amorphous and crystalline phases. To date, however, the atomic arrangement of the crystalline GST structure has not been fully resolved, resulting in a controversial understanding of the polymorphic transition mechanism. Here, the atomic and chemical arrangements of stable hexagonal structures of GST are determined by state-of-the-art aberration-corrected scanning transmission electron microscopy. A partially ordered Ge/Sb atomic stacking is resolved in the hexagonal structure to balance the enthalpy and entropy, differing from completely disordered Ge/Sb intermixing arrangement in a metastable rock-salt structure. The transition mechanism between these two phases is proposed, achieved by opening the van de Waals gap like a zipper triggered by the Ge/Sb hopping near vacancy grooves rather than the interplanar random atomic migration. The present results shed light on the understanding of the atomic arrangement and polymorphic phase transitions as well as the control of disorder in GST phase change memory.

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.

A Review of Germanium-Antimony-Telluride Phase Change Materials for Non-Volatile Memories and Optical Modulators

Chalcogenide phase change materials based on germanium-antimony-tellurides (GST-PCMs) have shown outstanding properties in non-volatile memory (NVM) technologies due to their high write and read speeds, reversible phase transition, high degree of scalability, low power consumption, good data retention, and multi-level storage capability. However, GST-based PCMs have shown recent promise in other domains, such as in spatial light modulation, beam steering, and neuromorphic computing. This paper reviews the progress in GST-based PCMs and methods for improving the performance within the context of new applications that have come to light in recent years.

Chalcogenide Phase Change Material for Active Terahertz Photonics.

The strikingly contrasting optical properties of various phases of chalcogenide phase change materials (PCM) has recently led to the development of novel photonic devices such as all-optical non-von Neumann memory, nanopixel displays, color rendering, and reconfigurable nanoplasmonics. However, the exploration of chalcogenide photonics is currently limited to optical and infrared frequencies. Here, a phase change material integrated terahertz metamaterial for multilevel nonvolatile resonance switching with spatial and temporal selectivity is demonstrated. By controlling the crystalline proportion of the PCM film, multilevel, non-volatile, terahertz resonance switching states with long retention time at zero hold power are realized. Spatially selective reconfiguration at sub-metamaterial scale is shown by delivering electrical stimulus locally through designer interconnect architecture. The PCM metamaterial also features ultrafast optical modulation of terahertz resonances with tunable switching speed based on the crystalline order of the PCM film. The multilevel nonvolatile, spatially selective, and temporally tunable PCM metamaterial will provide a pathway toward development of novel and disruptive terahertz technologies including spatio-temporal terahertz modulators for high speed wireless communication, neuromorphic photonics, and machine-learning metamaterials.

Designing crystallization in phase-change materials for universal memory and neuro-inspired computing

The global demand for data storage and processing has increased exponentially in recent decades. To respond to this demand, research efforts have been devoted to the development of non-volatile memory and neuro-inspired computing technologies. Chalcogenide phase-change materials (PCMs) are leading candidates for such applications, and they have become technologically mature with recently released competitive products. In this Review, we focus on the mechanisms of the crystallization dynamics of PCMs by discussing structural and kinetic experiments, as well as ab initio atomistic modelling and materials design. Based on the knowledge at the atomistic level, we depict routes to improve the parameters of phase-change devices for universal memory. Moreover, we discuss the role of crystallization in enabling neuro-inspired computing using PCMs. Finally, we present an outlook for future opportunities of PCMs, including all-photonic memories and processors, flexible displays with nanopixel resolution and nanoscale switches and controllers. Chalcogenide phase-change materials (PCMs) are leading candidates for non-volatile memory and neuro-inspired computing devices. This Review focuses on the crystallization mechanisms of PCMs as well as the design principles to achieve PCMs with high switching speeds and good data retention, which will aid the development of PCM-based universal memory and neuro-inspired devices.

Tunable near-infrared perfect absorber based on the hybridization of phase-change material and nanocross-shaped resonators

Ge2Sb2Te5 (GST) is a kind of non-volatile chalcogenide phase-change material, which has a significant difference in permittivity between its amorphous and crystalline states in the infrared range. On account of this remarkable property, the combination of GST and metamaterials has great potential in tunable meta-devices. In this paper, a perfect absorber based on a nanocross-resonator array stacked above a GST spacer layer and an Au mirror (i.e., a metal-dielectric-metal configuration) is designed and experimentally demonstrated. A thin indium tin oxide (ITO) protective layer is inserted between the GST spacer and the Au resonator to avoid heat-induced oxidation of the GST layer during phase transition. We found that the ITO layer not only can protect the GST layer from deterioration, but also allows a significant blue shift in the absorption peak from 1.808 μm to 1.559 μm by optimizing the thickness of the two dielectric layers without scaling down the size of the metal structure, which provides a more feasible idea in pushing the absorption peak to higher frequency. The LC circuit model is presented to explain this blue-shift phenomenon, which is mainly attributed to the engineering of the dielectric environment of the parallel plate capacitance. In addition, such good performance in dynamitic modulation makes this perfect absorber a robust candidate for optical switching and modulating in various situations.

Reduction of Rocksalt Phase in Ag -Doped Ge2Sb2Te5 : A Potential Material for Reversible Near-Infrared Window

Chalcogenide phase-change materials, which exhibit an amorphous-to-crystalline structural transition, are useful for applications in digital data storage, random-access memory, displays, and transmission windows for photonic devices. Here researchers dope the important chalcogenide Ge${}_{2}$Sb${}_{2}$Te${}_{5}$ with silver to lower its switching temperature, by drastically altering its electronic structure. This is of particular interest for developing photodetectors operating at near-infrared wavelengths.

Implementation of pulse timing discriminator functionality into a GeSbTe/GeCuTe double layer structure.

The functionality of a pulse timing discriminator, which is commonly required in optical communication systems and artificial neuromorphic engineering, was implemented into chalcogenide phase-change materials. GeSbTe (GST) and GeCuTe (GCT), which exhibit opposite refractive index behavior in their respective crystalline and amorphous phases, were employed. A GST/GCT double layer enabled the order of arrival of two counter-propagating femtosecond pulses to be encoded as a difference in the degree of amorphization of the GCT layer, i.e., either a brighter or darker contrast of the amorphized area with respect to the crystalline background. Nonthermal ultrafast amorphization contributed to a picosecond time resolution in the discrimination of the pulse arrival order.