Ge2Sb2Te5 Layer Research Articles

Inkjet‐Printed Phase Change Memory Devices

AbstractPhase change memory (PCM) is among the most promising candidates for the next generation of storage‐class and main memory systems in the computing era beyond Moore's law. However, the widespread installment of PCM devices is limited by the high price‐per‐bit and complex fabrication process. In this paper, it is shown that functional PCM memory devices can be printed, proving low‐cost avenues for non‐silicon memory technologies. Taking Ge‐Sb‐Te (GST) as a case study, PCM inks are prepared and optimize their structural, rheological, and printing parameters. GST layers are then printed onto PCM devices in the planar configuration, showing excellent performance, such as non‐volatility, resistivity contrast, low cycle‐to‐cycle variability, and endurance of at least 100 cycles. This paper provides a novel approach to liquid‐based engineered PCM devices through inkjet printing, enabling patterned memory layers, lower price‐per‐bit, and customizable multi‐material PCM arrays.

Design and validate a wide-bandwidth, high-performance tunable metamaterial based on cultural-innovation shell materials and its sensing applications

The application of metamaterials in controllable thermal emission devices is an interesting field. However, most of the demonstrated thermal emitters required continuous consumption of external energy (electrical or thermal) to provide an effective thermal emissivity. Here, a metamaterial containing phase change materials Ge2Sb2Te5 (GST) and shell materials with controllable thermal emission power was proposed and measured. Based on the completely amorphous state of the GST layer, an emissivity of 0.212 at wavelength 7.11 μm was achieved by this this metamaterial, while a thermal emission band (with an average amplitude of 0.857 and a bandwidth of 6.16 μm) was excited for the crystalline state. Moreover, numerous thermal emission states were excited by this metamaterial based on the intermediate states between completely amorphous and crystalline states of the GST layer. Tunability of the thermal emission window was obtained by this metamaterial sample. The temperature sensitivity of this metamaterial thermal emitter was 341 nm °C−1. By increasing the thickness of the GST or shell layers, the thermal emission performance of the metamaterial was enhanced. Since the phase transition of GST does not require the continuous consumption of external energy, the metamaterial has the potential to be used in the development of low-power heat emitters, as well as temperature sensors.

Dynamic and Polarization-Independent Wavefront Control Based on Hybrid Topological Metasurfaces.

Exceptional points (EPs), known as non-Hermitian singularities, have been observed and investigated in parity-time symmetric metasurfaces. However, the chirality and tunability in non-Hermitian metasurfaces still need to be explored. Here, we propose a dynamic topological metasurface with the meta-atom consisting of two orthogonally oriented nanorods, which are placed on the phase change material Ge2Sb2Te5 (GST) and SiO2 dielectric layer, respectively. When GST is converted from the amorphous state (a-GST) to the crystalline state (c-GST), an EP can be dynamically switched from the "ON" state to the "OFF" state in a parameter space. Moreover, based on the topologically protected phase and amplitude modulations of the cross-polarization component, the phase-only hologram and amplitude-only hologram are engineered in the a-GST case and concealed in the c-GST case. Finally, we explore the 2D-chiral symmetry of meta-atoms and further propose two spin-selective meta-deflectors and a hybrid meta-deflector operating with arbitrary polarizations. The GST-based hybrid metasurface offers richer possibilities to realize various wavefront controls.

Label-free biosensing with singular-phase-enhanced lateral position shift based on atomically thin plasmonic nanomaterials

Rapid plasmonic biosensing has attracted wide attention in early disease diagnosis and molecular biology research. However, it was still challenging for conventional angle-interrogating plasmonic sensors to obtain higher sensitivity without secondary amplifying labels such as plasmonic nanoparticles. To address this issue, we developed a plasmonic biosensor based on the enhanced lateral position shift by phase singularity. Such singularity presents as a sudden phase retardation at the dark point of reflection from resonating plasmonic substrate, leading to a giant position shift on reflected beam. Herein, for the first time, the atomically thin layer of Ge2Sb2Te5 (GST) on silver nanofilm was demonstrated as a novel phase-response-enhancing plasmonic material. The GST layer was not only precisely engineered to singularize phase change but also served as a protective layer for active silver nanofilm. This new configuration has achieved a record-breaking largest position shift of 439.3 μm measured in calibration experiments with an ultra-high sensitivity of 1.72 × 108 nm RIU−1 (refractive index unit). The detection limit was determined to be 6.97 × 10−7 RIU with a 0.12 μm position resolution. Besides, a large figure of merit (FOM) of 4.54 × 1011 μm (RIU∙°)−1 was evaluated for such position shift interrogation, enabling the labelfree detection of trace amounts of biomolecules. In targeted biosensing experiments, the optimized sensor has successfully detected small cytokine biomarkers (TNF-α and IL-6) with the lowest concentration of 1 × 10−16 M. These two molecules are the key proinflammatory cancer markers in clinical diagnosis, which cannot be directly screened by current clinical techniques. To further validate the selectivity of our sensing systems, we also measured the affinity of integrin binding to arginylglycylaspartic acid (RGD) peptide (a key protein interaction in cell adhesion) with different Mn2+ ion concentrations, ranging from 1 nM to 1 mM.

In situ atomic-scale observation of transformation from disordered to ordered layered structures in Ge-Sb-Te phase change memory thin films

Ge-Sb-Te (GST) thin films are emerging materials for various non-volatile memory applications. The compounds contain a huge number of vacancies that play important roles in structural and electronic transitions. Control of disorder and understanding the structural transformations in GST thin films are of great importance for the design and reliability of phase change memory devices. In this work, in situ heating atomic-resolution transmission electron microscopy is used to observe structural transformations from vacancy disordered (fcc) to layered ordered (vacancy-ordered (vo)) and van der Waals-bonded trigonal (vdW t)) structures. Starting from 160 °C, randomly distributed vacancies in fcc grains gradually accumulate into vacancy layers. Further heating to 220 and 240 °C results in the formation of vdW t-domains within the vo-GST grains and the formation of large <001>-oriented t-grains, in a good agreement with outcomes on in situ XRD heating of epitaxial vo-GST thin film. Moreover, vo and fcc GST structures can coexist with vdW t-structure in one grain. Full transformation to 〈112¯0〉-oriented t-grains occurs at 280 °C. Unlike other grains, the 〈001〉 t-grains show abnormal grain growth behaviour, while the electron beam irradiation accelerates the growth. Detailed characterizations of vo-grains reveal transient structures that indicate shear transformation mechanism and different defects as well as chemical changes during the transformations. Overall, this study provides new insights to understand the phase transition mechanisms and to optimize the microstructure in thin GST layers.

Modulation of insulator metal transition of VO₂ films grown on Al2O3 (001) and TiO2 (001) substrates by the crystallization of capping Ge2Sb2Te5 layer

We demonstrate modulation of insulator metal transition (IMT) of VO2 films grown on single crystalline substrates through the effect of in-plane compression with crystallization of capping chalcogenide layer on the targeted VO2 films. Chalcogenide germanium–antimony–telluride (Ge2Sb2Te5: GST), which shows large volume reduction of 6.8% with its phase change from amorphous to crystal, was deposited on VO2 films grown on Al2O3 (001) and TiO2 (001) substrates, where V–V atoms along the cR-axis in the tetragonal VO2 phase align parallel and perpendicular to the substrate surfaces, respectively. As a result, counter shifts in temperature-dependence of resistance characteristics, to lower and higher directions, were observed for VO2 films on Al2O3 (001) and TiO2 (001), consistent with the lattice modulation of VO2 films by the in-plane compression introduced by GST crystallization. The obtained results open a way to realize large resistance change of IMT under constant temperature by controlling GST phases.

Numerical investigation of optical bistability in a nonlinear plasmonic structure containing a phase change material

In this paper, a controllable nonlinear plasmonic structure is proposed based on a phase change material (PCM) layer to achieve tunable bistability characteristics. To this end, the Ge2Sb2Te5 (GST) layer (as a PCM) is sandwiched between a thin film of Ag and a Kerr material substrate. Then, this multilayered structure is used as a substrate for the ZnSiAs2 grating whose grooves are filled with the Kerr nonlinear material. Next, the grating is covered with a layer of CaF2. In this structure, we first calculate the reflection spectrum for different crystallization fractions using the finite element method (FEM) in the linear regime. The reflectance spectrum shows a dip in the near-infrared region, which is redshifted with increasing the crystallization fraction of the GST layer. This effect results from the movement of surface plasmon resonance to longer wavelengths with increasing the crystallization fraction. Then, we find that the dip in the reflectance spectrum is redshifted with enhancing the input intensity of the incident wave for different crystallization fractions in the nonlinear regime. This behavior confirms the existence of optical bistability through the proposed structure. So, we calculate the bistability curves at a fixed operating wavelength of 1550 nm for different crystallization fractions. Our results demonstrate that as the phase transition from the amorphous to the crystalline state occurs at a fixed operating wavelength, the bistability thresholds reduce while the hysteresis width also decreases and the bistability effect eventually disappears. Therefore, for each crystallization fraction of the GST layer we find a special wavelength at which a reasonable bistability curve with a reasonable hysteresis width is obtained. This operating wavelength is shifted by 33 nm as the crystallization fraction varies from 0.2 to 0.8. Finally, the effects of increasing the thickness of the GST layer on the bistability characteristics are examined. Our results show that stronger tunability of the operating wavelength by 50 nm with variation of crystallization degree from 0.2 to 0.8 is achieved when a thicker GST layer is used instead of a thinner one.

Multispectral Imaging with a Planar Cavity-Type Metasurface for Optical Security.

Multispectral imaging refers to capturing images in different wavelength ranges across the electromagnetic spectrum. Despite the potential impact of multispectral imaging, its widespread use has been limited by the poor spectral selectivity of naturally occurring materials beyond the visible range. In this study, we present a multilayered planar cavity structure to simultaneously record mutually independent visible and infrared (IR) images on solid surfaces. The structure consists of a color control unit (CCU) and an emission control unit (ECU). The visible color of the cavity is controlled by varying the thickness of the CCU, whereas its IR emission is spatially tuned by the laser-induced phase change of a Ge2Sb2Te5 layer embedded in the ECU. Because the CCU comprises only IR lossless layers, its thickness variation has negligible influence on the emission profile. This enables different color and thermal images to be printed in a single structure. The cavity structure can be fabricated on flexible substrates (plastic and paper) as well as rigid bodies. Furthermore, the printed images remain stable against bending. This study shows that the proposed multispectral metasurface is highly promising for use in the field of optical security, such as identification, authentication, and anti-counterfeiting.

A comparison of Ge, Sb and Te thermal diffusion through Ge2Sb2Te5

Chalcogenide Ge-Sb-Te alloys are used in a new generation of nonvolatile memories named Phase Change Memories which can be thermally switched from low resistivity to high resistivity states by feeding the memory cells with appropriate electrical pulses. During their lifetime, the heterogeneous redistribution of the elements within the active region of the cells is at the origin of the drift of their electrical characteristics and, ultimately, of their failure. Up to now, little is known about the relative diffusivities of the atomic elements that compose the alloys, what hampers the modeling and optimization of the materials and devices. Here, we report an experimental study of the thermal diffusion of Ge, Sb and Te through Ge2Sb2Te5. For this purpose, specific structures consisting of a layer of Ge, Sb or Te sandwiched between two Ge2Sb2Te5 layers have been fabricated and annealed at various temperatures and times. Using transmission electron microscopy based techniques, we evidence that Te diffusion can be activated from about 160 °C while Ge and Sb needs much higher temperatures, 220 °C and 350 °C, respectively. These results are in disagreement with previous theoretical simulations predicting that Ge is the most diffusive element while Te is the least.

Tunable pheromone interactions among microswimmers

Indirect interactions via shared memory deposited on the field ("field memory") play an essential role in collective motions. Some motile species, such as ants and bacteria, use attractive pheromones to complete many tasks. Mimicking these kinds of collective behavior at the laboratory scale, we present a pheromone-based autonomous agent system with tunable interactions. In this system, colloidal particles leave phase-change trails reminiscent of the process of pheromone deposition by individual ants, and the trails attract other particles and themselves. To implement this, we combine two physical phenomena: the phase change of a Ge2Sb2Te5 (GST) substrate by self-propelled Janus particles (pheromone deposition) and the AC (alternating current) electroosmotic (ACEO) flow generated by this phase change (pheromone attraction). Laser irradiation causes the GST layer to crystalize locally beneath the Janus particles, owing to the lens heating effect. Under AC field application, the high conductivity of the crystalline trail causes a field concentration and generates ACEO flow, and we introduce this flow as an attractive interaction between the Janus particles and the crystalline trail. By changing the AC frequency and voltage, we can tune the attractive flow, i.e., the sensitivity of the Janus particles to the trail, and the isolated particles undergo diverse states of motion, from self-caging to directional motion. A swarm of Janus particles also shows different states of collective motion, including colony formation and line formation. This tunability enables a reconfigurable system driven by a pheromone-like memory field.

Highly Flexible Infrared Emitter with Spatially Controlled Emissivity for Optical Security

AbstractEngineering the infrared (IR) emissivity of a material or structure is crucial for a variety of fields, including thermal camouflage, radiative cooling, personal thermal management, and optical security. For practical and wide‐spread use, the emitter should possess high mechanical flexibility while maintaining the capabilities of space‐selectively and dynamically controlling its emissivity. In this paper, an optical resonator consisting of a Ge2Sb2Te5 (GST) layer on top of a thin metal reflector stationed on a flexible substrate is presented as an IR emitter that satisfies the aforementioned requirements. A laser‐induced phase change from amorphous to crystalline GST enables dynamic tuning of the local emissivity of the resonator. This study illustrates that although GST is a brittle material, the emitters fabricated on plastic and paper substrates are highly robust against bending up to a radius of curvature of 0.5 cm. Moreover, visible light and IR images can be independently recorded in the same region by employing a spatially modulated laser beam owing to the multispectral properties of GST. The fact that a single emitter can exhibit different visible and IR images is particularly attractive for optical security applications, including anti‐forgery. This feature is experimentally demonstrated using white paper and color‐printed paper.

Laser-printed emissive metasurface as an optical security platform

Optical security is a promising application of metasurfaces because light has large degrees of freedom in metasurfaces. Although many different structures/materials have been proposed for this purpose, the fabrication of dynamic metasurfaces in a straightforward and scalable manner while maintaining a high security level remains a significant challenge. Herein, a metasurface consisting of a phase-changing Ge2Sb2Te5 (GST) layer and a thin metal back reflector is presented to space-selectively and dynamically control the infrared emission of the surface by a spatially modulated pulsed laser beam. Unlike conventional laser processes using a focused beam, the employed laser printing is an expanded beam-based parallel process that enables the fabrication of wafer-sized emission patterns. Owing to the multispectral responses of GST, mutually independent visible and infrared images can be printed in one region. Grayscale emission patterns can also be obtained by gradually modulating the spatial profile of the laser beam, which makes the replication of laser-printed emission patterns extremely difficult. We also demonstrate that colors images can be obtained by depositing IR lossless layer on the GST surface. All these features indicate that the presented emissive metasurface has the potential for use as an effective platform for anti-counterfeiting.

A tunable infrared emitter based on phase-changing material GST for visible-infrared compatible camouflage with thermal management.

Visible-infrared compatible camouflage is important to increase the counter-detection ability of a target due to the fast development of detection systems. However, most of the previously reported visible-infrared compatible camouflage structures are not suitable when the temperature of targets and type of background environment change. In this paper, we propose a tunable infrared emitter composed of ZnS/Ge/Ag/Ge2Sb2Te5/Ag films and numerically demonstrate visible-infrared compatible camouflage and radiation heat dissipation. Firstly, the proposed infrared emitter can produce different structural colors as the thickness of the ZnS film changes, which can be applied to visible camouflage. Secondly, the crystallization fraction of the Ge2Sb2Te5 (GST) layer could help to engineer the average emissivity of the proposed infrared emitter, achieving tunable mid-infrared (MIR) camouflage, radiation heat dissipation, and long-infrared (LIR) camouflage in wavelength ranges of 3-5 μm, 5-8 μm, and 8-14 μm, respectively. Finally, we numerically demonstrate the visible camouflage and infrared camouflage for different application scenarios by using the simulated visible and infrared images. This work has promising application potential in visible-infrared compatible camouflage technology.

Strong and Omnidirectional Light Absorption from Ultraviolet to Near‐Infrared Using GST Metasurface

AbstractFor the perfect light absorber, it has been a huge challenge to simultaneously realize ultra‐broadband and strong absorption of unpolarized light over a large angular range, though various materials and designs have been tried. The emergence of optical metasurface with powerful light field regulation ability provides a promising new strategy for achieving perfect absorbers. Here, a Ge2Sb2Te5 (GST)‐based metasurface with an average 92.5%/94.7% (experiment/simulation) absorptivity of unpolarized light, covering an ultra‐broadband from ultraviolet to near‐infrared region is designed and demonstrated. This GST‐based metasurface consists of double‐layer GST interspaced by SiO2 layer, and both field localizations in the top patterned GST layer and intrinsic loss of the bottom patternless GST layer together contribute to this broadband absorption over wide incident angles up to 70°. Strong absorbing ability, large angular responses, and feasibly scalable fabrication for such a perfect GST‐based metasurface absorber offer more promising application opportunities in integrated optoelectronic devices.

Development of Reflow Fill of CGeSbTe Films for Sub‐20 nm Cells in 3D Cross‐Point Memory

AbstractTo obtain reliable and scalable cells in 3D cross‐point memory, endurance failures caused by voiding from etching damage to memory cells should be resolved to preserve the properties of GeSbTe (GST) phase‐change materials. Herein, the fabrication of a damascene cell without patterning, which relies on the bottom‐up fill of carbon‐doped GST layers into confined cells, is proposed as a promising solution. The reflow fill of the sputtered carbon‐doped GST films into confined cells at high‐temperature and low‐power conditions is demonstrated, and a mechanism in which Sb and Te transfer into the cell through vapor transfer and viscous flow due to capillary pressure is verified. The aspect ratio increases from 2 to 7.7 under enhanced vaporization of Sb and Te as well as the capillary force. The advanced reflow‐fill process using a laser can overcome the limit of the conventional reflow‐fill technology by increasing the atomic mobility above the melting temperature. Based on the proposed method, 19 nm cross‐point memory devices are successfully integrated together with the Ovonyx threshold switch and appropriate memory windows are secured. Furthermore, the 9 nm cells exhibit reliable endurance cycles over 108.

Design, simulation and measurement of a active selector based on metamaterial in 5G frequency band

Functional devices based on electromagnetic (EM) tunable metasurfaces provide a feasible strategy for controlling of communication signals. Here, an EM metasurfaces based on Ge2Sb2Te5 (GST) layer is proposed and verified. At room temperature, this metasurface sample obtains a transmission window (resonant frequency 8.07 GHz, amplitude 0.92) based on the amorphous state of the GST layer. This transmission window is frequency selectable, which is achieved by changing the structural parameters (diameter of hole array, or thickness of dielectric layer). By changing the annealing temperature, this transmission window is always covered by the transmission valley, thus confirming the switchability of the metasurface sample. Finally, the sensitivity of the transmission window of this metasurface sample to liquid (water, acetone, ethanol, and PEG300) is measured and verified. The metasurface can be applied in communication, sensing, control and other fields.

Enhanced circular dichroism in Ge2Sb2Te5-loaded metasurface.

Circular dichroism (CD) is originally obtained from three-dimensional spiral structures by simultaneously exciting electric and magnetic resonances. To simplify construction, multilayer stacked asymmetric structures and the symmetric structures relying on oblique incidence are proposed for enhancing CD. Herein, we achieved the enhancement of dual-waveband CD by adding a Ge2Sb2Te5 (GST) layer on the top of a Z-shape gold array in a normally incident system. Benefited from the polarization selective excitations of electric and magnetic dipole resonances, the CD in a simple planar structure is immensely enhanced from near zero to 0.73 at 1.58 µm. Furthermore, the CD strengths is dynamically tuned by controlling the phase of GST. With the GST phase transition from amorphous (a-GST) to crystalline state (c-GST), CD magnitudes are switched by about 0.73 and 0.27 at dual wavebands respectively. The enhancement of CD by adding a layer on a simple planar array offers a new method for designing planar metasurfaces with strong chirality.

Ge[formula omitted]Sb[formula omitted]Te[formula omitted]-coated fiber optic SPR sensor: Improving sensitivity

This paper proposes and demonstrates a fiber optic surface plasmon resonance (SPR) sensor based on Au–Ge2 Sb2Te5 (GST). The GST is deposited on the Au film, which increases the dielectric constant of the metal layer due to the high refractive index (RI) of GST. Therefore, compared with conventional Au SPR sensor, the resonance wavelength of the Au–GST SPR sensor red-shifts, and the sensitivity is improved. When environmental RI varies in the range of 1.333∼1.403, the experimental results show that the sensor performs the best when the thicknesses of Au film is 50 nm and the GST layer’s thickness is 25 nm. The proposed sensor’s sensitivity is 5210 nm/RIU (refractive index unit), which is much higher than that of the conventional Au-based SPR sensor (2418 nm/RIU). The sensor achieves relatively high sensitivity based on a very simple structure, and it has potential applications in biological and biochemical such as DNA hybridization, cancer screenings, environmental temperature and humidity detection, and chemical analysis.

(Digital Presentation) Electrothermal Modeling of Interfacial Phase Change Memory

Phase change memory (PCM) is a high-speed non-volatile memory that utilizes the reversible and fast transition between highly conductive crystalline phase and highly resistive (dielectric) amorphous phase of the phase change material to store information and the large electrical resistivity contrast between the two phases for retrieval of the stored data [1,2]. One of the main challenges for PCM is the large power required to heat the active region above crystallization or melting temperature. Lower energy and higher speed operations have been demonstrated with thin film superlattice stacks of phase change materials known as interfacial phase change memory (iPCM) [3-6]. The mechanisms behind the improved performance of iPCM are still under investigation but recent work indicates similar crystallization and melt-quench operation of these devices [5].In this work, we perform electrothermal finite element simulations of reset and set operations on iPCM structures consisting of alternately stacked Ge2Sb2Te5 (GST) and GeTe layers using COMSOL multiphysics [7-10]. Electrical pulses are applied for reset and set processes utilizing an internal circuit model where a transistor is used as an access device. Coupled electric current and heat transfer physics are employed to incorporate Joule heating and thermoelectric effects (Thomson heat within a single material and Peltier heat at material interfaces) with temperature dependent Seebeck coefficients, thermal conductivities, electrical resistivities, heat capacities and thermal boundary resistances (TBR) for each material / material pairs. Latent heat of fusion is included in the amorphous-crystalline and solid-liquid transitions [10], giving rise to heat release at the crystal-amorphous boundaries during crystal growth and heat absorption at the grain boundaries during amorphization. Grain boundaries and material interfaces have high energy sites, making them easier to melt, described as heterogeneous melting [10].iPCM [5] structures utilize engineered interfaces formed between nanometer scale thin-film stacks, promoting amorphization through increased number of material interfaces and reduced thermal conduction due to TBR. Furthermore, the melting temperature, electrical conductivity and Seebeck coefficient of the different materials within an iPCM device differ. Hence, such layered structures may have the advantage of melting of only one of the alternating layers assisted by local heating or cooling due to Peltier effect at the interfaces.Our results on iPCM and conventional PCM structures of same dimensions and geometry (20 nm wide, 150 nm high pore-cells) show ~ 50% reduction in reset times and more consistent set times for iPCM cells due to lesser variations in grain sizes and location of boundaries.Acknowledgment: This work is partially supported by the National Science Foundation under award DMR-1710468.

Laser‐Printed Emissive Metasurface as an Anticounterfeiting Platform

AbstractOptical security is a promising application of metasurfaces because light has large degrees of freedom in metasurfaces. Although many different structures/materials are proposed for this purpose, the fabrication of dynamic metasurfaces in a straightforward and scalable manner, while maintaining a high security level, remains a significant challenge. Herein, a metasurface consisting of a phase‐changing GeSbTe layer and a metal back reflector is presented to space‐selectively and dynamically control the infrared emission of the surface by a spatially modulated pulsed laser beam. Unlike conventional laser processes using a focused beam, the employed laser printing is an expanded beam‐based parallel process that enables the fabrication of wafer‐sized emission patterns. Owing to the multispectral responses of GeSbTe, mutually independent visible and infrared images can be printed in one region. Grayscale emission patterns can also be obtained by gradually modulating the spatial profile of the laser beam, which makes the replication of laser‐printed emission patterns extremely difficult. These encouraging features indicate that the presented emissive metasurface has the potential for use as an effective platform for anticounterfeiting.