Ge2Sb2Te5 Phase Research Articles

Accelerating the development of phase‐change random access memory with in‐fab plasma profiling time‐of‐flight mass spectrometry

We demonstrate the potential of using plasma profiling time‐of‐flight mass spectrometry (PP‐TOFMS) to accelerate process developments for phase‐change random access memory (PCRAM) applications, which require advanced materials with composition‐driven properties. We assess the performances of PP‐TOFMS for the chemical depth‐profiling of GeSbTe phase change materials, first after deposition steps to investigate the top surface layer and the incorporation of silicon into the amorphous matrix, then after the thermal annealing step to refine in situ capping strategies, and finally in close loop with etching process steps. Comparison of reference‐free semiquantitative PP‐TOFMS analysis based on ion beam ratio with Rutherford backscattering spectrometry shows remarkable agreement (~10% relative). PP‐TOFMS proves to be a fast screening tool, which allows process monitoring and selection of samples that indeed need more complex analysis.

Investigations on transient regime of Ge2Sb2Te5-based vertical photodetector integrated with silicon-on-insulator waveguide

In this study, we conducted finite difference time domain simulation investigations of the transient responses of silicon-on-insulator waveguide-based vertical photodetectors using the phase-change material Ge2Sb2Te5 (GST) as a sensing element on the Si core. The charge transport characteristics comprising the carrier and displacement current were comprehensively studied based on variations in the dimension, pulse width, and frequency by solving the drift-diffusion model under constant biased conditions. The dynamic behaviors of two stable GST phases comprising amorphous (aGST) and crystalline (cGST) were compared in terms of the peak amplitude, peak arrival time, and rate of change in the current for both phases. The cGST based device was much faster, with the lowest peak arrival time of 21.5 ps and rate of arrival of 56.5 μA/ps at a wavelength of 1150 nm. The lowest value peak arrival time was 22 ps with a slope of 4 μA/ps for the aGST based device at a wavelength of 1850 nm. The results showed that cGST is suitable for lower wavelengths whereas aGST is appropriate for higher wavelengths in the near-infrared operating range to obtain instant dynamic response, thereby allowing the production of a tunable photodetector with a phase-change material.

Structural Transitions in Ge2Sb2Te5 Phase Change Memory Thin Films Induced by Nanosecond UV Optical Pulses.

Ge-Sb-Te-based phase change memory alloys have recently attracted a lot of attention due to their promising applications in the fields of photonics, non-volatile data storage, and neuromorphic computing. Of particular interest is the understanding of the structural changes and underlying mechanisms induced by short optical pulses. This work reports on structural changes induced by single nanosecond UV laser pulses in amorphous and epitaxial Ge2Sb2Te5 (GST) thin films. The phase changes within the thin films are studied by a combined approach using X-ray diffraction and transmission electron microscopy. The results reveal different phase transitions such as crystalline-to-amorphous phase changes, interface assisted crystallization of the cubic GST phase and structural transformations within crystalline phases. In particular, it is found that crystalline interfaces serve as crystallization templates for epitaxial formation of metastable cubic GST phase upon phase transitions. By varying the laser fluence, GST thin films consisting of multiple phases and different amorphous to crystalline volume ratios can be achieved in this approach, offering a possibility of multilevel data storage and realization of memory devices with very low resistance drift. In addition, this work demonstrates amorphization and crystallization of GST thin films by using only one UV laser with one single pulse duration and one wavelength. Overall, the presented results offer new perspectives on switching pathways in Ge-Sb-Te-based materials and show the potential of epitaxial Ge-Sb-Te thin films for applications in advanced phase change memory concepts.

Switchable Quarter-Wave Plate and Half-Wave Plate Based on Phase-Change Metasurface

Wave plates as important polarization control devices are widely used in the applications of biomedical imaging, fiber optical communication, astronomy, semiconductor industry and aerospace etc. To make the wave plate actively tunable, we propose a rectangular antenna-based metasurface based on the phase-changing material Ge2Sb2Te5 (GST) with high transmittance and polarization conversion rate. Before the GST phase transition, the metasurface operates as a quarter-wave plate in the wavelength range of 10.0–11.9 μm. After the GST turning into crystalline state, the metasurface operates as a half-wave plate in the wavelength range of 10.3–10.9 μm. The physical mechanism for the high transmittance efficiency and polarization conversion rate attributes to the combination of the electric dipole and magnetic dipole resonances. Moreover, the adjustable wavelength range can be further extended by changing the geometry of antenna-based GST to selectively enhancing or suppressing the electric and magnetic resonances. This work provides a new strategy for tunable wave plate, which would have the potential in the application of multi-functional integrated optical systems.

Ultra-wideband multi-level optical modulation in a Ge2Sb2Te5-based waveguide with low power consumption and small footprint

A Ge2Sb2Te5 (GST)-based four-level pulse amplitude optical modulator is proposed and its optical and electrical properties are studied. The four-level pulse amplitude modulation (PAM4) scheme is used as a solution to overcome the inherent speed limitation of the phase-change based optical modulators. The modulator is formed by implementing two GST-based active waveguide sections in a silicon ridge waveguide where the propagation loss of each active waveguide section can be varied by changing the GST phase. Optical simulation results show that the four different output states of the modulator are separated by more than 2.5dB in the ultra-wide wavelength range of 1.4μm<λ<1.7μm, while the small overall footprint of 3.28μm2, results in low insertion loss of 1.19dB. Electrical and thermal properties of the structure are also investigated which reveal the very small energy consumption of 11.97pJ/bit and high modulation speed of 0.428Gbit/sec for the proposed PAM4 modulator. These calculations show substantial improvement in the performance parameters of the proposed structure compared to the previously reported phase-change material (PCM)-based optical modulators in terms of modulation speed and energy consumption.

Towards a 3D GeSbTe phase change memory with integrated selector by non-aqueous electrodeposition.

We have recently reported a new method for the electrodeposition of thin film and nanostructured phase change memory (PCM) devices from a single, highly tuneable, non-aqueous electrolyte. The quality of the material was confirmed by phase cycling via electrical pulsed switching of both 100 nm nano-cells and thin film devices. This method potentially allows deposition into extremely small confined cells down to less than 5 nm, 3D lay-outs that require non-line-of-sight techniques, and seamless integration of selector devices. As electrodeposition requires a conducting substrate, the key condition for electronic applications based on this method is the use of patterned metal lines as the working electrode during the electrodeposition process. In this paper, we show the design and fabrication of a 2D passive memory matrix in which the word lines act as the working electrode and nucleation site for the growth of confined cells of Ge-Sb-Te. We will discuss the precursor requirement for deposition from non-aqueous, weakly coordinating solvents, show the transmission electron microscopy analysis of the electrodeposition growth process and elemental distribution in the deposits, and show the fabrication and characterisation of the Ge-Sb-Te memory matrix.

A phase-change thin film-tuned photonic crystal device

This paper reports a tunable photonic device that incorporates a thin layer of phase-change material, Ge2Sb2Te5 (GST), in a photonic crystal (PC) structure. The PC structure is based on a one-dimensional grating waveguide with a metal cladding. The metal-cladded PC structure supports a guided-mode resonance (GMR) that selectively absorbs light at a particular wavelength. Inserting the GST material into the gating waveguide makes it possible to control the GMR mode. Here, the GST-PC device was numerically designed and optimized to obtain significant tuning of the GMR mode around 1550 nm. The tuning phenomena were experimentally demonstrated by the heat-induced phase change between crystalline and amorphous phases of the GST thin film. A spectral shift of the resonant wavelength from 1440 to 1610 nm was achieved via the crystallization process. The phase tuning of GST exhibits good repeatability as demonstrated by switching between amorphous and crystalline phases of GST for multiple cycles. The GST-PC device represents a new approach for tuning optical resonances with potential applications including but not limited to integrated photonic circuits, optical communications, and high-performance optical filters.

Electrochemical metallization cell with solid phase tunable Ge2Sb2Te5 electrolyte

Electrochemical metallization (ECM) cell kinetics are strongly determined by the electrolyte and can hardly be altered after the cell has been fabricated. Solid-state property tunable electrolytes in response to external stimuli are therefore desirable to introduce additional operational degree of freedom to the ECM cells, enabling novel applications such as multistate memory and reconfigurable computation. In this work, we use Ge2Sb2Te5(GST) as the electrolyte material whose solid state is switched from the amorphous(a) to the crystalline(c) phase thermally. Electrical heating too is readily achievable. The resistive switching characteristics of the cells with different GST phases are examined. The magnitude of the high resistance, the SET voltage and the on/off ratio are found to be considerably affected by the solid phase of GST, whereas the magnitude of the low resistance is least affected. Moreover, a transition from volatile to nonvolatile SET switching is only observed for c-GST based cell under prolonged voltage sweep, but not for a-GST based cell. This work provides a springboard for more studies on the manipulation of the ECM cell kinetics by tunable electrolyte and the resulting unprecedented device functionalities.

A systematic evolution of optical band gap and local ordering in Ge1Sb2Te4 and Ge2Sb2Te5 materials revealed by in situ optical spectroscopy

A unique amalgam of fascinating potentials enabled the usage of phase change materials in rewritable optical and electrical storage technology. Furthermore, the identification of stoichiometric tuning of disorder in these materials opened up new pathways for the selection of materials for multi-bit data storage. Despite a reasonable understanding of structure and properties, an in situ study could aid in unravelling vital insights. Hence, we report on an analogy between the degree of disorder in Ge1Sb2Te4 and Ge2Sb2Te5 materials by employing novel temperature-dependent correlations of optical band gap (Eg), Tauc parameter (measure of disorder, B1/2 slope) and the change in band tail. Eg decreases from 0.8 eV (90 K) to 0.34 eV (480 K) for Ge1Sb2Te4 and from 0.82 eV (90 K) to 0.36 eV (480 K) for Ge2Sb2Te5. This trend reveals the semiconducting nature of both amorphous and crystalline phases. The change in disorder as exemplified by an increase in B1/2 of 2.7% and 14.9% during the phase transition of Ge1Sb2Te4 and Ge2Sb2Te5 respectively reveals a stronger degree of disorder in the cubic phase of Ge1Sb2Te4 as compared to the cubic phase of Ge2Sb2Te5. These findings are strongly supported by an unambiguous change in the width of the band tail. This confirms the origin of a pronounced disorder-induced localization in Ge1Sb2Te4 compared to the Ge2Sb2Te5 material.

Polarization switching of thermal emissions based on plasmonic structures incorporating phase-changing material Ge2Sb2Te5

The ability to control the polarization of thermal emissions is important for fundamental science and many applications such as multichannel infrared emitters and chemical sensing. Most previous works on controlling the polarization of thermal emission are based on changing geometric sizes of the structures. The active control remains elusive so far. Here, we propose a design to actively switch the polarization of thermal emission. A metal-insulator-metal plasmonic thermal emitter with phase changing material Ge2Sb2Te5 (GST) as the insulator is experimentally demonstrated. The thermal emitter with top GST and gold ellipses can excite third-order magnetic resonances with perpendicular polarization along both short radius and long radius. The polarization of the thermal emission can be rotated by 90° at 9.55 μm peak wavelength when GST phase changes from the amorphous phase to the 40% crystalline phase.

Optothermally controllable multiple high-order harmonics generation by Ge2Sb2Te5-mediated Fano clusters

Substantial enhancement of nonlinear high-order harmonics generation based on Fano-resonant nanostructures has received growing interest due to their promising potential for developing integrated and advanced next-generation nanophotonic devices. In this study, going beyond conventional subwavelength structures, we propose an optothermally functional hetero-metallodielectric asymmetric eight-member octamer cluster composed of a central silicon nanodisk and peripheral disks with a phase-change material (Ge2Sb2Te5). Using full electromagnetic calculations, we show that in the amorphous phase of the surrounding nanoparticles, the oligomer acts as an all-dielectric cluster, while in the crystalline regime, the octamer turns into a hybrid metallodielectric assembly. Exploiting the exquisite ability of supporting distinct Fano lineshapes at different wavelengths depending on the phase of Ge2Sb2Te5, we showed the generation of both second and third harmonics at amorphous and crystalline phases of GST nanodisks, respectively with the produced harmonic wavelengths of 425 nm and 317 nm, respectively. Our calculations for the corresponding conversion efficiencies revealed significant enhancements as ηSHG = 0.0081% and ηTHG = 0.012% for SHG and THG, respectively. Such an exquisite feature of multiresonant optothermally tunable cluster allows generation of several harmonics with substantial intensities using a single system for future photonics applications.

Improvement in cyclic operation of unit pixel device using Sb-excess Ge2Sb2Te5 thin films for hologram image implementation

The effects of the atomic composition of Ge2Sb2Te5 (GST), especially the Sb-excess phase, on device reliability were investigated for applications of spatial light modulator (SLM) devices using phase-change materials. Sb-excess GST phases were prepared by cosputtering with two target systems. For Sb-excess GST thin films, amorphous phases were found to be more directly crystallized to more conductive hexagonal-close-packing phases at higher crystallization temperatures without the appearance of a less conductive face-centered-cubic phase with increasing amount of incorporated Sb. The marked changes in crystallization behavior and improved thermal stability were suggested as critical benefits for enhancing the device durability during cyclic operations, considering that the interdiffusion within the GST could be the main origin of the final device failure. The number of cyclic operations of the unit pixel device was markedly improved to more than 103 when the amount of excess Sb was modulated from 0 to 21 at. %.

Thermal camouflage based on the phase-changing material GST

Camouflage technology has attracted growing interest for many thermal applications. Previous experimental demonstrations of thermal camouflage technology have not adequately explored the ability to continuously camouflage objects either at varying background temperatures or for wide observation angles. In this study, a thermal camouflage device incorporating the phase-changing material Ge2Sb2Te5 (GST) is experimentally demonstrated. It has been shown that near-perfect thermal camouflage can be continuously achieved for background temperatures ranging from 30 °C to 50 °C by tuning the emissivity of the device, which is attained by controlling the GST phase change. The thermal camouflage is robust when the observation angle is changed from 0° to 60°. This demonstration paves the way toward dynamic thermal emission control both within the scientific field and for practical applications in thermal information.

Design of ultra-low insertion loss active transverse electric-pass polarizer based [formula omitted] on silicon waveguide

An active transverse electric (TE) pass polarizer with ultra-low insertion loss (IL) is proposed based on asymmetrical directional coupler consisting of a silicon waveguide and a silicon hybrid waveguide with phase change material of Ge2Sb2Te5 (GST). The tunability of the polarizers relies on the considerable change of the optical properties of the GST between the amorphous and crystalline phases. We numerically demonstrate a 5.5-μm-long active polarizer has an ultra-high IL of above 28 dB for the unwanted transverse magnetic (TM) mode and the ultra-low IL of as little as 0.12 dB for TE mode with the crystalline phase of GST. When trigger to the amorphous phase, the device nearly exhibits transparent with the negligible IL for TE/TM mode. The proposed device possesses a broadband of ∼200 nm with a high extinction ratio (ER) as well as relative large fabrication tolerance.

Optothermally Tuned Charge Transfer Plasmons in Au-Ge2Sb2Te5 Core-Shell Assemblies

ABSTRACTTunable plasmonic resonances across the visible and near infrared spectra have provided novel ways to develop next-generation nanophotonic devices. Here, by utilizing optothermally controllable phase-changing material (PCM), we studied highly tunable charge transfer plasmon (CTP) resonance modes. To this end, we have designed a two-member dimer assembly including gold cores and Ge2Sb2Te5 (GST) shells in distant, touching, and overlapping conditions. We successfully demonstrated that toggling between amorphous (dielectric) and crystalline (conductive) phases of GST allows for achieving tunable dipolar and CTP resonances along the near-infrared spectrum. The proposed dimer structures can help forming optothermally controlled devices without further morphological variations in the geometry of the design, and having strong potential for advanced plasmon modulation and fast data routing.

Mapping the band structure of GeSbTe phase change alloys around the Fermi level

Phase change alloys are used for non-volatile random-access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin.

Optothermally Controlled Charge Transfer Plasmons in Au-Ge2Sb2Te5 Core-Shell Dimers

Functional and reversible plasmonic resonances across the visible and near-infrared spectrum have opened new avenues for developing advanced next-generation nanophotonic devices. In this study, by using optothermally controlled phase-change material (PCM) for plasmonic nanostructures, we successfully induced highly tunable charge transfer plasmon (CTP) resonance modes. To this end, we have chosen a two-member dimer assembly consisting of gold cores and Ge2Sb2Te5 (GST) shells in distant, touching, and overlapping regimes. We show that switching between amorphous (dielectric) and crystalline (conductive) phases of GST allows for achieving tunable dipolar and CTP resonances and enables an effective interplay between these modes along the near-infrared spectrum. By analyzing electromagnetically calculated spectral responses for the dimer antenna in tunneling and direct charge transfer regimes, we confirmed that the induced CTPs in touching and overlapping regimes are highly controllable and pronounced in comparison to the quantum tunneling regime. We also use the precise, fast, and controllable switching between dipolar and CTP resonant modes to develop a telecommunication switch based on a simple metallodielectric dimer. The proposed structures can help designing optothermally controlled devices without morphological variations in the geometry of the design, and having strong potential for advanced plasmon modulation and fast data routing.

Deflection angle switching with a metasurface based on phase-change nanorods [Invited

We propose the active metasurface using phase-change material Ge2Sb2Te5 (GST), which has two distinct phases so called amorphous and crystalline phases, for an ultrathin light path switching device. By arranging multiple anisotropic GST nanorods, the gradient metasurface, which has opposite directions of phase gradients at the two distinct phases of GST, is demonstrated theoretically and numerically. As a result, in the case of normal incidence of circularly polarized light at the wavelength of 1650 nm, the cross-polarized light deflects to −55.6° at the amorphous phase and +55.6° at the crystalline phase with the signal-to-noise ratio above 10 dB.

Tunable broadband near-infrared absorber based on ultrathin phase-change material

In this work, a tunable broadband near-infrared light absorber was designed and fabricated with a simple and lithography free approach by introducing an ultrathin phase-change material Ge2Sb2Te5 (GST) layer into the metal–dielectric multilayered film structure with the structure parameters as that: SiO2 (72.7 nm)/Ge2Sb2Te5 (6.0 nm)/SiO2 (70.2 nm)/Cu (>100.0 nm). The film structure exhibits a modulation depth of ∼72.6% and an extinction ratio of ∼8.8 dB at the wavelength of 1410 nm. The high light absorption (95%) of the proposed film structure at the wavelength of 450 nm in both of the amorphous and crystalline phase of GST, indicates that the intensity of the reflectance in the infrared region can be rapidly tuned by the blue laser pulses. The proposed planar layered film structure with layer thickness as the only controllable parameter and large reflectivity tuning range shows the potential for practical applications in near-infrared light modulation and absorption.

Optical Switching Using Transition from Dipolar to Charge Transfer Plasmon Modes in Ge2Sb2Te5 Bridged Metallodielectric Dimers

Capacitive coupling and direct shuttling of charges in nanoscale plasmonic components across a dielectric spacer and through a conductive junction lead to excitation of significantly different dipolar and charge transfer plasmon (CTP) resonances, respectively. Here, we demonstrate the excitation of dipolar and CTP resonant modes in metallic nanodimers bridged by phase-change material (PCM) sections, material and electrical characteristics of which can be controlled by external stimuli. Ultrafast switching (in the range of a few nanoseconds) between amorphous and crystalline phases of the PCM section (here Ge2Sb2Te5 (GST)) allows for designing a tunable plasmonic switch for optical communication applications with significant modulation depth (up to 88%). Judiciously selecting the geometrical parameters and taking advantage of the electrical properties of the amorphous phase of the GST section we adjusted the extinction peak of the dipolar mode at the telecommunication band (λ~1.55 μm), which is considered as the OFF state. Changing the GST phase to crystalline via optical heating allows for direct transfer of charges through the junction between nanodisks and formation of a distinct CTP peak at longer wavelengths (λ~1.85 μm) far from the telecommunication wavelength, which constitutes the ON state.