Phase-change Material Ge2Sb2Te5 Research Articles

Ruthenium doped Ge2Sb2Te5 nanomaterial as fast speed phase-change materials with good thermal stability

Ge2Sb2Te5 (GST) as phase-change material has the advantage of good thermal stability and long cycle life, however large power consumption and lower phase change speed partly limit large-scale application for phase-change materials (PCM). In this paper, a comprehensive research is investigated on Ru doped GST phase-change material, from properties to performances for PCM application. Ru doping can efficiently improve the thermal stability of GST alloy and the thickness change of Ru0.05(GST)0.95 film is 3.3%. In addition, high 10-year data retention temperature (∼151 °C) and small crystal grain reflect the advantages of data retention and low power consumption for Ru0.05(GST)0.95 film. Furthermore, the structure characteristics show the stable face-centred cubic structure at crystalline state after annealing. As for electrical properties, Ru0.05(GST)0.95-based PCM cell shows excellent reversible SET-RESET properties with a suitable operation window and good endurance up to 106 electrical cycles with a resistance ratio of about 10. And also, it has a dramatically lower power consumption of 1.46 × 10-11 J compared with GST-based cell (4.76 × 10-10 J), making it a promising material for the Internet of Things (IOT) field applications.

Observation of ultrafast amorphization dynamics in GeCu2Te3 thin films using echelon-based single-shot transient absorbance spectroscopy

The compound GeCu2Te3 (GCT) has attracted considerable attention because of its several advantages for next-generation nonvolatile memories, including its higher thermal stability and lower volume change, with large optical contrast between the crystalline and amorphous phases. In this study, we demonstrate the ultrafast amorphization dynamics that occur in GCT by utilizing echelon-based single-shot transient absorbance spectroscopy and coherent phonon spectroscopy. We find that the timescale of the absorbance change accompanying amorphization is ∼2 ps, which is close to the dephasing time of the A1 optical phonons. Based on the observed results and the robust structural network of crystalline GCT, we discuss the amorphization dynamics in GCT by comparing it with that in the typical phase-change material Ge2Sb2Te5.

Rewritable and Tunable Laser-Induced Optical Gratings in Phase-Change Material Films.

Laser-induced periodic surface structures (LIPSS) can be fabricated in virtually all types of solid materials and show great promise for efficient and scalable production of surface patterns with applications in various fields from photonics to engineering. While the majority of LIPSS manifest as modifications of the surface relief, in special cases, laser impact can also lead to periodic modulation of the material phase state. Here, we report on the fabrication of high-quality periodic structures in the films of phase-change material Ge2Sb2Te5 (GST). Due to considerable contrast of the refractive index of GST in its crystalline and amorphous states, the fabricated structures provide strong spatial modulation of the optical properties, which facilitates their applications. By changing the excitation laser wavelength, we observe the scaling of the grating period as well as transition between formation of different types of LIPSS. We optimize the laser exposure routine to achieve large-scale high-quality phase-change gratings with controllable period and demonstrate their reversible tunability through intermediate amorphization steps. Our results reveal the prospects of fast and rewritable fabrication of high-quality periodic structures for photonics and can serve as a guideline for further development of phase-change material-based optical elements.

Adaptive infrared camouflage based on quasi-photonic crystal with [formula omitted

In this study, a novel adaptive infrared (IR) camouflage film based on one-dimensional (1D) quasi-photonic crystal (PC) with phase-change material Ge2Sb2Te5 (GST), is theoretically demonstrated. The designed film shows high emissivity state at low temperature and low emissivity state at high temperature, via the transition between amorphous and crystalline phase of GST. Moreover, the quasi-PC structure, designed with top optimized layers, possesses low emissivity property by splicing constructive interference (CI) peaks. In particular, the designed quasi-PC film is more conducive to the IR camouflage with variable object temperature (To), compared to those with static emissivity. This study provides a significant pathway to 1DPC application for adaptive IR camouflage and other technologies that rely on controlling the transfer of thermal radiation.

Point defects in disordered and stable GeSbTe phase-change materials

Phase-change materials (PCMs) are promising candidates for efficient storage-class memory exploiting the pronounced resistivity contrast between their amorphous and crystalline phase. GeSbTe compounds, the prototypical class of PCMs, are theoretically predicted to be small-gap semiconductors in their disordered, cubic and ordered, hexagonal phase, but in experiment they show p-type conduction. While this is attributed to self-doping, the very defect types responsible have not been entirely identified. Here, we present an ab-initio study of point defects and their formation energies in GeSb2Te4 and Ge2Sb2Te5 in their disordered and ordered crystalline phases. Our simulations indicate that GeSb, rather than VGe or SbTe, is the most important defect to explain p-doping in the hexagonal structure. In the disordered phase, on the other hand, standard gradient-corrected functionals yield low-formation energies for defects associated with n-type conduction, in apparent contradiction with experimental data. We discuss possible sources for this discrepancy and argue that it stems from the underestimation of the band gap.

Direct Visualization of the Earliest Stages of Crystallization.

Investigating the earliest stages of crystallization requires the transmission electron microscope (TEM) and is particularly challenging for materials which can be affected by the electron beam. Typically, when imaging at magnifications high enough to observe local crystallinity, the electron beam's current density must be high to produce adequate image contrast. Yet, minimizing the electron dose is necessary to reduce the changes caused by the beam. With the advent of a sensitive, high-speed, direct-detection camera for a TEM that is corrected for spherical aberration, it is possible to probe the early stages of crystallization at the atomic scale. High-quality images with low contrast can now be analyzed using new computing methods. In the present paper, this approach is illustrated for crystallization in a Ge2Sb2Te5 (GST-225) phase-change material which can undergo particularly rapid phase transformations and is sensitive to the electron beam. A thin (20 nm) film of GST-225 has been directly imaged in the TEM and the low-dose images processed using Python scripting to extract details of the nanoscale nuclei. Quantitative analysis of the processed images in a video sequence also allows the growth of such nuclei to be followed.

Structural transition on doping rare earth Sm to Ge2Sb2Te5 phase change material

The effect of doping rare earth element Sm on the structure of Ge2Sb2Te5 (GST) phase change material is studied. The X-ray diffraction (XRD) spectra of (Ge2Sb2Te5)100−xSmx(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) bulk alloys are analyzed to understand the impact of Sm doping. The Rietveld refinement of XRD results discloses the change in lattice parameters of the hexagonal phase of GST on Sm doping whereas no significant change in the fcc phase is observed. The quantitative phase analysis of GST with Sm doping is analyzed and the fraction of hexagonal phase shows an increase, while the fraction of the fcc phase reduces up to x = 0.4% Sm addition. The crystallite size and strain of GST obtained using the WH-plot increases with Sm content up to x = 0.4%. There is an increase in d103-spacing of the hexagonal phase on Sm incorporation. The change in the local structure of GST on Sm addition is studied employing Raman Spectroscopy and Far-IR spectroscopy. The local structure around Ge and Sb is observed to change as there is an increase in the fraction of octahedral Ge in comparison to the tetrahedral Ge with Sm content from x = 0 to x = 0.4 . This results in the lowering of homopolar Ge—Ge and Ge—Sb bonds due to the decrease in Ge rich tetrahedra from x = 0 to x = 0.4 of Sm addition.

Electrically controlled 1 × 2 tunable switch using a phase change material embedded silicon microring.

Phase change material Ge2Sb2Te5 (GST) has recently emerged as a highly promising candidate for photonic device applications owing to its high optical contrast, self-holding bi-stability, and fast material response. Here, we propose and analyze a 1×2 tunable switch using a GST embedded silicon microring resonator exploiting high optical contrast during GST phase change and a high thermo-optic coefficient of amorphous phase GST. Our device exhibits high extinction ratios of 25.57 dB and 18.75 dB at through and drop ports, respectively, with just a 1 µm long GST layer. The two states of the switch are realizable by electrically inducing phase change in GST. For post phase change from amorphous to crystalline and vice versa, the fall time down the 80% of phase transition temperature is ∼66ns and ∼45ns, respectively. The resonance wavelength shift per unit active length is 0.661 nm/µm, and the tuning efficiency is 1.16 nm/mW. The large wavelength tunability (4.63 nm) of the proposed switch makes it an attractive option for reconfigurable photonic integrated circuits.

Effect of local structure on the optical and dielectric behaviour of Sm doped GeSbTe phase change material

Abstract Ge2Sb2Te5 (GST) is considered to be one of the most prominent phase change materials for data storage applications. The UV–Vis–NIR transmittance spectra of (Ge2Sb2Te5)100-xSmx (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) thin films are utilized to analyze the effect of rare earth element Sm on the optical behaviour of GST. The linear and nonlinear optical studies of GST on Sm incorporation provide the understanding of electron transport properties and band structural details. The optical parameters such as absorption coefficient (α) and the refractive index are obtained through transmission data and are analyzed for Sm doped GST thin films. There is significant change of the absorption coefficient near the optical band gap for (Ge2Sb2Te5)100-xSmx thin films. The optical dispersion parameters are analyzed to understand the material's structure. The optical band gap, Urbach energy, and disorder of Sm doped GST thin films are analyzed for band structure modification details on Sm addition. The free career concentration to electron effective mass ratio and high-frequency dielectric constant for (Ge2Sb2Te5)100-xSmx thin films are analyzed. Optical conductivity, dielectric constant, surface and bulk energy loss function and tan δ loss function for (Ge2Sb2Te5)100-xSmx thin films are obtained. The optical non-linear parameters are analyzed to understand the dependence of the polarizability of GST on Sm addition. The incorporation of Sm leads to the change in local bonding environment which may improve the switching mechanism of phase change memory devices.

Obtaining glasses in the extremely crystallizing Ge–Sb–Te phase change material

Using thermal co-evaporation techniques, we show that various glassy compositions can be obtained along the GexSbxTe100-2x join in the ternary Ge–Sb–Te system which is known to display dramatic crystallization tendencies. Earlier attempts to produce bulk glasses have been limited to Sb-poor compositions close to the eutectic GeTe6. Results indicate a weak variation of Tg with composition x and also a thermal stability that is weak for most systems, and especially for compositions close to the domain where Ge2Sb2Te5 can be formed. Our results are put in perspective with other network-forming chalcogenides and the Tg variation suggests the preferential formation of Te–Sb–Te, at variance with isochemical compounds such as Ge–Sb–Se. Data are discussed within the framework of topological based approaches which not only indicate that optimal glass formation is achieved for compositions satisfying the Maxwell stability criterion but also predict a flexible to rigid transition at x=8.5%. Most of our glasses could be formed around this composition.

Enhancing the surface morphology for improved phase change mechanism by Sm doping in Ge2Sb2Te5 thin films

The effect of doping of rare earth element Sm to Ge2Sb2Te5 (GST) phase change material (PCM) on the surface morphology in (Ge2Sb2Te5)100−xSmx (x = 0, 0.6, 1.2) thin films is investigated employing atomic force microscopy (AFM). The incorporation of Sm to GST thin films indicates composition dependent grain size distribution and surface roughness. The Sm doping impacts the nucleation dominated crystallization mechanism of GST phase change material. The root mean square roughness value in AFM micrographs of the thin films decreases with Sm content. The RMS roughness (Rq) to average roughness (Ra) ratio value 1.119 for x = 0.6% of Sm-doped GST thin films, points to the Gaussian distribution of grain sizes. The average grain size of (Ge2Sb2Te5)100−xSmx (x = 0, 0.6, 1.2) thin films shows a decrease on Sm incorporation which may be attributed to the compositional dependence of crystallization activation energy for phase change. Few sharp spikes on the thin film for x = 1.2 of Sm addition to GST suggest an increase in the hexagonal phase as compared to the fcc phase which may alter the mechanism of phase transition in these PCMs.

High-efficiency reflection phase tunable metasurface at near-infrared frequencies* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0200400, 2016YFA0200800, and 2016YFA0300601), the National Natural Science Foundation of China (Grant Nos. 61888102,11674387, 11974386, and 61905274), the Strategic Priority Research Program and Key Research Program of Frontier Sciences of Chinese Academy of Sciences (Grant Nos. XDB33000000, XDB28000000,

The realization of active modulation of reflection phase based on metasurfaces is of great significance for flexible control of electromagnetic wavefront, which makes metasurfaces have practical application values in polarization conversion, beam steering, metalens, etc. In this paper, a reflection phase tunable gap-surface plasmon (GSP) metasurface based on phase change materials Ge2Sb2Te5 (GST) is designed and experimentally demonstrated. By virtue of the characteristics of large permittivities difference before and after GST phase transition and the existence of stable intermediate states, the continuous modulation of near-infrared reflection phase larger than 200° has been realized. At the same time, through the reasonable design of the structure sizes, the reflection has been maintained at about 0.4 and basically does not change with the GST phase transition, which improved the working efficiency of the metasurface significantly. In addition, the coupled-mode theory (CMT) is introduced to make a full analysis of the modulation mechanism of the reflection phase, which proves that the phase transition of GST can induce the transition of metasurface working state from overcoupling mode to critical coupling mode. The improvement of the metasurface working efficiency has practical values for wavefront modulation.

Mechanical characterization and properties of continuous wave laser irradiated Ge2Sb2Te5 stripes

Crystalline micrometer size stripes in 2.2 μm thick Ge2Sb2Te5 phase-change material films were produced by irradiation with a Continuous Wave Laser of 405 nm wavelength. The shape and the dimensions of the crystallized regions were investigated by Transmission Electron Microscopy and then compared with simulations based on temperature-crystal growth velocity literature data. The temperature-time profile was determined taking into account the laser power, the optical and thermal properties of both the amorphous and crystalline phase. The mechanical properties of the amorphous and of the crystallized regions were characterized by an ultra nano-indentation technique. This procedure allows a direct and local measurement of hardness and Young's modulus in the amorphous and in the contiguous crystalline regions on the micrometer scale. The following values for Young's modulus and for hardness were obtained: 33±4 GPa and 2.00±0.3 GPa for the amorphous phase, and 51±8 GPa and 2.90±0.45 GPa for the crystalline phase. The stresses induced by the density increase in the crystallized region cause, on the irradiated surface, a series of fracture whose characteristic behavior depends on the laser power and on the spacing between two contiguous scans. These results are of relevance for the mechanical failure mechanisms in potential phase-change devices.

Nonvolatile and reconfigurable tuning of surface lattice resonances using phase-change Ge2Sb2Te5 thin films

We propose a novel configuration for nonvolatile and reconfigurable tuning of surface lattice resonances (SLRs) based on thin films of phase-change material Ge2Sb2Te5 (GST). The configuration is composed of gold nanorod array embedded in a thin GST film. Results show that, an extremely wide SLR wavelength tuning range reaching 1600 nm, large tuning figure of merit of 21, and high SLR quality factors of 47 and 17 can be achieved in the mid-infrared regime when GST transits from the amorphous state into the crystalline state. The effects of the GST layer thicknesses on the SLR tuning performance are discussed, and the performances of several modified configurations are compared. We expect this work will advance the engineering of SLR tuning based on phase-change materials and promote its applications especially in nanolasing and narrowband filtering.

Tunable Thermal Camouflage Based on GST Plasmonic Metamaterial.

Thermal radiation control has attracted increasing attention in a wide range of field, including infrared detection, radiative cooling, thermal management, and thermal camouflage. Previously reported thermal emitters for thermal camouflage presented disadvantages of lacking either tunability or thermal stability. In this paper, we propose a tunable thermal emitter consisting of metal-insulator-metal (MIM) plasmonic metamaterial based on phase-change material Ge2Sb2Te5 (GST) to realize tunable control of thermal radiation in wavelength ranges from 3 μm to 14 μm. Meanwhile, the proposed thermal emitter possesses near unity emissivity at the wavelength of 6.3 μm to increase radiation heat dissipation, maintaining the thermal stability of the system. The underlying mechanism relies on fundamental magnetic resonance and the interaction between the high-order magnetic resonance and anti-reflection resonance. When the environmental background is blackbody, the tunable emitter maintains signal reduction rates greater than 80% in middle-IR and longer-IR regions from 450 K to 800 K and from room temperature to 800 K, respectively. The dependences of thermal camouflage on crystallization fraction of GST, incident angles and polarization angles have been investigated in detail. In addition, the thermal emitter can continuously realize thermal camouflage for various background temperatures and environmental background in atmospheric window in the range of 3–5 μm.

Programmable phase-change metasurfaces on waveguides for multimode photonic convolutional neural network

Neuromorphic photonics has recently emerged as a promising hardware accelerator, with significant potential speed and energy advantages over digital electronics for machine learning algorithms, such as neural networks of various types. Integrated photonic networks are particularly powerful in performing analog computing of matrix-vector multiplication (MVM) as they afford unparalleled speed and bandwidth density for data transmission. Incorporating nonvolatile phase-change materials in integrated photonic devices enables indispensable programming and in-memory computing capabilities for on-chip optical computing. Here, we demonstrate a multimode photonic computing core consisting of an array of programable mode converters based on on-waveguide metasurfaces made of phase-change materials. The programmable converters utilize the refractive index change of the phase-change material Ge2Sb2Te5 during phase transition to control the waveguide spatial modes with a very high precision of up to 64 levels in modal contrast. This contrast is used to represent the matrix elements, with 6-bit resolution and both positive and negative values, to perform MVM computation in neural network algorithms. We demonstrate a prototypical optical convolutional neural network that can perform image processing and recognition tasks with high accuracy. With a broad operation bandwidth and a compact device footprint, the demonstrated multimode photonic core is promising toward large-scale photonic neural networks with ultrahigh computation throughputs.

Comments on the electronic transport mechanisms in the crystalline state of Ge—Sb—Te phase-change materials

It is known that phase-change materials, such as Ge–Sb—Te ternary system, are promising resistive non-volatile random-access memory applications with ultra-rapid reversible transformations between the crystalline and amorphous phases. This class of electronic transition is categorized to be the metal-insulator transition (MIT). The Anderson-type MIT has been discussed extensively in phase-change materials (PCMs) and isothermal annealing of amorphous PCMs (a-PCMs) which, above a certain temperature, leads to the metallic (crystalline) phase. In the insulator regime near the MI transition, Mott-type variable-range hopping (VRH) and/or Efros-Shkolvskii hopping (ESH) at low temperatures below 20 K (and down to 1 K) in Ge1Sb2Te4 (GST124) have been discussed extensively, however, we criticize the above argument through a detailed discussion of physical parameters that support the VRH mechanism. It is not clear whether or not the density-of-states (DOS) near the Fermi level is localized (like the Fermi glass) in the crystalline phase. It is also suggested that grain boundaries are expected to interfere with the electronic transport in the crystalline state. We should take into account the grain boundary effects on the electronic transport in the crystalline phase of Ge—Sb—Te system.

Switching between singular points in non-PT-symmetric multilayer structures using phase-change materials

We investigate the switching between singular points in non-parity-time-symmetric multilayer structures using phase-change materials at the optical communication wavelength. We first show that absorbing singularities can be switched to exceptional points (EPs) in a two-layer structure consisting of a phase-change material layer and a lossy layer by switching the phase-change material Ge2Sb2Te5 (GST) from its crystalline to its amorphous phase. We also show that spectral singularities (SSs) can be switched to EPs in a three-layer structure consisting of a lossless dielectric layer sandwiched between a GST layer and a gain layer by switching the GST from its crystalline to its amorphous phase. We then show that self-dual SSs can be switched to unidirectional spectral singularities in a three-layer structure consisting of a lossy layer sandwiched between a GST layer and a gain layer by switching the GST from its amorphous to its crystalline phase. In addition, at the unidirectional spectral singularity, zero reflection from one side and infinite reflection from the opposite side are simultaneously realized. We finally show that we can design an active device with large modulation depth achieved by a very small variation of the imaginary part of the refractive index of the active absorbing material in the lossy layer. Our results could potentially contribute to the development of a new generation of singularity-enhanced switchable optical devices.

Nonvolatile, Reconfigurable and Narrowband Mid-Infrared Filter Based on Surface Lattice Resonance in Phase-Change Ge2Sb2Te5.

We propose a nonvolatile, reconfigurable, and narrowband mid-infrared bandpass filter based on surface lattice resonance in phase-change material GeSbTe. The proposed filter is composed of a two-dimensional gold nanorod array embedded in a thick GeSbTe film. Results show that when GeSbTe transits from the amorphous state to the crystalline state, the narrowband reflection spectrum of the proposed filter is tuned from 3.197 m to 4.795 m, covering the majority of the mid-infrared regime, the peak reflectance decreases from 72.6% to 25.8%, and the corresponding quality factor decreases from 19.6 to 10.3. We show that the spectral tuning range can be adjusted by varying the incidence angle or the lattice period. By properly designing the gold nanorod sizes, we also show that the quality factor can be greatly increased to 70 at the cost of relatively smaller peak reflection efficiencies, and that the peak reflection efficiency can be further increased to 80% at the cost of relatively smaller quality factors. We expect that this work will advance the engineering of GeSbTe-based nonvalatile tunable surface lattice resonances and will promote their applications especially in reconfigurable narrowband filters.

Progress in metasurfaces based on Ge–Sb–Te phase-change materials

Recently, metasurfaces based on phase-change materials (PCMs) have attracted increasing attention due to the dramatic optical properties contrast between amorphous and crystalline states. The chalcogenide PCMs can be reversibly switched by electrical or optical pulses, offering tunability and reconfigurability for the metasurfaces. In this Perspective, the latest achievements and ongoing development in reconfigurable metasurfaces based on chalcogenide PCMs are presented, including the applications in nonlinear optics, anapole control, beam steering, perfect absorbers, and polaritons. This Perspective ends with perspectives for the growing demands of PCMs based on metasurfaces.