Germanium Antimony Research Articles

Reversible modulations of insulator–metal transition in an epitaxial VO2 film through thermal crystallization and femtosecond laser-induced-amorphization of capping Ge2Sb2Te5 layer

We demonstrate reversible modulation of an insulator–metal transition (IMT) of a VO2 film grown on an Al2O3 (001) substrate through crystallization and re-amorphization of a chalcogenide germanium–antimony–telluride (Ge2Sb2Te5: GST) capping layer. After succeeding in the negative shift of IMT temperature (Tr) of the VO2 film through the crystallization of the GST layer accompanied by volume reduction, we performed re-amorphization of the crystalline GST by femtosecond laser irradiation. Under the optimized conditions of laser irradiation considering the penetration depth, re-amorphization of the GST layer was fully achieved, resulting in the shift-back of Tr toward a high-temperature side. Such a reversal of IMT through the crystallization and re-amorphization of the capping GST layer was demonstrated over two cycles. It was suggested that capping GST effectively induces interfacial strain modifications in the VO2 film underneath. Although the shifts in the IMT are still small, reversible modulation of IMT shown here will be beneficial for applications of VO2 films with controllable IMT.

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.

Device-scale atomistic modelling of phase-change memory materials

Computer simulations can play a central role in the understanding of phase-change materials and the development of advanced memory technologies. However, direct quantum-mechanical simulations are limited to simplified models containing a few hundred or thousand atoms. Here we report a machine-learning-based potential model that is trained using quantum-mechanical data and can be used to simulate a range of germanium–antimony–tellurium compositions—typical phase-change materials—under realistic device conditions. The speed of our model enables atomistic simulations of multiple thermal cycles and delicate operations for neuro-inspired computing, specifically cumulative SET and iterative RESET. A device-scale (40 × 20 × 20 nm3) model containing over half a million atoms shows that our machine-learning approach can directly describe technologically relevant processes in memory devices based on phase-change materials.

Systematic Study on Electronic, Mechanical, and Thermal Transport Properties of Germanium Antimony Selenide Telluride Alloy by a First-Principles Approach

Systematic Study on Electronic, Mechanical, and Thermal Transport Properties of Germanium Antimony Selenide Telluride Alloy by a First-Principles Approach

Discrete thermokinetic computational model of laser-induced phase transitions in phase-changing materials

This paper presents a thermokinetic computational model of phase transitions in GST225 (germanium–antimony–tellurium) thin films [as well as other phase change materials (PCMs)] induced and initiated by the impact of nano- and femtosecond laser pulses in a wide energy fluence range according to the results of experimental studies using Raman spectra and thin-film samples of TEM cross-sectional image analysis. Applying this phase transition model makes it possible to understand the mechanism of the induced phase transition regarding the usage of PCMs in photonics and optoelectronic devices, which require precise control of the phase state of their PCM-based active elements for their functioning. The proposed model shows the internal structure of the sample, generating both the profile of the crystalline fraction distribution over the sample's depth, providing images of virtual TEM sections, as well as the volume distribution of the crystalline phase.

Biomarker detection using GST-based permittivity-asymmetric metasurface

Non-invasive detection of biomarkers can help in the early screening of lethal diseases, such as cancer and Alzheimer's. In this paper, we present a numerical study of a high-quality resonant permittivity-asymmetric metasurface that offers tunability in the mid-infrared range by employing the phase change property of Germanium Antimony Telluride, Ge3Sb2Te6 (GST326). In this approach, we used dissimilar materials of ellipse-shaped nanopillars, and shifted the nanopillars from their center positions to break the symmetry-protected bound state and generate a high-Q resonance sufficient for detecting trace amounts of analytes. The peak location of the resonance can be controlled by tuning the crystallization state of GST326 for broadband retrieval of analyte signatures. We performed a grid search to optimize the design parameters to obtain a tuning range that matches the absorption band of amide I and II groups. As a proof of concept, we demonstrate sensing of a thin protein layer using the proposed metasurface and we show how our structure is sensitive to the imaginary part of the analyte refractive index and how the permittivity-asymmetry resonance is robust against crystallization losses. We also propose a layout for a practical device that can be used for biomarker detection applications.

Endotaxial Intergrowth of Copper Telluride in GeTe-Rich Germanium Antimony Tellurides Leads to High Thermoelectric Performance

In composite materials with nominal compositions Cu2GexSb2Tex+4 (11 ≤ x ≤ 18, i.e., between Cu6.7Ge36.7Sb6.7Te50 and Cu4.5Ge40.9Sb4.5Te50), precipitates consisting of copper tellurides are endotaxially intergrown in a matrix of Cu-doped germanium antimony tellurides. The precipitates as well as the matrix material undergo various phase transitions as shown by temperature-dependent X-ray diffraction and X-ray absorption contrast imaging. Eventually, the precipitates dissolve in the matrix at temperatures exceeding 580 °C. The temperature-dependent behavior was also traced by photoemission electron microscopy up to 460 °C. At high temperatures, the thermoelectric properties are superior to those of pure germanium antimony tellurides obtained by comparable syntheses; a maximal zT value of 1.83 for Cu2Ge16Sb2Te20 is reached at 500 °C. The application of an effective mass model reveals optimal charge carrier concentrations for all three compositions investigated. The p-type Cu2Ge16Sb2Te20 material was used in combination with PbTe:In (n-type) to construct a thermoelectric module. Concludingly, the measurement of the Hall effect that suggests no significant changes in Cu-doping levels of the matrix with temperature application of grain boundary optimization and a temperature-induced reset of the microstructure are proposed as strategies for overcoming material degradation upon applying electrical currents.

Preparation of high-purity chalcogenide glasses containing gallium(III) sulfide

A technique for preparation of high-purity Ga-Ge-S and Ga-Sb-S glasses is developed. The method includes two main stages: synthesis of gallium(III) sulfide by interaction of gallium(III) iodide with sulfur in an evacuated silica-glass reactor; synthesis and loading of germanium(II) sulfide and antimony(III) sulfide in the reactor. Thermodynamic modeling of Ge-S and Sb-S systems is carried out. Optimum conditions for the synthesis of germanium and antimony sulfides as components of the chalcogenide charge are theoretically and experimentally determined. High-purity Ga5Ge35S60 and Ga8Sb32S60 glasses with the 0.2–0.6 ppm(at) hydrogen impurity content in the form of SH-groups and heterogeneous micron and submicron-sized impurity inclusions of less than 102 pieces/cm3 are prepared.

A Systematic Study on Electronic, Mechanical, and Thermal Transport Properties of Germanium Antimony Selenide Telluride Alloy by First-Principles Approach

A Systematic Study on Electronic, Mechanical, and Thermal Transport Properties of Germanium Antimony Selenide Telluride Alloy by First-Principles Approach

Impacts from triple phases of a germanium–antimony–tellurium film coating on thermal emission from SiO2 and boron doped Si

This work experimentally demonstrates mid-infrared emittance spectra of dielectric and semi-conductor substrates with and without a germanium–antimony–tellurium (GST) film coating. The film experiences non-volatile phase changes at 140°C and 300°C. Impacts from amorphous, face-centered cubic, and hexagonal close packed phases on spectral emittance are demonstrated within the spectral range from 4 μm to 18 μm. The spectra are measured at 100°C, 200°C, 300°C, and 400°C to show temperature dependence. Close-to-total emittance is calculated for comparison. The GST film can reduce emittance from a SiO2 substrate, but it raises close-to-normal emittance as well as the spectral emittance at wavelengths 5 μm ≤ λ ≤ 18 μm for the doped Si substrate.

Thermal conductivity of (Ge2Sb2Te5)1−xCx phase change films

Germanium–antimony–telluride has emerged as a nonvolatile phase change memory material due to the large resistivity contrast between amorphous and crystalline states, rapid crystallization, and cyclic endurance. Improving thermal phase stability, however, has necessitated further alloying with optional addition of a quaternary species (e.g., C). Here, the thermal transport implications of this additional species are investigated using frequency-domain thermoreflectance in combination with structural characterization derived from x-ray diffraction and Raman spectroscopy. Specifically, the room temperature thermal conductivity and heat capacity of (Ge2Sb2Te5)1−xCx are reported as a function of carbon concentration (x≤0.12) and anneal temperature (T≤350°C) with results assessed in reference to the measured phase, structure, and electronic resistivity. Phase stability imparted by the carbon comes with comparatively low thermal penalty as materials exhibiting similar levels of crystallinity have comparable thermal conductivity despite the addition of carbon. The additional thermal stability provided by the carbon does, however, necessitate higher anneal temperatures to achieve similar levels of structural order.

Review on recent progress in patterning phase change materials

This review discusses critical aspects of patterning phase change materials (PCMs), including dry etching, wet clean, and encapsulation, as they dictate the reliability and functionality of the phase change random access memory devices. Specifically, alloys of germanium–antimony–tellurium are used as a model system, and the importance of PCM composition control, critical dimension control, high fidelity pattern transfer, and a system level of ambient control to avoid oxidation that can alter the materials’ functionality are highlighted. The research findings motivate the development of a state-of-the-art integrated system that combines dry etch, wet clean, and encapsulation into one platform to realize consistent and successful patterning of PCMs for future generations of the memory devices.

“All-crystalline” phase transition in nonmetal doped germanium–antimony–tellurium films for high-temperature non-volatile photonic applications

Phase-change materials have attracted much attention in the past three years due to its wide applications in non-volatile optical fields. Although the amorphous-cubic phase transition of Ge2Sb2Te5 is used most commonly, it has problems such as poor thermal stability and large density variation because the amorphous phase is metastable and of low density. Since these issues have not been well addressed, non-volatile optics are currently limited to applications in mild environments (≤120 °C). In this paper, we replace the traditional phase transition (amorphous-cubic phase) with the “all-crystalline” phase transition (cubic-hexagonal phase) that has been ignored in the past, achieving combination of high optical contrast (ΔR=10.3%), high thermal stability (TL = 240 °C) and ultra-small density variation (Δρ = 0.5%). The maximum service temperature of non-volatile optics increases from 120 to 240 °C. Our results from experiments, theoretical calculations and spectral fittings are in good agreement, proving that the key to achieve “all-crystalline” phase transition with high optical contrast is to increase the difference in structural disorder between cubic and hexagonal phases. An effective method for increasing this disorder difference is the small atom (N or C) doping rather than large atom (Ag) doping that previously thought. The new insights have been well explained by discussing relationships among doping, formation energy, structural disorder and optical contrast. Therefore, our research not only proposes new methods and mechanisms for achieving high-performance “all-crystalline” phase transition, but also extends the applications of non-volatile optical devices from mild to high-temperature environments in the fields of aerospace and military.

Investigation of crystallization behavior and structure of nanocomposite multilayer phase change thin films with zinc antimony and germanium antimony

Multilayer Zn50Sb50/Ge8Sb92 (ZS/GS) thin films were proposed and fabricated by alternate sputtering. The crystallization behaviors of nanocomposite ZS/GS thin films induced by heating were investigated by in situ resistance measurement. The crystallization behaviors of ZS/GS thin films can be regulated by tuning the thickness ratio and periods. The multilayer [ZS (8 nm)/GS (4 nm)]4 ([ZS(8)/GS(4)]4) thin film exhibits a high crystallization temperature (~ 245 °C), a large crystallization activation energy (2.61 eV), and high data retention ability (168 °C). The phase structure and thickness variation of the [ZS(8)/GS(4)]4 thin film were analyzed through x-ray diffraction (XRD) and x-ray reflection (XRR), respectively. The volume shrinkage during crystallization was as small as 3.9%. The multilayer structure and interface stability were confirmed by using transmission electron microscopy (TEM). A picosecond laser pump-probe system was used to evaluate the transition speed. The crystallization and amorphization periods of the [ZS(8)/GS(4)]4 thin film were only about 6.2 ns and 3.9 ns, respectively.

Germanium antimony quantum dots morphology and Raman spectroscopy fabricated by inert gas condensation

Abstract Phase change materials (PCM) based on nanostructure are attractive interest for non-volatile memory due to their high data–storage density and low power consumption. Germanium antimony (Ge-Sb) quantum dots (QDs) with tunable size and density have been fabricated on mica, silicon, and quartz glass substrates by magnetron sputtering with inert gas condensation. The deposition time rate plays an important role in the formation of the monodisperse or aggregate quantum dots. The dots’ morphology, in terms of size and density as observed by atomic force microscopy (AFM) strongly depends on the deposition time rate. The size of QDs was observed by (AFM) images topography between 1.6 and 3.2 nm. Raman spectroscopy measurements reveal that Ge-Sb phonon at 250 cm−1 (Lo mode) and 220 cm−1 (To mode). Also, strong Sb-Sb crystallized peaks positioned at about 145 cm−1 and 110 cm−1 is observed.

Investigation of optical and electrical properties of Cobalt-doped Ge-Sb-S thin film

Abstract Amorphous Germanium Antimony Sulphide (Ge-Sb-S) doped with Cobalt (Co) have been deposited on glass substrates by thermal evaporation technique on a glass substrate. The films deposited onto glass substrates are characterized by Energy Dispersive X-ray Fluorescence Spectrometer, UV–VIS spectrophotometer, Raman spectroscopy, and Capacitance-Voltage Keithley meter. The optical band gap was calculated from the UV–Visible spectrum and found to be 2.05 eV. Raman spectroscopy measurements reveal that a wide band spectrum from 300 to 410 cm−1 centered at 355 cm−1. The Raman shift peaks at 325 cm−1 and 350 cm−1 are as-signed to the bond stretching mode Sb-S and Ge-S, respectively. In addition, from the obtained Raman spectra it is concluded that the presence of Co doped with Ge-Sb-S. The capacitance and conductance versus voltage measurements were performed at different temperatures. The results show a slight increase in the capacitance with temperature and it reaches a maximum value around 150 °C, and eventually it becomes negative. This behavior is interpreted in terms of the nucleation growth process and the thermally activated conduction process with measured activation energy of 0.79 eV.

Tuning the Vacancy Concentration in Lithium Germanium Antimony Tellurides—Influence on Phase Transitions, Lithium Mobility, and Thermoelectric Properties

In the solid solution series Li2–xGe3+1/2xSb2Te7 and Li2–xGe11+1/2xSb2Te15 (0 ≤ x ≤ 2), the heterovalent substitution gradually changes the vacancy concentration on the cation position from 0% (for...

Power Factor of Germanium Antimony Tellurium Thin Film on Al<sub>2</sub>O<sub>3</sub> Ceramic Substrate Deposited by Pulsed–DC<i></i> Magnetron Sputtering

Germanium–Antimony–Telluride (Ge–Sb–Te) has low electrical resistivity and thermal conductivity for good thermoelectric properties. The Ge–Sb–Te thin films were deposited on Al2O3 ceramic substrate by pulsed–dc magnetron sputtering system using a 99.99 % Ge:Sb:Te of 1:1:1 composite target and annealed at 573, 623, 673, and 723 K for 1 hour in vacuum. The phase identification, atomic composition, morphology and film thickness (d), carrier concentration (n), mobility (µ), Seebeck coefficient (S) and electrical resistivity (ρ) of the as–deposited and the annealed samples were investigated by X–ray diffraction (XRD), energy dispersive X–ray spectroscopy (EDX), field–emission scanning electron microscopy (FE–SEM), Hall–effect measurement, steady state method and calculation of from n and µ, respectively. The results demonstrated that the as–deposited Ge–Sb–Te film showed amorphous phase and annealing changed the phase crystalline. Morphologies of annealed Ge–Sb–Te films showed very large grain size and porosity to obtaining good n and µ. The approximately maximum power factor (P) was 4.22×10−4 W m−1 K−2 at annealing temperature of 723 K.

Optically reconfigurable metasurfaces and photonic devices based on phase change materials

Photonic components with adjustable parameters, such as variable-focal-length lenses or spectral filters, which can change functionality upon optical stimulation, could offer numerous useful applications. Tuning of such components is conventionally achieved by either micro- or nanomechanical actuation of their constituent parts, by stretching or by heating. Here, we report a novel approach for making reconfigurable optical components that are created with light in a non-volatile and reversible fashion. Such components are written, erased and rewritten as two-dimensional binary or greyscale patterns into a nanoscale film of phase-change material by inducing a refractive-index-changing phase transition with tailored trains of femtosecond pulses. We combine germanium–antimony–tellurium-based films with a diffraction-limited resolution optical writing process to demonstrate a variety of devices: visible-range reconfigurable bichromatic and multi-focus Fresnel zone plates, a super-oscillatory lens with subwavelength focus, a greyscale hologram, and a dielectric metamaterial with on-demand reflection and transmission resonances.

High Thermoelectric Figure of Merit Values of Germanium Antimony Tellurides with Kinetically Stable Cobalt Germanide Precipitates.

Heterostructures that consist of a germanium antimony telluride matrix and cobalt germanide precipitates can be obtained by straightforward solid-state synthesis including simple annealing and quenching procedures. The microscale precipitates are homogeneously distributed in a matrix with pronounced "herringbone-like" nanostructure associated with very low thermal conductivities. In comparison to the corresponding pure tellurides, the figure of merit (ZT) values of heterostructured materials are remarkably higher. This is mostly due to an increase of the Seebeck coefficient with only little impact on the electrical conductivity. In addition, the phononic part of the thermal conductivity is significantly reduced in some of the materials. As a result, ZT values of ca. 1.9 at 450 °C are achieved. Temperature-dependent changes of the thermoelectric properties are well-understood and correlate with complex phase transitions of the telluride matrix. However, the high ZT values are retained in multiple measurement cycles.