Editorial

3 pages, 191 KiB

Open AccessEditorial

Synthesis, Properties and Applications of Germanium Chalcogenides

by Stefania M. S. Privitera

Nanomaterials 2022, 12(17), 2925; https://doi.org/10.3390/nano12172925 - 25 Aug 2022

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Germanium (Ge) chalcogenides are characterized by unique properties which make these materials interesting for a very wide range of applications, from phase change memories to ovonic threshold switches, from photonics to thermoelectric and photovoltaic devices [...] Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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9 pages, 2284 KiB

Open AccessArticle

Structural and Electrical Properties of Annealed Ge₂Sb₂Te₅ Films Grown on Flexible Polyimide

by Marco Bertelli, Adriano Díaz Fattorini, Sara De Simone, Sabrina Calvi, Riccardo Plebani, Valentina Mussi, Fabrizio Arciprete, Raffaella Calarco and Massimo Longo

Nanomaterials 2022, 12(12), 2001; https://doi.org/10.3390/nano12122001 - 10 Jun 2022

Cited by 4 | Viewed by 1725

Abstract

The morphological, structural, and electrical properties of as-grown and annealed Ge₂Sb₂Te₅ (GST) layers, deposited by RF-sputtering on flexible polyimide, were studied by means of optical microscopy, atomic force microscopy, X-ray diffraction, Raman spectroscopy, and electrical characterization. The X-ray [...] Read more.

The morphological, structural, and electrical properties of as-grown and annealed Ge₂Sb₂Te₅ (GST) layers, deposited by RF-sputtering on flexible polyimide, were studied by means of optical microscopy, atomic force microscopy, X-ray diffraction, Raman spectroscopy, and electrical characterization. The X-ray diffraction annealing experiments showed the structural transformation of GST layers from the as-grown amorphous state into their crystalline cubic and trigonal phases. The onset of crystallization of the GST films was inferred at about 140 °C. The vibrational properties of the crystalline GST layers were investigated via Raman spectroscopy with mode assignment in agreement with previous works on GST films grown on rigid substrates. The electrical characterization revealed a good homogeneity of the amorphous and crystalline trigonal GST with an electrical resistance contrast of 8 × 10⁶. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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14 pages, 14743 KiB

Open AccessArticle

Phase Separation in Ge-Rich GeSbTe at Different Length Scales: Melt-Quenched Bulk versus Annealed Thin Films

by Daniel Tadesse Yimam, A. J. T. Van Der Ree, Omar Abou El Kheir, Jamo Momand, Majid Ahmadi, George Palasantzas, Marco Bernasconi and Bart J. Kooi

Nanomaterials 2022, 12(10), 1717; https://doi.org/10.3390/nano12101717 - 18 May 2022

Cited by 4 | Viewed by 2045

Abstract

Integration of the prototypical GeSbTe (GST) ternary alloys, especially on the GeTe-Sb₂Te₃ tie-line, into non-volatile memory and nanophotonic devices is a relatively mature field of study. Nevertheless, the search for the next best active material with outstanding properties is still [...] Read more.

Integration of the prototypical GeSbTe (GST) ternary alloys, especially on the GeTe-Sb₂Te₃ tie-line, into non-volatile memory and nanophotonic devices is a relatively mature field of study. Nevertheless, the search for the next best active material with outstanding properties is still ongoing. This search is relatively crucial for embedded memory applications where the crystallization temperature of the active material has to be higher to surpass the soldering threshold. Increasing the Ge content in the GST alloys seems promising due to the associated higher crystallization temperatures. However, homogeneous Ge-rich GST in the as-deposited condition is thermodynamically unstable, and phase separation upon annealing is unavoidable. This phase separation reduces endurance and is detrimental in fully integrating the alloys into active memory devices. This work investigated the phase separation of Ge-rich GST alloys, specifically Ge₅Sb₂Te₃ or GST523, into multiple (meta)stable phases at different length scales in melt-quenched bulk and annealed thin film. Electron microscopy-based techniques were used in our work for chemical mapping and elemental composition analysis to show the formation of multiple phases. Our results show the formation of alloys such as GST213 and GST324 in all length scales. Furthermore, the alloy compositions and the observed phase separation pathways agree to a large extent with theoretical results from density functional theory calculations. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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16 pages, 5816 KiB

Open AccessArticle

Structural Assessment of Interfaces in Projected Phase-Change Memory

by Valeria Bragaglia, Vara Prasad Jonnalagadda, Marilyne Sousa, Syed Ghazi Sarwat, Benedikt Kersting and Abu Sebastian

Nanomaterials 2022, 12(10), 1702; https://doi.org/10.3390/nano12101702 - 17 May 2022

Cited by 2 | Viewed by 1746

Abstract

Non-volatile memories based on phase-change materials have gained ground for applications in analog in-memory computing. Nonetheless, non-idealities inherent to the material result in device resistance variations that impair the achievable numerical precision. Projected-type phase-change memory devices reduce these non-idealities. In a projected phase-change [...] Read more.

Non-volatile memories based on phase-change materials have gained ground for applications in analog in-memory computing. Nonetheless, non-idealities inherent to the material result in device resistance variations that impair the achievable numerical precision. Projected-type phase-change memory devices reduce these non-idealities. In a projected phase-change memory, the phase-change storage mechanism is decoupled from the information retrieval process by using projection of the phase-change material’s phase configuration onto a projection liner. It has been suggested that the interface resistance between the phase-change material and the projection liner is an important parameter that dictates the efficacy of the projection. In this work, we establish a metrology framework to assess and understand the relevant structural properties of the interfaces in thin films contained in projected memory devices. Using X-ray reflectivity, X-ray diffraction and transmission electron microscopy, we investigate the quality of the interfaces and the layers’ properties. Using demonstrator examples of Sb and Sb₂Te₃ phase-change materials, new deposition routes as well as stack designs are proposed to enhance the phase-change material to a projection-liner interface and the robustness of material stacks in the devices. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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10 pages, 3554 KiB

Open AccessArticle

Interface Analysis of MOCVD Grown GeTe/Sb₂Te₃ and Ge-Rich Ge-Sb-Te/Sb₂Te₃ Core-Shell Nanowires

by Arun Kumar, Seyed Ariana Mirshokraee, Alessio Lamperti, Matteo Cantoni, Massimo Longo and Claudia Wiemer

Nanomaterials 2022, 12(10), 1623; https://doi.org/10.3390/nano12101623 - 10 May 2022

Cited by 1 | Viewed by 1675

Abstract

Controlling material thickness and element interdiffusion at the interface is crucial for many applications of core-shell nanowires. Herein, we report the thickness-controlled and conformal growth of a Sb₂Te₃ shell over GeTe and Ge-rich Ge-Sb-Te core nanowires synthesized via metal-organic chemical [...] Read more.

Controlling material thickness and element interdiffusion at the interface is crucial for many applications of core-shell nanowires. Herein, we report the thickness-controlled and conformal growth of a Sb₂Te₃ shell over GeTe and Ge-rich Ge-Sb-Te core nanowires synthesized via metal-organic chemical vapor deposition (MOCVD), catalyzed by the Vapor–Liquid–Solid (VLS) mechanism. The thickness of the Sb₂Te₃ shell could be adjusted by controlling the growth time without altering the nanowire morphology. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were employed to examine the surface morphology and the structure of the nanowires. The study aims to investigate the interdiffusion, intactness, as well as the oxidation state of the core-shell nanowires. Angle-resolved X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface chemistry of the nanowires. No elemental interdiffusion between the GeTe, Ge-rich Ge-Sb-Te cores, and Sb₂Te₃ shell of the nanowires was revealed. Chemical bonding between the core and the shell was observed. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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12 pages, 1672 KiB

Open AccessEditor’s ChoiceArticle

Growth, Electronic and Electrical Characterization of Ge-Rich Ge–Sb–Te Alloy

by Adriano Díaz Fattorini, Caroline Chèze, Iñaki López García, Christian Petrucci, Marco Bertelli, Flavia Righi Riva, Simone Prili, Stefania M. S. Privitera, Marzia Buscema, Antonella Sciuto, Salvatore Di Franco, Giuseppe D’Arrigo, Massimo Longo, Sara De Simone, Valentina Mussi, Ernesto Placidi, Marie-Claire Cyrille, Nguyet-Phuong Tran, Raffaella Calarco and Fabrizio Arciprete

Nanomaterials 2022, 12(8), 1340; https://doi.org/10.3390/nano12081340 - 13 Apr 2022

Cited by 6 | Viewed by 2558

Abstract

In this study, we deposit a Ge-rich Ge–Sb–Te alloy by physical vapor deposition (PVD) in the amorphous phase on silicon substrates. We study in-situ, by X-ray and ultraviolet photoemission spectroscopies (XPS and UPS), the electronic properties and carefully ascertain the alloy composition to [...] Read more.

In this study, we deposit a Ge-rich Ge–Sb–Te alloy by physical vapor deposition (PVD) in the amorphous phase on silicon substrates. We study in-situ, by X-ray and ultraviolet photoemission spectroscopies (XPS and UPS), the electronic properties and carefully ascertain the alloy composition to be GST 29 20 28. Subsequently, Raman spectroscopy is employed to corroborate the results from the photoemission study. X-ray diffraction is used upon annealing to study the crystallization of such an alloy and identify the effects of phase separation and segregation of crystalline Ge with the formation of grains along the [111] direction, as expected for such Ge-rich Ge–Sb–Te alloys. In addition, we report on the electrical characterization of single memory cells containing the Ge-rich Ge–Sb–Te alloy, including I-V characteristic curves, programming curves, and SET and RESET operation performance, as well as upon annealing temperature. A fair alignment of the electrical parameters with the current state-of-the-art of conventional (GeTe)_n-(Sb₂Te₃)_m alloys, deposited by PVD, is found, but with enhanced thermal stability, which allows for data retention up to 230 °C. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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12 pages, 3083 KiB

Open AccessArticle

Interface Formation during the Growth of Phase Change Material Heterostructures Based on Ge-Rich Ge-Sb-Te Alloys

by Caroline Chèze, Flavia Righi Riva, Giulia Di Bella, Ernesto Placidi, Simone Prili, Marco Bertelli, Adriano Diaz Fattorini, Massimo Longo, Raffaella Calarco, Marco Bernasconi, Omar Abou El Kheir and Fabrizio Arciprete

Nanomaterials 2022, 12(6), 1007; https://doi.org/10.3390/nano12061007 - 18 Mar 2022

Cited by 3 | Viewed by 2205

Abstract

In this study, we present a full characterization of the electronic properties of phase change material (PCM) double-layered heterostructures deposited on silicon substrates. Thin films of amorphous Ge-rich Ge-Sb-Te (GGST) alloys were grown by physical vapor deposition on Sb₂Te₃ and [...] Read more.

In this study, we present a full characterization of the electronic properties of phase change material (PCM) double-layered heterostructures deposited on silicon substrates. Thin films of amorphous Ge-rich Ge-Sb-Te (GGST) alloys were grown by physical vapor deposition on Sb₂Te₃ and on Ge₂Sb₂Te₅ layers. The two heterostructures were characterized in situ by X-ray and ultraviolet photoemission spectroscopies (XPS and UPS) during the formation of the interface between the first and the second layer (top GGST film). The evolution of the composition across the heterostructure interface and information on interdiffusion were obtained. We found that, for both cases, the final composition of the GGST layer was close to Ge₂SbTe₂ (GST212), which is a thermodynamically favorable off-stoichiometry GeSbTe alloy in the Sb-GeTe pseudobinary of the ternary phase diagram. Density functional theory calculations allowed us to calculate the density of states for the valence band of the amorphous phase of GST212, which was in good agreement with the experimental valence bands measured in situ by UPS. The same heterostructures were characterized by X-ray diffraction as a function of the annealing temperature. Differences in the crystallization process are discussed on the basis of the photoemission results. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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13 pages, 3425 KiB

Open AccessArticle

Crystallization and Electrical Properties of Ge-Rich GeSbTe Alloys

by Stefano Cecchi, Iñaki Lopez Garcia, Antonio M. Mio, Eugenio Zallo, Omar Abou El Kheir, Raffaella Calarco, Marco Bernasconi, Giuseppe Nicotra and Stefania M. S. Privitera

Nanomaterials 2022, 12(4), 631; https://doi.org/10.3390/nano12040631 - 14 Feb 2022

Cited by 11 | Viewed by 2890

Abstract

Enrichment of GeSbTe alloys with germanium has been proposed as a valid approach to increase the crystallization temperature and therefore to address high-temperature applications of non-volatile phase change memories, such as embedded or automotive applications. However, the tendency of Ge-rich GeSbTe alloys to [...] Read more.

Enrichment of GeSbTe alloys with germanium has been proposed as a valid approach to increase the crystallization temperature and therefore to address high-temperature applications of non-volatile phase change memories, such as embedded or automotive applications. However, the tendency of Ge-rich GeSbTe alloys to decompose with the segregation of pure Ge still calls for investigations on the basic mechanisms leading to element diffusion and compositional variations. With the purpose of identifying some possible routes to limit the Ge segregation, in this study, we investigate Ge-rich Sb₂Te₃ and Ge-rich Ge₂Sb₂Te₅ with low (<40 at %) or high (>40 at %) amounts of Ge. The formation of the crystalline phases has been followed as a function of annealing temperature by X-ray diffraction. The temperature dependence of electrical properties has been evaluated by in situ resistance measurements upon annealing up to 300 °C. The segregation and decomposition processes have been studied by scanning transmission electron microscopy (STEM) and discussed on the basis of density functional theory calculations. Among the studied compositions, Ge-rich Ge₂Sb₂Te₅ is found to be less prone to decompose with Ge segregation. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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10 pages, 2492 KiB

Open AccessArticle

Phase Change Ge-Rich Ge–Sb–Te/Sb₂Te₃ Core-Shell Nanowires by Metal Organic Chemical Vapor Deposition

by Arun Kumar, Raimondo Cecchini, Claudia Wiemer, Valentina Mussi, Sara De Simone, Raffaella Calarco, Mario Scuderi, Giuseppe Nicotra and Massimo Longo

Nanomaterials 2021, 11(12), 3358; https://doi.org/10.3390/nano11123358 - 10 Dec 2021

Cited by 4 | Viewed by 2552

Abstract

Ge-rich Ge–Sb–Te compounds are attractive materials for future phase change memories due to their greater crystallization temperature as it provides a wide range of applications. Herein, we report the self-assembled Ge-rich Ge–Sb–Te/Sb₂Te₃ core-shell nanowires grown by metal-organic chemical vapor deposition. [...] Read more.

Ge-rich Ge–Sb–Te compounds are attractive materials for future phase change memories due to their greater crystallization temperature as it provides a wide range of applications. Herein, we report the self-assembled Ge-rich Ge–Sb–Te/Sb₂Te₃ core-shell nanowires grown by metal-organic chemical vapor deposition. The core Ge-rich Ge–Sb–Te nanowires were self-assembled through the vapor–liquid–solid mechanism, catalyzed by Au nanoparticles on Si (100) and SiO₂/Si substrates; conformal overgrowth of the Sb₂Te₃ shell was subsequently performed at room temperature to realize the core-shell heterostructures. Both Ge-rich Ge–Sb–Te core and Ge-rich Ge–Sb–Te/Sb₂Te₃ core-shell nanowires were extensively characterized by means of scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, Raman microspectroscopy, and electron energy loss spectroscopy to analyze the surface morphology, crystalline structure, vibrational properties, and elemental composition. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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12 pages, 5896 KiB

Open AccessArticle

Tailoring the Structural and Optical Properties of Germanium Telluride Phase-Change Materials by Indium Incorporation

by Xudong Wang, Xueyang Shen, Suyang Sun and Wei Zhang

Nanomaterials 2021, 11(11), 3029; https://doi.org/10.3390/nano11113029 - 12 Nov 2021

Cited by 9 | Viewed by 2565

Abstract

Chalcogenide phase-change materials (PCMs) based random access memory (PCRAM) enter the global memory market as storage-class memory (SCM), holding great promise for future neuro-inspired computing and non-volatile photonic applications. The thermal stability of the amorphous phase of PCMs is a demanding property requiring [...] Read more.

Chalcogenide phase-change materials (PCMs) based random access memory (PCRAM) enter the global memory market as storage-class memory (SCM), holding great promise for future neuro-inspired computing and non-volatile photonic applications. The thermal stability of the amorphous phase of PCMs is a demanding property requiring further improvement. In this work, we focus on indium, an alloying ingredient extensively exploited in PCMs. Starting from the prototype GeTe alloy, we incorporated indium to form three typical compositions along the InTe-GeTe tie line: InGe₃Te₄, InGeTe₂ and In₃GeTe₄. The evolution of structural details, and the optical properties of the three In-Ge-Te alloys in amorphous and crystalline form, was thoroughly analyzed via ab initio calculations. This study proposes a chemical composition possessing both improved thermal stability and sizable optical contrast for PCM-based non-volatile photonic applications. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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13 pages, 3545 KiB

Open AccessArticle

High-Throughput Calculations on the Decomposition Reactions of Off-Stoichiometry GeSbTe Alloys for Embedded Memories

by Omar Abou El Kheir and Marco Bernasconi

Nanomaterials 2021, 11(9), 2382; https://doi.org/10.3390/nano11092382 - 13 Sep 2021

Cited by 13 | Viewed by 2405

Abstract

Chalcogenide GeSbTe (GST) alloys are exploited as phase change materials in a variety of applications ranging from electronic non-volatile memories to neuromorphic and photonic devices. In most applications, the prototypical Ge

_{2}

Sb

_{2}

Te

_{5}

compound along the GeTe-Sb

_{2}

Te

_{3}

[...] Read more.

Chalcogenide GeSbTe (GST) alloys are exploited as phase change materials in a variety of applications ranging from electronic non-volatile memories to neuromorphic and photonic devices. In most applications, the prototypical Ge

_{2}

Sb

_{2}

Te

_{5}

compound along the GeTe-Sb

_{2}

Te

_{3}

pseudobinary line is used. Ge-rich GST alloys, off the pseudobinary tie-line with a crystallization temperature higher than that of Ge

_{2}

Sb

_{2}

Te

_{5}

, are currently explored for embedded phase-change memories of interest for automotive applications. During crystallization, Ge-rich GST alloys undergo a phase separation into pure Ge and less Ge-rich alloys. The detailed mechanisms underlying this transformation are, however, largely unknown. In this work, we performed high-throughput calculations based on Density Functional Theory (DFT) to uncover the most favorable decomposition pathways of Ge-rich GST alloys. The knowledge of the DFT formation energy of all GST alloys in the central part of the Ge-Sb-Te ternary phase diagram allowed us to identify the cubic crystalline phases that are more likely to form during the crystallization of a generic GST alloy. This scheme is exemplified by drawing a decomposition map for alloys on the Ge-Ge

_{1}

Sb

_{2}

Te

_{4}

tie-line. A map of decomposition propensity is also constructed, which suggests a possible strategy to minimize phase separation by still keeping a high crystallization temperature. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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15 pages, 3326 KiB

Open AccessEditor’s ChoiceArticle

Effect of Nitrogen Doping on the Crystallization Kinetics of Ge₂Sb₂Te₅

by Minh Anh Luong, Nikolay Cherkashin, Béatrice Pecassou, Chiara Sabbione, Frédéric Mazen and Alain Claverie

Nanomaterials 2021, 11(7), 1729; https://doi.org/10.3390/nano11071729 - 30 Jun 2021

Cited by 14 | Viewed by 2761

Abstract

Among the phase change materials, Ge₂Sb₂Te₅ (GST-225) is the most studied and is already integrated into many devices. N doping is known to significantly improve some key characteristics such as the thermal stability of materials and the resistance [...] Read more.

Among the phase change materials, Ge₂Sb₂Te₅ (GST-225) is the most studied and is already integrated into many devices. N doping is known to significantly improve some key characteristics such as the thermal stability of materials and the resistance drift of devices. However, the origin, at the atomic scale, of these alterations is rather elusive. The most important issue is to understand how N doping affects the crystallization characteristics, mechanisms and kinetics, of GST-225. Here, we report the results of a combination of in situ and ex situ transmission electron microscopy (TEM) investigations carried out on specifically designed samples to evidence the influence of N concentration on the crystallization kinetics and resulting morphology of the alloy. Beyond the known shift of the crystallization temperature and the observation of smaller grains, we show that N renders the crystallization process more “nucleation dominated” and ascribe this characteristic to the increased viscosity of the amorphous state. This increased viscosity is linked to the mechanical rigidity and the reduced diffusivity resulting from the formation of Ge–N bonds in the amorphous phase. During thermal annealing, N hampers the coalescence of the crystalline grains and the cubic to hexagonal transition. Making use of AbStrain, a recently invented TEM-based technique, we evidence that the nanocrystals formed from the crystallization of N-doped amorphous GST-225 are under tension, which suggests that N is inserted in the lattice and explains why it is not found at grain boundaries. Globally, all these results demonstrate that the origin of the effect of N on the crystallization of GST-225 is not attributed to the formation of a secondary phase such as a nitride, but to the ability of N to bind to Ge in the amorphous and crystalline phases and to unbind and rebind with Ge along the diffusion path of this atomic species during annealing. Full article

(This article belongs to the Special Issue Synthesis, Properties and Applications of Germanium Chalcogenides)

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