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Modi G, Meng AC, Rajagopalan S, Thiruvengadam R, Davies PK, Stach EA, Agarwal R. Controlled Self-Assembly of Nanoscale Superstructures in Phase-Change Ge-Sb-Te Nanowires. NANO LETTERS 2024; 24:5799-5807. [PMID: 38701332 DOI: 10.1021/acs.nanolett.4c00878] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2024]
Abstract
Controlled growth of semiconductor nanowires with atomic precision offers the potential to tune the material properties for integration into scalable functional devices. Despite significant progress in understanding the nanowire growth mechanism, definitive control over atomic positions of its constituents, structure, and morphology via self-assembly remains challenging. Here, we demonstrate an exquisite control over synthesis of cation-ordered nanoscale superstructures in Ge-Sb-Te nanowires with the ability to deterministically vary the nanowire growth direction, crystal facets, and periodicity of cation ordering by tuning the relative precursor flux during synthesis. Furthermore, the role of anisotropy on material properties in cation-ordered nanowire superstructures is illustrated by fabricating phase-change memory (PCM) devices, which show significantly different growth direction dependent amorphization current density. This level of control in synthesizing chemically ordered nanoscale superstructures holds potential to precisely modulate fundamental material properties such as the electronic and thermal transport, which may have implications for PCM, thermoelectrics, and other nanoelectronic devices.
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Affiliation(s)
- Gaurav Modi
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Andrew C Meng
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Srinivasan Rajagopalan
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Rangarajan Thiruvengadam
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Peter K Davies
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Eric A Stach
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
| | - Ritesh Agarwal
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
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2
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Kumar A, Cecchini R, Wiemer C, Mussi V, De Simone S, Calarco R, Scuderi M, Nicotra G, Longo M. Phase Change Ge-Rich Ge-Sb-Te/Sb 2Te 3 Core-Shell Nanowires by Metal Organic Chemical Vapor Deposition. NANOMATERIALS (BASEL, SWITZERLAND) 2021; 11:3358. [PMID: 34947707 PMCID: PMC8707013 DOI: 10.3390/nano11123358] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/02/2021] [Revised: 12/01/2021] [Accepted: 12/08/2021] [Indexed: 12/03/2022]
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. 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.
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Affiliation(s)
- Arun Kumar
- CNR—Institute for Microelectronics and Microsystems, Via C. Olivetti 2, 20864 Agrate Brianza, Italy; (A.K.); (C.W.)
| | - Raimondo Cecchini
- CNR—Institute for Microelectronics and Microsystems, Via Gobetti 101, 40129 Bologna, Italy;
| | - Claudia Wiemer
- CNR—Institute for Microelectronics and Microsystems, Via C. Olivetti 2, 20864 Agrate Brianza, Italy; (A.K.); (C.W.)
| | - Valentina Mussi
- CNR—Institute for Microelectronics and Microsystems, Via del Fosso del Cavaliere 100, 00133 Rome, Italy; (V.M.); (S.D.S.); (R.C.)
| | - Sara De Simone
- CNR—Institute for Microelectronics and Microsystems, Via del Fosso del Cavaliere 100, 00133 Rome, Italy; (V.M.); (S.D.S.); (R.C.)
| | - Raffaella Calarco
- CNR—Institute for Microelectronics and Microsystems, Via del Fosso del Cavaliere 100, 00133 Rome, Italy; (V.M.); (S.D.S.); (R.C.)
| | - Mario Scuderi
- CNR—Institute for Microelectronics and Microsystems, Strada VIII 5, 95121 Catania, Italy; (M.S.); (G.N.)
| | - Giuseppe Nicotra
- CNR—Institute for Microelectronics and Microsystems, Strada VIII 5, 95121 Catania, Italy; (M.S.); (G.N.)
| | - Massimo Longo
- CNR—Institute for Microelectronics and Microsystems, Via del Fosso del Cavaliere 100, 00133 Rome, Italy; (V.M.); (S.D.S.); (R.C.)
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3
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Cecchini R, Gajjela RSR, Martella C, Wiemer C, Lamperti A, Nasi L, Lazzarini L, Nobili LG, Longo M. High-Density Sb 2 Te 3 Nanopillars Arrays by Templated, Bottom-Up MOCVD Growth. SMALL (WEINHEIM AN DER BERGSTRASSE, GERMANY) 2019; 15:e1901743. [PMID: 31222940 DOI: 10.1002/smll.201901743] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/05/2019] [Revised: 05/17/2019] [Indexed: 06/09/2023]
Abstract
Sb2 Te3 exhibits several technologically relevant properties, such as high thermoelectric efficiency, topological insulator character, and phase change memory behavior. Improved performances are observed and novel effects are predicted for this and other chalcogenide alloys when synthetized in the form of high-aspect-ratio nanostructures. The ability to grow chalcogenide nanowires and nanopillars (NPs) with high crystal quality in a controlled fashion, in terms of their size and position, can boost the realization of novel thermoelectric, spintronic, and memory devices. Here, it is shown that highly dense arrays of ultrascaled Sb2 Te3 NPs can be grown by metal organic chemical vapor deposition (MOCVD) on patterned substrates. In particular, crystalline Sb2 Te3 NPs with a diameter of 20 nm and a height of 200 nm are obtained in Au-functionalized, anodized aluminum oxide (AAO) templates with a pore density of ≈5 × 1010 cm-2 . Also, MOCVD growth of Sb2 Te3 can be followed either by mechanical polishing and chemical etching to produce Sb2 Te3 NPs arrays with planar surfaces or by chemical dissolution of the AAO templates to obtain freestanding Sb2 Te3 NPs forests. The illustrated growth method can be further scaled to smaller pore sizes and employed for other MOCVD-grown chalcogenide alloys and patterned substrates.
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Affiliation(s)
| | - Raja S R Gajjela
- CNR-IMM, via C. Olivetti 2, 20864, Agrate Brianza, MB, Italy
- Dipartimento di Chimica, Materiali e Ingegneria Chimica "Giulio Natta,", Politecnico di Milano, Via Mancinelli 7, 20131, Milano, Italy
| | | | - Claudia Wiemer
- CNR-IMM, via C. Olivetti 2, 20864, Agrate Brianza, MB, Italy
| | | | - Lucia Nasi
- CNR-IMEM, Parco Area delle Scienze 37/A, 43124, Parma, Italy
| | - Laura Lazzarini
- CNR-IMEM, Parco Area delle Scienze 37/A, 43124, Parma, Italy
| | - Luca G Nobili
- Dipartimento di Chimica, Materiali e Ingegneria Chimica "Giulio Natta,", Politecnico di Milano, Via Mancinelli 7, 20131, Milano, Italy
| | - Massimo Longo
- CNR-IMM, via C. Olivetti 2, 20864, Agrate Brianza, MB, Italy
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4
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Abstract
Metastable rocksalt structured Ge2Sb2Te5 is the most widely used phase-change material for data storage, yet the atomic arrangements of which are still under debate. In this work, we have proposed metastable stacking-polymorphism in cubic Ge2Sb2Te5 based on first-principles calculations. Our results show that cubic Ge2Sb2Te5 is actually polymorphic, varying from randomly distributed vacancies to highly ordered vacancy layers; consequently, the electrical property varies between metallic and semiconducting. These different atomic stackings of cubic Ge2Sb2Te5 can be obtained at different experimental synthetic conditions. The concept of stacking-polymorphic Ge2Sb2Te5 provides important fundamentals for metastable Ge2Sb2Te5 and is useful for tuning the performance of the phase-change materials.
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Affiliation(s)
- Shixiong He
- School of Materials Science and Engineering, and Center for Integrated Computational Materials Engineering, International Research Institute for Multidisciplinary Science, Beihang University , Beijing 100191, China
| | - Linggang Zhu
- School of Materials Science and Engineering, and Center for Integrated Computational Materials Engineering, International Research Institute for Multidisciplinary Science, Beihang University , Beijing 100191, China
| | - Jian Zhou
- School of Materials Science and Engineering, and Center for Integrated Computational Materials Engineering, International Research Institute for Multidisciplinary Science, Beihang University , Beijing 100191, China
| | - Zhimei Sun
- School of Materials Science and Engineering, and Center for Integrated Computational Materials Engineering, International Research Institute for Multidisciplinary Science, Beihang University , Beijing 100191, China
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Momand J, Wang R, Boschker JE, Verheijen MA, Calarco R, Kooi BJ. Dynamic reconfiguration of van der Waals gaps within GeTe-Sb 2Te 3 based superlattices. NANOSCALE 2017. [PMID: 28621784 DOI: 10.1039/c7nr01684k] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
Phase-change materials based on GeSbTe show unique switchable optoelectronic properties and are an important contender for next-generation non-volatile memories. Moreover, they recently received considerable scientific interest, because it is found that a vacancy ordering process is responsible for both an electronic metal-insulator transition and a structural cubic-to-trigonal transition. GeTe-Sb2Te3 based superlattices, or specifically their interfaces, provide an interesting platform for the study of GeSbTe alloys. In this work such superlattices have been grown with molecular beam epitaxy and they have been characterized extensively with transmission electron microscopy and X-ray diffraction. It is shown that the van der Waals gaps in these superlattices, which result from vacancy ordering, are mobile and reconfigure through the film using bi-layer defects and Ge diffusion upon annealing. Moreover, it is shown that for an average composition that is close to GeSb2Te4 a large portion of 9-layered van der Waals systems is formed, suggesting that still a substantial amount of random vacancies must be present within the trigonal GeSbTe layers. Overall these results illuminate the structural organization of van der Waals gaps commonly encountered in GeSbTe alloys, which are intimately related to their electronic properties and the metal-insulator transition.
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Affiliation(s)
- Jamo Momand
- Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
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Mio AM, Privitera SMS, Bragaglia V, Arciprete F, Bongiorno C, Calarco R, Rimini E. Chemical and structural arrangement of the trigonal phase in GeSbTe thin films. NANOTECHNOLOGY 2017; 28:065706. [PMID: 28050966 DOI: 10.1088/1361-6528/28/6/065706] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
The thermal and electrical properties of phase change materials, mainly GeSbTe alloys, in the crystalline state strongly depend on their phase and on the associated degree of order. The switching of Ge atoms in superlattice structures with trigonal phase has been recently proposed to develop memories with reduced switching energy, in which two differently ordered crystalline phases are the logic states. A detailed knowledge of the stacking plane sequence, of the local composition and of the vacancy distribution is therefore crucial in order to understand the underlying mechanism of phase transformations in the crystalline state and to evaluate the retention properties. This information is provided, as reported in this paper, by scanning transmission electron microscopy analysis of polycrystalline and epitaxial Ge2Sb2Te5 thin samples, using the Z-contrast high-angle annular dark field method. Electron diffraction clearly confirms the presence of compositional mixing with stacking blocks of 11, 9 or 7 planes corresponding to Ge3Sb2Te6, Ge2Sb2Te5, and GeSb2Te4, alloys respectively in the same trigonal phase. By increasing the degree of order (according to the annealing temperature, the growth condition, etc) the spread in the statistical distribution of the blocks reduces and the distribution of the atoms in the cation planes also changes from a homogenous Ge/Sb mixing towards a Sb-enrichment in the planes closest to the van der Waals gaps. Therefore we show that the trigonal phase of Ge2Sb2Te5, the most studied chalcogenide for phase-change memories, is actually obtained in different configurations depending on the distribution of the stacking blocks (7-9-11 planes) and on the atomic occupation (Ge/Sb) at the cation planes. These results give an insight in the factors determining the stability of the trigonal phase and suggest a dynamic path evolution that could have a key role in the switching mechanism of interfacial phase change memories and in their data retention.
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Affiliation(s)
- Antonio M Mio
- Istituto per la Microelettronica e Microsistemi-Consiglio Nazionale delle Ricerche, Zona Industriale VIII Strada 5, I-95121 Catania, Italy
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7
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Berlin K, Trampert A. Liquid-solid phase transition of Ge-Sb-Te alloy observed by in-situ transmission electron microscopy. Ultramicroscopy 2016; 178:27-32. [PMID: 27839868 DOI: 10.1016/j.ultramic.2016.10.010] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2016] [Revised: 09/30/2016] [Accepted: 10/18/2016] [Indexed: 11/17/2022]
Abstract
Melting and crystallization dynamics of the multi-component Ge-Sb-Te alloy have been investigated by in-situ transmission electron microscopy (TEM). Starting point of the phase transition study is an ordered hexagonal Ge1Sb2Te4 thin film on Si(111) where the crystal structure and the chemical composition are verified by scanning TEM and electron energy-loss spectroscopy, respectively. The in-situ observation of the liquid phase at 600°C including the liquid-solid and liquid-vacuum interfaces and their movements was made possible due to an encapsulation of the TEM sample. The solid-liquid interface during melting displays a broad and diffuse transition zone characterized by a vacancy induced disordered state. Although the velocities of interface movements are measured to be in the nanometer per second scale, both, for crystallization and solidification, the underlying dynamic processes are considerably different. Melting reveals linear dependence on time, whereas crystallization exhibits a non-linear time-dependency featuring a superimposed start-stop motion. Our results may provide valuable insight into the atomic mechanisms at interfaces during the liquid-solid phase transition of Ge-Sb-Te alloys.
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Affiliation(s)
- Katja Berlin
- Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany.
| | - Achim Trampert
- Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany
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8
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Lee JY, Kim JH, Jeon DJ, Han J, Yeo JS. Atomic Migration Induced Crystal Structure Transformation and Core-Centered Phase Transition in Single Crystal Ge 2Sb 2Te 5 Nanowires. NANO LETTERS 2016; 16:6078-6085. [PMID: 27657176 DOI: 10.1021/acs.nanolett.6b02188] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
A phase change nanowire holds a promise for nonvolatile memory applications, but its transition mechanism has remained unclear due to the analytical difficulties at atomic resolution. Here we obtain a deeper understanding on the phase transition of a single crystalline Ge2Sb2Te5 nanowire (GST NW) using atomic scale imaging, diffraction, and chemical analysis. Our cross-sectional analysis has shown that the as-grown hexagonal close-packed structure of the single crystal GST NW transforms to a metastable face-centered cubic structure due to the atomic migration to the pre-existing vacancy layers in the hcp structure going through iterative electrical switching. We call this crystal structure transformation "metastabilization", which is also confirmed by the increase of set-resistance during the switching operation. For the set to reset transition between crystalline and amorphous phases, high-resolution imaging indicates that the longitudinal center of the nanowire mainly undergoes phase transition. According to the atomic scale analysis of the GST NW after repeated electrical switching, partial crystallites are distributed around the core-centered amorphous region of the nanowire where atomic migration is mainly induced, thus potentially leading to low power electrical switching. These results provide a novel understanding of phase change nanowires, and can be applied to enhance the design of nanowire phase change memory devices for improved electrical performance.
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Affiliation(s)
- Jun-Young Lee
- School of Integrated Technology and ‡Yonsei Institute of Convergence Technology, Yonsei University , Incheon 406-840, Republic of Korea
| | - Jeong-Hyeon Kim
- School of Integrated Technology and ‡Yonsei Institute of Convergence Technology, Yonsei University , Incheon 406-840, Republic of Korea
| | - Deok-Jin Jeon
- School of Integrated Technology and ‡Yonsei Institute of Convergence Technology, Yonsei University , Incheon 406-840, Republic of Korea
| | - Jaehyun Han
- School of Integrated Technology and ‡Yonsei Institute of Convergence Technology, Yonsei University , Incheon 406-840, Republic of Korea
| | - Jong-Souk Yeo
- School of Integrated Technology and ‡Yonsei Institute of Convergence Technology, Yonsei University , Incheon 406-840, Republic of Korea
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9
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Local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures. Sci Rep 2016; 6:26724. [PMID: 27220411 PMCID: PMC4879703 DOI: 10.1038/srep26724] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2016] [Accepted: 05/05/2016] [Indexed: 11/17/2022] Open
Abstract
Insights into the local atomic arrangements of layered Ge-Sb-Te compounds are of particular importance from a fundamental point of view and for data storage applications. In this view, a detailed knowledge of the atomic structure in such alloys is central to understanding the functional properties both in the more commonly utilized amorphous–crystalline transition and in recently proposed interfacial phase change memory based on the transition between two crystalline structures. Aberration-corrected scanning transmission electron microscopy allows direct imaging of local arrangement in the crystalline lattice with atomic resolution. However, due to the non-trivial influence of thermal diffuse scattering on the high-angle scattering signal, a detailed examination of the image contrast requires comparison with theoretical image simulations. This work reveals the local atomic structure of trigonal Ge-Sb-Te thin films by using a combination of direct imaging of the atomic columns and theoretical image simulation approaches. The results show that the thin films are prone to the formation of stacking disorder with individual building blocks of the Ge2Sb2Te5, Ge1Sb2Te4 and Ge3Sb2Te6 crystal structures intercalated within randomly oriented grains. The comparison with image simulations based on various theoretical models reveals intermixed cation layers with pronounced local lattice distortions, exceeding those reported in literature.
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Campi D, Baldi E, Graceffa G, Sosso GC, Bernasconi M. Electron-phonon interaction and thermal boundary resistance at the interfaces of Ge2Sb2Te5 with metals and dielectrics. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2015; 27:175009. [PMID: 25873568 DOI: 10.1088/0953-8984/27/17/175009] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
The Ge2Sb2Te5 compound is of interest for applications in phase change non-volatile memories. First-principles calculations of phonon dispersion relations and electron-phonon coupling constant provide an estimate of the electron-phonon contribution to the thermal boundary resistance at the interfaces of Ge2Sb2Te5 with dielectrics (silica) and metal electrodes (Al and TiN). The diffuse mismatch model including full phononic dispersion has been used to compute the phononic contribution to the thermal boundary resistance. The calculated value of the electron-phonon contribution to the TBR at 300 K of about 14 m(2)K GW(-1) would dominate the TBR at the interfaces of hexagonal Ge2Sb2Te5 with the surrounding dielectrics and metals considered here once interdiffusion at the boundaries could be minimized.
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Affiliation(s)
- D Campi
- Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via R. Cozzi 55, I-20125 Milano, Italy
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