1
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Lee C, Kim D, Lim H, Seong Y, Kim H, Park JH, Yang D, Shin HJ, Wuttig M, Choi BJ, Cho MH. Ultrahigh Stability and Operation Performance in Bi-doped GeTe/Sb 2Te 3 Superlattices Achieved by Tailoring Bonding and Structural Properties. ACS NANO 2024; 18:25625-25635. [PMID: 39223725 DOI: 10.1021/acsnano.4c06909] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/04/2024]
Abstract
Changes in bond types and the reversible switching process between metavalent and covalent bonds are related to the operating mechanism of the phase-change (PC) behavior. Thus, controlling the bonding characteristics is the key to improving the PC memory performance. In this study, we have controlled the bonding characteristics of GeTe/Sb2Te3 superlattices (SLs) via bismuth (Bi) doping. The incorporation of Bi into the GeTe sublayers tailors the metavalent bond. We observed significant improvement in device reliability, set speed, and power consumption induced upon increasing Bi incorporation. The introduction of Bi was found to suppress the change in density between the SET and RESET states, resulting in a significant increase in device reliability. The reduction in Peierls distortion, leading to a more octahedral-like atomic arrangement, intensifies electron-phonon coupling with increased bond polarizability, which are responsible for the fast set speed and low power consumption. This study demonstrates how the structural and thermodynamic changes in phase change materials alter phase change characteristics due to systematic changes of bonding and provides an important methodology for the development of PC devices.
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Affiliation(s)
- Changwoo Lee
- Department of Physics, Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea
| | - Dasol Kim
- Department of Physics, Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea
- Institute of Physics, Physics of Novel Materials, RWTH Aachen University, 52056 Aachen, Germany
| | - Hyeonwook Lim
- Department of Physics, Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea
| | - Yeonwoo Seong
- Department of Physics, Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea
| | - Hyunwook Kim
- Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea
| | - Ju Hwan Park
- Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea
| | - Dogeon Yang
- Department of Physics, Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea
| | - Hee Jun Shin
- POSTECH, Pohang Accelerator Laboratory, 80, Jigokro-127-beongil, Nam-gu, Pohang, Gyeongbuk 37673, Republic of Korea
| | - Matthias Wuttig
- Institute of Physics, Physics of Novel Materials, RWTH Aachen University, 52056 Aachen, Germany
| | - Byung Joon Choi
- Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea
| | - Mann-Ho Cho
- Department of Physics, Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea
- Department of System Semiconductor Engineering, Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea
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2
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d’Acapito F, Rehman MA. Effectiveness of ab initio molecular dynamics in simulating EXAFS spectra from layered systems. JOURNAL OF SYNCHROTRON RADIATION 2024; 31:1078-1083. [PMID: 39042581 PMCID: PMC11371032 DOI: 10.1107/s1600577524005484] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/01/2024] [Accepted: 06/07/2024] [Indexed: 07/25/2024]
Abstract
The simulation of EXAFS spectra of thin films via ab initio methods is discussed. The procedure for producing the spectra is presented as well as an application to a two-dimensional material (WSe2) where the effectiveness of this method in reproducing the spectrum and the linear dichroic response is shown. A series of further examples in which the method has been employed for the structural determination of materials are given.
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Affiliation(s)
- F. d’Acapito
- Consiglio Nazionale delle Ricerche, Istituto Officina dei Materiali – OGG, c/o ESRF, Grenoble, France
| | - M. A. Rehman
- Department of Chemical and Materials Engineering, New Uzbekistan University, Tashkent, Uzbekistan
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3
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Sun T, Liu F, Guo J, Han G, Zhang Y. A Phase-Change Mechanism of GST-SL Based Superlattices upon Sb Flipping. MATERIALS 2021; 14:ma14020360. [PMID: 33450936 PMCID: PMC7828381 DOI: 10.3390/ma14020360] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 11/10/2020] [Revised: 01/07/2021] [Accepted: 01/09/2021] [Indexed: 11/16/2022]
Abstract
Reversible phase-change behaviors of Ge–Sb–Te based superlattices (GST-SL) were studied by ab initio molecular dynamics (AIMD) simulations based on three models containing Ge/Sb intermixing, namely the Petrov-mix, Ferro-mix, and Kooi-mix models. The flipping behavior of Sb atoms was found in all the three GST-SL models in the melting process. Among them the Kooi-mix model exhibited the best stability, and the analyses of bond length distribution and electron localization function provided a better explanation on the phase transition of GST-SL. Finally, we proposed a fast switching model for GST-SL based on Sb flipping.
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Affiliation(s)
- Teng Sun
- Key Laboratory of Trans-Scale Laser Manufacturing Technology, Beijing University of Technology, Ministry of Education, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China; (T.S.); (J.G.); (G.H.); (Y.Z.)
- Beijing Engineering Research Center of Laser Technology, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
- Institute of Laser Engineering, Faculty of Materials and Manufacturing, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
| | - Furong Liu
- Key Laboratory of Trans-Scale Laser Manufacturing Technology, Beijing University of Technology, Ministry of Education, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China; (T.S.); (J.G.); (G.H.); (Y.Z.)
- Beijing Engineering Research Center of Laser Technology, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
- Institute of Laser Engineering, Faculty of Materials and Manufacturing, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
- Correspondence: ; Tel.: +86-010-67396559; Fax: +86-010-67392773
| | - Jicheng Guo
- Key Laboratory of Trans-Scale Laser Manufacturing Technology, Beijing University of Technology, Ministry of Education, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China; (T.S.); (J.G.); (G.H.); (Y.Z.)
- Beijing Engineering Research Center of Laser Technology, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
- Institute of Laser Engineering, Faculty of Materials and Manufacturing, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
| | - Gang Han
- Key Laboratory of Trans-Scale Laser Manufacturing Technology, Beijing University of Technology, Ministry of Education, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China; (T.S.); (J.G.); (G.H.); (Y.Z.)
- Beijing Engineering Research Center of Laser Technology, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
- Institute of Laser Engineering, Faculty of Materials and Manufacturing, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
| | - Yongzhi Zhang
- Key Laboratory of Trans-Scale Laser Manufacturing Technology, Beijing University of Technology, Ministry of Education, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China; (T.S.); (J.G.); (G.H.); (Y.Z.)
- Beijing Engineering Research Center of Laser Technology, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
- Institute of Laser Engineering, Faculty of Materials and Manufacturing, Beijing University of Technology, 100 Ping Leyuan, Chaoyang District, Beijing 100124, China
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4
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Ribaldone C, Dragoni D, Bernasconi M. A first-principles study of the switching mechanism in GeTe/InSbTe superlattices. NANOSCALE ADVANCES 2020; 2:5209-5218. [PMID: 36132039 PMCID: PMC9418462 DOI: 10.1039/d0na00577k] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 07/15/2020] [Accepted: 09/11/2020] [Indexed: 06/15/2023]
Abstract
Interfacial Phase Change Memories (iPCMs) based on (GeTe)2/Sb2Te3 superlattices have been proposed as an alternative candidate to conventional PCMs for the realization of memory devices with superior switching properties. The switching mechanism was proposed to involve a crystalline-to-crystalline structural transition associated with a rearrangement of the stacking sequence of the GeTe bilayers. Density functional theory (DFT) calculations showed that such rearrangement could be achieved by means of a two-step process with an activation barrier for the flipping of Ge and Te atoms which is sensitive to the biaxial strain acting on GeTe bilayers. Within this picture, strain-engineering of GeTe bilayers in the GeTe-chalcogenide superlattice can be exploited to further improve the iPCM switching performance. In this work, we study GeTe-InSbTe superlattices with different compositions by means of DFT, aiming at exploiting the large mismatch (3.8%) in the in-plane lattice parameter between GeTe and In3SbTe2 to reduce the activation barrier for the switching with respect to the (GeTe)2-Sb2Te3 superlattice.
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Affiliation(s)
- Chiara Ribaldone
- Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca Via R. Cozzi 55 I-20125 Milano Italy
| | - Daniele Dragoni
- Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca Via R. Cozzi 55 I-20125 Milano Italy
| | - Marco Bernasconi
- Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca Via R. Cozzi 55 I-20125 Milano Italy
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5
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Han G, Liu F, Li W, Huang Y, Sun N, Ye F. Local structure and phase change behavior in interfacial intermixing GeTe-Sb 2Te 3 superlattices. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2020; 32:255401. [PMID: 32050167 DOI: 10.1088/1361-648x/ab7577] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Ge/Sb atomic intermixing in interfacial cationic layers is a common phenomenon for GeTe-Sb2Te3 superlattice (GST-SL) used in memory devices. In this paper, we explored the effect of Ge/Sb intermixing on the phase change behavior of GST-SL upon the heating-quenching procedure. Four interfacial intermixing models of Kooi, Ferro, Petrov and inverted Petrov with different Ge/Sb intermixing ratios (25/75, 50/50 and 75/25) were developed based on the ab initio molecular dynamics. The structural evolution indicated that the Ge/Sb interfacial intermixing could facilitate the structure changes especially for 50/50 Ge/Sb intermixed models. When quenching from 1500 K, more 4-fold Ge-centered octahedrons were produced than tetrahedrons, and the electron localization function further proved that the distorted of Ge(Sb)-centered 6-fold octahedrons were caused by the asymmetrical interactions of Ge-Ge/Sb and Ge-Te. A relatively large Te p orbital contribution in coexisted Ge/Te layer led to a narrower bandgap. In addition, different Ge/Sb atom intermixed ratio which affected the electronic local structure, led to the discrepancy in the initial atom movement of Sb or Ge movement near the gap. The present studies enrich the understanding of Ge/Sb interfacial atomic intermixing effects in GST-SL structural changes.
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Affiliation(s)
- Gang Han
- Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, People's Republic of China. Beijing Engineering Research Center of Applied Laser Technology, Beijing University of Technology, Beijing 100124, People's Republic of China
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6
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Lotnyk A, Dankwort T, Hilmi I, Kienle L, Rauschenbach B. In situ observations of the reversible vacancy ordering process in van der Waals-bonded Ge-Sb-Te thin films and GeTe-Sb 2Te 3 superlattices. NANOSCALE 2019. [PMID: 31135011 DOI: 10.1016/j.scriptamat.2019.03.024] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
Chalcogenide-based thin films are employed in data storage and memory technology whereas van der Waals-bonded layered chalcogenide heterostructures are considered to be a main contender for memory devices with low power consumption. The reduction of switching energy is due to the lowering of entropic losses governed by the restricted motion of atoms in one dimension within the crystalline states. The investigations of switching mechanisms in such superlattices have recently attracted much attention and the proposed models are still under debate. This is partially due to the lack of direct observation of atomic scale processes, which might occur in these chalcogenide systems. This work reports direct, nanoscale observations of the order-disorder processes in van der Waals bonded Ge-Sb-Te thin films and GeTe-Sb2Te3-based superlattices using in situ experiments inside an aberration-corrected transmission electron microscope. The findings reveal a reversible self-assembled reconfiguration of the structural order in these materials. This process is associated with the ordering of randomly distributed vacancies within the studied materials into ordered vacancy layers and with readjustment of the lattice plane distances within the newly formed layered structures, indicating the high flexibility of these layered chalcogenide-based systems. Thus, the ordering process results in the formation of vacancy-bonded building blocks intercalated within van der Waals-bonded units. Moreover, vacancy-bonded building blocks can be reconfigured to the initial structure under the influence of an electron beam, while in situ exposure of the recovered layers to a targeted electron beam leads to the reverse process. Overall, the outcomes provide new insights into local structure and switching mechanism in chalcogenide superlattices.
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Affiliation(s)
- Andriy Lotnyk
- Leibniz Institute of Surface Engineering (IOM), Permoserstr. 15, 04318, Leipzig, Germany.
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7
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Cojocaru-Mirédin O, Hollermann H, Mio AM, Wang AYT, Wuttig M. Role of grain boundaries in Ge-Sb-Te based chalcogenide superlattices. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2019; 31:204002. [PMID: 30769332 DOI: 10.1088/1361-648x/ab078b] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Interfacial phase change memory devices based on a distinct nanoscale structure called superlattice have been shown to outperform conventional phase-change devices. This improvement has been attributed to the hetero-interfaces, which play an important role for the superior device characteristics. However, the impact of grain boundaries (GBs), usually present in large amounts in a standard sputter-deposited superlattice film, on the device performance has not yet been investigated. Therefore, in the present work, we investigate the structure and composition of superlattice films by high resolution x-ray diffraction (XRD) cross-linked with state-of-the art methods, such as correlative microscopy, i.e. a combination of high-resolution transmission electron microscopy and atom probe tomography to determine the structure and composition of GBs at the nanometer scale. Two types of GBs have been identified: high-angle grain boundaries (HAGBs) present in the upper part of a 340 nm-thick film and low-angle grain boundaries present in the first 40 nm of the bottom part of the film close to the substrate. We demonstrate that the strongest intermixing takes place at HAGBs, where heterogeneous nucleation of Ge2Sb2Te5 can be clearly determined. Yet, the Ge1Sb2Te4 phase could also be detected in the near vicinity of a low-angle grain boundary. Finally, a more realistic view of the intermixing phenomenon in Ge-Sb-Te based chalcogenide superlattices will be proposed. Moreover, we will discuss the implications of the presence of GBs on the bonding states and device performance.
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8
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Saito Y, Fons P, Mitrofanov KV, Makino K, Tominaga J, Robertson J, Kolobov AV. Chalcogenide van der Waals superlattices: a case example of interfacial phase-change memory. PURE APPL CHEM 2019. [DOI: 10.1515/pac-2019-0105] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Abstract
2D van der Waals chalcogenides such as topological insulators and transition-metal dichalcogenides and their heterostructures are now at the forefront of semiconductor research. In this paper, we discuss the fundamental features and advantages of van der Waals bonded superlattices over conventional superlattices made of 3D materials and describe in more detail one practical example, namely, interfacial phase change memory based on GeTe–Sb2Te3 superlattice structures.
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Affiliation(s)
- Yuta Saito
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology , Tsukuba , Japan
- Department of Engineering , University of Cambridge , Cambridge , United Kingdom of Great Britain and Northern Ireland
| | - Paul Fons
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology , Tsukuba , Japan
| | - Kirill V. Mitrofanov
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology , Tsukuba , Japan
| | - Kotaro Makino
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology , Tsukuba , Japan
| | - Junji Tominaga
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology , Tsukuba , Japan
| | - John Robertson
- Department of Engineering , University of Cambridge , Cambridge , United Kingdom of Great Britain and Northern Ireland
| | - Alexander V. Kolobov
- National Institute of Advanced Industrial Science and Technology , Tsukuba , Japan
- Herzen State Pedagogical University , Faculty of Physics , St. Petersburg , Russian Federation
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9
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Makino K, Kato K, Saito Y, Fons P, Kolobov AV, Tominaga J, Nakano T, Nakajima M. Terahertz generation measurements of multilayered GeTe-Sb 2Te 3 phase change materials. OPTICS LETTERS 2019; 44:1355-1358. [PMID: 30874649 DOI: 10.1364/ol.44.001355] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/16/2019] [Accepted: 02/08/2019] [Indexed: 06/09/2023]
Abstract
Multilayered structures of GeTe and Sb2Te3 phase change material, also referred to as interfacial phase change memory (iPCM), provide superior performance for nonvolatile electrical memory technology in which the atomically controlled structure plays an important role in memory operation. Here, we report on terahertz (THz) wave generation measurements. Three- and 20-layer iPCM samples were irradiated with a femtosecond laser, and the generated THz radiation was observed. The emitted THz pulse was found to be always p polarized independent of the polarization of the excitation pulse. Based on the polarization dependence as well as the flip of the THz field from photoexcited Sb2Te3 and Bi2Te3, the THz emission process can be attributed to the surge current flow due to the built-in surface depletion layer formed in p-type semiconducting iPCM materials.
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10
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Inoue N, Nakamura H. Structural transition pathway and bipolar switching of the GeTe-Sb 2Te 3 superlattice as interfacial phase-change memory. Faraday Discuss 2019; 213:303-319. [PMID: 30378622 DOI: 10.1039/c8fd00093j] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
We investigated the resistive switching mechanism between the high-resistance state (HRS) and the low-resistance state (LRS) of the GeTe-Sb2Te3 (GST) superlattice. First-principles calculations were performed to identify the structural transition pathway and to evaluate the current-voltage (I-V) characteristics of the GST device cell. After determining the atomistic structures of the stable structural phases of the GST superlattice, we found the structural transition pathways and the transition states of possible elementary processes in the device, which consisted of a thin film of GST superlattice and semi-infinite electrodes. The calculations of the I-V characteristics were examined to identify the HRS and the LRS, and the results reasonably agreed with those of our previous study (H. Nakamura, et al., Nanoscale, 2017, 9, 9286). The calculated HRS/LRS and analysis of the transition states of the pathways suggest that a bipolar switching mode dominated by the electric-field effect is possible.
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Affiliation(s)
- Nobuki Inoue
- CD-FMat, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba Central 2, Tsukuba, Ibaraki 305-8568, Japan.
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11
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Jones RO. Bonding in phase change materials: concepts and misconceptions. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2018; 30:153001. [PMID: 29480162 DOI: 10.1088/1361-648x/aab22e] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Bonding concepts originating in chemistry are surveyed from a condensed matter perspective, beginning around 1850 with 'valence' and the word 'bond' itself. The analysis of chemical data in the 19th century resulted in astonishing progress in understanding the connectivity and stereochemistry of molecules, almost without input from physicists until the development of quantum mechanics in 1925 and afterwards. The valence bond method popularized by Pauling and the molecular orbital methods of Hund, Mulliken, Bloch, and Hückel play major roles in the subsequent development, as does the central part played by the kinetic energy in covalent bonding (Ruedenberg and others). 'Metallic' (free electron) and related approaches, including pseudopotential and density functional theories, have been remarkably successful in understanding structures and bonding in molecules and solids. We discuss these concepts in the context of phase change materials, which involve the rapid and reversible transition between amorphous and crystalline states, and note the confusion that some have caused, in particular 'resonance' and 'resonant bonding'.
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Affiliation(s)
- R O Jones
- Peter-Grünberg-Institut PGI-1 and JARA/HPC, Forschungszentrum Jülich, D-52425 Jülich, Germany
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12
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Yang Z, Xu M, Cheng X, Tong H, Miao X. Manipulation of dangling bonds of interfacial states coupled in GeTe-rich GeTe/Sb 2Te 3 superlattices. Sci Rep 2017; 7:17353. [PMID: 29229978 PMCID: PMC5725461 DOI: 10.1038/s41598-017-17671-w] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2017] [Accepted: 11/28/2017] [Indexed: 11/09/2022] Open
Abstract
Superlattices consisting of stacked nano-sized GeTe and Sb2Te3 blocks have attracted considerable attention owing to their potential for an efficient non-melting switching mechanism, associated with complex bonding between blocks. Here, we propose possible atomic models for the superlattices, characterized by different interfacial bonding types. Based on interplanar distances extracted from ab initio calculations and electron diffraction measurements, we reveal possible intercalation of dangling bonds as the GeTe content in the superlattice increases. The dangling bonds were further confirmed by X-ray photoelectron spectroscopy, anisotropic temperature dependent resistivity measurements down to 2 K and magnetotransport analysis. Changes of partially coherent decoupled topological surfaces states upon dangling bonds varying contributed to the switching mechanism. Furthermore, the topological surface states controlled by changing the bonding between stacking blocks may be optimized for multi-functional applications.
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Affiliation(s)
- Zhe Yang
- Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074, China
- School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China
| | - Ming Xu
- Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074, China
- School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China
| | - Xiaomin Cheng
- Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074, China
- School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China
| | - Hao Tong
- Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074, China.
- School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China.
| | - Xiangshui Miao
- Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074, China
- School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China
- Wuhan National High Magnetic Field Centre, Huazhong University of Science and Technology, Wuhan, 430074, China
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Choi MS, Cheong BK, Ra CH, Lee S, Bae JH, Lee S, Lee GD, Yang CW, Hone J, Yoo WJ. Electrically Driven Reversible Phase Changes in Layered In 2 Se 3 Crystalline Film. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2017; 29:1703568. [PMID: 28977703 DOI: 10.1002/adma.201703568] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/27/2017] [Revised: 08/13/2017] [Indexed: 06/07/2023]
Abstract
An unconventional phase-change memory (PCM) made of In2 Se3 , which utilizes reversible phase changes between a low-resistance crystalline β phase and a high-resistance crystalline γ phase is reported for the first time. Using a PCM with a layered crystalline film exfoliated from In2 Se3 crystals on a graphene bottom electrode, it is shown that SET/RESET programmed states form via the formation/annihilation of periodic van der Waals' (vdW) gaps (i.e., virtual vacancy layers) in the stack of atomic layers and the concurrent reconfiguration of In and Se atoms across the layers. From density functional theory calculations, β and γ phases, characterized by octahedral bonding with vdW gaps and tetrahedral bonding without vdW gaps, respectively, are shown to have energy bandgap value of 0.78 and 1.86 eV, consistent with a metal-to-insulator transition accompanying the β-to-γ phase change. The monolithic In2 Se3 layered film reported here provides a novel means to achieving a PCM based on melting-free, low-entropy phase changes in contrast with the GeTe-Sb2 Te3 superlattice film adopted in interfacial phase-change memory.
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Affiliation(s)
- Min Sup Choi
- SKKU Advanced Institute of Nano-Technology (SAINT), Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 16419, Korea
- Department of Mechanical Engineering, Columbia University, New York, NY, 10027, USA
| | - Byung-Ki Cheong
- Korea Institute of Science and Technology (KIST), Hwarang-ro, 14-gil, Seongbuk-gu, Seoul, 02792, Korea
| | - Chang Ho Ra
- SKKU Advanced Institute of Nano-Technology (SAINT), Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 16419, Korea
| | - Suyoun Lee
- Korea Institute of Science and Technology (KIST), Hwarang-ro, 14-gil, Seongbuk-gu, Seoul, 02792, Korea
| | - Jee-Hwan Bae
- School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 16419, Korea
| | - Sungwoo Lee
- Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul, 151-742, Korea
| | - Gun-Do Lee
- Department of Materials Science and Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul, 151-742, Korea
| | - Cheol-Woong Yang
- School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 16419, Korea
| | - James Hone
- Department of Mechanical Engineering, Columbia University, New York, NY, 10027, USA
| | - Won Jong Yoo
- SKKU Advanced Institute of Nano-Technology (SAINT), Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 16419, Korea
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Kolobov AV, Fons P, Saito Y, Tominaga J. Atomic Reconfiguration of van der Waals Gaps as the Key to Switching in GeTe/Sb 2Te 3 Superlattices. ACS OMEGA 2017; 2:6223-6232. [PMID: 31457867 PMCID: PMC6644333 DOI: 10.1021/acsomega.7b00812] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/18/2017] [Accepted: 09/12/2017] [Indexed: 05/09/2023]
Abstract
Nonvolatile memory, of which phase-change memory (PCM) is a leading technology, is currently a key element of various electronics and portable systems. An important step in the development of conceptually new devices is the class of van der Waals (vdW)-bonded GeTe/Sb2Te3 superlattices (SLs). With their order of magnitude faster switching rates and lower energy consumption compared to those of alloy-based devices, they are widely regarded as the next step in the implementation of PCM. In contrast to conventional PCM, where the SET and RESET states arise from the crystalline and amorphous phases, in SLs, both the SET and RESET states remain crystalline. In an earlier work, the superior performance of SLs was attributed to the reduction of entropic losses associated with the one-dimensional motion of interfacial Ge atoms located in the vicinity of Sb2Te3 quintuple layers. Subsequent experimental studies using transmission electron microscopy revealed that GeTe and Sb2Te3 blocks strongly intermix during the growth of the GeTe phase, challenging the original proposal but at the same time raising new fundamental issues. In this work, we propose a new approach to switching in SLs associated with the reconfiguration of vdW gaps accompanied by local deviation of stoichiometry from the GeTe/Sb2Te3 quasibinary alloys. The model resolves in a natural way the existing controversies, explains the large conductivity contrast between the SET and RESET crystalline states, is not compromised by Ge/Sb intermixing, and provides a new perspective for the industrial development of memory devices based on such SLs. The proposed concept of vdW gap reconfiguration may also be applicable to designing a broad variety of engineered two-dimensional vdW solids.
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15
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Nakamura H, Rungger I, Sanvito S, Inoue N, Tominaga J, Asai Y. Resistive switching mechanism of GeTe-Sb 2Te 3 interfacial phase change memory and topological properties of embedded two-dimensional states. NANOSCALE 2017; 9:9386-9395. [PMID: 28657077 DOI: 10.1039/c7nr03495d] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
A theoretical study of an interfacial phase change memory made of a GeTe-Sb2Te3 superlattice with W electrodes is presented to identify the high and low resistance states and the switching mechanism. The ferro structure of the GeTe layer block in the Te-Ge-Te-Ge sequence can be in the low resistance state only if the SET/RESET mode consists of a two step dynamical process, corresponding to a vertical flip of the Ge layer with respect to the Te layer, followed by lateral motion driven by thermal relaxation. The importance of spin-orbit coupling at the GeTe/Sb2Te3 interface to the "bias polarity-dependent" SET/RESET operation is shown, and an analysis of the two-dimensional states confined at the GeTe/Sb2Te3 interface inside the resistive switching layer is presented. Our results allow us to propose a phase diagram for the transition from a topologically nontrivial to a trivial gap state of these two-dimensional compounds.
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Affiliation(s)
- Hisao Nakamura
- CD-FMat, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba Central 2, Tsukuba, Ibaraki 305-8568, Japan.
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16
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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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17
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Makino K, Kuromiya S, Takano K, Kato K, Nakajima M, Saito Y, Tominaga J, Iida H, Kinoshita M, Nakano T. THz Pulse Detection by Multilayered GeTe/Sb 2Te 3. ACS APPLIED MATERIALS & INTERFACES 2016; 8:32408-32413. [PMID: 27933830 DOI: 10.1021/acsami.6b11418] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
We proposed and demonstrated terahertz (THz) pulse detection by means of multilayered GeTe/Sb2Te3 phase-change memory materials that are also known as a multilayer topological insulator-normal insulator (MTN) system. THz time-domain spectroscopy measurement was performed for MTN films with different multilayer repetitions as well as a conventional as-grown Ge-Te-Sb (GST) alloy film. It was found that MTNs absorb THz waves and that the absorption coefficient depends on the number of layers, while the as-grown GST alloy film was almost transparent for THz waves. Simple MTN-based THz detection devices were fabricated, and the THz-induced change in the current signal was measured when a DC bias voltage was applied between the electrodes. We confirmed that irradiation of THz pulse causes a decrease in the resistance of the MTNs. This result indicates that our devices are capable of THz detection.
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Affiliation(s)
- Kotaro Makino
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) , Tsukuba, Ibaraki 305-8565, Japan
| | - Shota Kuromiya
- Institute of Laser Engineering, Osaka University , Suita, Osaka 565-0871, Japan
| | - Keisuke Takano
- Institute of Laser Engineering, Osaka University , Suita, Osaka 565-0871, Japan
| | - Kosaku Kato
- Institute of Laser Engineering, Osaka University , Suita, Osaka 565-0871, Japan
| | - Makoto Nakajima
- Institute of Laser Engineering, Osaka University , Suita, Osaka 565-0871, Japan
| | - Yuta Saito
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) , Tsukuba, Ibaraki 305-8565, Japan
| | - Junji Tominaga
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) , Tsukuba, Ibaraki 305-8565, Japan
| | - Hitoshi Iida
- National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST) , Tsukuba, Ibaraki 305-8563, Japan
| | - Moto Kinoshita
- National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology (AIST) , Tsukuba, Ibaraki 305-8563, Japan
| | - Takashi Nakano
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) , Tsukuba, Ibaraki 305-8565, Japan
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18
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Atomic Layering, Intermixing and Switching Mechanism in Ge-Sb-Te based Chalcogenide Superlattices. Sci Rep 2016; 6:37325. [PMID: 27853289 PMCID: PMC5112535 DOI: 10.1038/srep37325] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2016] [Accepted: 10/24/2016] [Indexed: 11/08/2022] Open
Abstract
GeSbTe-based chalcogenide superlattice (CSLs) phase-change memories consist of GeSbTe layer blocks separated by van der Waals bonding gaps. Recent high resolution electron microscopy found two types of disorder in CSLs, a chemical disorder within individual layers, and SbTe bilayer stacking faults connecting one block to an adjacent block which allows individual block heights to vary. The disorder requires a generalization of the previous switching models developed for CSL systems. Density functional calculations are used to describe the stability of various types of intra-layer disorder, how the block heights can vary by means of SbTe-based stacking faults and using a vacancy-mediated kink motion, and also to understand the nature of the switching process in more chemically disordered CSLs.
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Kalikka J, Zhou X, Behera J, Nannicini G, Simpson RE. Evolutionary design of interfacial phase change van der Waals heterostructures. NANOSCALE 2016; 8:18212-18220. [PMID: 27759127 DOI: 10.1039/c6nr05539g] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
We use an evolutionary algorithm to explore the design space of hexagonal Ge2Sb2Te5; a van der Waals layered two dimensional crystal heterostructure. The Ge2Sb2Te5 structure is more complicated than previously thought. Predominant features include layers of Ge3Sb2Te6 and Ge1Sb2Te4 two dimensional crystals that interact through Te-Te van der Waals bonds. Interestingly, (Ge/Sb)-Te-(Ge/Sb)-Te alternation is a common feature for the most stable structures of each generation's evolution. This emergent rule provides an important structural motif that must be included in the design of high performance Sb2Te3-GeTe van der Waals heterostructure superlattices with interfacial atomic switching capability. The structures predicted by the algorithm agree well with experimental measurements on highly oriented, and single crystal Ge2Sb2Te5 samples. By analysing the evolutionary algorithm optimised structures, we show that diffusive atomic switching is probable by Ge atoms undergoing a transition at the van der Waals interface from layers of Ge3Sb2Te6 to Ge1Sb2Te4 thus producing two blocks of Ge2Sb2Te5. Evolutionary methods present an efficient approach to explore the enormous multi-dimensional design parameter space of van der Waals bonded heterostructure superlattices.
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Affiliation(s)
- Janne Kalikka
- Singapore University of Technology and Design, 8 Somapah Road, Singapore.
| | - Xilin Zhou
- Singapore University of Technology and Design, 8 Somapah Road, Singapore.
| | - Jitendra Behera
- Singapore University of Technology and Design, 8 Somapah Road, Singapore.
| | | | - Robert E Simpson
- Singapore University of Technology and Design, 8 Somapah Road, Singapore.
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Guo S, Xu L, Zhang J, Hu Z, Li T, Wu L, Song Z, Chu J. Enhanced Crystallization Behaviors of Silicon-Doped Sb2Te Films: Optical Evidences. Sci Rep 2016; 6:33639. [PMID: 27640336 PMCID: PMC5027526 DOI: 10.1038/srep33639] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2016] [Accepted: 08/31/2016] [Indexed: 11/09/2022] Open
Abstract
The optical properties and structural variations of silicon (Si) doped Sb2Te (SST) films as functions of temperature (210–620 K) and Si concentration (0–33%) have been investigated by the means of temperature dependent Raman scattering and spectroscopic ellipsometry experiments. Based upon the changes in Raman phonon modes and dielectric functions, it can be concluded that the temperature ranges for intermediates and transition states are estimated to 150, 120, 90, and 0 K, corresponding to ST, SST25%, SST28%, and SST33% films, respectively. The phenomenon also can be summarized by the thermal evolutions of interband electronic transition energies (En) and partial spectral weight integral (I). The disappearance of intermediate (INT) state for SST33% film between amorphous (AM) and hexagonal (HEX) phases can be attributed to the acceleratory crystallization of HEX phase by Si introduction. It illustrates that the risk of phase separation (Sb and Te) during the cyclic phase-change processes decreases with the increasing Si concentration. The enhanced crystallization behaviors can optimize the data retention ability and the long term stability of ST by Si doping, which are important indicators for phase change materials. The performance improvement has been analyzed qualitatively from the optical perspective.
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Affiliation(s)
- Shuang Guo
- Department of Electronic Engineering, East China Normal University, Shanghai 200241, China
| | - Liping Xu
- Department of Electronic Engineering, East China Normal University, Shanghai 200241, China
| | - Jinzhong Zhang
- Department of Electronic Engineering, East China Normal University, Shanghai 200241, China
| | - Zhigao Hu
- Department of Electronic Engineering, East China Normal University, Shanghai 200241, China
| | - Tao Li
- State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
| | - Liangcai Wu
- State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
| | - Zhitang Song
- State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
| | - Junhao Chu
- Department of Electronic Engineering, East China Normal University, Shanghai 200241, China
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Morphology and Electric Conductance Change Induced by Voltage Pulse Excitation in (GeTe)2/Sb2Te3 Superlattices. Sci Rep 2016; 6:33223. [PMID: 27618797 PMCID: PMC5020502 DOI: 10.1038/srep33223] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2016] [Accepted: 08/23/2016] [Indexed: 11/08/2022] Open
Abstract
Chalcogenide superlattice (SL) phase-change memory materials are leading candidates for non-volatile, energy-efficient electric memory where the electric conductance switching is caused by the atom repositioning in the constituent layers. Here, we study the time evolution of the electric conductance in [(GeTe)2/(Sb2Te3)1]4 SLs upon the application of an external pulsed electric field by analysing the structural and electrical responses of the SL films with scanning probe microscopy (SPM) and scanning probe lithography (SPL). At a low pulse voltage (1.6–2.3 V), a conductance switching delay of a few seconds was observed in some SL areas, where the switch to the high conductance state (HCS) is accompanied with an SL expansion under the strong electric field of the SPM probe. At a high pulse voltage (2.5–3.0 V), the HCS current was unstable and decayed in a few seconds; this is ascribed to the degradation of the HCS crystal phase under excessive heating. The reversible conductance change under a pulse voltage of opposite polarity emphasised the role of the electric field in the phase-transition mechanism.
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22
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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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