1
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Kolobov AV, Kuznetsov VG, Krbal M, Zabotnov SV. Lone-Pair-Enabled Polymorphism and Photostructural Changes in Chalcogenide Glasses. MATERIALS (BASEL, SWITZERLAND) 2023; 16:6602. [PMID: 37834739 PMCID: PMC10574025 DOI: 10.3390/ma16196602] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/03/2023] [Revised: 09/24/2023] [Accepted: 10/06/2023] [Indexed: 10/15/2023]
Abstract
S- and Se-based chalcogenide glasses are intrinsically metastable and exhibit a number of photo-induced effects unique to this class of materials, reversible photostructural changes and photo-induced anisotropy being major examples. These effects are usually interpreted in terms of the formation of valence alternation pairs and 'wrong' bonds. In this work, using density functional theory simulations, we demonstrate for the case example of As2S3 that a strong decrease in the optical band gap can be achieved if a polymorphic transformation of the local structure from orpiment to that of tetradymite takes place. For the formation of the latter, the presence of lone-pair electrons in near-linear atomic configurations is crucial. Our results represent a novel approach to understanding the photo-induced structural changes in chalcogenide glasses as being due to the presence of polymorphism, and will lead to their wider use in various photonic devices.
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Affiliation(s)
- Alexander V. Kolobov
- Institute of Physics, Herzen State Pedagogical University of Russia, 48 Moïka Emb., St. Petersburg 191186, Russia
| | | | - Milos Krbal
- Center of Materials and Nanotechnologies, Faculty of Chemical Technology, University of Pardubice, Nam. Čs. Legii 565, 530 02 Pardubice, Czech Republic;
| | - Stanislav V. Zabotnov
- Faculty of Physics, Lomonosov Moscow State University, 1/2 Leninskie Gory, Moscow 119991, Russia
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2
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Rules of hierarchical melt and coordinate bond to design crystallization in doped phase change materials. Nat Commun 2021; 12:6473. [PMID: 34753920 PMCID: PMC8578292 DOI: 10.1038/s41467-021-26696-9] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2020] [Accepted: 09/27/2021] [Indexed: 11/23/2022] Open
Abstract
While alloy design has practically shown an efficient strategy to mediate two seemingly conflicted performances of writing speed and data retention in phase-change memory, the detailed kinetic pathway of alloy-tuned crystallization is still unclear. Here, we propose hierarchical melt and coordinate bond strategies to solve them, where the former stabilizes a medium-range crystal-like region and the latter provides a rule to stabilize amorphous. The Er0.52Sb2Te3 compound we designed achieves writing speed of 3.2 ns and ten-year data retention of 161 °C. We provide a direct atomic-level evidence that two neighbor Er atoms stabilize a medium-range crystal-like region, acting as a precursor to accelerate crystallization; meanwhile, the stabilized amorphous originates from the formation of coordinate bonds by sharing lone-pair electrons of chalcogenide atoms with the empty 5d orbitals of Er atoms. The two rules pave the way for the development of storage-class memory with comprehensive performance to achieve next technological node. In phase-change memory, writing speed and data retention are two seemingly conflicting performances. Here the authors report hierarchical melt and coordinate bond strategies to stabilize a medium-range crystal-like region and amorphous region, respectively.
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3
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Zhao X, Song K, Huang H, Yan Y, Su Y, Qian P. Effect of Alloying Elements on the Stacking Fault Energy and Ductility in Mg 2Si Intermetallic Compounds. ACS OMEGA 2021; 6:20254-20263. [PMID: 34395974 PMCID: PMC8358948 DOI: 10.1021/acsomega.1c02099] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/20/2021] [Accepted: 07/19/2021] [Indexed: 06/13/2023]
Abstract
Alloying elements can pronouncedly change the mechanical properties of intermetallic compounds. However, the effect mechanism of this in Mg2Si alloys is not clear yet. In this paper, systematic first-principles calculations were performed to investigate the effect of alloying elements on the ductility of Mg-Si alloys. It was found that some alloying elements such as In, Cu, Pd, etc. could improve the ductility of Mg2Si alloys. Moreover, the interatomic bonding mechanisms were analyzed through the electron localization functional. Simultaneously, the machine-learning method was employed to help identify the most important features associated with the toughening mechanisms. It shows that the ground state atomic volume (V GS) is strongly related to the stacking fault energy (γus) of Mg2Si alloys. Interestingly, the alloying elements with appropriate V GS and higher Allred-Rochow electronegativity (En) would reduce the γus in the Mg-Si-X system and yield a better ductility. This work demonstrates how a fundamental theoretical understanding at the atomic and electronic levels can rationalize the mechanical properties of Mg2Si alloys at a macroscopic scale.
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Affiliation(s)
- Xinpeng Zhao
- Institute
for Advanced Materials and Technology, University
of Science and Technology Beijing, Beijing 100083, China
| | - Keke Song
- School
of Mathematics and Physics, University of
Science and Technology Beijing, Beijing 100083, China
| | - Haiyou Huang
- Institute
for Advanced Materials and Technology, University
of Science and Technology Beijing, Beijing 100083, China
| | - Yu Yan
- Institute
for Advanced Materials and Technology, University
of Science and Technology Beijing, Beijing 100083, China
| | - Yanjing Su
- Institute
for Advanced Materials and Technology, University
of Science and Technology Beijing, Beijing 100083, China
- Beijing
Advanced Innovation Center for Materials Genome Engineering, University of Science and Technology Beijing, Beijing 100083, China
| | - Ping Qian
- School
of Mathematics and Physics, University of
Science and Technology Beijing, Beijing 100083, China
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4
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Wang J, Cui D, Kong Y, Shen L. Unusual Force Constants Guided Distortion-Triggered Loss of Long-Range Order in Phase Change Materials. MATERIALS (BASEL, SWITZERLAND) 2021; 14:3514. [PMID: 34202545 PMCID: PMC8269605 DOI: 10.3390/ma14133514] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 05/11/2021] [Revised: 06/11/2021] [Accepted: 06/21/2021] [Indexed: 11/24/2022]
Abstract
Unusual force constants originating from the local charge distribution in crystalline GeTe and Sb2Te3 are observed by using the first-principles calculations. The calculated stretching force constants of the second nearest-neighbor Sb-Te and Ge-Te bonds are 0.372 and -0.085 eV/Å2, respectively, which are much lower than 1.933 eV/Å2 of the first nearest-neighbor bonds although their lengths are only 0.17 Å and 0.33 Å longer as compared to the corresponding first nearest-neighbor bonds. Moreover, the bending force constants of the first and second nearest-neighbor Ge-Ge and Sb-Sb bonds exhibit large negative values. Our first-principles molecular dynamic simulations also reveal the possible amorphization of Sb2Te3 through local distortions of the bonds with weak and strong force constants, while the crystalline structure remains by the X-ray diffraction simulation. By identifying the low or negative force constants, these weak atomic interactions are found to be responsible for triggering the collapse of the long-range order. This finding can be utilized to guide the design of functional components and devices based on phase change materials with lower energy consumption.
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Affiliation(s)
- Jiong Wang
- Powder Metallurgy Research Institute, Central South University, Changsha 410083, China; (J.W.); (D.C.)
| | - Dongyu Cui
- Powder Metallurgy Research Institute, Central South University, Changsha 410083, China; (J.W.); (D.C.)
| | - Yi Kong
- Powder Metallurgy Research Institute, Central South University, Changsha 410083, China; (J.W.); (D.C.)
| | - Luming Shen
- School of Civil Engineering, The University of Sydney, Sydney, NSW 2006, Australia;
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5
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Kang L, Chen L. First-principles study of the liquid and amorphous phases of Sb 2Te phase change memory material. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2021; 33:165703. [PMID: 33740774 DOI: 10.1088/1361-648x/abf077] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/04/2021] [Accepted: 03/19/2021] [Indexed: 06/12/2023]
Abstract
We have investigated the local structure of liquid and amorphous phases of Sb2Te phase change memory material by the means of density functional theory-molecular dynamics simulations. The models of liquid and amorphous states were generated by quenching from the melt. The results show that the local environment of liquid Sb2Te is a mixed bonding geometry, where the average coordination numbers (CNs) of Sb and Te atoms are 4.93 and 4.23, respectively. Compared with crystalline state, there are more Sb-Sb bonds (∼53%) and less Sb-Te bonds (∼42%) with the presence of Te-Te bonds (∼5%) in liquid Sb2Te. Therefore, the formation of homopolar bonds and the breaking of heteropolar bonds are important structural transformations in melt process. For amorphous Sb2Te, the average CNs of Sb and Te atoms are 4.54 and 3.57, respectively. They are mostly in an octahedral environment, similar to the case in crystalline phase. The fractions of Sb-Sb, Te-Te, and Sb-Te bonds are ∼52%, ∼2%, and ∼46%, respectively. Thus, the increase in the fraction of octahedron accompanied with the decrease in average CN is the major structural changes in quenching process. Furthermore, the octahedral geometry in both the crystalline and amorphous Sb2Te increases the local structural similarity, facilitating the rapid low-energy crystallization.
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Affiliation(s)
- Lei Kang
- School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People's Republic of China
| | - Leng Chen
- School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People's Republic of China
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6
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Martinez JC, Simpson RE. Relation Between Resistance Drift and Optical Gap in Phase Change Materials. ADVANCED THEORY AND SIMULATIONS 2020. [DOI: 10.1002/adts.202000117] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Affiliation(s)
- Jose C. Martinez
- Engineering and Production Development Singapore University of Technology and Design 8 Somapah Rd Singapore 487372 Singapore
| | - Robert E. Simpson
- Engineering and Production Development Singapore University of Technology and Design 8 Somapah Rd Singapore 487372 Singapore
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7
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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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8
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Raghuwanshi M, Cojocaru-Mirédin O, Wuttig M. Investigating Bond Rupture in Resonantly Bonded Solids by Field Evaporation of Carbon Nanotubes. NANO LETTERS 2020; 20:116-121. [PMID: 31804085 DOI: 10.1021/acs.nanolett.9b03435] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Carbon nanotubes, which possess an atomic arrangement that closely resembles graphene, form a class of nanomaterials with an exceptional application portfolio including electronics, batteries, sensors, etc. Both carbon nanotubes and graphene have exceptional mechanical and electronic properties. These exceptional properties of graphene are attributed to the combined effect of σ and π bonds which form upon sp2 hybridization, resulting in what is known as resonant bonding. Here, we use atom probe tomography (APT, a technique based on controlled desorption of atoms under high electric field) to observe its bond-rupture characteristics. Results show that the bond rupture of carbon nanotubes, which are resonantly bonded, is similar to that observed for covalently bonded systems. However, a significant difference is observed when compared with those solids which are metavalently bonded. This clearly justifies that resonant bonding, a sub-branch of covalent bonding, is very different from "metavalent" bonding.
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Affiliation(s)
- Mohit Raghuwanshi
- I. Physikalisches Institut IA , RWTH Aachen , Sommerfeldstrasse 14 , 52074 Aachen , Germany
| | - Oana Cojocaru-Mirédin
- I. Physikalisches Institut IA , RWTH Aachen , Sommerfeldstrasse 14 , 52074 Aachen , Germany
| | - Matthias Wuttig
- I. Physikalisches Institut IA , RWTH Aachen , Sommerfeldstrasse 14 , 52074 Aachen , Germany
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9
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Mawale R, Halenkovič T, Bouška M, Gutwirth J, Nazabal V, Bora PL, Pečinka L, Prokeš L, Havel J, Němec P. Mass spectrometric investigation of amorphous Ga-Sb-Se thin films. Sci Rep 2019; 9:10213. [PMID: 31308483 PMCID: PMC6629872 DOI: 10.1038/s41598-019-46767-8] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2019] [Accepted: 07/05/2019] [Indexed: 11/11/2022] Open
Abstract
Amorphous chalcogenide thin films are widely studied due to their enhanced properties and extensive applications. Here, we have studied amorphous Ga-Sb-Se chalcogenide thin films prepared by magnetron co-sputtering, via laser ablation quadrupole ion trap time-of-flight mass spectrometry. Furthermore, the stoichiometry of the generated clusters was determined which gives information about individual species present in the plasma plume originating from the interaction of amorphous chalcogenides with high energy laser pulses. Seven different compositions of thin films (Ga content 7.6–31.7 at. %, Sb content 5.2–31.2 at. %, Se content 61.2–63.3 at. %) were studied and in each case about ~50 different clusters were identified in positive and ~20–30 clusters in negative ion mode. Assuming that polymers can influence the laser desorption (laser ablation) process, we have used parafilm as a material to reduce the destruction of the amorphous network structure and/or promote the laser ablation synthesis of heavier species from those of lower mass. In this case, many new and higher mass clusters were identified. The maximum number of (40) new clusters was detected for the Ga-Sb-Se thin film containing the highest amount of antimony (31.2 at. %). This approach opens new possibilities for laser desorption ionization/laser ablation study of other materials. Finally, for selected binary and ternary clusters, their structure was calculated by using density functional theory optimization procedure.
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Affiliation(s)
- Ravi Mawale
- Department of Graphic Arts and Photophysics, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 53210, Pardubice, Czech Republic
| | - Tomáš Halenkovič
- Department of Graphic Arts and Photophysics, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 53210, Pardubice, Czech Republic.,Institut des Sciences Chimiques de Rennes, UMR-CNRS 6226, Equipe Verres et Céramiques, Université de Rennes 1, 35042, Rennes, France
| | - Marek Bouška
- Department of Graphic Arts and Photophysics, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 53210, Pardubice, Czech Republic
| | - Jan Gutwirth
- Department of Graphic Arts and Photophysics, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 53210, Pardubice, Czech Republic
| | - Virginie Nazabal
- Department of Graphic Arts and Photophysics, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 53210, Pardubice, Czech Republic.,Institut des Sciences Chimiques de Rennes, UMR-CNRS 6226, Equipe Verres et Céramiques, Université de Rennes 1, 35042, Rennes, France
| | - Pankaj Lochan Bora
- Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic.,CEITEC-Central European Institute of Technology Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Lukáš Pečinka
- Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Lubomír Prokeš
- Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic.,Department of Physical Electronics, Faculty of Science, Masaryk University, Kotlářská 2, 61137, Brno, Czech Republic.,CEPLANT, R&D Centre for Low-Cost Plasma and Nanotechnology Surface Modification, Masaryk University, Kotlářská 2, 61137, Brno, Czech Republic
| | - Josef Havel
- Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Petr Němec
- Department of Graphic Arts and Photophysics, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 53210, Pardubice, Czech Republic.
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10
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Konstantinou K, Mocanu FC, Lee TH, Elliott SR. Revealing the intrinsic nature of the mid-gap defects in amorphous Ge 2Sb 2Te 5. Nat Commun 2019; 10:3065. [PMID: 31296874 PMCID: PMC6624207 DOI: 10.1038/s41467-019-10980-w] [Citation(s) in RCA: 25] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/2018] [Accepted: 06/03/2019] [Indexed: 11/09/2022] Open
Abstract
Understanding the relation between the time-dependent resistance drift in the amorphous state of phase-change materials and the localised states in the band gap of the glass is crucial for the development of memory devices with increased storage density. Here a machine-learned interatomic potential is utilised to generate an ensemble of glass models of the prototypical phase-change alloy, Ge2Sb2Te5, to obtain reliable statistics. Hybrid density-functional theory is used to identify and characterise the geometric and electronic structures of the mid-gap states. 5-coordinated Ge atoms are the local defective bonding environments mainly responsible for these electronic states. The structural motif for the localisation of the mid-gap states is a crystalline-like atomic environment within the amorphous network. An extra electron is trapped spontaneously by these mid-gap states, creating deep traps in the band gap. The results provide significant insights that can help to rationalise the design of multi-level-storage memory devices.
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Affiliation(s)
| | - Felix C Mocanu
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK
| | - Tae-Hoon Lee
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK
| | - Stephen R Elliott
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK
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11
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Wang G, Li C, Shi D, Zhang Y, Shen X. Laser-induced metastable phase in crystalline phase-change films by confocal Raman spectrometer. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2018; 205:551-556. [PMID: 30075435 DOI: 10.1016/j.saa.2018.07.077] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/04/2018] [Revised: 07/24/2018] [Accepted: 07/26/2018] [Indexed: 06/08/2023]
Abstract
Understanding crystallization process in phase-change materials is very important for data storage application. Especially, accurately controlling the metastable phase transition as well as characterizing its structure evolution is still under investigation. In this study, phase transformations have occurs from amorphous to crystalline phases when the phase-change films were irradiated continuously by the 785 nm laser irradiation. By adjusting the laser power, the different metastable phases in conventional Ge2Sb2Te5, Sb2Te3, ZnSb, ZnSb-Al2O3 and ZnSb-ZnO were obtained and distinguished by their different Raman vibration modes. The effect of laser power on the phase-change threshold of these films was studied systematically. Large structural differences induced by laser irradiation were revealed based on the changes in Raman profiles. Our study may offer a new insight into an accurate control of distinct metastable state to realize optical multilevel memory.
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Affiliation(s)
- Guoxiang Wang
- Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo, Zhejiang 315211, China; Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo, Zhejiang 315211, China.
| | - Chao Li
- Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo, Zhejiang 315211, China; Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo, Zhejiang 315211, China
| | - Daotian Shi
- Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo, Zhejiang 315211, China; Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo, Zhejiang 315211, China
| | - Yawen Zhang
- Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo, Zhejiang 315211, China; Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo, Zhejiang 315211, China
| | - Xiang Shen
- Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies, Ningbo University, Ningbo, Zhejiang 315211, China; Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo, Zhejiang 315211, China.
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12
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Singh J, Singh G, Kaura A, Tripathi S. Effect of gradual ordering of Ge/Sb atoms on chemical bonding: A proposed mechanism for the formation of crystalline Ge2Sb2Te5. J SOLID STATE CHEM 2018. [DOI: 10.1016/j.jssc.2018.01.021] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
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13
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Guo YR, Dong F, Qiao C, Wang JJ, Wang SY, Xu M, Zheng YX, Zhang RJ, Chen LY, Wang CZ, Ho KM. Structural signature and transition dynamics of Sb2Te3 melt upon fast cooling. Phys Chem Chem Phys 2018; 20:11768-11775. [DOI: 10.1039/c8cp00142a] [Citation(s) in RCA: 26] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Sb-centered defective octahedrons connect with each other via four-fold rings, a close link of the four-fold ring and defective octahedrons in amorphous Sb2Te3 has been revealed.
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Affiliation(s)
- Y. R. Guo
- Shanghai Ultra-Precision Optical Manufacturing Engineering Center and Department of Optical Science and Engineering, Fudan University
- Shanghai
- China
| | - F. Dong
- Shanghai Ultra-Precision Optical Manufacturing Engineering Center and Department of Optical Science and Engineering, Fudan University
- Shanghai
- China
| | - C. Qiao
- Shanghai Ultra-Precision Optical Manufacturing Engineering Center and Department of Optical Science and Engineering, Fudan University
- Shanghai
- China
| | - J. J. Wang
- Shanghai Ultra-Precision Optical Manufacturing Engineering Center and Department of Optical Science and Engineering, Fudan University
- Shanghai
- China
| | - S. Y. Wang
- Shanghai Ultra-Precision Optical Manufacturing Engineering Center and Department of Optical Science and Engineering, Fudan University
- Shanghai
- China
- Key Laboratory for Information Science of Electromagnetic Waves (MoE)
- Shanghai 200433
| | - Ming Xu
- School of Optical & Electronic Information, Huazhong University of Science & Technology
- Wuhan
- China
| | - Y. X. Zheng
- Shanghai Ultra-Precision Optical Manufacturing Engineering Center and Department of Optical Science and Engineering, Fudan University
- Shanghai
- China
| | - R. J. Zhang
- Shanghai Ultra-Precision Optical Manufacturing Engineering Center and Department of Optical Science and Engineering, Fudan University
- Shanghai
- China
| | - L. Y. Chen
- Shanghai Ultra-Precision Optical Manufacturing Engineering Center and Department of Optical Science and Engineering, Fudan University
- Shanghai
- China
| | - C. Z. Wang
- Ames Laboratory, U. S. Department of Energy and Department of Physics and Astronomy, Iowa State University
- Ames
- USA
| | - K. M. Ho
- Ames Laboratory, U. S. Department of Energy and Department of Physics and Astronomy, Iowa State University
- Ames
- USA
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14
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Song WX, Liu ZP, Liu LM. Inherent Simple Cubic Lattice Being Responsible for Ultrafast Solid-Phase Change of Ge 2Sb 2Te 5. J Phys Chem Lett 2017; 8:2560-2564. [PMID: 28535351 DOI: 10.1021/acs.jpclett.7b00913] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Crystallization of solid is generally slow in kinetics for atoms trapped in solids. Phase-change materials (PCMs) challenge current theory on its ultrafast reversible amorphous-to-crystal transition. Here by using the stochastic surface walking global optimization method, we establish the first global potential energy surface (PES) for Ge2Sb2Te5. By analyzing all structures on the global PES, we show that an inherent structural pattern of simple cubic lattice is present universally in low-energy structures, either globally in a newly found metastable simple cubic crystal phase or locally in the amorphous structures. Our solid-to-solid reaction pathway sampling reveals that this simple cubic lattice plays a critical role in the rapid amorphous-to-crystal transition, which occurs via dynamic vacancy creation/annihilation, Martensitic-type {100} shearing, and diffusionless local relaxation. This knowledge from global PES allows the prediction of PCMs by linking the phase-change kinetics with the geometry of metastable phases.
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Affiliation(s)
- Wen-Xiong Song
- Collaborative Innovation Center of Chemistry for Energy Material, Key Laboratory of Computational Physical Science (Ministry of Education), Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University , Shanghai 200433, China
- Beijing Computational Science Research Center , Beijing 100193, China
| | - Zhi-Pan Liu
- Collaborative Innovation Center of Chemistry for Energy Material, Key Laboratory of Computational Physical Science (Ministry of Education), Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University , Shanghai 200433, China
- Beijing Computational Science Research Center , Beijing 100193, China
| | - Li-Min Liu
- Beijing Computational Science Research Center , Beijing 100193, China
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15
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Lee TH, Elliott SR. The Relation between Chemical Bonding and Ultrafast Crystal Growth. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2017; 29. [PMID: 28417576 DOI: 10.1002/adma.201700814] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/09/2017] [Revised: 03/07/2017] [Indexed: 05/09/2023]
Abstract
Glasses are often described as supercooled liquids, whose structures are topologically disordered like a liquid, but nevertheless retain short-range structural order. Structural complexity is often associated with complicated electron-charge distributions in glassy systems, making a detailed investigation challenging even for short-range structural order, let alone their atomic dynamics. This is particularly problematic for lone-pair-rich, semiconducting materials, such as phase-change materials (PCMs). Here, this study shows that analytical methods for studying bonding, based on the electron-charge density, rather than a conventional atomic pair-correlation-function approach, allows an in-depth investigation into the chemical-bonding network, as well as lone pairs, of the prototypical PCM, Ge2 Sb2 Te5 (GST). It is demonstrated that the structurally flexible building units of the amorphous GST network, intimately linked to the presence of distinctly coexisting weak covalent and lone-pair interactions, give rise to cooperative structural-ordering processes, by which ultrafast crystal growth becomes possible. This finding may universally apply to other PCMs.
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Affiliation(s)
- Tae Hoon Lee
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK
| | - Stephen R Elliott
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK
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Kolobov AV, Fons P, Tominaga J. Understanding Phase-Change Memory Alloys from a Chemical Perspective. Sci Rep 2015; 5:13698. [PMID: 26323962 PMCID: PMC4555180 DOI: 10.1038/srep13698] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2015] [Accepted: 08/03/2015] [Indexed: 11/09/2022] Open
Abstract
Phase-change memories (PCM) are associated with reversible ultra-fast low-energy crystal-to-amorphous switching in GeTe-based alloys co-existing with the high stability of the two phases at ambient temperature, a unique property that has been recently explained by the high fragility of the glass-forming liquid phase, where the activation barrier for crystallisation drastically increases as the temperature decreases from the glass-transition to room temperature. At the same time the atomistic dynamics of the phase-change process and the associated changes in the nature of bonding have remained unknown. In this work we demonstrate that key to this behavior is the formation of transient three-center bonds in the excited state that is enabled due to the presence of lone-pair electrons. Our findings additionally reveal previously ignored fundamental similarities between the mechanisms of reversible photoinduced structural changes in chalcogenide glasses and phase-change alloys and offer new insights into the development of efficient PCM materials.
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Affiliation(s)
- A V Kolobov
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8562, Japan
| | - P Fons
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8562, Japan
| | - J Tominaga
- Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8562, Japan
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Xu M, Zhang W, Mazzarello R, Wuttig M. Disorder Control in Crystalline GeSb 2Te 4 Using High Pressure. ADVANCED SCIENCE (WEINHEIM, BADEN-WURTTEMBERG, GERMANY) 2015; 2:1500117. [PMID: 27708999 PMCID: PMC5034799 DOI: 10.1002/advs.201500117] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 04/04/2015] [Revised: 05/15/2015] [Indexed: 06/06/2023]
Abstract
Electronic phase-change memory devices take advantage of the different resistivity of two states, amorphous and crystalline, and the swift transitions between them in active phase-change materials (PCMs). In addition to these two distinct phases, multiple resistive states can be obtained by tuning the atomic disorder in the crystalline phase with heat treatment, because the disorder can lead to the localization of the electronic states and, thus, hamper the electron transport. The goal of this work is to achieve and explore multiple disordered configurations in PCMs by applying high pressure. Large-scale ab initio molecular dynamics simulations demonstrate that pressure can lower the energy barrier for the antisite migration in crystalline PCMs. The accumulation of these antisite atoms largely increases the compositional disorder, adding localized electronic states near the conduction band. The disorder-induced electron localization triggered by pressure is a novel way to modulate the properties of materials. Furthermore, the random distortion of the lattice induced by the compositional disorder provides a new mechanism that contributes to the amorphization of crystalline PCMs at high pressure.
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Affiliation(s)
- Ming Xu
- Institute of Physics (IA) RWTH Aachen University Aachen 52074 Germany
| | - Wei Zhang
- Institute of Physics (IA) RWTH Aachen University Aachen 52074 Germany
| | - Riccardo Mazzarello
- Institute for Theoretical Solid State Physics RWTH Aachen University Aachen 52074 Germany
| | - Matthias Wuttig
- Institute of Physics (IA) RWTH Aachen University Aachen 52074 Germany; JARA-FIT and JARA-HPC RWTH Aachen University Aachen 52056 Germany
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18
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Jang MH, Jeong KS, Park SJ, Park SJ, Cho MH, Song JY. Phase-change-induced martensitic deformation and slip system in GeSbTe. RSC Adv 2015. [DOI: 10.1039/c4ra16946h] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Phase-change stress induced martensitic deformation on GeSbTe.
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Affiliation(s)
- Moon Hyung Jang
- Institute of Physics and Applied Physics
- Yonsei University
- Seoul
- Republic of Korea
| | - Kwang Sik Jeong
- Institute of Physics and Applied Physics
- Yonsei University
- Seoul
- Republic of Korea
| | - Seung Jong Park
- Institute of Physics and Applied Physics
- Yonsei University
- Seoul
- Republic of Korea
| | - Sung Jin Park
- Institute of Physics and Applied Physics
- Yonsei University
- Seoul
- Republic of Korea
| | - Mann-Ho Cho
- Institute of Physics and Applied Physics
- Yonsei University
- Seoul
- Republic of Korea
| | - Jae Yong Song
- Center for Nanocharacterization
- Korea Research Institute of Standards and Science
- Daejeon
- Republic of Korea
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Deringer VL, Zhang W, Lumeij M, Maintz S, Wuttig M, Mazzarello R, Dronskowski R. Bonding Nature of Local Structural Motifs in Amorphous GeTe. Angew Chem Int Ed Engl 2014; 53:10817-20. [DOI: 10.1002/anie.201404223] [Citation(s) in RCA: 107] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2014] [Indexed: 11/11/2022]
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Deringer VL, Zhang W, Lumeij M, Maintz S, Wuttig M, Mazzarello R, Dronskowski R. Bindungseigenschaften lokaler Strukturmotive in amorphem GeTe. Angew Chem Int Ed Engl 2014. [DOI: 10.1002/ange.201404223] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
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21
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Ding J, Xu M, Guan PF, Deng SW, Cheng YQ, Ma E. Temperature effects on atomic pair distribution functions of melts. J Chem Phys 2014; 140:064501. [DOI: 10.1063/1.4864106] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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22
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Schröder T, Schwarzmüller S, Stiewe C, de Boor J, Hölzel M, Oeckler O. The solid solution series (GeTe)x(LiSbTe2)2 (1 ≤ x ≤ 11) and the thermoelectric properties of (GeTe)11(LiSbTe2)2. Inorg Chem 2013; 52:11288-94. [PMID: 24093486 DOI: 10.1021/ic401516m] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Exchanging one Ge(2+) with two Li(+) per formula unit in (GeTe)n(Sb2Te3) (n = 1, 2, 3, ...) eliminates cation vacancies, because it leads to an equal number of cations and anions. This substitution results in the solid solution (GeTe)x(LiSbTe2)2 (with x = n - 1, but n not necessarily an integer). For x < 6, these stable compounds crystallize in a rock-salt-type structure with random cation disorder. Neutron data show that a small fraction of Ge occupies tetrahedral voids for x = 2 and 3. For x > 6, (GeTe)x(LiSbTe2)2 forms a GeTe-type structure that shows a phase transition to a cubic high-temperature phase at ca. 280 °C. The thermoelectric properties of (GeTe)11(LiSbTe2)2 have been investigated and show that this compound is a promising thermoelectric material with a ZT value of 1.0 at 450 °C. The high ZT value of the thermodynamically stable compound is caused by a low phononic contribution to the thermal conductivity; probably, Li acts as a "pseudo-vacancy".
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Affiliation(s)
- Thorsten Schröder
- LMU Munich , Department of Chemistry, Butenandtstr. 5-13 (D), 81377 Munich, Germany
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23
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Piccione B, Agarwal R, Jung Y, Agarwal R. Size-dependent chemical transformation, structural phase-change, and optical properties of nanowires. PHILOSOPHICAL MAGAZINE (ABINGDON, ENGLAND) 2013; 93:2089-2121. [PMID: 23997656 PMCID: PMC3755783 DOI: 10.1080/14786435.2013.765981] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/04/2023]
Abstract
Nanowires offer a unique approach for the bottom up assembly of electronic and photonic devices with the potential of integrating photonics with existing technologies. The anisotropic geometry and mesoscopic length scales of nanowires also make them very interesting systems to study a variety of size-dependent phenomenon where finite size effects become important. We will discuss the intriguing size-dependent properties of nanowire systems with diameters in the 5 - 300 nm range, where finite size and interfacial phenomena become more important than quantum mechanical effects. The ability to synthesize and manipulate nanostructures by chemical methods allows tremendous versatility in creating new systems with well controlled geometries, dimensions and functionality, which can then be used for understanding novel processes in finite-sized systems and devices.
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Affiliation(s)
- Brian Piccione
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, U.S.A
| | - Rahul Agarwal
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, U.S.A
| | - Yeonwoong Jung
- Department of Electrical Engineering, Yale University, New Haven, CT, U.S.A
| | - Ritesh Agarwal
- Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA, U.S.A
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Gabardi S, Caravati S, Bernasconi M, Parrinello M. Density functional simulations of Sb-rich GeSbTe phase change alloys. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2012; 24:385803. [PMID: 22945279 DOI: 10.1088/0953-8984/24/38/385803] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
We generated models of the amorphous phase of Sb-rich GeSbTe phase change alloys by quenching from the melt within density functional molecular dynamics. We considered the two compositions Ge(1)Sb(1)Te(1) and Ge(2)Sb(4)Te(5). Comparison with previous results on the most studied Ge(2)Sb(2)Te(5) allowed us to draw some conclusions on the dependence of the structural properties of the amorphous phase on the alloy composition. Vibrational and electronic properties were also scrutinized. Phonons at high frequencies above 200 cm(-1) are localized in tetrahedra around Ge atoms in Sb-rich compounds as well as in Ge(2)Sb(2)Te(5). All compounds are semiconducting in the amorphous phase, with a band gap in the range 0.7-1.0 eV.
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Affiliation(s)
- S Gabardi
- Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Milano, Italy
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26
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Xu M, Cheng YQ, Wang L, Sheng HW, Meng Y, Yang WG, Han XD, Ma E. Pressure tunes electrical resistivity by four orders of magnitude in amorphous Ge2Sb2Te5 phase-change memory alloy. Proc Natl Acad Sci U S A 2012; 109:E1055-62. [PMID: 22509004 PMCID: PMC3344948 DOI: 10.1073/pnas.1119754109] [Citation(s) in RCA: 51] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Ge-Sb-Te-based phase-change memory is one of the most promising candidates to succeed the current flash memories. The application of phase-change materials for data storage and memory devices takes advantage of the fast phase transition (on the order of nanoseconds) and the large property contrasts (e.g., several orders of magnitude difference in electrical resistivity) between the amorphous and the crystalline states. Despite the importance of Ge-Sb-Te alloys and the intense research they have received, the possible phases in the temperature-pressure diagram, as well as the corresponding structure-property correlations, remain to be systematically explored. In this study, by subjecting the amorphous Ge(2)Sb(2)Te(5) (a-GST) to hydrostatic-like pressure (P), the thermodynamic variable alternative to temperature, we are able to tune its electrical resistivity by several orders of magnitude, similar to the resistivity contrast corresponding to the usually investigated amorphous-to-crystalline (a-GST to rock-salt GST) transition used in current phase-change memories. In particular, the electrical resistivity drops precipitously in the P = 0 to 8 GPa regime. A prominent structural signature representing the underlying evolution in atomic arrangements and bonding in this pressure regime, as revealed by the ab initio molecular dynamics simulations, is the reduction of low-electron-density regions, which contributes to the narrowing of band gap and delocalization of trapped electrons. At P > 8 GPa, we have observed major changes of the average local structures (bond angle and coordination numbers), gradually transforming the a-GST into a high-density, metallic-like state. This high-pressure glass is characterized by local motifs that bear similarities to the body-centered-cubic GST (bcc-GST) it eventually crystallizes into at 28 GPa, and hence represents a bcc-type polyamorph of a-GST.
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Affiliation(s)
- M. Xu
- Department of Materials Science and Engineering, the Johns Hopkins University, Baltimore, MD 21218
| | - Y. Q. Cheng
- Department of Materials Science and Engineering, the Johns Hopkins University, Baltimore, MD 21218
- Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN 37381
| | - L. Wang
- High Pressure Synergetic Consortium, Carnegie Institution of Washington, Argonne, IL 60439
- State Key Laboratory of Superhard Materials, Jilin University, Changchun, 130012, China
| | - H. W. Sheng
- School of Physics, Astronomy and Computational Sciences, George Mason University, Fairfax, VA 22030
| | - Y. Meng
- High Pressure Collaborative Access Team, Carnegie Institution of Washington, Argonne, IL 60439; and
| | - W. G. Yang
- High Pressure Synergetic Consortium, Carnegie Institution of Washington, Argonne, IL 60439
| | - X. D. Han
- Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100022, China
| | - E. Ma
- Department of Materials Science and Engineering, the Johns Hopkins University, Baltimore, MD 21218
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27
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Lee TH, Elliott SR. Ab Initio computer simulation of the early stages of crystallization: application to Ge(2)Sb(2)Te(5) phase-change materials. PHYSICAL REVIEW LETTERS 2011; 107:145702. [PMID: 22107213 DOI: 10.1103/physrevlett.107.145702] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/12/2011] [Revised: 07/30/2011] [Indexed: 05/31/2023]
Abstract
By virtue of the ultrashort phase-transition time of phase-change memory materials, e.g., Ge(2)Sb(2)Te(5), we successfully reproduce the early stages of crystallization in such a material using ab initio molecular-dynamics simulations. A stochastic distribution in the crystallization onset time is found, as generally assumed in classical nucleation theory. The critical crystal nucleus is estimated to comprise 5-10 (Ge,Sb)(4)Te(4) cubes. Simulated growth rates of crystalline clusters in amorphous Ge(2)Sb(2)Te(5) are consistent with extrapolated experimental measurements. The formation of ordered planar structures in the amorphous phase plays a critical role in lowering the interfacial energy between crystalline clusters and the amorphous phase, which explains why Ge-Sb-Te materials exhibit ultrafast crystallization.
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Affiliation(s)
- T H Lee
- Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom
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28
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Caravati S, Colleoni D, Mazzarello R, Kühne TD, Krack M, Bernasconi M, Parrinello M. First-principles study of nitrogen doping in cubic and amorphous Ge2Sb2Te5. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2011; 23:265801. [PMID: 21673401 DOI: 10.1088/0953-8984/23/26/265801] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
We investigated the structural, electronic and vibrational properties of amorphous and cubic Ge(2)Sb(2)Te(5) doped with N at 4.2 at.% by means of large scale ab initio simulations. Nitrogen can be incorporated in molecular form in both the crystalline and amorphous phases at a moderate energy cost. In contrast, insertion of N in the atomic form is very energetically costly in the crystalline phase, though it is still possible in the amorphous phase. These results support the suggestion that N segregates at the grain boundaries during the crystallization of the amorphous phase, resulting in a reduction in size of the crystalline grains and an increased crystallization temperature.
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Affiliation(s)
- S Caravati
- Computational Science, Department of Chemistry and Applied Biosciences, ETH Zurich, USI Campus, Via Giuseppe Buffi 13, CH-6900 Lugano, Switzerland
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Pressure-induced reversible amorphization and an amorphous-amorphous transition in Ge₂Sb₂Te₅ phase-change memory material. Proc Natl Acad Sci U S A 2011; 108:10410-4. [PMID: 21670255 DOI: 10.1073/pnas.1107464108] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Ge(2)Sb(2)Te(5) (GST) is a technologically very important phase-change material that is used in digital versatile disks-random access memory and is currently studied for the use in phase-change random access memory devices. This type of data storage is achieved by the fast reversible phase transition between amorphous and crystalline GST upon heat pulse. Here we report pressure-induced reversible crystalline-amorphous and polymorphic amorphous transitions in NaCl structured GST by ab initio molecular dynamics calculations. We have showed that the onset amorphization of GST starts at approximately 18 GPa and the system become completely random at approximately 22 GPa. This amorphous state has a cubic framework (c-amorphous) of sixfold coordinations. With further increasing pressure, the c-amorphous transforms to a high-density amorphous structure with trigonal framework (t-amorphous) and an average coordination number of eight. The pressure-induced amorphization is investigated to be due to large displacements of Te atoms for which weak Te-Te bonds exist or vacancies are nearby. Upon decompressing to ambient conditions, the original cubic crystalline structure is restored for c-amorphous, whereas t-amorphous transforms to another amorphous phase that is similar to the melt-quenched amorphous GST.
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Lencer D, Salinga M, Wuttig M. Design rules for phase-change materials in data storage applications. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2011; 23:2030-2058. [PMID: 21469218 DOI: 10.1002/adma.201004255] [Citation(s) in RCA: 146] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/18/2011] [Indexed: 05/30/2023]
Abstract
Phase-change materials can rapidly and reversibly be switched between an amorphous and a crystalline phase. Since both phases are characterized by very different optical and electrical properties, these materials can be employed for rewritable optical and electrical data storage. Hence, there are considerable efforts to identify suitable materials, and to optimize them with respect to specific applications. Design rules that can explain why the materials identified so far enable phase-change based devices would hence be very beneficial. This article describes materials that have been successfully employed and dicusses common features regarding both typical structures and bonding mechanisms. It is shown that typical structural motifs and electronic properties can be found in the crystalline state that are indicative for resonant bonding, from which the employed contrast originates. The occurence of resonance is linked to the composition, thus providing a design rule for phase-change materials. This understanding helps to unravel characteristic properties such as electrical and thermal conductivity which are discussed in the subsequent section. Then, turning to the transition kinetics between the phases, the current understanding and modeling of the processes of amorphization and crystallization are discussed. Finally, present approaches for improved high-capacity optical discs and fast non-volatile electrical memories, that hold the potential to succeed present-day's Flash memory, are presented.
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Affiliation(s)
- Dominic Lencer
- I. Physikalisches Institut IA, RWTH Aachen University, Germany
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31
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Liu XQ, Li XB, Zhang L, Cheng YQ, Yan ZG, Xu M, Han XD, Zhang SB, Zhang Z, Ma E. New structural picture of the Ge2Sb2Te5 phase-change alloy. PHYSICAL REVIEW LETTERS 2011; 106:025501. [PMID: 21405239 DOI: 10.1103/physrevlett.106.025501] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/11/2010] [Indexed: 05/30/2023]
Abstract
Using electron microscopy and diffraction techniques, as well as first-principles calculations, we demonstrate that as much as 35% of the total Ge atoms in the cubic phase of Ge2Sb2Te5 locate in tetrahedral environments. The Ge-vacancy interactions play a crucial stabilizing role, leading to Ge-vacancy pairs and the sharing of vacancies that clusters tetrahedral Ge into domains. The Ge2Sb2Te5 structure with coexisting octahedral and tetrahedral Ge produces optical and structural properties in good agreement with experimental data and explains the property contrast as well as the rapid transformation in this phase-change alloy.
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Affiliation(s)
- X Q Liu
- Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100022, China
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Caravati S, Bernasconi M, Parrinello M. First principles study of the optical contrast in phase change materials. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2010; 22:315801. [PMID: 21399368 DOI: 10.1088/0953-8984/22/31/315801] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
We study from first principles the optical properties of the phase change materials Ge(2)Sb(2)Te(5) (GST), GeTe and Sb(2)Te(3) in the crystalline phase and in realistic models of the amorphous phase generated by quenching from the melt in ab initio molecular dynamics simulations. The calculations reproduce the strong optical contrast between the crystalline and amorphous phases measured experimentally and exploited in optical data storage. It is demonstrated that the optical contrast is due to a change in the optical matrix elements across the phase change in all the compounds. It is concluded that the reduction of the optical matrix elements in the amorphous phases is due to angular disorder in p-bonding which dominates the amorphous network in agreement with previous proposals (Huang and Robertson 2010 Phys. Rev. B 81 081204) based on calculations on crystalline models.
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Affiliation(s)
- S Caravati
- Department of Chemistry and Applied Biosciences, ETH Zurich, USI Campus, Via Giuseppe Buffi 13, 6900 Lugano, Switzerland
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Cho E, Im J, Park C, Son WJ, Kim DH, Horii H, Ihm J, Han S. Atomic and electronic structures of amorphous Ge(2)Sb(2)Te(5); melt-quenched versus ideal glasses. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2010; 22:205504. [PMID: 21393709 DOI: 10.1088/0953-8984/22/20/205504] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
To investigate an amorphous structure of Ge(2)Sb(2)Te(5) that satisfies the 8-N rule (so-called 'ideal glass'), we perform alternative melt-quench simulations on Si(2)As(2)Se(5) and replace atoms in the final structure with Ge-Sb-Te. The resulting structures have salient features of the 8-N rule such as the tetrahedral configuration for all Ge atoms and the localized Te lone pairs at the valence top. In addition, the average Ge-Te and Sb-Te distances are in good agreement with experiment. The energetic stability of the ideal glass supports the existence of this amorphous structure that is distinct from the melt-quenched glass. From the analysis of electronic structures and optical dielectric constants, it is concluded that the electronic character of the melt-quenched amorphous Ge(2)Sb(2)Te(5) lies in between the resonant p-bonding of the crystalline phase and the covalent bonding of the ideal glass.
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Affiliation(s)
- E Cho
- Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea
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