Ab initio molecular dynamics and materials design for embedded phase-change memory

AbstractThe Ge2Sb2Te5 alloy has served as the core material in phase-change memories with high switching speed and persistent storage capability at room temperature. However widely used, this composition is not suitable for embedded memories—for example, for automotive applications, which require very high working temperatures above 300 °C. Ge–Sb–Te alloys with higher Ge content, most prominently Ge2Sb1Te2 (‘212’), have been studied as suitable alternatives, but their atomic structures and structure–property relationships have remained widely unexplored. Here, we report comprehensive first-principles simulations that give insight into those emerging materials, located on the compositional tie-line between Ge2Sb1Te2 and elemental Ge, allowing for a direct comparison with the established Ge2Sb2Te5 material. Electronic-structure computations and smooth overlap of atomic positions (SOAP) similarity analyses explain the role of excess Ge content in the amorphous phases. Together with energetic analyses, a compositional threshold is identified for the viability of a homogeneous amorphous phase (‘zero bit’), which is required for memory applications. Based on the acquired knowledge at the atomic scale, we provide a materials design strategy for high-performance embedded phase-change memories with balanced speed and stability, as well as potentially good cycling capability.

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An engineering model for high-speed switching in GeSbTe phase-change memory

Applied Physics Express ◽

10.35848/1882-0786/ac4a11 ◽

2022 ◽

Author(s):

Junji TOMINAGA

Keyword(s):

Phase Change ◽

High Speed ◽

High Performance ◽

Ab Initio Molecular Dynamics ◽

Switching Speed ◽

Non Volatile Memory ◽

Crystal Transition ◽

Dynamics Simulations ◽

High Speed Switching ◽

Successful Phase

Abstract Ge2Sb2Te5 is the most successful phase-change alloy in non-volatile memory using the amorphous-crystal phase transition. In deriving further high performance in switching, especially SET speed (from amorphous to crystal transition) should still be modified. In this work, It was examined an ideal Ge2Sb2Te5 alloy based on the Kolobov model using ab-initio molecular dynamics simulations. As a result, it was cleared that a uniaxial exchange between vacancies and Ge atoms is the crucial role in realizing high-speed switching and a large contrast in the resonance bonding state in the alloy. The vacancy engineering enables the alloy switching speed extremely faster.

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A high performance phase change memory with fast switching speed and high temperature retention by engineering the GexSbyTez phase change material

2011 International Electron Devices Meeting ◽

10.1109/iedm.2011.6131481 ◽

2011 ◽

Cited By ~ 11

Author(s):

H. Y. Cheng ◽

T. H. Hsu ◽

S. Raoux ◽

J.Y. Wu ◽

P. Y. Du ◽

...

Keyword(s):

High Temperature ◽

Phase Change ◽

Phase Change Material ◽

High Performance ◽

Phase Change Memory ◽

Switching Speed ◽

Fast Switching ◽

Change Material ◽

Change Memory ◽

Fast Switching Speed

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Correlating the electronic structures of metallic/semiconducting MoTe2 interface to its atomic structures

National Science Review ◽

10.1093/nsr/nwaa087 ◽

2020 ◽

Author(s):

Bo Han ◽

Chen Yang ◽

Xiaolong Xu ◽

Yuehui Li ◽

Ruochen Shi ◽

...

Keyword(s):

Phase Boundary ◽

Electronic Structures ◽

2D Materials ◽

Atomic Scale ◽

Contact Interface ◽

Structure Property ◽

Atomic Structures ◽

Structure Property Relationships ◽

Scanning Transmission ◽

Unit Cells

Abstract Contact interface properties are important in determining the performances of devices that are based on atomically thin two-dimensional (2D) materials, especially for those with short channels. Understanding the contact interface is therefore important to design better devices. Herein, we use scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles calculations to reveal the electronic structures within the metallic (1T′)-semiconducting (2H) MoTe2 coplanar phase boundary across a wide spectral range and correlate its properties to atomic structures. We find that the 2H-MoTe2 excitonic peaks cross the phase boundary into the 1T′ phase within a range of approximately 150 nm. The 1T′-MoTe2 crystal field can penetrate the boundary and extend into the 2H phase by approximately two unit-cells. The plasmonic oscillations exhibit strong angle dependence, that is a red-shift of π+σ (approximately 0.3–1.2 eV) occurs within 4 nm at 1T′/2H-MoTe2 boundaries with large tilt angles, but there is no shift at zero-tilted boundaries. These atomic-scale measurements reveal the structure–property relationships of the 1T′/2H-MoTe2 boundary, providing useful information for phase boundary engineering and device development based on 2D materials.

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Complex Atomic-Scale Structures in Solids by a Combination of Theory and Microscopy

Microscopy and Microanalysis ◽

10.1017/s1431927600023928 ◽

1998 ◽

Vol 4 (S2) ◽

pp. 760-761

Author(s):

S. T. Pantelides ◽

S. J. Pennycook ◽

M. F. Chisholm ◽

A. Maiti ◽

Y. Yan ◽

...

Keyword(s):

High Performance ◽

Atomic Scale ◽

Extended Defects ◽

Atomic Structures ◽

Spatially Resolved ◽

Transmission Electron ◽

Scanning Transmission ◽

Synergistic Approach ◽

Comprehensive Picture ◽

Electron Microscopes

Modern high-performance computers are now capable of calculations that can be used to determine the preferred atomic arrangements in complex systems. Both first-principles and semiempirical approaches have been developed with complementary capabilities. At the same time, powerful transmission electron microscopes have been developed that yield direct atomic-scale images of crystals and extended defects such as dislocations, grain boundaries and buried interfaces. This paper presents several examples where a synergistic approach combining theoretical results, Z-contrast scanning transmission electron microscopy, and spatially resolved electron energy loss spectroscopy (EELS) have led to the elucidation of complex atomic structures. In some cases, theory predicts, experiment confirms and expands and theory revisits; in other cases, observations come first and theory helps put together a comprehensive picture or goes beyond the original observations with new predictions. In both cases, equilibrium structures, impurity or stressinduced structural transformations, and dynamical processes such as diffusion, segregation or precipitation are elucidated in great detail.

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The mechanism for the enhanced piezoelectricity in multi-elements doped (K,Na)NbO3 ceramics

Nature Communications ◽

10.1038/s41467-021-21202-7 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Xiaoyi Gao ◽

Zhenxiang Cheng ◽

Zibin Chen ◽

Yao Liu ◽

Xiangyu Meng ◽

...

Keyword(s):

High Performance ◽

Functional Materials ◽

Atomic Scale ◽

Ferroelectric Ceramics ◽

Structure Property ◽

X Ray Diffraction ◽

Temperature Dependent ◽

Transmission Electron ◽

Scanning Transmission ◽

High Dielectric

Abstract(K,Na)NbO3 based ceramics are considered to be one of the most promising lead-free ferroelectrics replacing Pb(Zr,Ti)O3. Despite extensive studies over the last two decades, the mechanism for the enhanced piezoelectricity in multi-elements doped (K,Na)NbO3 ceramics has not been fully understood. Here, we combine temperature-dependent synchrotron x-ray diffraction and property measurements, atomic-scale scanning transmission electron microscopy, and first-principle and phase-field calculations to establish the dopant–structure–property relationship for multi-elements doped (K,Na)NbO3 ceramics. Our results indicate that the dopants induced tetragonal phase and the accompanying high-density nanoscale heterostructures with low-angle polar vectors are responsible for the high dielectric and piezoelectric properties. This work explains the mechanism of the high piezoelectricity recently achieved in (K,Na)NbO3 ceramics and provides guidance for the design of high-performance ferroelectric ceramics, which is expected to benefit numerous functional materials.

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Systematic materials design for phase-change memory with small density changes for high-endurance non-volatile memory applications

Applied Physics Express ◽

10.7567/1882-0786/ab1301 ◽

2019 ◽

Vol 12 (5) ◽

pp. 051008 ◽

Cited By ~ 2

Author(s):

Yuta Saito ◽

Shogo Hatayama ◽

Yi Shuang ◽

Satoshi Shindo ◽

Paul Fons ◽

...

Keyword(s):

Phase Change ◽

Phase Change Memory ◽

Materials Design ◽

Non Volatile Memory ◽

Small Density ◽

Volatile Memory ◽

Memory Applications ◽

Change Memory

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Phase change materials for building construction: An overview of nano-/micro-encapsulation

Nanotechnology Reviews ◽

10.1515/ntrev-2020-0067 ◽

2020 ◽

Vol 9 (1) ◽

pp. 896-921

Author(s):

Amende Sivanathan ◽

Qingqing Dou ◽

Yuxuan Wang ◽

Yunfeng Li ◽

Jorge Corker ◽

...

Keyword(s):

Energy Consumption ◽

Phase Change ◽

High Performance ◽

Phase Change Materials ◽

Structural Integrity ◽

Renewable Energy Sources ◽

Building Construction ◽

Core Material ◽

Energy Efficient Buildings ◽

Change Material

AbstractBuildings contribute to 40% of total global energy consumption, which is responsible to 38% of greenhouse gas emissions. It is critical to enhance the energy efficiency of buildings to mitigate global warming. In the last decade, advances in thermal energy storage (TES) techniques using phase change material (PCM) have gained much attention among researchers, mainly to reduce energy consumption and to promote the use of renewable energy sources such as solar energy. PCM technology is one of the most promising technologies available for the development of high performance and energy-efficient buildings and, therefore, considered as one of the most effective and on-going fields of research. The main limitation of PCM is its leakage problem which limits its potential use in building construction and other applications such as TES and textiles, which can be overcome by employing nano-/micro-encapsulation technologies. This paper comprehensively overviews the nano-/micro-encapsulation technologies, which are mainly classified into three categories including physical, physiochemical and chemical methods, and the properties of microcapsules prepared. Among all encapsulation technologies available, the chemical method is commonly used since it offers the best technological approach in terms of encapsulation efficiency and better structural integrity of core material. There is a need to develop a method for the synthesis of nano-encapsulated PCMs to achieve enhanced structural stability and better fracture resistance and, thus, longer service life. The accumulated database of properties/performance of PCMs and synthesised nano-/micro-capsules from various techniques presented in the paper should serve as the most useful information for the production of nano-/micro-capsules with desirable characteristics for building construction application and further innovation of PCM technology.

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Quasiperiodic interfaces: HREM observations of grain and phase boundaries

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100174795 ◽

1990 ◽

Vol 48 (4) ◽

pp. 332-333

Author(s):

K. L. Merkle

Keyword(s):

Atomic Structure ◽

Direct Determination ◽

Lattice Parameter ◽

Atomic Scale ◽

Misfit Dislocations ◽

Oxide Systems ◽

Atomic Structures ◽

Periodic Arrays ◽

Parameter Ratio ◽

Metal Metal

The atomic structures of internal interfaces have recently received considerable attention, not only because of their importance in determining many materials properties, but also because the atomic structure of many interfaces has become accessible to direct atomic-scale observation by modem HREM instruments. In this communication, several interface structures are examined by HREM in terms of their structural periodicities along the interface.It is well known that heterophase boundaries are generally formed by two low-index planes. Often, as is the case in many fcc metal/metal and metal/metal-oxide systems, low energy boundaries form in the cube-on-cube orientation on (111). Since the lattice parameter ratio between the two materials generally is not a rational number, such boundaries are incommensurate. Therefore, even though periodic arrays of misfit dislocations have been observed by TEM techniques for numerous heterophase systems, such interfaces are quasiperiodic on an atomic scale. Interfaces with misfit dislocations are semicoherent, where atomically well-matched regions alternate with regions of misfit. When the misfit is large, misfit localization is often difficult to detect, and direct determination of the atomic structure of the interface from HREM alone, may not be possible.

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Probing vacancies and structural distortions at individual defects in ceramics using EELS

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100165859 ◽

1996 ◽

Vol 54 ◽

pp. 678-679

Author(s):

D. J. Wallis ◽

N. D. Browning

Keyword(s):

Structural Information ◽

Core Loss ◽

Nearest Neighbors ◽

Ceramic Materials ◽

Scanning Transmission Electron Microscope ◽

Real Space ◽

Atomic Scale ◽

Structure Property ◽

Scanning Transmission ◽

Key Issues

In electron energy loss spectroscopy (EELS), the near-edge region of a core-loss edge contains information on high-order atomic correlations. These correlations give details of the 3-D atomic structure which can be elucidated using multiple-scattering (MS) theory. MS calculations use real space clusters making them ideal for use in low-symmetry systems such as defects and interfaces. When coupled with the atomic spatial resolution capabilities of the scanning transmission electron microscope (STEM), there therefore exists the ability to obtain 3-D structural information from individual atomic scale structures. For ceramic materials where the structure-property relationships are dominated by defects and interfaces, this methodology can provide unique information on key issues such as like-ion repulsion and the presence of vacancies, impurities and structural distortion.An example of the use of MS-theory is shown in fig 1, where an experimental oxygen K-edge from SrTiO3 is compared to full MS-calculations for successive shells (a shell consists of neighboring atoms, so that 1 shell includes only nearest neighbors, 2 shells includes first and second-nearest neighbors, and so on).

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