Electrical Wind Force–Driven and Dislocation-Templated Amorphization in Phase-Change Nanowires

Nam, Sung Wook; Chung, Hee Suk; Lo, Yu Chieh; Qi, Lida; Li, Ju; Lu, Ye; Johnson, A. T. Charlie; Jung, Yeonwoong; Nukala, Pavan; Agarwal, Ritesh

doi:10.1126/science.1220119

Cited by 161 publications

(172 citation statements)

References 32 publications

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“…More recently, the observations of in situ microstructural changes using transmission electron microscopy (TEM) have shown the important role of extended defects such as line defects, formed by vacancy condensation during the amorphization of single-crystalline GST nanowires. 13 The reversible phase transition process in PCM material is mainly controlled by the thermal conduction; the heat can be generated either from a laser light or voltage pulse leads to Joule heating. Thus, it is important to understand the contribution of phonons to the thermal transport and its energetics in PCM based materials at low dimensions.…”

Section: Introductionmentioning

confidence: 99%

The role of atomic vacancies on phonon confinement in α-GeTe

Kalra

Murugavel

2015

AIP Advances

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Atomic defects and their dynamics play a vital role in controlling the behavior of non-volatile phase change memory materials used in advanced optical storage devices. Synthesis and structural analysis by XRD and Raman spectroscopy on α-GeTe single crystal with different sizes are reported. The spectroscopic measurements on micron and nano sized α-GeTe single crystal reveal the evolution of phonon confinement with crystal sizes of few hundred nanometers. The characteristic vibrational modes of bulk α-GeTe structure are found to downshift and asymmetrically broaden to lower frequency with decreasing the single crystal size. We attribute the observed downshift of Raman lines in α-GeTe is largely due to the presence of high concentration of atomic vacancies. The crystal size and temperature dependent Raman spectra provide explicitly the dynamics of vacancies on optical phonon confinement in α-GeTe structure. Thus, the observed large concentration of vacancies and their size dependency might influence the phase change phenomenon in GeTe based alloys.

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Section: Introductionmentioning

confidence: 99%

The role of atomic vacancies on phonon confinement in α-GeTe

Kalra

Murugavel

2015

AIP Advances

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“…S1). Such priming effects are largely difficult to detect through conventional material characterization, for instance electron diffraction of the gradual changes in the structure of GST caused by multiple electrical pulses (13), but are evidenced in this study via a simple approach comprising in situ electricalconductivity measurements of both pronounced and fast variations in the GST structure upon applying a single electrical pulse. Such high electrical sensitivity is the key to revealing the microscopic origin of an s-to-l transition in this material.…”

Section: Significancementioning

confidence: 99%

Ultrafast phase-change logic device driven by melting processes

Loke

Skelton

Wang

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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The ultrahigh demand for faster computers is currently tackled by traditional methods such as size scaling (for increasing the number of devices), but this is rapidly becoming almost impossible, due to physical and lithographic limitations. To boost the speed of computers without increasing the number of logic devices, one of the most feasible solutions is to increase the number of operations performed by a device, which is largely impossible to achieve using current silicon-based logic devices. Multiple operations in phase-change-based logic devices have been achieved using crystallization; however, they can achieve mostly speeds of several hundreds of nanoseconds. A difficulty also arises from the tradeoff between the speed of crystallization and long-term stability of the amorphous phase. We here instead control the process of melting through premelting disordering effects, while maintaining the superior advantage of phase-change-based logic devices over silicon-based logic devices. A melting speed of just 900 ps was achieved to perform multiple Boolean algebraic operations (e.g., NOR and NOT). Ab initio molecular-dynamics simulations and in situ electrical characterization revealed the origin (i.e., bond buckling of atoms) and kinetics (e.g., discontinuouslike behavior) of melting through premelting disordering, which were key to increasing the melting speeds. By a subtle investigation of the well-characterized phase-transition behavior, this simple method provides an elegant solution to boost significantly the speed of phase-change-based in-memory logic devices, thus paving the way for achieving computers that can perform computations approaching terahertz processing rates.computing | chalcogenides T he extremely high and ever-increasing demand for faster computers is currently addressed by traditional methods, such as miniaturization (to increase the number of devices), but this is rapidly becoming almost impossible, due to physical and lithographic constraints (1-4). The speed of computers is known to be determined almost solely by the performance of logic devices, i.e., their speed and number, which control most computations (or processes) in computers (5). Logic devices are mostly required to operate around 1 ns for achieving fast computations, and to transfer information between alternate devices, such as random-access memories, without delays (5). To increase the speed of computers without increasing the number of logic devices, one of the most feasible methods is to increase the number of operations performed by a device, which is largely unachievable using current silicon-based logic devices (6, 7).Multiple operations in a logic device are currently best achieved using devices comprised of phase-change nonvolatile memory materials-based on the reversible and multilevel switching of a phase-change material (PCM) between crystalline and glassy states having a contrast in physical properties, e.g., electrical resistivity (8, 9)-which can perform more than three times the number of operations than can s...

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“…However, most in situ TEM experiments have been carried out using model structures, for example, suspended bridge or free-standing wire forms of GST. 10,17,18 Although useful, the information obtained from such experiments is not directly applicable to the practical design of PCRAM cells, as the actual current density and associated Joule heat in a confined cell geometry is quite different.…”

Section: Introductionmentioning

confidence: 99%

Microstructure-dependent DC set switching behaviors of Ge–Sb–Te-based phase-change random access memory devices accessed by in situ TEM

et al. 2015

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Phase-change random access memory (PCRAM) is one of the most promising nonvolatile memory devices. However, inability to secure consistent and reliable switching operations in nanometer-scale programing volumes limits its practical use for highdensity applications. Here, we report in situ transmission electron microscopy investigation of the DC set switching of Ge-Sb-Te (GST)-based vertical PCRAM cells. We demonstrate that the microstructure of GST, particularly the passive component surrounding the dome-shaped active switching volume, plays a critical role in determining the local temperature distribution and is therefore responsible for inconsistent cell-to-cell switching behaviors. As demonstrated by a PCRAM cell with a highly crystallized GST matrix, the excessive Joule heat can cause melting and evaporation of the switching volume, resulting in device failure. The failure occurred via two-step void formation due to accelerated phase separation in the molten GST by the polaritydependent atomic migration of constituent elements. The presented real-time observations contribute to the understanding of inconsistent switching and premature failure of GST-based PCRAM cells and can guide future design of reliable PCRAM. INTRODUCTIONPhase-change random access memory (PCRAM) devices store and erase information by utilizing a large resistivity difference between the crystalline and amorphous states of chalcogenide materials. 1 Among various chalcogenide materials, the Ge-Sb-Te (GST) alloy system is currently considered the most promising candidate material for PCRAM owing to its fast and reversible phase-transition capability, in both the amorphous-to-crystalline (set) and the reverse crystallineto-amorphous (reset) switching. 2,3 Moreover, the set switching of GST occurs through an abrupt increase in the current density at a critical electric field, which is known as threshold switching. 1,4 The threshold switching provides local Joule heat and thereby facilitates the crystallization process at a relatively low electric field, ranging from 30 to 50 V μm − 1 . 5,6 Many modern PCRAM cell designs adopt a nanometer-scale hemispherical or cylindrical shape for programing volume to increase the cell density and reduce the operation power. 7 In such device structures, only a portion of the programing volume makes direct contact with a heater electrode, and the other sides are surrounded by passive GST that remains in the crystalline state and acts as an electrical conductivity path during the set and reset switching. One of the critical reliability issues related to these cell structures is the compositional change, or phase separation, caused by electric field-

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Electrical Wind Force–Driven and Dislocation-Templated Amorphization in Phase-Change Nanowires

Cited by 161 publications

References 32 publications

The role of atomic vacancies on phonon confinement in α-GeTe

The role of atomic vacancies on phonon confinement in α-GeTe

Ultrafast phase-change logic device driven by melting processes

Microstructure-dependent DC set switching behaviors of Ge–Sb–Te-based phase-change random access memory devices accessed by in situ TEM

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