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Chalcogenide-based phase change materials (PCMs) are promising candidates for the active element in novel electrical nonvolatile memories and have been applied successfully in rewritable optical disks. Nanostructured PCMs are considered as the next generation building blocks for their low power consumption, high storage density, and fast switching speed. Yet their crystallization kinetics at high temperature, the rate-limiting property upon switching, faces great challenges due to the short time and length scales involved. Here we present a facile method to synthesize highly controlled, ligand-free GeTe nanoparticles, an important PCM, with an average diameter under 10 nm. Subsequent crystallization by slow and ultrafast rates allows unravelling of the crystallization kinetics, demonstrating the breakdown of Arrhenius behavior for the crystallization rate and a fragile-to-strong transition in the viscosity as well as the overall crystal growth rate for the as-deposited GeTe nanoparticles. The obtained results pave the way for further development of phase-change memory based on GeTe with sub-lithographic sizes. [ABSTRACT FROM AUTHOR]

A phase change nanowire holds a promise for nonvolatile memory applications, but its transition mechanism has remained unclear due to the analytical difficulties at atomic resolution. Here we obtain a deeper understanding on the phase transition of a single crystalline Ge2Sb2Te5 nanowire (GST NW) using atomic scale imaging, diffraction, and chemical analysis. Our cross-sectional analysis has shown that the as-grown hexagonal close-packed structure of the single crystal GST NW transforms to a metastable face-centered cubic structure due to the atomic migration to the pre-existing vacancy layers in the hcp structure going through iterative electrical switching. We call this crystal structure transformation "metastabilization", which is also confirmed by the increase of set-resistance during the switching operation. For the set to reset transition between crystalline and amorphous phases, high-resolution imaging indicates that the longitudinal center of the nanowire mainly undergoes phase transition. According to the atomic scale analysis of the GST NW after repeated electrical switching, partial crystallites are distributed around the core-centered amorphous region of the nanowire where atomic migration is mainly induced, thus potentially leading to low power electrical switching. These results provide a novel understanding of phase change nanowires, and can be applied to enhance the design of nanowire phase change memory devices for improved electrical performance. [ABSTRACT FROM AUTHOR]

Phasechange materials are widely considered for application innonvolatile memories because of their ability to achieve phase transformationin the nanosecond time scale. However, the knowledge of fast crystallizationdynamics in these materials is limited because of the lack of fastand accurate temperature control methods. In this work, we have developedan experimental methodology that enables ultrafast characterizationof phase-change dynamics on a more technologically relevant melt-quenchedamorphous phase using practical device structures. We have extractedthe crystallization growth velocity (U) in a functionalcapped phase change memory (PCM) device over 8 orders of magnitude(10–10< U< 10–1m/s) spanning a wide temperature range (415 < T< 580 K). We also observed direct evidence of non-Arrhenius crystallizationbehavior in programmed PCM devices at very high heating rates (>108K/s), which reveals the extreme fragility of Ge2Sb2Te5in its supercooled liquid phase. Furthermore,these crystallization properties were studied as a function of deviceprogramming cycles, and the results show degradation in the cell retentionproperties due to elemental segregation. The above experiments areenabled by the use of an on-chip fast heater and thermometer calledas microthermal stage (MTS) integrated with a vertical phase changememory (PCM) cell. The temperature at the PCM layer can be controlledup to 600 K using MTS and with a thermal time constant of 800 ns,leading to heating rates ∼108K/s that are closeto the typical device operating conditions during PCM programming.The MTS allows us to independently control the electrical and thermalaspects of phase transformation (inseparable in a conventional PCMcell) and extract the temperature dependence of key material propertiesin real PCM devices. [ABSTRACT FROM AUTHOR]