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Chapter 3

Nanowire Phase-Change Memory

Phase-change materials (PCMs) are an important class of materials that reversibly and rapidly change their structure from a stable crystalline to a metastable amorphous phase under the influence of an optical or electrical pulse. This chapter provides a comprehensive review of research in nanowire phase-change memory, a prospective candidate for universal memory. Traditionally, crystal-to-amorphous transformation in PCM thin-film devices was carried out through the application of electrical pulses, which joule heat to melt and quench the crystalline phase, whereas the amorphous-to-crystalline transformation was carried out by the application of electric pulses that heat the amorphous phase above the crystallization temperature. Problems with this ‘thermal’ strategy of switching in thin-film PCM that restrict continued scaling are briefly reviewed, and research in self-assembled 1D-PCM devices that show better scaling properties than their thin-film counterparts is highlighted. Upon repeated switching, device performance of all the PCM devices degrades owing to issues of electromigration, and this effect is enhanced with top-down processing and polycrystallinity of thin-films. Bottom-up synthesized single-crystalline nanowires, owing to their lateral and confined geometry show better scaling, retention, and endurance behavior (switching properties) in comparison to thin-film PCM devices. A detailed review on the synthesis of single-crystalline nanowires and switching properties of nanowire devices is provided. Revolutionary and exciting applications such as multilevel switching using core/shell nanowire devices are discussed, and it is shown that the drift of electrical properties in amorphous phase – an important challenge to overcome to realize multilevel switching capability – is stress dependent, and is less pronounced in nanowire devices owing to their better stress–relaxation mechanisms. The melt-quench mechanism of crystal to amorphous switching in PCM is challenged, and a dislocation-templated and carrier-wind force driven amorphization based mechanism, which directly makes use of the unique bonding and structure in crystalline PCM for phase-change, is discussed through a detailed review of in situ electron microscopy based experiments on PCM nanowire devices.

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05 Dec 2014
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From the book series:
Smart Materials Series