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HH2.06 - Electrical Wind Force-Driven and Dislocation-Templated Amorphization in Phase-Change Materials 
Date/Time:
April 22, 2014   4:00pm - 4:30pm
 
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Electrical wind force is an important element in switching behaviors of phase-change materials. It is hypothesized that the electrical wind force influences the degree of order-disorder existing in phase-change materials, by coupling with the motions of one-dimensional defect structures, namely, dislocations. We discuss electrical wind force-driven behaviors occurring in phase-change materials. First, we report mass transport behaviors when DC voltage biases are applied in line-shape Ge2Sb2Te5 (GST) devices. As the electrical current density reached 3-4 MA/cm2 by DC voltage bias, a directional mass transport was identified by forming asymmetric surface morphology. Joule-heating by the current density raised the temperature up to ~300 oC, implying that the mass transport of GST occurs in hexagonal phase (solid state) regime. Second, we extend the understanding about the presence of electrical wind force to the electrical switching behaviors of GST. We studied the effect of electrical voltage pulses on crystalline-to-amorphous phase transition of GST by in situ transmission electron microscopy (TEM). The electrical voltage pulse plays a critical role by creating dislocations through heat shock process: The rising edge of the pulse produces vacancies by heating, whereas during rapid cooling, atomic vacancies are condensed into dislocation loops. As the dislocations feel the electrical wind force, they become mobile and glide in the direction of hole-carrier motion. The continuous increase in the density of dislocations moving unidirectionally leads to dislocation jamming, which eventually induces the crystalline-to-amorphous phase transition. We interpreted it through one-dimensional traffic model in which an increase of the dislocation density exceeding a certain threshold point induces a catastrophic jamming of dislocations. Density functional theory (DFT) calculations of generalized-stacking-fault (GSF) energy showed that the basal plane of GST hexagonal phase provides favorable pathways of dislocation motions. Our understanding about the dislocation-templated amorphization suggests that outstanding capability of incorporating dislocations attributed to GST-layered structure is one of the origins of the fast switching behaviors.
 


 
 
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