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In Situ Electron Microscopy of Electromigration in Nanomaterials

Author: ZhaoJiong
Tutor: ZhuJing
School: Tsinghua University
Course: Materials Science and Engineering
Keywords: In situ TEM Electromigration Defects Nanomaterials
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Type: PhD thesis
Year: 2012
Downloads: 200
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Abstract


We focused on the in situ electron microscopy of electromigration in variousnanomaterials, including nanowires, nanotubes, thin films, liquid, etc. Weperformed the in situ observations of the electromigration phenomena invacancies, surfaces, interfaces, dislocations and amorphous materials, where thedefects in crystals exist. In specific, the thesis can be divided into five chapters,they are respectively:Instantaneous electrical breakdown measurements of semiconductor andmetallic nanowires are performed by an in situ transmission electron microscopymethod. Our results directly reveal the mechanism that typical thermally heatedsemiconductor nanowires break at the midpoint, while metallic nanowiresbreakdown near the two ends due to the stress induced by electromigration. Thedifferent breakdown mechanisms for the nanowires are caused by the differentthermal and electrical properties of the materials.The mass transportation mechanism in electric-biased carbon nanotubes(CNTs) is investigated experimentally. Except for the widely acceptedelectromigration mechanism, we find out the thermal effect can also induce themass transportation in the form of thermomigration or thermal evaporation.Moreover, the convincing in situ transmission electron microscope experimentresults show the thermal gradient force overrides the electromigration force inmost conditions, according to specific parameters of the CNTs and “cargos”. Afull analysis on the thermal gradient force and electromigration force imposedon the cargos is given, thus our experimental results are well explained andunderstood.The electromigration of W surface atoms and edges are demonstrated by insitu TEM techniques. The surface grooving, faceting, and Rayleigh instability isdemonstrated. The probability distribution of fractal conductance change andtime interval during the electromigration of W wires obeys the scaling law, andit shows that the principle of intermittency dominates the whole surfaceelectromigration process. The electromigration of Al films was performed by in situ TEM technique.The diffraction contrast image can both show the structure of dislocations andthe dynamical movement of the dislocations. The electrical current could varythe size of the dislocation loops, which can be due to the atomic diffusion drivenby the electromigration force and the Ostwald Ripening force.The upwind self-electromigration of liquid metal is demonstrated by in situtransmission electron microscopy technique. Our direct observation shows that adc current through a liquid Gallium Ga nanobridge can maintain a steadynanofluidic flow along the liquid bridge itself. Among all the liquid bridges, theminimum width we observed in the experiments reaches3.5nm. It has beenrevealed that the onset migration current density is directly related to the widthof the bridge.These experimental observations and out analysis provide insights into thephysical picture of electromigration as well as opportunities for potentialapplications, such as protection design of nanodevices, nano-scale masstransportation, electroplastic engineering of metallic materials, etc.

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