Abstract
Steps on macroscopic surfaces provide a useful model system for quantifying electron scattering at defects in nanostructures, where the large surface/volume ratio will cause surface effects to dominate. Here, the effects of electron scattering at surface steps are quantified using thin silver films with (111) surface orientation. Using real-time scanning tunneling microscopy (STM) measurements while large current densities are applied to the films, changes in step fluctuations and island motion are observed and quantified. Applying the tools of the continuum step model, the observations are analyzed in terms of step free energies and kinetics, yielding quantitative values of the electromigration force driving the observed mass displacements. The derived magnitudes are surprisingly large in comparison with classical calculations of the force due to electron scattering at the internal surface of a conductor. This result indicates that the specific atomistic characteristics of the scattering sites, in this case kinks at the step edge, may greatly enhance the electromigration force. Within the classical ballistic picture of ballistic momentum transfer, specific mechanisms for such enhancement include enhanced geometric “blocking” at the kinked step edges, changes in carrier density near kinks, and current crowding. Quantum transmission effects at atomic-scale defect sites may also be responsible for the observed enhancement. The nature of classical current crowding as a function of the shape and size of defect was characterized using magnetic force microscopy (MFM) of fabricated micron-scale model structures. Techniques were developed to remove the effects of instrumental broadening using deconvolution, so that full three-dimensional maps of the magnetic fields above the current line are determined. A Green function inversion technique is then used to invert the field distribution to determine the spatial variations in the current density in the sample. Current enhancement is highly localized near defects and is maximized by sharp variations in geometry that require strong deflections of the current path. Current enhancements up to a factor of 4 are found at the most strongly deflecting defects, while small notches of various shapes typically cause local enhancements of tens of percent to a factor of 2. The perpendicular component of the current flow around defects forms a dipole pattern with length scale determined by the length of the defect along the direction of the current flow. The shape and localization of the dipole pattern vary with the sharpness and symmetry of the defect. The current crowding affect alone is not sufficient to explain the greatly enhanced electromigration force observed for scattering at kink sites at steps.
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Acknowledgments
This work has been supported by the University of Maryland NSF MRSEC under grant # DMR 05-20471, including use of the Shared Experimental Facilities. Infrastructure support is also provided by the UMD NanoCenter and CNAM.
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Yongsunthon, R., Tao, C., Rous, P., Williams, E. (2011). Surface Electromigration and Current Crowding. In: Michailov, M. (eds) Nanophenomena at Surfaces. Springer Series in Surface Sciences, vol 47. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-16510-8_5
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