Electromigration induced changes in structure result from a diffusion bias caused by an imposed external electric field. The mechanisms by which the diffusion bias is induced (wind force or direct force) are similar in the surface and the bulk. Thus in temperature regimes where surface diffusion is more readily activated than bulk diffusion, surface electromigration should play an important role in changes in morphology of a current-carrying structure. Surface electromigration occurs on Si(111) surfaces at elevated temperatures. The signature is the spontaneous evolution of non-equilibrium configurations of the steps with one direction of the current, and the return to equilibrium with the opposite direction of current with respect to the "down-hill" step direction. Quantification of the nature of the surface electromigration force has been performed by STM-measurement of the shapes and rates of the evolving non-equilibrium morphologies, and by comparison of the rates of decay in the presence and absence of the stabilizing direct current. The basic mechanism of both formation and decay is the motion of individual steps, which occurs with a rate governed by the rate of equilibrium fluctuations of the steps. During formation of non-equilibrium structures, individual fluctuations couple to the external field, yielding single-point step collisions which then "zip" up to form step bunches. The curvature at the "zip" point has been quantified in terms of the competing effects of energetic costs of step bending and the electromigration force. During decay, steps move in a nearly one-dimensional mode which has been quantified in terms of the step-step repulsions and the electromigration force. The results are consistent with an effective charge less than 0.01 electron units. The applications of this approach to studying broader problems in electromigration will be discussed.
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@footnote 1@Supported by the U. of MD NSF-MRSEC