Nonequilibrium Interface Dynamics:
Fundamental Physical Issues in Nonequilibrium Interface Dynamics

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Current-induced Instabilities on Si Surfaces

Dr. John Weeks

Institute for Physical Science and Technology and Department of Chemistry and Biochemistry,
University of Maryland

Abstract:   Crystal surfaces with atomic steps can exhibit a number of different morphological instabilities that may be important in crystal growth and nano-scale device fabrication. Particularly interesting step bunching and step wandering patterns arise on Si surfaces when heated by a direct electric current. These patterns have a strong dependence on both the current direction and the temperature. We discuss a novel two-region diffusion model that describes the interplay between driven diffusion of adatoms induced by the electric field and the different kinds of surface reconstruction found on terraces and in a finite region around a step. In particular we argue that the step wandering instability seen on Si(111) surfaces at temperatures between 1050C and 1150C can be understood by assuming faster diffusion in step regions than on the terraces. By taking the proper limit of a model with discrete diffusion jumps, we show that in the conventional continuum BCF picture with boundary conditions at a sharp step this corresponds to a model with a negative kinetic coefficient. Effective negative kinetic coefficients also play an important role in rationalizing the very different behavior seen for Si(001) surfaces. We also use a geometric representation in terms of arc length and curvature to derive a nonlinear evolution equation for a step in the presence of an electric field oriented at an angle to the average step direction. This work is supported by the National Science Foundation under the Materials Research Science and Engineering Center grant DMR-00-80008.