AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS3-MoM

Paper SS3-MoM8
New Diffusion Mechanisms on Amorphous Surfaces

Monday, November 3, 2003, 10:40 am, Room 328

Session: Surface Diffusion and Wetting
Presenter: A.S. Dalton, University of Illinois at Urbana Champaign
Authors: A.S. Dalton, University of Illinois at Urbana Champaign
D. Llera-Hurlburt, University of Illinois at Urbana Champaign
E.G. Seebauer, University of Illinois at Urbana Champaign
K.R. Bray, North Carolina State University
G.N. Parsons, North Carolina State University
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The structural and energetic heterogeneity of amorphous surfaces should lead to effective values of the diffusivity that differ significantly from those on crystalline surfaces. However, little work has been done to investigate this possibility. The present work employs a combination of molecular dynamics simulations and experiments involving fractal analysis of surface topography to examine diffusion mechanisms on amorphous silicon (a-Si). Simulations indicate that surface diffusion on a-Si involves substantial exchange with the underlying bulk, but is dominated primarily by short-lived atoms generated from strained three-membered ring structures. The total effective activation energy for mass transport is 2.3 eV, and the formation energy for the most mobile species is about 0.8 eV. Experiments probed crystalline grains grown on a-Si surfaces by simple annealing at 635 to 665°C. Atomic force microscopy images were examined using dimensional fractal analysis to extract the static scaling coefficient and lateral correlation length. The transport rate varied with the length scale over which it was measured--a new result that represents the first experimental measurement of this theoretically predicted phenomenon. At short length scales near 50 nm on amorphous fields, surface diffusion was found to obey an Arrhenius law with an activation energy of 0.9 eV. Measurements over larger length scales including multiple crystal grains exhibited a larger activation energy of 2.4 eV. The significance of the correspondence between the computational and experimental values is discussed.