Kinetic Roughening of Fe/Fe(100) Epitaxial Thin Films
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M.C. Bartelt* and J.W. Evans"* IPRT*, Department of Mathematics", and Ames Laboratory, Iowa State University, Ames, Iowa 50011
ABSTRACT
We model kinetic roughening during Fe(100) homoepitaxy, where the formation of mounds with selected slope has been observed. Our model incorporates irreversible nucleation and growth of two-dimensional square islands in each layer, and a step-edge barrier to diffusive downward transport (which exceeds the barrier, Ed, to terrace diffusion by Esch). We estimate that ESCh-45meV compared with EdA=450meV. To reproduce observed behavior, it is also essential for the model to incorporate "downward funneling" of depositing atoms to four-fold hollow adsorption sites, as this controls slope selection. Finally, we discuss model predictions for the non-monotonic temperature dependence of kinetic roughening. INTRODUCTION
Homoepitaxial growth on single crystal surfaces is often initiated via nucleation and growth of two-dimensional islands, a process mediated by diffusion of deposited atoms across perfect terraces'. Subsequently, these islands coalesce and percolate. As growth proceeds further to the multilayer regime, islands nucleate and grow on top of lower layer islands, sometimes well before the lower layers are complete. Villain 2 has noted that in such systems where the activation barrier to downward transport of diffusing atoms at island edges exceeds the barrier for terrace diffusion, there is an instability in the evolution of the surface morphology. The instability is induced by the reflection of diffusing atoms from descending steps and capture at ascending steps. This produces a lateral mass flux3 or "Schwoebel current", J-sh, in the uphill direction (see Fig.l), and results in the formation of "mounds" or "pyramids". Such mound formation has now been observed during both semiconductor 4 and metal 5 -' homoepitaxial growth, where significant additional step-edge or Ehrlich-Schwoebel barriers are known to occur8 "°. Furthermore, often these mounds have been observed to coarsen slowly in time, while maintaining a roughly constant local slope".
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