Growth Properties of the Si(100) Steps: A Molecular Dynamics Study
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GROWTH PROPERTIES OF THE Si(100) STEPS: A MOLECULAR DYNAMICS STUDY
Christopher Roland and George H. Gilmer AT&T Bell Laboratories, Murray Hill, New Jersey 07974. ABSTRACT We have mapped out the energy surfaces seen by a single silicon adatom over the Si(100) surface and Si(100) steps, using Molecular Dynamics methods. This identifies the most likely binding sites as well as the activation energies for diffusion over the terraces and steps. We find that only the SB step with no rebonded atoms is a good sink for adatoms - the SA, rebonded SB and DB steps are weak sinks. Because of a higher density of binding sites and lower activation energies for surface diffusion along the step edge, we expect that growth at the SB and DB steps take place much more readily than at the SA step. I. INTRODUCTION The microscopic structure of surfaces, as they change during molecular-beam epitaxy (MBE) is of considerable fundamental and technological interest. At relatively high temperatures, the predominant mode of epitaxial growth is step flow: adatoms adsorbed on the flat terraces make their way via diffusion to the step edges, where they are more easily incorporated into the bulk crystal. At intermediate temperatures, this process is in competition with the growth of islands, which nucleate on the flat terraces. Recently, much attention has focused on growth on the vicinal Si(100) surface, mainly because it is hoped that overlayer growth at the Si(100) steps will facilitate the fabrication of device structures [1]. The Si(100) surface is subject to reconstruction: the surface can lower its energy by forming dimers of 2x1 or Ix2 symmetry, arranged in parallel rows. The formation of these dimers is accompanied by considerable strain. As shown by Alerhand and coworkers [2], the system can decrease its elastic energy, at the cost of introducing steps. Because of the diamond structure of silicon, the dimer orientation on the terraces separated by single-height steps is forced to alternate, while double-height steps separate terraces on which the dimers have the same orientation. Following Chadi [3], SA will denote a single-height step with dimers on the upper terrace oriented perpendicular to the step edge, while SB will denote a single-height step with dirners on the upper terrace oriented parallel to the step edge. The double-height steps, DA and D B, are defined in an analogous fashion. To date much of the theoretical work has focused on the equilibrium structures of the vicinal Si(100) surface - in particular, on the single to double step transition. However, a detailed knowledge of the kinetics is also important, especially under such nonequilibrium conditions as growth and evaporation. Since step growth is determined by the rate at which an adatom can reach a step edge, and how fast it can then become
Mat. Res. Soc. Symp. Proc. Vol. 237. ยง1992 Materials Research Society
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part of the step edge, we have mapped out the potental energy surface seen by a silicon adatom over the flat Si(100) surface and the Si(100) steps, using s
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