Physics Colloquium - Friday, March 7th, 2008, 4:00 P.M.

E300 Math/Science Center; Refreshments at 3:30 P.M. in Room E200

Jacques Amar
Department of Physics and Astronomy
University of Toledo

Effects of Steering and Shadowing in Epitaxial Growth

While a variety of surface relaxation processes are usually assumed to determine the surface morphology in epitaxial growth, the deposition process can also play an important role. For example, in the case of unstable metal epitaxial growth, the steering of depositing atoms due to short-range (SR) and long-range (LR) attraction can significantly enhance the surface roughness and mound angle,[1] while for large angles of incidence shadowing can also play an important role. In particular, in recent experiments on grazing incidence Cu/Cu(100) growth,[2] a series of morphological transitions was observed as the angle between the beam and substrate normal was increased, from symmetric to asymmetric mounds at moderate deposition angles, to asymmetric ripples oriented perpendicular to the beam at larger deposition angles, and finally to symmetric pyramids with (111) facets at very large angles. Here we discuss the roles of steering and shadowing in determining the observed surface morphology.[3] We first present results obtained using a simplified model which includes the effects of shadowing but not SR and LR attraction. Our results indicate that many of the features observed in Cu(100) growth, including the existence of a transition from asymmetric mounds to ripples, can be explained primarily by geometrical effects. We also show that the formation of (111) facets is crucial to the development of ripples at large angles of incidence. A second transition to 'rods' with (111) facets oriented parallel to the beam is also found at high deposition angles and film thicknesses. When steering due to SR and LR attraction is included, we find two main effects. In the multilayer regime 'flux-focusing' due to attraction tends to enhance the anisotropy and reduce the critical thickness for the ripple transition. In contrast, near the transition from ripples to rods, sideways attraction tends to stabilize the formation of symmetric pyramids as is observed experimentally. The scaling of the surface roughness and anisotropy as a function of film thickness and deposition angle and the effects of growth temperature and surface relaxation processes such as edge-diffusion on the surface morphology will also be discussed.

1 J. Yu and J. G. Amar, Phys. Rev. Lett. 89, 286103 (2002).
2 S. van Dijken et al, Phys. Rev. B 61, 14047 (2000).
3 Y. Shim and J.G. Amar, Phys. Rev. Lett. 98, 046103 (2007).