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Atomic Steering Committee

Atomic Steering Committee By manipulating the process by which films grow on surfaces, one can obtain smooth coatings. The example shows a 10-atom thick layer of silicon,grown on a type of germanium surface known as Ge(001). The left panel is the normal growth of the film, while the right panel shows the case in which the growth has been manipulated to make it smoother.

Even the smoothest-looking coatings are very rough on the atomic scale, with islands of atoms peppered abundantly across the microscopic landscape. Roughness is not always a bad thing. In fact, it's often beneficial in adhesives, aerodynamic materials, and surfaces that "catalyze" or speed up chemical reactions. Understanding its root causes will help researchers to make surfaces smoother or rougher.

Depositing copper atoms on a Cu surface, researchers at the University of Twente in the Netherlands identified a largely ignored mechanism which contributes to introducing roughness when a beam of atoms is deposited onto a surface. Known as steering, it arises when surface atoms, including already deposited ones, exert chemical forces on incoming atoms and cause them to veer towards the surface. Steering causes incoming atoms to arrive preferentially on the top of protruding islands of atoms on the surface.

Because of the properties of their electrons, metal surfaces exert the greatest steering forces, while semiconductors and insulators are expected to exert weaker forces. The steering effect is most markedly pronounced when the beam of incoming atoms is at grazing angles with respect to a metal surface. This effect is less pronounced for beams of atoms aimed perpendicularly at a surface.

Steering Simulations

Simulations of the steering effect for three separate situations, shown in a through c. In each case, researchers calculated the influx of atoms onto a surface (shown in red) with a structure (red protrusion) already top of it. a) The atom flux on a surface with a one-atom high structure on top of it when a beam of atoms comes in at a grazing angle, namely 80° with respect to the perpendicular of the surface. Demonstrating the case when the beam of atoms comes in from the right side, the graph shows that the atom flux is enhanced above the structure (mostly close to the front side) and decreased behind the structure. b) The atom flux on a surface with a one atom high structure on top of it and a deposition angle of 0° (perpendicular to the surface). Now the flux enhancement on top of the structure is much smaller. c) The atom flux on a surface with a three-atom structure on top of it and a deposition angle of 80°. An increase in the height of the structures increases the focusing of atom flux on top of the structures considerably. Overall, this figure shows that steering effect increases with deposition angle and surface roughness.

Besides providing insights into the causes of roughness, understanding this effect may help researchers to prepare arrays of surface ridges, which could serve as templates for making magnetic nanowires and other customized materials.

This research is reported by Sebastiaan van Dijken, Louis C. Jorritsma, and Bene Poelsema in the 17 May 1999 issue of Physical Review Letters.