Title: Nanoscale Pattern Formation Produced by Ion Bombardment of Solid Surfaces

Abstract: Bombarding a solid surface with a broad ion beam can produce a remarkable variety of nanoscale patterns.  Oblique-incidence bombardment, for example, can lead to the formation of surface ripples with wavelengths as short as 10 nanometers. The anisotropic Kuramoto-Sivashinsky (AKS) equation has traditionally been used to model the formation of these ripples.

The equation of motion for the surface can be rigorously derived for angles of ion incidence just above the threshold angle for ripple formation. In the case of two diametrically opposed, obliquely-incident ion beams, the equation of motion close to threshold and at long times is a simplified version of the AKS equation, and the ripples that form are disordered. In contrast, if the surface is bombarded with a single obliquely-incident beam, the behavior is dramatically different: highly ordered ripples can emerge at sufficiently long times.  This order results from the combined effect of the nonlinearity and strong linear dispersion.

Experiments show that nanoscale patterns can also form if a solid is bombarded with ions that have an energy too low to produce sputtering.  If the solid is bombarded with two diametrically opposed, obliquely-incident beams of this kind, highly ordered, faceted ripples emerge for angles of incidence just above threshold.  The equation of motion in this case is a generalized, anisotropic version of the Cahn-Hilliard equation.

Biographical Sketch: R. Mark Bradley received his B.Sc. in physics and mathematics from the University of Toronto in 1979, and his doctorate in theoretical condensed matter physics from Stanford University in 1985.  After postdoctoral work at the IBM T.J. Watson Research Center, he joined the faculty of the Department of Physics at Colorado State University in 1987, where he now holds the rank of professor and has a joint appointment in the Department of Mathematics.  Bradley is the author of 125 refereed publications, one of which has been cited over 1,200 times.  His research on nanoscale patterns produced by ion bombardment of solids is supported by the U.S. National Science Foundation.