When
Joint Colloquium with Physics Department * Note 3:00 start time and location in the Physics Building
Speaker: R. Mark Bradley, Colorado State University, Ft Collins
Title: Nanoscale Self-Organization Induced by Ion Bombardment of Solid Surfaces
Abstract: Bombarding a solid surface with a broad ion beam can produce a remarkable variety of self-assembled nanoscale patterns, including periodic height modulations or “ripples” and hexagonal arrays of nanodots. The emergence of these patterns is not just fascinating --- in the future, ion bombardment may become an important tool for fabricating nanostructures.
As an introduction to the field, the question of why ion oblique-incidence bombardment often produces ripples on a solid surface will be addressed. As we will discuss, the anisotropic Kuramoto-Sivashinsky (AKS) equation has traditionally been used to model ripple formation. Intriguing phenomena are observed that are not reproduced by this model, however. An improved approximation to the sputter yield leads to an equation of motion that differs from the AKS equation by the inclusion of a cubic nonlinearity. This term can lead to the formation of a terraced topography, in accord with experimental observations for high ion incidence angles. The rapid variation in the slope at the edge of a terrace results from the formation of a non-classical shock that violates the Lax entropy condition. Finally, a method of rigorously deriving the continuum equation of motion for the surface near the threshold angle for pattern formation will be discussed. In this regime, dispersive effects can lead to the formation of highly ordered ripples, a fact that may prove useful in applications.