Physics (Internal)

Irradiation-Induced Morphology Evolution

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract:

Ion beam irradiation is an essential basic tool in many industries, used widely for chemical doping and surface processing. For over 50 years, spontaneously-forming ripple-like structures have been occasionally observed during this process, but the observation in 1999 of highly-ordered, hexagonal arrays of nanoscale dots on irradiated GaSb has sparked an intense renewed interest in ion-induced surface modification. It has been hoped that this process could form the basis of a "bottom-up" process to cheaply and rapidly manufacture arrays of nanostructures, or induce surface coatings with tunable opto-electronic pproperties. However, despite many years of accumulated study of ion irradiation, and thorough knowledge of the generic ingredients for ordered structures, a definitive explanation for their origin in this system has remained elusive.

In this talk we will describe several lines of mathematical inquiry into this problem: (a) the use of molecular dynamics simulations of individual ion impacts to inform a multi-scale continuum theory, (b) a continuum mechanical description of the effect of beam-induced stress on the dynamics of the irradiated film, and (c) investigations on the effects of chemistry via a model of phase separation. Although a robust mathematical model of the basic physics, that accurately predicts experimental results across a range of parameter values, appears to remain just beyond reach, each of these lines of inquiry has improved our fundamental understanding of this fascinating system within certain parameter regimes, and several patterns simpler than hexagonal dots now appear explainable with reasonable experimental agreement.