NSLS-II Friday Lunchtime Seminar

Karl Ludwig*, Mahsa Mokhtarzadeh*, Jeffrey Ulbrandt#, Peco Myint*, Suresh Narayanan+, and Randall Headrick# *Boston University / #University of Vermont +Advanced Photon Source, Argonne National Laboratory Enabled by the continued increases in brilliance available from synchrotron x-ray sources, our goal is to develop coherent x-ray scattering as a powerful new tool for the investigation of surface dynamics during growth and patterning processes. In particular, our research has been developed new approaches in X-ray Photon Correlation Spectroscopy (XPCS), using it to examine the dynamics of kinetic roughening in amorphous thin film growth, the step structure evolution in polycrystalline epitaxial growth and self-organized ion beam nanopatterning. Broad-beam low-energy ion bombardment can lead to the spontaneous formation of nanoscale surface structures, but the dominant mechanisms driving evolution remain controversial. Here I will describe our studies of the classic case of ion-beam rippling of SiO2 surfaces, in which we examine the relationship between the average kinetics of ripple formation and the underlying ?uctuation dynamics. In the early stage growth of ?uctuations we find a novel behavior with memory stretching back to the beginning of the bombardment. For a given length scale, correlation times do not grow signi?cantly beyond the characteristic time associated with the early-stage ripple growth. In the late stages of patterning, when the average surface structure on a given length scale is no longer evolving, dynamical processes continue on the surface. Nonlinear processes dominate at long length scales, leading to compressed exponential decay of the speckle correlation functions, while at short length scales the dynamics appears to approach a linear behavior consistent with viscous ?ow relaxation. This behavior is found to be consistent with simulations of a recent nonlinear growth model. In addition