arXiv subjects

Karl F. Ludwig Jr.

Publications and source records attributed to Karl F. Ludwig Jr..

4 recordsLinked to original sources

arXiv2021

Nanoscale dynamics during self-organized ion beam patterning of Si: I. Ar$^+$ Bombardment

Coherent grazing-incidence small-angle X-ray scattering is used to investigate the average kinetics and the fluctuation dynamics during self-organized nanopatterning of silicon by Ar$^+$ bombardment at 65$^{\circ}$ polar angle. At early times, the surface behavior can be understood within the framework of linear theory. The transition away from the linear theory behavior is observed in the dynamics through the intensity correlation function. It quickly evolves to exhibit stretched exponential decay on short length scales and compressed exponential decay on length scales corresponding the dominant structural length scale - the ripple wavelength. The correlation times also peak strongly at the ripple length scale. This behavior has notable similarities but also significant differences with the phenomenon of de Gennes narrowing. Overall, this dynamics behavior is found to be consistent with simulations of a nonlinear growth model.

cond-mat.mtrl-sci↗

arXiv2021

Nanoscale dynamics during self-organized ion beam patterning of Si: II. Kr$^+$ Bombardment

Despite extensive study, fundamental understanding of self-organized patterning by broad-beam ion bombardment is still incomplete and controversial. Understanding the nanopatterning of elemental semiconductors, particularly silicon, is both foundational for the broader field and of intrinsic scientific and technological interest itself. This is the second component of a two-part investigation of the kinetics and fluctuation dynamics of self-organized nanoscale ripple development on silicon during 1 keV Ar$^+$ (Part I) and Kr$^+$ bombardment. Here, it's found that the ion-enhanced viscous flow relaxation is essentially equal for Kr$^+$-induced patterning as previously found for Ar$^+$ patterning. The magnitude of the surface curvature dependent roughening rate in the early stage kinetics is larger for Kr$^+$ than for Ar$^+$, qualitatively consistent with expectations for erosive and mass redistributive contributions to the initial surface instability. As with the Ar$^+$ case, fluctuation dynamics in the late stage show a peak in correlation time at the length scale corresponding to the dominant structural feature on the surface -- the ripples. Analogy is made to the phenomenon of de Gennes narrowing in liquids, but significant differences are also pointed out. Finally, it's shown that speckle motion during the surface evolution can be analyzed to determine spatial inhomogeneities in erosion rate and ripple velocity on the surface. This allows the direction and speed of ripple motion to be measured in real time, a unique capability for measuring these fundamental parameters outside the specialized environment of FIB/SEM systems.

cond-mat.mtrl-sci↗

arXiv2015

Using coherent X-rays to directly measure the propagation velocity of defects during thin film deposition

The properties of artificially grown thin films are often strongly affected by the dynamic relationship between surface growth processes and subsurface structure. Coherent mixing of X-ray signals promises to provide an approach to better understand such processes. Here, we demonstrate the continuously variable mixing of surface and bulk scattering signals during real-time studies of sputter deposition of a-Si and a-WiS2 films by controlling the X-ray penetration and escape depths in coherent grazing incidence small angle X-ray scattering (Co-GISAXS). Under conditions where the X-ray signal comes from both the growth surface and the thin film bulk, oscillations in temporal correlations arise from coherent interference between scattering from stationary bulk features and from the advancing surface. We also observe evidence that elongated bulk features propagate upward at the same velocity as the surface. Additionally, a highly surface sensitive mode is demonstrated that can access the surface dynamics independently of the subsurface structure.

cond-mat.mtrl-sci↗

arXiv2006

Wavelength Tunability of Ion-bombardment Induced Ripples on Sapphire

A study of ripple formation on sapphire surfaces by 300-2000 eV Ar+ ion bombardment is presented. Surface characterization by in-situ synchrotron grazing incidence small angle x-ray scattering and ex-situ atomic force microscopy is performed in order to study the wavelength of ripples formed on sapphire (0001) surfaces. We find that the wavelength can be varied over a remarkably wide range-nearly two orders of magnitude-by changing the ion incidence angle. Within the linear theory regime, the ion induced viscous flow smoothing mechanism explains the general trends of the ripple wavelength at low temperature and incidence angles larger than 30. In this model, relaxation is confined to a few-nm thick damaged surface layer. The behavior at high temperature suggests relaxation by surface diffusion. However, strong smoothing is inferred from the observed ripple wavelength near normal incidence, which is not consistent with either surface diffusion or viscous flow relaxation.

cond-mat.mtrl-sci↗