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

Abstract: 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.