Nanoscale dynamics during self-organized ion beam patterning of Si. II. Kr+ bombardment

Understanding the self-organized ion beam nanopatterning of elemental semiconductors, particularly silicon, is of intrinsic scientific and technological interest. This is the second component of a two-part coherent x-ray scattering and x-ray photon correlation spectroscopy (XPCS) investigation of the kinetics and fluctuation dynamics of nanoscale ripple development on silicon during 1 keV ${\mathrm{Ar}}⁺$ (part I) and ${\mathrm{Kr}}⁺$ bombardment at ${65}^{\ensuremath{\circ}}$ polar angle. Here it is found that the ion-enhanced viscous flow relaxation is essentially equal for ${\mathrm{Kr}}⁺$-induced patterning as previously found for ${\mathrm{Ar}}⁺$ patterning despite the difference in ion masses. However, the magnitude of the surface curvature-dependent roughening rate in the early-stage kinetics is larger for ${\mathrm{Kr}}⁺$ than for ${\mathrm{Ar}}⁺$, consistent with expectations that the heavier ion gives an increased mass redistributive contribution to the initial surface instability. As with the ${\mathrm{Ar}}⁺$ case, fluctuation dynamics in the late stage show a peak in correlation times at the length scale corresponding to the dominant structural feature on the surface---the ripples. Finally, it is shown that speckle motion during the surface evolution can be analyzed to determine spatial inhomogeneities in erosion rate and ripple velocity. This allows the direction and speed of ripple motion to be measured in a real time experiment. In the present case, ripple motion is found to be into the projected direction of the ion source, in contrast to expectations from an existing sputter erosion driven model with parameters derived from binary collision approximation simulations.