  We have investigated the formation of laser-induced periodic surface
structures (LIPSS or ripples) on silicon upon excitation with p-polarized
excimer laser pulses in the deep ultraviolet region (wavelength = 193 nm, pulse
duration = 20 ns). Well-pronounced ripples with a period close to the laser
wavelength were observed for pulse numbers N = 100 or higher, and the ripple
period increased with the angle of incidence. While these results seem to be
qualitatively consistent with the standard Sipe-theory, we observed a
fundamentally different ripple formation mechanism and ripple morphology. At
low pulse numbers, isolated nanoparticles with a size of a few tens of
nanometers are observed at the silicon surface, which then start to agglomerate
in 2D and self-organize to form ripples with a very shallow modulation depth as
the pulse number increases. Employing a recently developed plasmonic model
based on the propagation of a surface plasmon polariton on a rough surface, we
demonstrate excellent quantitative agreement of the evolution of the ripple
period with incidence angle. Finally, we show that surface regions exposed to
lower laser fluence feature micro- and nanopores, which give rise to pronounced
photoluminescence (PL) emission in the visible spectral region, as opposed to
the nanoparticle-based ripples not showing PL.
