  We have investigated the nanostructure and microstructure resulting from ns
laser irradiation simultaneous with deposition of Co films on Si(001)
substrates. The spatial order and length scales of the resulting nanopatterns
and their crystalline microstructure were investigated as a function of film
thickness h and laser energy density E using a combination of atomic force,
scanning electron and transmission electron microscopies. The results could be
classified into two distinct categories based on the laser energy density used.
It was observed that the thickness-dependent E required to melt the Co film
(E_{Co}) was lower than Si (E_{Si}) primarily because of the higher
reflectivity of Si. Consequently, for energy densities E_{Co}<E_{1}<E_{Si} that
preferentially melted the Co film, spatially ordered nanoparticles were formed
and were attributed to capillary-driven transport in the liquid phase. The
ordering length scale corresponded to the interference fringe spacing \Lambda
and the microstructure was primarily Co metal and a metal-rich silicide phase.
The native oxide layer played an important role in minimizing the Co-Si
reaction. For laser energy densities E_{2}\geq E_{Si}, spatially ordered
patterns with periodic length scales L<\Lambda were observed and resulted from
interference of an incident laser beam with a beam transmitted into the Si
substrate. These nanopatterns showed one- as well as two-dimensional spatial
ordering of the nanostructures. The microstructure in this laser energy regime
was dominated by silicide formation. These results suggest that various
nanostructures and microstructures, ranging from nearly pure metal to a Si-rich
silicide phase can be formed by appropriate choice of the laser energy
simultaneous with film growth.
