  We have studied diffusion pathways of a silyl radical adsorbed on the
hydrogenated Si (100)-(2x1) surface by density-functional theory. The process
is of interest for the growth of crystalline silicon by plasma-enhanced
chemical vapor deposition. Preliminary searches for migration mechanisms have
been performed using metadynamics simulations. Local minima and transition
states have been further refined by using the nudged-elastic-band method.
Barriers for diffusion from plausible adsorption sites as low as 0.2 eV have
been found, but trap states have also been spotted, leading to a more stable
configuration, with escape barriers of 0.7 eV. Diffusion among weakly bound
physisorbed states is also possible with very low activation barriers (<50
meV). However, desorption mechanisms (either as SiH3 or as SiH4) from
physisorbed or more strongly bound adsorption configurations turn out to have
activation energies similar to diffusion barriers. Kinetic Monte Carlo
simulations based on ab initio activation energies show that the silyl radical
diffuses at most by a few lattice spacing before desorbing at temperatures in
the range 300-1000 K.
