  Classical molecular-dynamics simulations have been carried out to investigate
densification mechanisms in silicon dioxide thin films deposited on an
amorphous silica surface, according to a simplified ion-beam assisted
deposition (IBAD) scenario. We compare the structures resulting from the
deposition of near-thermal (1 eV) SiO$_{2}$ particles to those obtained with
increasing fraction of 30 eV SiO$_{2}$ particles. Our results show that there
is an energy interval - between 12 and 15 eV per condensing SiO$_2$ unit on
average - for which the growth leads to a dense, low-stress amorphous
structure, in satisfactory agreement with the results of low-energy ion-beam
experiments. We also find that the crossover between low- and high-density
films is associated with a tensile to compressive stress transition, and a
simultaneous healing of structural defects of the {\em a-}SiO$_2$ network,
namely three- and four-fold rings. It is observed, finally, that densification
proceeds through significant changes at intermediate length scales (4--10 \AA),
leaving essentially unchanged the ``building blocks'' of the network, viz. the
Si(O$_{1/2}$)$_{4}$ tetrahedra. This latter result is in qualitative agreement
with the mechanism proposed to explain the irreversible densification of
amorphous silica recovered from high pressures ($\sim$ 15--20 GPa).
