Structural properties of silicon dioxide thin films densified by medium-energy particles

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).