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) SiO2 particles to those obtained with increasing fraction of 30 eV SiO2 particles. Our results show that there is an energy interval -between 12 and 15 eV per condensing SiO2 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 a-SiO2 network, namely threeand four-fold rings. It is observed, finally, that densification proceeds through significant changes at intermediate length scales (4-10Å), 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 (∼ 15-20 GPa).

Using classical molecular-dynamics simulations, we worked out a simple model of Ion Beam Assisted Deposition (IBAD) of silicon dioxide on an amorphous silica substrate, in view to in- vestigate the modifications of the structural and vibrational properties induced by medium-energy bombardment. Atoms are assumed to interact via the two- and three-body potential developed by Nakano et al.[1]. Analysis of the films grown with increasing ratio, R, of medium-(30 eV) to low- (1 eV) kinetic energy SiO2 particles shows that the density rises rapidly, from 1.3 g/cm3 for R =0 to about 2.3 g/cm3 for R = 0.7. This effect can be associated primarily with structural changes occurring at an intermediate length scale (4-10 Å), as it manifests itself by changes in the so-called first sharp diffraction peak (FSDP), the finger-print of medium range order (MRO) in a-SiO2 glass [2, 3]. We found also that the densification results in a significant decrease of the number of “soft” vibrational modes, occurring in the 0.5-3.5 THz frequency range.