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 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, threefold and fourfold rings. It is observed, finally, that densification proceeds through significant changes at intermediate length scales (4–10 Å ), 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).