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

Abstract

ABSTRACTUsing 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.