Structural and vibrational properties of silicon dioxide thin films densified by medium-energy particles bombardment
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Structural and vibrational properties of silicon dioxide thin films densified by medium-energy particles bombardment Alexis Lef`evre1 , Laurent J. Lewis1, Ludvik Martinu2, and Michael R. Wertheimer2 (1)D´epartement de Physique et Groupe des Couches Minces (GCM), Universit´e de Montr´eal, Case Postale 6128, Succursale Centre-Ville, Montr´eal Qu´ebec H3C 3J7, Canada ; e-mail: [email protected] (2)D´epartement de G´enie Physique et de G´enie des Mat´eriaux et Groupe des Couches Minces ´ (GCM), Ecole Polytechnique de Montr´eal, Case Postale 6079, Succursale Centre-Ville, Montr´eal, Qu´ebec H3C 3A7, Canada ABSTRACT 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 investigate 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/cm 3 for R = 0 to about 2.3 g/cm3 for R = 0.7. This effect can be associated primarily with structural changes ˚ as it manifests itself by changes in the so-called occurring at an intermediate length scale (4-10 A), 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. INTRODUCTION In recent years, continuing progress in techniques such as ion beam assisted deposition (IBAD), plasma-enhanced chemical vapor deposition (PECVD), or sputter deposition, often referred to as ion-assisted deposition, makes it possible to grow thin films with a satisfactory control of their structural properties and chemical composition[4, 5]. Moreover, as a bonus, ion energy and fluxes enter as new parameters, allowing experimentalists to grow material over a broad range of thermodynamic conditions, thereby overcoming the limits inherent to equilibrium growth. However, despite widespread use, the fundamental aspects of the mechanisms by which energetic particles modify the growth process are poorly understood. This knowledge is very important for materials processing, since a number of films’ properties are found to depend critically on the ion energy and fluxes[6]. These properties are strongly related to the atomic structure of the deposited material. However, for the case of silicon dioxide (SiO2 ), and more generally for amorphous thin films, conventional characterization methods only provide an average picture of the atomic correlations, thereby losing important information about ion-bombardment induced modifications. In this paper, we report the results of classical molecular-dynamics (MD) simulations of the growth of amorphous silicon dioxide (SiO 2 ) on an amorpho
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