Structural properties of erbium-activated silica-titania glasses: modeling by molecular dynamics method

The SiO2-TiO2 binary system is of significant technological importance because it offers the possibility of producing planar waveguides with a controlled refractive index depending on the TiO2/SiO2 molar ratio. Moreover, titania-silica films are suitable hosts for doping by rare-earth ions. Recently, the preparation of erbium and europium-activated SiO2-TiO2 planar waveguides by radio-frequency-sputtering as well as its characterization by means of different spectroscopic techniques have been reported by some of us. Raman scattering measurements and optical properties suggest an essentially homogeneous structure of the film with a network consisting of intermingled TiO4 and SiO4 units. However, quenching of the luminescence, a signature of rare-earth clustering, has been found effective especially for erbium-doped films. Here, we use Molecular Dynamics simulation to reconstruct a silica-titania glass with a Ti/Si atomic ratio of 8.5% activated by 0.7 at% of erbium. These quantities are chosen because they give both refractive index and optically ions concentration suitable for applications. We use a modified Born-Mayer-Huggins potential taking into account a three-body interaction. The distribution of TiO4 and SiO4 units as well as the bridging to non-bridging oxygen ratio are evaluated. The local environment of rare-earth ions is also analyzed. In particular, the clustering of erbium is discussed. From the simulated structure, the crystal-field strength is computed and discussed according to the Er3+ local environment. Finally, results are compared with information obtained by Raman and photoluminescence spectra.