Optical spectroscopy and dispersion parameters of Ge15Se60X25 (X = As or Sn) amorphous thin filmsHeba E. Atyia and N.A. HegabEur. Phys. J. Appl. Phys., 63 1 (2013) 10301DOI: https://doi.org/10.1051/epjap/2013130099