Optical spectroscopy and dispersion parameters of Ge15Se60X25 (X = As or Sn) amorphous thin films Heba E. Atyia and N.A. Hegab Eur. Phys. J. Appl. Phys., 63 1 (2013) 10301 Published online: 05 July 2013 DOI: 10.1051/epjap/2013130099