Optical contrast formation in amorphous silicon-carbon alloy films by ion implantation with Ar and group IV elements has been studied by different methods. The increased disorder and additional chemical modification of a-SiC : H, induced by the implantation of Ge or Sn, results in a considerable change of the optical properties as demonstrated by optical transmission spectra. The accompanying change in the optical gap and absorption coefficient has been derived from optical transmission and reflection measurements. The results have been related to the introduction of additional disorder and creation of new bonding configurations as suggested by Raman, infrared (IR), Mossbauer and X-ray photo-electron spectroscopy (XPS). (C) 2001 Elsevier Science Ltd. All rights reserved.