Fourier transform infrared (FTIR) spectroscopy was employed to characterize the formation process of conducting and/or insulating layers in silicon by arsenic or oxygen ion implantation, respectively. Two methods of buried insulating layer formation were studied. The first involved implantation of 200 keV oxygen ions at a dose of 1.8 x 10(18) cm(-2) at implantation temperature in the range 500-550 degreesC followed by annealing at 1300 degreesC for 5 h. The second involved 190 keV oxygen implantation in three cycles, each cycle followed by annealing at 1315 degreesC for 2 h. The Si overlayer of these substrates as well as bulk Si wafers were then implanted with 70 keV As+ ions at a nominal dose of 5 x 10(15) cm(-2). Annealing at 950 or 1150 degreesC led to dopant activation and the formation of conducting layers. The optical multilayer modeling of such inhomogeneous structures is given in detail. Depth profiles of oxygen atomic concentration or free carrier concentration as well as the corresponding refractive index depth profiles are quantified in a fast, cheap, accurate, and contactless way using FTIR spectroscopy. Furthermore, layer thickness, chemical composition, crystallinity, interface quality, and the electrical and transport properties are also evaluated. The results are in good agreement with ion beam analysis and electrical measurements and it is demonstrated that FTIR spectroscopy can act as a complementary technique to ion beam analysis techniques, taking over the role of the electrical methods (which are destructive) and giving much more information.