Although, your question is insufficient itself, but I think, I got your view point for asking this question. While looking at the IR spectra of a doped materials, you have to take pristine material as the reference. In most of the cases, doping leads to degrade the bonding strength of the atoms, and bands become broader. A broad band indicates interaction with the surrounding species (may be your dopant atoms). But in some cases, band position changes drastically, which indicate attachment or substitution. If you provide your model system, it would be much easier to provide our input.