Electro optical absorption in hydrogenated amorphous silicon (α-Si:H) – amorphous silicon carbonitride (α-SiCxNy) multilayers have been studied in two different planar multistack waveguides. The waveguides were realized by plasma enhanced chemical vapour deposition (PECVD), a technology compatible with the standard microelectronic processes. Light absorption is induced at λ = 1.55 µm through the application of an electric field which induces free carrier accumulation across the multiple insulator/semiconductor device structure.