Preparation and optical properties of nanocrystalline diamond coatings for infrared planar waveguides

The high index of refraction planar optical waveguides operating in the near infrared spectrum were prepared from the intrinsic hydrogenated amorphous silicon (a-Si:H) layer coated by nanocrystalline diamond (NCD). The a-Si:H layers were grown by radio frequency plasma enhanced chemical vapor deposition on glass substrates from silane and hydrogen mixture at the substrate temperature about 250 °C followed at 210 °C by the NCD coating grown by the microwave plasma enhanced chemical vapor deposition from hydrogen rich plasma with adding methane and carbon dioxide at the substrate temperature. The optical absorptance spectra, Raman spectra and scanning electron microscopy reveal the diamond character of the NCD coatings as well as the stability of a-Si:H layer during NCD deposition. The critical issue of the NCD coated a-Si:H optical waveguide is the optical absorption in NCD coating. The Finite Element Method simulations show that the near infrared optical absorption coefficient of the NCD coating must be reduced below 10/cm to reduce the optical attenuation below 1 dB/cm in the planar waveguide with the 50 nm thick NCD coating.