Thermo-optical properties of hydrogenated amorphous carbon and nitrogen-modified carbon layers from in situ ellipsometric studies

Abstract

Hydrogenated amorphous carbon and nitrogen-modified carbon layers, respectively a-C:H and a-C:N:H, were fabricated by plasma-assisted chemical vapor deposition and subjected to ellipsometric measurements in the temperature range 25 ÷ 300 °C. The results show that the layers are metastable and above 200 °C they undergo a transformation from dielectric to a more stable semiconducting structure. It is confirmed by a rapid drop of the optical gap (from 2.3 eV to 1.2 eV for a-C:N:H; from 2.4 eV to 1.8 eV for a-C:H) and an increase of refractive index, n, and extinction coefficient, k. The changes are permanent, followed by hydrogen/nitrogen release as it results from the investigation of Fourier transform infrared spectra. Keywords: Amorphous materials, Coatings, Chemical vapor deposition, Annealing, Optical spectroscopy