Raman scattering and photoluminescence of nitrogenated amorphous carbon films

Abstract

The structural and optical properties of nitrogenated amorphous carbon films, grown by rf-magnetron sputtering on silicon substrates, were studied by Raman and photoluminescence (PL) spectroscopy as a function of the nitrogen concentration and the substrate bias voltage $V_b.$ For films deposited with $V_b\text{=10\hspace{0.17em}}\mathrm{V},$ the photoluminescence emission was most intense at nitrogen concentrations in the carrier gas of 25% (75% Ar), while the intensity ratio $\mathrm{I(D)/I(G)}$ of the Raman bands of disordered graphite $\mathrm{(D}$ band) and graphite $\mathrm{(G}$ band) partially substituted by nitrogen exhibited a minimum simultaneously observed with a minimum of $G$ -band frequency and a maximum of $G$ -band width. Changes in spectral characteristics of Raman scattering at a concentration of 25% $\text{(≅30\hspace{0.17em}}\mathrm{at}\mathrm{ \%)}$ are indicative of an increase of ${\mathrm{sp}}^3$ -bonded fraction and disorder. PL-enhancement coincides, in this case, with structural changes and is probably correlated to the substitution of nitrogen in the tetrahedraly bonded amorphous matrix. In the case of films deposited in a pure nitrogen atmosphere, ${\mathrm{N}}_2\text{=100\%,}$ no significant PL-intensity changes appeared to exist between films deposited at low positive (10 V) and highly negative $\text{(−200\hspace{0.17em}}\mathrm{V})$ substrate bias. After several months of sample storage in air, samples grown at negative $V_b$ were found to preserve their structural and optical properties, while films grown at positive bias ${\mathrm{(V}}_b\text{=10)}$ and nitrogen concentrations in the carrier gas above 70% $\text{(≅40\hspace{0.17em}}\mathrm{at}\mathrm{ \%)}$ delaminated.