Ex situ spectroscopic ellipsometry investigations of chemical vapor deposited nanocomposite carbon thin films

Abstract

Sulfur-incorporated nanocrystalline carbon (n-C:S) thin films were grown on molybdenum substrates by hot-filament chemical vapor deposition technique using gas mixtures of methane, hydrogen and a range of hydrogen sulfide (H 2S) concentrations (100–500 ppm with an interval of 100 ppm) at a fixed substrate temperature of 900 °C. They were optically characterized using ex situ spectroscopic phase modulated ellipsometry from near IR to near UV (1.5–5.0 eV) obtaining their pseudo-dielectric function as a function of [H 2S]. The ellipsometry data (〈ε r ( E)〉, 〈ε i ( E)〉) were modeled using Bruggeman effective medium theory and dispersion relations for the amorphous semiconductors: Forouhi and Bloomer (FB) parameterization model. A simplified two-layer model consisting of a top layer comprising an aggregate mixture of sp 3 C+sp 2 C+void and a bulk layer ( L 2), defined as a dense amorphized FB-modeled material, was found to simulate the data reasonably well. Through these simulations, it was possible to estimate the dielectric function of our n-C:S material, along with the optical bandgap ( E g), film thickness ( d) and an overlayer thickness ( L 1), which is equivalent to surface roughness layer (σ SE) as a function of H 2S concentration. The physical interpretation of the five modeling parameters obtained in the amorphous dispersion model applied to the case of n-C:S thin films is discussed. The optical bandgap was found to decrease systematically with increasing H 2S concentration, indicating an enhancement of π-bonded carbon (sp 2 C), in agreement with RS results. These results are compared to those obtained for films grown without sulfur (n-C), in order to demonstrate the influence of sulfur addition on film microstructure. These analyses led to a correlation between the film microstructure and its electronic properties.