Ex situ spectroscopic ellipsometry and Raman spectroscopy investigations of chemical vapor deposited sulfur incorporated nanocrystalline carbon thin films

Abstract

Sulfur incorporated nanocrystalline carbon (n-C:S) thin films were grown on molybdenum substrates by a hot-filament chemical vapor deposition technique using gas mixtures of methane, hydrogen, and a range of hydrogen sulfide (H2S) concentrations (100–500 ppm with an interval of 100 ppm) at a fixed substrate temperature of 900 °C. They were optically characterized using Raman spectroscopy (RS) and ex situ spectroscopic phase modulated ellipsometry from near-infrared to near UV (1.5–5.0 eV) obtaining their vibrational frequencies and pseudodielectric function, respectively, as a function of [H2S]. The ellipsometry data [〈εr(E)〉,〈εi(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 sp3C+sp2C+void and a bulk layer (L2), 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 band gap (Eg), film thickness (d), void fraction (fv), and roughness layer (σ) as a function of H2S 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 Raman and ellipsometry results indicate that the average size of nanocrystallites in the sulfur-incorporated carbon thin films becomes smaller with increasing H2S concentration, consistent with atomic force microscopy measurements where the distribution of grain size yielded a gamma around 20 nm. The band gap was found to decrease systematically with increasing H2S concentration, indicating an enhancement of π-bonded carbon (sp2C), in agreement with RS results. These results are compared to those obtained for films grown without sulfur (n-C), in order to study the influence of sulfur addition on film microstructure. These analyses led to a correlation between the film microstructure and its electronic properties.