Optical Characterization and Modeling of Sulfur Incorporated Nanocrystalline Carbon Thin Films Deposited By Hot Filament
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Optical Characterization and Modeling of Sulfur Incorporated Nanocrystalline Carbon Thin Films Deposited By Hot Filament CVD S. Guptaa, B. R. Weinerb and G. Morellc a Department of Physics, University of Puerto Rico, San Juan, PO Box 23343, PR00931, USA b Department of Chemistry, University of Puerto Rico, San Juan, PO Box 23346, PR00931, USA c Dept. of Physical Science, University of Puerto Rico, San Juan, PO Box 23323, PR00931, USA ABSTRACT Sulfur incorporated nanocrystalline carbon (n-C:S) thin films grown on molybdenum substrates by hot-filament chemical vapor deposition (HFCVD) using gas mixtures of methane, hydrogen and a range of hydrogen sulfide (H2S) concentrations are optically examined using Raman spectroscopy (RS) and ex situ spectroscopic phase modulated ellipsometry (SPME) from near IR to near UV (1.5-5.0 eV) obtaining their vibrational frequencies and pseudodielectric function, respectively. The ellipsometry data (, ) were modeled using Bruggeman effective-medium theory (BEMT) and five parameters Forouhi and Bloomer (FB) dispersion 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 bandgap (Eg), film thickness (d), and roughness layer (σ) as a function of [H2S]. The physical interpretation(s) of the modeling parameters obtained were 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 AFM measurements. The bandgap was found to decrease systematically with increasing H2S concentration, indicating the enhancement of midgap states and sp2 C network, in agreement with RS results. These results are compared to those obtained for the films grown without sulfur (n-C), in order to study the influence of sulfur addition to the CVD process. This analysis led to a correlation between the film microstructure and its electronic properties. INTRODUCTION A great deal of attention has been given to diamond and diamond-like carbon (DLC) thin films since their advent owing to a wide range of desired and unique mechanical, optical and electronic properties (such as: high mechanical hardness, chemical inertness, negative electron affinity, and very high electron and hole mobilities) [1,2]. This combination of superlative properties paves their way to several potential and technological applications: optical coatings, wide-band IR transmissive windows, and flat panel displays (FPDs) to name a few [3]. It is also well known that the optical and electronic properties of these carbon materials are controlled by the ratio of sp3/sp2 coordinated carbon bonds [4,5]. Films having a high fraction of sp3 C exhibit a higher optical band gap and hardness as compared to films ri
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