Spectroscopic ellipsometry studies of nanocrystalline carbon thin films deposited by HFCVD

Abstract

Nanocrystalline carbon thin films were grown by hot-filament chemical vapor deposition (HFCVD) using a relatively high concentration of methane in hydrogen. The films were deposited on molybdenum substrates at 900°C, and under various substrate-biasing conditions. The optical properties were examined ex situ using spectroscopic phase-modulated ellipsometry (SPME) from the near IR to the near UV region (1.5–5.0 eV). The ellipsometry data [ψ(λ i), Δ(λ i)] were modeled using Bruggeman effective-medium approximation (EMA) and the dispersion relation for the amorphous semiconductor (Forouhi and Bloomer Model; Phys. Rev. B 34, 7018, 1986). We performed these simulations by least-square regression analysis (LRA) and obtained the true dielectric function of our nanocrystalline carbon material and the energy band-gap ( E g ), along with the film thickness, bulk void fraction and roughness layer. We discuss the possible physical meaning of the five parameters in the amorphous dispersion model applied to the case of nanocrystalline carbon. Micro-Raman spectroscopy and profilometry were used to guide and validate the simulations.