Band structure of diamond-like carbon films assessed from optical measurements in wide spectral range

Abstract

The optical measurements of thin films on substrates, such as ellipsometry and spectrophotometry, can be efficiently used for obtaining information about film material structure if their dielectric function is parametrized by a proper dispersion model. The sample of hydrogenated diamond-like carbon (DLC) film, prepared by plasma enhanced chemical vapor deposition on silicon single crystal substrate, was investigated by ellipsometry in the NIR–UV range (0.6–6.5 eV), synchrotron ellipsometry in the UV–XUV range (4–20 eV) and reflectometry in NIR–UV range (1.24–6.5 eV). Various dispersion models based on the parametrization of density of states (PDOS) were investigated concerning their application in the wide spectral range. The previously used simple PDOS model was found to be insufficient. It resulted in poor separation of π and σ bands in the short spectral range while in the wide range it could not reflect the film optical properties because transitions to higher excited states were neglected. The inclusion of these transitions led to a considerable improvement of the fit above 16 eV. However, the realistic value of the band gap of σ electrons was obtained only after refinement of the previously expected parabolic shape of the DOS of π and σ electrons. The further developed PDOS model can be used for any other disordered material and it will, together with the optical measurements extended to XUV, supply important information about its band structure.