Optimized Choice of Light Incidence Angles for the Determination of Optical Constants from Strongly Absorbing Thin Solid Films in a Narrow Spectral Range

The determination of the linear optical constants of solids is an important part of solid state optical characterization. Reflection spectroscopy and ellipsometry of surfaces or thin solid films represent established techniques to access those optical constants; however, they may suffer from ambiguity in the obtained optical constants. We discuss methods for identifying the physically meaningful solution from the solution multiplicity, making use of a proper combination of independent measurements. Elaborating contours of constant reflectance (iso-reflectance curves) facilitates the reliable identification of correct optical constants. A numerical criterion is further provided to select suitable combinations of measurements. The procedure is demonstrated through its application to simulated spectra of a Nb2O5 film in the spectral region where the onset of the fundamental absorption edge is observed.