Abstract: tIn this study, thickness related changes of the optical properties of doped tin oxide were studied. Twodifferent sets of samples were prepared. The first set was doped with iron or nickel on silicon substratewith thicknesses of 29–56 nm, the second was iron doped on gold/glass substrate with 1.6–6.3 nm. Theoptical constants were determined by using spectral ellipsometry (SE) followed by modelling of thedielectric function with an oscillator model using Gaussian peaks. The analysis of the optical constantsshows a dependence of the refraction and the absorption on the thickness of the doped tin oxide coating.In addition to the tin oxide absorption in the UV, one additional absorption peak was found in the near-IR/red which is related to plasmonic effects due to the doping. This peak shifts from the near-IR tothe red part of the visible spectrum and becomes stronger by reducing the thickness, probably due tothe formation of metal nanoparticles in this layer. These results were found for two different sets ofsamples by using the same optical model. Afterwards the second sample set was tested in the SurfacePlasmon Resonance Enhanced Ellipsometric (SPREE) gas measurement with CO gas. It was found that thethickness has significant influence on the sensitivity and thus the adsorption of the CO gas. By increasingthe thickness from 1.6 nm to 5.1 nm, the sensing ability is enhanced due to a higher coverage of thesurface with the over coating. This is explained by the high affinity of CO molecules to the incorporatedFe-nanoparticles in the tin oxide coating. By increasing the thickness further to 6.3 nm, the sensing abilitydrops because the layer disturbs the SPR sensing effect too much.

Fischer D., Hertwig A., Beck U., Negendank D., Lohse V., Kormunda M., Esser N.: “Dependence of the optical constants and the performance in the SPREE gas measurement on the thickness of doped tin oxide over coatings“ Applied Surface Science (2016); http://dx.doi.org/10.1016/j.apsusc.2016.11.188