Optical constants of bulk SrTiO_3, Al₂O₃ on SrTiO_3, and BaSnO₃ on SrTiO₃ using ellipsometry

We measured the ellipsometric angles Ψ and △ for bulk SrTiO₃ (STO) before and after cleaning (isopropanol and ozone cleaning at 150°C for 20 minutes) on a Variable Angle Spectroscopic Ellipsometer (VASE) from 0.5 to 6.6 eV with 0.01 eV steps at four different angles of incidence from 60° to 75° with a step size of 5°. We found that ozone cleaning is a better cleaning method than isopropanol. Due to the surface roughness and remaining surface layers, the imaginary part of the pseudodielectric function below the band gap of STO (3.2 eV) is positive and is used to calculate the thickness of the surface layers on STO as 19.7 Å. Using these results, we developed a VASE model for bulk STO to describe the optical constants of STO.

Al₂O₃/STO and BaSnO₃/STO samples were ozone cleaned and then we measured the ellipsometric angles under the same conditions. The clean removes most of the water and chemical molecules attached to the Al₂O₃ and BaSnO₃ (BSO) surface_. We used this STO model to find the surface layer thickness and optical constants of Al₂O₃/STO using the Al₂O₃ layer thickness of 49 Å from XRR and of BSO/STO using the BSO layer thickness of 136 Å from XRR. We also compared the optical constants of bulk STO with STO on Al₂O₃/STO and BSO/STO.