Optical constants and lattice vibrations of bulk SrTiO₃ and BaSnO₃ on SrTiO₃ using spectroscopic ellipsometry from 0.03 to 6.6 eV.

We measured the ellipsometric angles Ψ and Δ for bulk SrTiO₃ (STO) before and after cleaning (first isopropyl cleaning and then ozone cleaning at 150°C for 20 minutes) on a J. A. Woollam Variable Angle Spectroscopic Ellipsometer 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 isopropyl cleaning. We calculated the thickness of the surface layer considering the <ε₂> below the band gap of STO as 19.7 Å. Then we measured the ellipsometric angles Ψ and Δ for ozone cleaned bulk STO on J. A. Woollam IR VASE MARK II Ellipsometer from 250 to 8000 cm^-1 at four different angles of incidence from 60° to 75° with a step size of 5° at 8 cm^-1 resolution. Lattice vibrations of STO was descrided by Lorentz and Gaussian oscillators. We developed a VASE model for bulk STO using the obtained oxide layer thickness to describe the optical constants of STO and IR VASE model to describe the lattice vibrations of STO. Then BaSnO₃/STO sample was ozone cleaned (after isopropyl cleaning) and we measured Ψ and Δ under the same conditions on both ellipsometers. Ozone cleaning removes most of the water and chemical molecules attached to BaSnO₃ (BSO) surface. This VASE model was used to find the surface layer thickness and optical constants of BSO (band gap of ∼ 3.1 eV) using the thin film layer thickness (136 Å) from XRR measurement and IR VASE model was used to describe the lattice vibrations of BSO/STO using two Lorentz oscillators.