Tensile strain induced changes in the optical spectra of SrTiO3 epitaxial thin films

Effect of biaxial tensile strains on optical functions and band edge transitions of ultra thin epitaxial films was studied using as an example a 13 nm thick SrTiO3 films deposited on KTaO3(100) single-crystal substrates. Optical functions were determined by spectroscopic ellipsometry technique. It was found that tensile strains result in a shift of the low energy band gap optical transitions to higher energies and decrease the refractive index in the visible region. Comparison of the optical spectra for strained SrTiO3 films and for homoepitaxial strain-free SrTiO3:Cr (0.01 %at.) films deposited on SrTiO3(100) single crystalline substrates showed that this shift could not be related to technological imperfections or to reduced thickness. The observed effect is connected with changes in the lowest conduction and in the top valence bands that are due to increase of the in-plane lattice constant and/or onset of polar phase in the tensile strain-induced ultra-thin epitaxial SrTiO3 films.