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

TL;DR

This study investigates how biaxial tensile strain affects the optical properties and band structure of ultra-thin SrTiO3 epitaxial films, revealing strain-induced shifts in band gap and refractive index changes.

Contribution

It provides experimental evidence linking tensile strain to modifications in optical spectra and electronic band structure in SrTiO3 thin films, highlighting the role of lattice expansion and polar phase onset.

Findings

Tensile strain shifts low energy optical transitions to higher energies.

Refractive index decreases in the visible region under tensile strain.

Strain effects are due to changes in conduction and valence bands, not technological imperfections.

Abstract

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.