Strain-induced optical band gap variation of SnO₂ films

Thickness dependent strain relaxation effects are utilized to study the impact of crystal anisotropy on the optical band gap of epitaxial SnO₂ films grown by pulsed laser deposition on (0001)-oriented sapphire substrates. An X-ray diffraction analysis reveals that all films are under tensile biaxial in-plane strain and that strain relaxation occurs with increasing thickness. Variable angle spectroscopic ellipsometry shows that the optical band gap of the SnO₂ films continuously increases with increasing film thickness. This increase in the band gap is linearly related to the strain state of the films, which indicates that the main origin of the band gap change is strain relaxation. The experimental observation is in excellent agreement with results from density functional theory for biaxial in-plane strain. This work demonstrates that strain is an effective way to tune the band gap of SnO₂ films and suggests that strain engineering is an appealing route to tailor the optical properties of oxide semiconductors.