Refractive index and interband transitions in strain modified NaNbO3 thin films grown by MOCVD

Abstract

The influence of lattice strain on the refractive index and optical band gap of NaNbO3 thin films, deposited by the liquid-delivery spin metalorganic chemical vapor deposition method, was investigated by spectroscopic ellipsometry. Epitaxial growth of coherently strained NaNbO3 films was confirmed by high-resolution x-ray diffraction and transmission electron microscopy. Incorporated lattice strain in the films was varied by the use of the oxide substrates NdGaO3, SrTiO3 and DyScO3, which exhibit lattice mismatches to NaNbO3 with different sign, magnitude and anisotropy. The Sellmeier dispersion was employed to analyze the ellipsometry data in energy region of 1.49–2.75 eV. The refractive index at 632.8 nm of the pseudomorphically grown NaNbO3 films critically depends on the incorporated elastic lattice strain and results in a continuous decrease from 2.46 to 2.18 by varying the in-plane strain from compressive to tensile. Band gap energies for films grown under compressive and tensile lattice strain were determined by collecting spectroscopic ellipsometry data in a larger energy range between 0.73–6.48 eV and evaluating them by the Tauc-Lorentz dispersion. We observed that for tensily strained NaNbO3 films deposited on DyScO3 and SrTiO3, the band gap energies increased to and eV, respectively. For the compressively strained NaNbO3 film deposited on NdGaO3 the band gap is shifted to still higher energies ( eV).