Complex dielectric function in lead-free NKN films

We use optical transmission spectroscopy and spectral ellipsometry (SE) to determine the real and imaginary part of the complex dielectric function in both ceramic samples and thin films of lead-free Na_xK_1-xNbO₃ (NKN). Thin films of NKN were prepared by pulsed laser deposition (PLD) from ceramic NKN targets. The optical band gap from transmission measurements in thin films yield an optical band gap of 3.94 or 3.55 eV, depending on whether direct or indirect transitions, respectively, are assumed. The fit procedure of SE results, based on the Tauc-Lorentz model, resulted in a band gap for films of 3.66 eV, whereas the band gap of the thick ceramic samples was 3.79 eV. Examples of amorphous and highly polycrystalline thin films, deposited at 450 and 600 °C, respectively, are discussed.