Dielectric function and interband critical points of compressively strained ferroelectric K 0.85 Na 0.15 NbO 3 thin film with monoclinic and orthorhombic symmetry

The dielectric function and interband critical points of compressively strained ferroelectric K 0.85 Na 0.15 NbO 3 thin film grown by metal -organic vapor phase epitaxy (MOVPE) are studied in broad spectral and temperature ranges by spectroscopic ellipsometry (SE). The temperature dependence of the measured pseudodielectric functions is strongly affected by a structural phase transition from the monoclinic M c -phase to the orthorhombic c -phase at about 428 K. Using a parametric optical constant model, the corresponding dielectric functions as well as the interband optical transitions of the film are determined in the spectral range of 0.73-6.00 eV. Standard critical point (SCP) analysis of the 2 nd derivatives of the dielectric functions identified three and four critical points for monoclinic and orthorhombic symmetries, respectively. A systematic redshift of the threshold energies with increasing temperatures was observed.