Spectroscopic ellipsometry of SrBi2Ta2−xNbxO9 ferroelectric thin films

Abstract

Optical properties of the ferroelectric thin-film SrBi2Ta2−xNbxO9(0⩽x⩽2) solid-solution system were investigated by spectroscopic ellipsometry from the infrared to the ultraviolet-visible region. Optical constants and the band-gap energies were determined by multilayer analysis of the respective pseudodielectric functions. With increasing x, it is found that the refractive index slightly increases in the infrared and rises from 2.0 to 2.3 in the visible region, and the band-gap energy shifts from 4.17 to 3.61 eV at room temperature. A possible explanation for the experimental observations, the reduced extension of the Nb 4d orbital, is mentioned.