Optical spectra and Li-impurity effect in energy structure of KTaO3 quantum paraelectric

Optical constant n and k absorption spectra of pure KTaO3 and KTaO3:Li (1%, 3 %, 5% and 10% mol) in 2.5-5eV spectral and 4.2-300 K temperature area we re studied with a J.A. Woollam spectral ellipsometer. The energy structure of KTaO3 and KTaO3:Li was calculated using a semi-empirical Hartree-Fock (m odified INDO) method. Theory predicts inconsiderable effect of Li doping an d related oxygen displacement on the structure of the bottom conductive and top valence bands. The fundamental bandgap is indirect since valence band maximum lies at R- and the conduction band minimum is at Gamma-point of Bri llouin zone. Weak Li effect was observed in absorption for Li-doped KTaO3 d own to 40 K. However, optical absorption increases in KTaO:Li at T < 40 K, i.e. in low temperature dipolar state.