UV light‐induced absorption in pure KTaO 3 at low temperature

Abstract

Pure KTaO3 with cubic symmetry down to lowest temperatures is a model incipient ferroelectric exhibiting the existence of intrinsic inversion symmetry breaking defects in a wide range of experiments. At low temperature a strong second harmonic generation (SHG) of Nd:YAG laser light is observed, despite the overall cubic symmetry of the crystal. Under excitation with UV-light in the band gap region (about 330 nm (3.7 eV)) a strong green luminescence (GL, at 510 nm (2.43 eV)) appears with excitation maximum near 330 nm (3.7 eV). Both, SHG and GL depend strongly on previous oxidation/reduction treatments, increasing with the reduction. In absorption at low temperature broad bands peaking at 1.2 eV and at 1.5 eV are observed in reduced KTaO3 samples. Additional UV-illumination in the band gap region at low temperature gives rise to broad light-induced absorption bands in the same 1–2 eV region. These observations are being discussed on the basis of polaronic holes, of charge transfer vibronic exitons (CTVEs) and of CTVE clusters interacting with oxygen vacancies. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)