Luminescence and second harmonic generation at low temperature in reduced, pure and doped KTaO₃

Abstract

In the incipient ferroelectric system KTaO3 the green luminescence and an SHG-intensity at low temperature are found to depend drastically on oxidation or reduction treatments. The intensity of both signals is varying with the duration of the reduction in parallel over orders of magnitude. This is seen not only for nominally pure samples but also for Li-doped specimen ([Li]<2%). As-grown samples display a behaviour corresponding to a slightly reduced state. Only fully oxidized samples provide a fully characterized starting material. Under applied electric field a contribution of the bulk polarization and the alignment of dipolar microregions both of oxygen defect-and of intentionally Li-doped defect origin are observed. The oxygen related defects are discussed on the basis of charge transfer vibronic excitons or oxygen vacancy related centers.