Photoinduced phenomena and shallow levels in K1-xLixTaO3

Li admixture greatly enhances n-type photoconductivity of KTaO3 crystals. Thermally stimulated current experiments indicate that this phenomenon is related with the presence of hole-trapping levels with depths of 50 - 100 meV. This is in good agreement with recent energy structure calculations showing the existence of levels at these energies above the top of the VB, due to perturbed O2- p-states. The study of visible photoluminescence and thermoluminescence shows that the hole-trapping centre also controls the emission process. We conclude in favour of radiative recombination involving O--Ta(4+)polaron pairs.