Light-induced defects in KTaO3

Photoconductivity (PC), thermally stimulated conductivity (TSC), photoluminescence (PL), thermoluminescence (TL), and electron spin resonance (ESR) measurements have been made on single crystals of potassium tantalate over the temperature range 4.2–290 K. We revealed two sorts of O− shallow hole centers which are responsible for the two temperature regions of PL and PC enhancement: T<70 K and 100–150 K. Both O− centers were identified by their ESR spectra. While at low temperatures PL and PC have a rather intrinsic origin, i.e., they do not depend essentially on the sort or quality of crystals, at 100–150 K both quantities strongly depend on the defect content and vanish in well-oxidized crystals. We show that O− centers serve as radiative electron–hole recombination centers. Their energy levels are situated at 0.08 and 0.16 eV above the top of the valence band. Measurements of TSC and TL after UV irradiation revealed several glow peaks at temperatures 18–30 K and 65–70 K. There is a good correlation between TSC and TL intensity in different samples as well as after annealing in O2 and H2 atmospheres. Because electrons are mobile species in KTaO3, we attribute both TSC and TL to the thermal ionization of the same shallow donor centers related with isolated oxygen vacancies. The experimental data were treated in a one-trap/one-recombination center model, which takes into account the presence of “thermally disconnected” deep electron traps.