Transient hole traps in PZT

Pb(Zr1-xTix)O3 (PZT) that has been equilibrated with an oxidizing atmosphere contains a stoichiometric excess of oxygen, and, as a result, is a p-type semiconductor at high temperatures. It becomes an insulator near room temperature because of the strong trapping of holes by cation vacancies and/or acceptor impurity centers. The trapped holes are not detected by electron paramagnetic resonance suggesting that they are trapped in diamagnetic pairs according to the so-called “negative U” effect. Illumination by band gap light produces two metastable EPR active species, identified as Pb+3 and Ti+3. Near room temperature these centers gradually decay due to recombination and the drift of holes to deep, stable centers. The transient trapping of holes at shallow centers delays the return to the stable, insulating state, and also leads to a thermally-activated hole mobility. The trapping of holes as Pb+3 centers is attributed to the electronic structure of the Pb+2 ion with its “inert pair” 6s2 electrons.