High retention of the polarization and depolarization field in polycrystalline metal/Pb(ZrTi)O_3/metal capacitors

摘要

Long term relaxation of the photovoltaic current in poled and then short-circuited capacitors with polycrystalline Pb(ZrTi)O_3 (PZT) film, where (111) oriented PZT grains are separated by ultrathin semiconductor PbO phase is studied. The quantum energy of the excitation light is lower than PZT band gap but higher than PbO band gap. The measured photovoltaic current differs entirely from well-known photovoltaic current observed in single-crystal ferroelectrics, which flows in polarization direction and originates due to asymmetry of impurity potential caused by the polarization. The photocurrent, which we measure, is always directed against the polarization and is not related to the depolarization of the film. The driving force of the measured photocurrent is the depolarization field generated by polarization charge on PZT grain boundaries [Delimova et al., Appl. Phys. Lett. 91, 112907 (2007)]. Photoexcited in PbO interlayers free carriers drift in this field toward electrodes producing the photocurrent, which can serve as a criterion of existence of the depolarization field. It is shown that the steady-state photovoltaic current in poled M/PZT/M capacitors, measured for a year without their repolarization, demonstrates only 30% decrease. This means that during the year the depolarization field has remained in the film, the polarization charge generating the depolarization field has not been compensated, thereby indicating that the polarization is also conserved. The calculation of the photocurrent, depolarization field and polarization, performed using two-dimensional phenomenological model, shows that the polarization charge on grain boundaries cannot be compensated completely neither by free carriers nor charged dopants, which confirms the experimental result.