High Retention of Polarization in Polycrystalline M/PZT/M Capacitors in the Presence of Depolarization Field Near Grain Boundaries
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control (Impact Factor: 1.51). 11/2011; 58(10):2252 - 2258. DOI: 10.1109/TUFFC.2011.2075
Source: IEEE Xplore
Recently, we reported on the photovoltaic current observed in poled capacitors with polycrystalline Pb(ZrTi)O3 (PZT) films, where (111)-oriented PZT grains are separated by an ultrathin semiconductor PbO phase. This photocurrent is driven by the depolarization field, which is generated by residual uncompensated polarization charge located on grain boundaries near electrodes. We showed that the photocurrent can serve as a criterion of existence of the depolarization field and demonstrated that this field is retained in the film for at least one year. Here, we present new experimental and numerical results which confirm the proposed conception of the photovoltaic effect. We study the photocurrent depending on the kind of electrodes, preliminary illumination in an open-circuit regime, and light intensity of LED, and give evidence of retention of the depolarization field in the films for at least for one and one-half years. The numerical study of the photovoltaic effect at extremely high photogeneration rate shows that total compensation of the polarization charge by photoexcited carriers in these structures is impossible. This photovoltaic effect can be used for nondestructive readout in ferroelectric memory.
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ABSTRACT: The experimentally measured dependence of the photo-emf on the remanent polarization in thin-film M/Pb(Zr,Ti)O3(PZT)/M capacitors correlates well with the model developed by us for the intergrain photovoltaic effect for films with columnar structure of PZT grains and heterophase intergrain boundaries. In this case, the photo-emf is determined by the depolarization field generated by the uncompensated polarization charge at PZT grain boundaries. It is shown that the magnitude and orientation of the built-in field in an intergrain channel of such PZT films can be derived from measurements of the photo-emf at zero polarization with a sensitivity on the order of a few millivolts.
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ABSTRACT: The polarization dependence of the current in epitaxial and polycrystalline (with conductive grain boundaries) Pb(Zr,Ti)O3 (PZT) films is studied using direct-current (dc) measurements and scanning spreading current microscopy. Both methods show identical results in micro- and nanoscale ranges. The current response from the film to the applied bias contains a long relaxation component that depends on the bias rise rate and polarization direction, exhibiting current peaks near the coercive force value. The polarization dependences of the current for polycrystalline and epitaxial films are found to be fundamentally different. The current of the polycrystalline film is much higher when the bias is directed against the polarization, whereas the current of the epitaxial film is higher if the bias and polarization directions coincide. All films exhibit current hysteresis of non-ferroelectric (clockwise) direction with decreasing bias. It is also shown that the polarization dependences of the transient current in both polycrystalline and epitaxial films are similar to the polarization dependence of the photovoltaic current in these films.
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