High retention of polarization in polycrystalline M/PZT/M capacitors in the presence of depolarization field near grain boundaries
ABSTRACT 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 operation of the ferroelectric nonvolatile memory field effect transistor is theoretically examined extensively for the first time. The ferroelectric transistor device properties are derived by combining the silicon charge‐sheet model of metal‐oxide‐semiconductor field‐effect transistor device operation with Maxwell’s first equation which describes the properties of the ferroelectric film. The model we present describes ferroelectric transistor I‐V and C‐V behavior when time‐dependent voltages are applied which result in hysteresis due to ferroelectric switching. The theoretical results provide unique insight into the effects of geometrical and material parameters on the electrical properties of the transistor. These parameters include the ferroelectric spontaneous and remanent polarization, the coercive field, and dielectric layer thicknesses. We have found that the conventional concept of threshold voltage is no longer useful, and that increasing the spontaneous polarization has only a minor impact on memory operation due to reverse dipole switching of the ferroelectric layer. The application of the model to optimize design and fabrication parameters is illustrated with a virtual prototyping example. The model is also used to develop a practical testing methodology for this unique device.Journal of Applied Physics 01/1993; · 2.21 Impact Factor
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ABSTRACT: An anomalous increase of electric conduction with decreasing temperature was found in the current through Pb(Ti,Zr)O3 ferroelectric/SrTiO3 epitaxial heterostructures. Comparison with current-voltage characteristics of other ferroelectric epitaxial heterostructures and the analysis using the band diagram indicate that the current is tunneling through the p-n junction formed by Pb(Ti,Zr)O3 and SrTiO3, which have been regarded as insulators so far.Physical Review B - PHYS REV B. 01/1998; 57(10).
Conference Proceeding: Recent progress in FET-type ferroelectric memories[show abstract] [hide abstract]
ABSTRACT: The recent progress in FET-type ferroelectric memories is reviewed. Improvement of the data retention characteristics in MFIS (M; metal, F; ferroelectric, I; insulator, S; semiconductor)- and MFMIS-FETs is first described and recent experimental results are presented. Then, it is shown that a 1T2C-type cell, in which two ferroelectric capacitors with the same area are connected to the gate of a MOSFET, is useful in improving the data retention characteristics, and operation and integration characteristics of the cells are discussed.Electron Devices Meeting, 2003. IEDM '03 Technical Digest. IEEE International; 01/2004