A modified model of metal-semiconductor contacts is applied to analyze the capacitancevoltage and current-voltage characteristics of metal-ferroelectric-metal structures. The ferroelectric polarization is considered as a sheet of surface charge situated at a fixed distance from the interface. The presumable high concentration of structural defects acting as active electric traps is taken into account by introducing a deep acceptor-like level. The model is applied on a set of metal-Pb(Zr,Ti)O 3 -metal samples with different Zr/Ti ratios, deposited by different methods, and having different thickness, electrode materials, and electrode areas. Values around 10 18 cm -3 were estimated for the hole concentration from capacitance-voltage measurements. The space charge density near the electrode, estimated from current-voltage measurements, is in the 10 20 -10 21 cm -3 range. The total thickness of the interface layer ranges from 3 nm to 35 nm, depending on the Zr/Ti ratio, on the shape of the hysteresis loop, and on the electrode material. The simulated I-V characteristics is fitted to the experimental one using the potential barrier and Richardson's constant as parameters. The potential barrier is determined to be in the 1.09-1.37 eV range and the Richardson's constant is 520 Acm -2 K -2 .

Asymmetric metal-ferroelectric-metal ͑MFM͒ structures were manufactured by sol-gel deposition of a lead zirconate-titanate ͑PZT with Zr/Ti ratio 65/35͒ film on Pt-coated Si, with a Au top electrode. The average remnant polarization of 9 C/cm 2 and the coercive field of 39 kV/cm were obtained from the hysteresis loop measurements. A detailed analysis of the polarization-electric field ( P -E), capacitance-voltage (C -V), and current-voltage (I -V) measurement results allowed us to estimate the near-electrode space-charge region thickness ͑roughly half of the film thickness at zero voltage͒, net doping concentration ͑around 10 18 cm Ϫ3 ), built-in potential ͑in the 0.4 -0.8 V range, depending on the injecting electrode͒, and dynamic dielectric constant ͑5.2͒. The current logarithm-voltage dependence for the field-enhanced Schottky emission obeys a ''1/4'' law. The spectral distribution of the short circuit current measured under continuous light illumination in the 290-800 nm range exhibits a cutoff wavelength at 370 nm and a maximum sensitivity at about 340 nm. The estimated band-gap energy of the PZT material is 3.35 eV. The MFM structure is discussed in terms of two back-to-back Schottky diodes with a ferroelectric material in between. It is concluded that the semiconductor properties of the films are not negligible and, in certain conditions, are dominating over the ferroelectric ones.

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The abnormal polarization offset observed in case of graded ferroelectric structures is explained assuming the presence of a nonreversible part of polarization due to the imposed polarization gradient. It is shown that an upper limit for the offset should exist, fixed by the remnant polarization of the component materials considered as independent layers. This is valid for the structures in which polarization increases or decreases in steps from one layer to the other, but should also be valid for the structures with continuous gradient. However, large values of the charge offset could be achieved in the last case. The electric field dependence of the polarization offset is predicted, together with the possibility of obtaining large nonconventional pyroelectric coefficients. The simulation developed in the case of a bilayer ferroelectric structure and using data from the lead–lanthanum–zirconate–titanate system describes well the observed features of the graded ferroelectric structures.