A model for metal-ferroelectric-metal heterostructures with Schottky contacts is proposed. The model adapts the general theories of metal-semiconductor rectifying contacts for the particular case of metal-ferroelectric contact by introducing the ferroelectric polarization as a sheet of surface charge located at a finite distance from the electrode interface, a deep trapping level of high concentration, and the static and dynamic values of the dielectric constant. Consequences of the proposed model on relevant quantities of the Schottky contact such as builtin voltage, charge density, and depletion width, as well as on the interpretation of the current-voltage and capacitance-voltage characteristics are discussed in detail.

Leakage current measurements were performed on epitaxial, single-crystal quality $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_3$ films with thicknesses in the $50–300\mathrm{nm}$ range. It was found that the voltage behavior of the leakage current has a minor dependence on thickness, which rules out the space-charge limited currents as main leakage source. Temperature-dependent measurements were performed to obtain more information on the transport mechanism through the metal-ferroelectric-metal (MFM) structure. The results are analyzed in the frame of interface-controlled Schottky emission. A surprisingly low value of only $0.12–0.13\mathrm{eV}$ was obtained for the potential barrier, which is much smaller than the reported value of $0.87\mathrm{eV}$ [I. Stolichnov et al., Appl. Phys. Lett. 75, 1790 (1999)]. The result is explained by the effect of the ferroelectric polarization on the potential barrier height. The low value of the effective Richardson constant, of the order of ${10}^{-7}–{10}^{-6}\mathrm{A}∕{\mathrm{cm}}^2{\mathrm{K}}^2$, suggests that the pure thermionic emission is not the adequate conduction mechanism for epitaxial MFM structures. The true mechanism might be interface-controlled injection, followed by a low mobility drift through the film volume.

Photovoltaic properties of the metal-ferroelectric-metal structures, having $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$ metal oxide electrodes and $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_3$ (PZT) as ferroelectric layer, are investigated by the short-circuit photocurrent (SC-PHC) in the $200–800\mathrm{nm}$ wavelength domain. The band-gap dependence on the Zr content was determined from the spectral distribution of the SC-PHC signal. It was found that the band-gap value increases linearly with the Zr content, from about $$ $3.9\mathrm{eV}$ to about $4.4\mathrm{eV}$ . It is shown that the sign and the magnitude of the signal depend on the internal bias and on the spontaneous polarization direction and value. The photocurrent describes a hysteresis loop similar to that of the ferroelectric polarization and can be used as a nondestructive readout of the nonvolatile memories based on PZT films. The existence of a significant SC-PHC signal at wavelengths corresponding to subgap energies is attributed to the presence of charged, deep levels in the forbidden band. It is also shown that the epitaxial PZT films have the potential for solid-state UV detectors, with current responsivity as high as $1\mathrm{mA}∕\mathrm{W}$ . The results are not entirely consistent with a bulk photovoltaic effect and are discussed in the frame of a Schottky barrier model for the metal-ferroelectric interface.

A modified model of metal-semiconductor contacts is applied to analyze the capacitance-voltage 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 acceptorlike level. The model is applied on a set of metal-$\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_3$ -metal samples with different $\mathrm{Zr}∕\mathrm{Ti}$ ratios, deposited by different methods, and having different thicknesses, electrode materials, and electrode areas. Values around ${10}^{18}{\mathrm{cm}}^{-3}$ 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 ${10}^{20}–{10}^{21}{\mathrm{cm}}^{-3}$ range. The total thickness of the interface layer ranges from $3\text{to}35\mathrm{nm}$ , depending on the $\mathrm{Zr}∕\mathrm{Ti}$ ratio, on the shape of the hysteresis loop, and on the electrode material. The simulated $I\text{-}V$ 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 $1.09–1.37\mathrm{eV}$ range and Richardson’s constant is $520\mathrm{A}{\mathrm{cm}}^{-2}{\mathrm{K}}^{-2}$ .