An overview of the state of art in ferroelectric thin films is presented. First, we review applications: microsystems' applications, applications in high frequency electronics, and memories based on ferroelectric materials. The second section deals with materials, structure ͑domains, in particular͒, and size effects. Properties of thin films that are important for applications are then addressed: polarization reversal and properties related to the reliability of ferroelectric memories, piezoelectric nonlinearity of ferroelectric films which is relevant to microsystems' applications, and permittivity and loss in ferroelectric films-important in all applications and essential in high frequency devices. In the context of properties we also discuss nanoscale probing of ferroelectrics. Finally, we comment on two important emerging topics: multiferroic materials and ferroelectric one-dimensional nanostructures.

In bulk ferroelectric ceramics, extrinsic contributions associated with motion of domain walls and phase boundaries are a significant component of the measured dielectric and piezoelectric response. In thin films, the small grain sizes, substantial residual stresses, and the high concentration of point and line defects change the relative mobility of these boundaries. One of the consequences of this is that thin films typically act as hard piezoelectrics. This paper reviews the literature in this field, emphasizing the difference between the nonlinearities observed in the dielectric and piezoelectric properties of films. The effect of ac field excitation levels, dc bias fields, temperature, and applied mechanical stress are discussed.

Interferometric measurements of electric field-induced displacements in piezoelectric thin films using single-beam and double-beam optical detection schemes are reported. It is shown that vibrational response measured with a single-beam interferometer includes a large contribution of the bending motion of substrate. Therefore, it is difficult to apply single-beam technique for piezoelectric measurements in thin films. To suppress the bending effect a high-resolution double-beam interferometer is proposed. The sensitivity of the interferometer is significantly improved in comparison with previously reported system. The interferometer is shown to resolve small displacements without using a lock-in technique. An example of the interferometric capabilities is demonstrated with experimental results on electric field, frequency, and time dependences of piezoelectric response for quartz and Pb(Zr,Ti)O3 thin film.

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The longitudinal d33 piezoelectric coefficient was studied in rhombohedral Pb(Zr0.6Ti0.4)O3 thin films with (111), (100), and “random” orientation. The largest d33 was found in (100)-oriented films and the smallest along the polarization direction in (111)-oriented films. These results are in a good qualitative agreement with recent theoretical predictions [Du, Zheng, Belegundu, and Uchino, Appl. Phys. Lett. 72, 2421 (1998)]. The field dependence of d33 was also investigated as a function of crystallographic orientation of the films. It was found that (100)-oriented films with the highest piezoelectric coefficient exhibit the weakest nonlinearity. Observed variation in the piezoelectric nonlinearity with film orientation can be fully explained by taking into account domain-wall contributions, which are dependent on film orientation.

Through the use of relations analogous to that of the Rayleigh law, it is demonstrated that the ac electric field dependence of the permittivity of ferroelectric thin films can be described. It is further shown that both reversible and irreversible components of the permittivity decrease linearly with the logarithm of the frequency of the ac field. The results demonstrate that the models describing the interaction of domain walls and randomly distributed pinning centers in magnetic materials can be extended to the displacement of domain walls in ferroelectric thin films.

The size, shape, and polarization orientation of fatigued areas formed during the suppression of the switchable polarization (Prs) (fatigue) in Pt–PZT–Pt ferroelectric capacitors (FECAPs), were observed by means of atomic force microscopy and by imaging the phase of the piezoelectric vibration induced by a low ac field applied between the top and bottom electrodes. In the virgin state (FECAP as prepared), the pattern of the polarization domains with opposite orientation was randomly distributed with typical sizes of 1–3 μm. The application of a dc field larger than the coercive field (Ec) enabled to fully orient the polarization of the regions in either directions. During the initial fatigue (<35% of suppressed Prs), polarized regions with frozen orientation and size ranging between 100 nm and 1 μm became visible. In the fatigued state (>65% of suppressed Prs), two main configurations of the frozen polarization domains were distinguished. One was characterized by a strong preferential direction (top to bottom electrode) and the other by randomly distributed regions of opposite oriented frozen polarization. The degrees of fatigue obtained by analyzing the vibration phase images are in good agreement with those obtained by standard polarization measurements. It is concluded that the Prs suppression (fatigue) is due to “region by region” or “grain by grain” freezing of Prs and that the frozen Prs can have a preferential orientation.

A simple and reliable method which allows one to distinguish between the two major microscopic scenarios for the suppression of the switching polarization (Prs), i.e., pinning of ferroelectric domain walls (DWs) through the Pb(Zr,Ti)O3 film (PZT) (bulk scenario) and inhibition of the growth of opposite domains due to the nucleus suppression at the electrode interfaces (interface scenario), is proposed. In addition, a new electric treatment able to significantly suppress Prs in Pt–PZT–Pt ferroelectric capacitors (FECAPs) of thicknesses above 1.4 μm, was discovered and studied. It consists of the application of an external alternating electric field (Ee) which cycles the polarization at very low frequency (1.7 mHz). After only 10–20 cycles, Prs can be suppressed by a factor of 10. The same FECAP, when subjected to Ee at higher frequency (30 kHz), endures at least 108 switches, before attaining an equivalent Prs suppression (hereafter called fatigue). The fatigued states obtained with the two different procedures appear to be different. In the first case (slow cycling) it is suggested that the suppression of Prs is related to the DW pinning scenario, whereas in the second case—which corresponds to the normal fatiguing procedure—it is related to the interface scenario.