Multiferroic response of nanocrystalline lithium niobate J. Appl. Phys. 111, 07D907 (2012) Non-radiative complete surface acoustic wave bandgap for finite-depth holey phononic crystal in lithium niobate Offsets of hysteresis loops along the polarization axis have been observed on a step graded Pb͑Zr,Ti͒O 3 ͑PZT͒ thin film using a Sawyer-Tower ͑ST͒ circuit. However, the same effect may be artificially reproduced by adding adequate resistors and diodes in parallel with a nongraded PZT thin film. The hypothesis that the offsets were mainly due to the asymmetrical charging up of the standard capacitor used in the ST circuit, allows us to establish that the graded ferroelectric sample behaves as a kind of rectifying device. It is concluded that the presence of asymmetrical leakage currents in compositionally graded devices may allow the elucidation of the origin of the offsets often observed in these structures. Correlatively, it is demonstrated that such offsets do not represent an abnormal static polarization but a dc voltage. The E m 4 power law dependence of the offsets ͑where E m is the amplitude of the electric field͒ was found to be attributable to the nonlinear increase of the net leakage current.

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The ferroelectric properties of Pt∕Pb(Zr0.6Ti0.4)O3∕Pt∕TiO2∕SiO2∕Si thin-film capacitors with different thicknesses are investigated. According to the literature data, tilting of the hysteresis loops and marked increase of the coercive fields are observed when the thickness of the film is reduced. The degradation of the switching properties is fully reproduced by simulations including nonferroelectric space-charge layers at both ferroelectric/electrode interfaces. Based on the theoretical results, a converse model is constructed from which the overall interface capacitance, the total interface built-in potential, and both dielectric permittivity and polarization of the bulk ferroelectric layer are determined for each film. Remarkably the polarization loop due to the switching domains, calculated for each Pb(Zr,Ti)O3 (PZT) capacitor, exhibited a squarelike shape with coercive fields in agreement with the bulk value. Moreover, a unique set of parameters was found whatever the film thickness. From our results it is concluded that the degradation of the ferroelectric properties of metal-PZT thin-film-metal capacitors, often encountered when their thickness is reduced, probably arises from a mechanism of modulation of density and sign of the space charge at both interfaces.

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An epitaxial oxygen deficient Pb(Zr0.25,Ti0.75)O3 (PZT) thin film, which presented hysteresis loop with significant shift along the electric field axis and apparent polarization suppression, is investigated. Loop deformations are studied and entirely explained, both qualitatively and quantitatively by simulations including the effect of an ultrathin interfacial layer uniformly charged. The method developed in this paper is suitable to calculate not only the polarization due to the switching domains, but also all the characteristics of the space charge layer. The determination of the linear dielectric constant of the bulk ferroelectric layer does not require preparation of films with different thicknesses, unlike most of the methods proposed to date. Linear dielectric constant and thickness of the interfacial layer are in the range εil=80–130 and dil=8–12nm, respectively. On the other hand, a very large interfacial charge concentration (Nil of few 1026m−3) is obtained. For the studied PZT sample, hysteresis deformations are not attributable to pinning of domain walls, despite the large value of Nil, but rather to the interfacial space charge layer that screens the applied electric field and prevents the full switching of the ferroelectric domains.

Hysteresis measurements performed on graded Pt/lead zirconate titanate (PZT)/Pt structures with well oxygenated PZT films do not display any shift along the polarization axis (Voffset) as previously reported. On the other hand, when the PZT graded films were grown under low oxygen pressure, an offset voltage was measured. This shift was systematically enhanced after cycling the film as for fatigue measurements. It was also observed that the Voffset is independent of the value of the reference capacitor used in the Sawyer–Tower circuit. We propose an asymmetry in the leakage current of the structure to be at the origin of the shift along the polarization axis.

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Dielectric, hysteresis and fatigue measurements are performed on Pb(Zr0.54Ti0.46)O3 (PZT) thin film capacitors with different thicknesses and different electrode configurations, using platinum and LaNiO3 conducting oxide. The data are compared with those collected in a previous work devoted to study of size effect by R. Bouregba et al., [J. Appl. Phys. 106, 044101 (2009)]. Deterioration of the ferroelectric properties, consecutive to fatigue cycling and thickness downscaling, presents very similar characteristics and allows drawing up a direct correlation between the two phenomena. Namely, interface depolarization field (Edep) resulting from interface chemistry is found to be the common denominator, fatigue phenomena is manifestation of strengthen of Edep in the course of time. Change in dielectric permittivity, in remnant and coercive values as well as in the shape of hysteresis loops are mediated by competition between degradation of dielectric properties of the interfaces and possible accumulation of interface space charge. It is proposed that presence in the band gap of trap energy levels with large time constant due to defects in small nonferroelectric regions at the electrode—PZT film interfaces ultimately governs the aging process. Size effect and aging process may be seen as two facets of the same underlying mechanism, the only difference lies in the observation time of the phenomena.