G. Poullain

Asymmetrical leakage currents as a possible origin of the polarization offsets observed in compositionally graded ferroelectric films

Offsets of hysteresis loops along the polarization axis have been observed on a step graded Pb(Zr,Ti)O3 (PZT) thin film using a Sawyer–Tower (ST) 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 Em4 power law dependence of the offsets (where Em is the amplitude of the electric field) was found to be attributable to the nonlinear increase of the net leakage current.

Graded ferroelectric thin films: Possible origin of the shift along the polarization axis

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.

A model for fatigue in ferroelectric thin films based on trapping of carriers at interfacial states

Experimental data on fatigue in the Metal∕Ferroelectric∕Metal thin film structures are reported. A model is proposed based on the trapping and the releasing of the free carriers in the band-gap states located at the interfaces between the electrodes and the ferroelectric film. Fits of the experimental data with the plots calculated from the model show very good agreement. In particular, the fatigue dependence on both the frequency and the magnitude of the applied voltage is well reproduced by the model. Saturation of fatigue for a large number of cycles is also predicted.

Investigation of thickness dependence of the ferroelectric properties of Pb(Zr0.6Ti0.4)O3 thin-film capacitors

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 result...

Epitaxial PZT thin films on TiOx covered Pt/MgO substrate by RF magnetron sputtering

Abstract C-axis epitaxial ferroelectric Pb(Zr0.25Ti0.75)O3 (PZT) thin films with high crystalline quality structure have been prepared at a temperature of 500 °C on MgO(200) single crystal covered with epitaxial Pt(200) thin film. PZT and Pt depositions were performed by RF magnetron sputtering and PZT growth was ensured provided that an ultra thin 0.3–3 nm TiOx upper layer was sputtered on Pt electrode prior to PZT deposition. Moreover it was found that in situ crystallization of a perovskite phase with epitaxial c-axis microstructure required to control precisely the O2/(Ar+O2) ratio in the plasma gas during both PZT and TiOx upper layer depositions. Otherwise, depositions led to crystallization of PZT with high c-axis orientation but simply textured microstructure. Growth of perovskite with either (101) or (111) preferred orientation has also been demonstrated at the same temperature. Additionally strong hysteresis loop deformations, likely rising from oxygen vacancies, are reported for the less oxidized samples.

Polarization loop deformations of an oxygen deficient Pb(Zr0.25, Ti0.75)O3 ferroelectric thin film

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 eil=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, hysteresi...

Interface depolarization field as common denominator of fatigue and size effect in Pb(Zr0.54Ti0.46)O3 ferroelectric thin film capacitors

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 i...

Numerical Extraction of the True Ferroelectric Polarization Due to Switching Domains from Hysteresis Loops Measured Using A Sawyer-Tower Circuit

Investigating polarization loops of ferroelectric capacitors using a Sawyer-Tower test circuit may lead to some difficulties. It is indeed well-known that the plot of D-E loops may be subject to strong deformations hindering the ferroelectric polarization due to switching domains and preventing the accurate evaluation of remanent and spontaneous polarization ( P r and P s ) and of the coercive field ( E c ). These distortions originate from the unavoidable presence of circuit elements and/or from the leaky character of the ferroelectric capacitor under test. In this paper, a suitable expression for the release of the deformations and the computation of the polarization due to switching domains is proposed including both the measurement data and the parasitic effects of all parameters of a Sawyer-Tower circuit. A numerical procedure is then presented allowing not only to restore the true shape of the ferroelectric polarization but also to determine both the resistivity and the linear dielectric constant of a ferroelectric sample. Lastly the merits of this method is demonstrated through a successful application to a ferroelectric PZT thin film.

Nanoscale investigations of switching properties and piezoelectric activity in ferroelectric thin films using piezoresponse force microscopy

With respect to nanoscale ferroelectric thin films research, piezoresponse force microscopy (PFM) for domain imaging and local piezoelectric spectroscopy for switching properties and piezoelectric activity measurements require the control of experimental conditions associated to the experimental set-up. To avoid any misinterpretation of the results, stiffness of the cantilever, or frequency and amplitude of the driving AC voltage have to be carefully investigated. In this paper, optimal working conditions determined with our experimental set-up are described in order to evidence the architecture of domains, to measure the coercive voltage and to evaluate the piezoelectric activity of Pb(Zr, Ti)O3 thin films. Illustrations are carried out on highly oriented, unetched and ion beam etched tetragonal films; explanations of the behaviour of the ferroelectric material at the nanoscale level are given.

Crystallographic and optical properties of epitaxial Pb(Zr0.6,Ti0.4)O3 thin films grown on LaAlO3 substrates

Pb(Zr0.6,Ti0.4)O3 (PZT) thin films are grown in situ on LaAlO3 substrates by rf magnetron sputtering. The relationship between structural and optical properties is investigated as a function of growth temperature. The ferroelectric films exhibit satisfying crystallization with epitaxial growth from 475 °C. The optical refractive index value is 2.558, in agreement with the bulk value. The films show homogeneous structure and the squarelike shape of the index profile along with the PZT thickness suggests a good interface quality with the substrate. The crystallographic and optical properties measured on our films tend to demonstrate the suitability of in situ grown PZT films for optical applications.