Hartmut Gundel

Preparation of multi-coating PZT thick films by sol–gel method onto stainless steel substrates

PbZr0.45Ti0.55O3 ferroelectric films have been prepared by sol–gel method, using alkoxide precursor compounds and multi-layer technique. The gel films were deposited by spin-coating onto stainless steel substrates. In order to obtain crystallization in the perovskite phase, the samples were annealed at 600–700°C for 1 min. The dependence of the electric properties on the heat-treatment temperature is studied, and the coercive electric field as a function of the material thickness is determined. By SEM photography, the microstructure of the films could be shown to be homogeneous.

Preparation of antiferroelectric PbZrxTi1−xO3 thin films on LaSrMnO3-coated steel substrates

Antiferroelectric lead zirconate titanate (PZT) thin films have been prepared by chemical solution deposition using oxide powders. The films were processed onto steel substrates coated with a conducting oxide layer of LaSrMnO3. Homogeneous and uniform films up to 3.4 μm thick and with polarization saturation values up to 35 μC/cm2 have been obtained. The compositional transition from the ferroelectric to the antiferroelectric phase at the high zirconium side of the PZT phase diagram was studied by x-ray diffraction and hysteresis loop measurements as a function of the Zr/Ti ratio. Film thickness and annealing temperature dependencies are reported.

The role of electrode material and polarization fatigue on electron emission from ferroelectric Pb(ZrxTi1−x)O3 cathodes

Electron emission from bulk Pt/PZT/Pt and RuO2/PZT/RuO2 ferroelectric cathodes is reported. The total emission current (0.8 A) and polarization fatigue are high for ferroelectric cathodes with Pt electrodes, while emission current (0.18 A) and polarization fatigue are significantly lower for cathodes with RuO2 electrodes. The data correlate with earlier results from research on ferroelectric memories, which suggest that oxygen vacancies near the electrode/ferroelectric interface and through the bulk of the ferroelectric layer play a major role in controlling the polarization switching of PZT-based capacitors and consequently in controlling the electron emission process in PZT cathodes. The strong correlation between polarization fatigue and emission observed in the present investigation provides evidence for the role of polarization inversion on the electron emission phenomenon. In addition, the data provide evidence for the role that oxygen vacancies play in controlling electron emission from PZT cathodes.

Description of the low field nonlinear dielectric properties of ferroelectric and multiferroic materials

Ferroelectric and multiferroic materials present a nonlinear variation in their permittivity due to domain wall motion. Currently, this variation is described either by the Rayleigh law for fields above a threshold or by a power law for soft ferroelectrics. We propose a hyperbolic law based on the contributions of domain walls and intrinsic lattice which includes the two classic approaches. The threshold field is clearly defined by considering reversible and irreversible components of the permittivity. A good agreement between the hyperbolic law and experimental data is obtained. Moreover, we show that the threshold field obeys to the Volgel–Fulcher law.

Sol-Gel Derived PZT/RuO2 Multilayer Films on Stainless Steel Substrates

The structural and electric properties of PZT thin films deposited by the sol-gel process on RuO2 coated stainless steel as well as on bare stainless steel were studied and compared. As-deposited amorphous RuO2 thin films on stainless steel were transformed to pure rutile-type RuO2 at temperatures ranging from 400°C to 600°C, the resistivity of which had a value less than 200 μΩ ° cm. The PZT films processed on RuO2 needed slightly higher temperature (610°C) in order to be completely transformed into the perovskite phase than PZT on stainless steel (600°C). The films showed best ferroelectric properties when depositing it on the 100 nm thick RuO2 bottom electrodes fired at 400°C for 10 minutes. The hysteresis loops of the PZT films were more slim when being processed on RuO2 coated stainless steel than on bare stainless steel. For the RuO2 bottom electrodes the values of coercive field decreased to about 55% of those without a RuO2 layer.

Optical characterization of PZT thin films for waveguide applications

In order to develop an electro-optic waveguide, Pb(Zr, Ti)O3 ceramic ferroelectric thin films were elaborated by a modified sol-gel process on glass substrate. In the aim to study the optical properties of the PZT films, an accurate refractive index and thickness measurement apparatus was set up, which is called M-lines device. An evaluation of experimental uncertainty and calculation of the precision of the refractive index and thickness were developed on PZT layers. Two different processes of PZT elaboration were made and studied with this apparatus. The reproducibility of one fabrication process was tested and results are presented in this paper.

Effect of Manganese Doping of BaSrTiO 3 on Diffusion and Domain Wall Pinning

In the present paper, the influence of manganese doping on the dielectric properties of BaSrTiO 3 thin films is presented. The real and imaginary parts of the materials permittivity have been measured in a large frequency range (100 Hz – 1 MHz) and as a function of the electric field. The tunability and the figure of merit of the material have been obtained from the measurement of the permittivity under an applied DC bias electric field. For the undoped material, the dielectric losses become important for a large DC bias which leads to breakdown. At a suitable dopant rate, this effect disappears. In order to better understand the origin of the related phenomena, we measure the permittivity as a function of the AC excitation amplitude and we decompose the obtained permittivity with the hyperbolic law. This enables to extract the different contributions of the bulk (low frequency diffusion and high frequency lattice relaxation) and of the domain wall motions (vibration and pinning/unpinning) to the materials dielectric permittivity and to understand the effect of manganese doping on each contribution. Knowledge of the related mechanisms allows us to establish the optimum dopant rate (mainly conditioned by the lattice contribution) and to reduce the domain wall motion, which finally is beneficial for the desired properties of the ferroelectric thin film. A particular attention is paid to low frequency diffusion, an especially harmful effect when a DC biasing is mandatory (tunable electronic component in mobile telecommunication devices for example).

Optical properties of PZT thin films deposited on a ZnO buffer layer

The optical properties of lead zirconate titanate (PZT) thin films deposited on ZnO were studied by m-lines spectroscopy. In order to retrieve the refractive index and the thickness of both layers from the m-lines spectra, we develop a numerical algorithm for the case of a two-layer system and show its robustness in the presence of noise. The sensitivity of the algorithm of the two-layer model allows us to relate the observed changes in the PZT refractive index to the PZT structural change due to the ZnO interface of the PZT/ZnO optical waveguide.

INFLUENCE OF THE MORPHOLOGY OF BARIUM STRONTIUM TITANATE THIN FILMS ON THE FERROELECTRIC AND DIELECTRIC PROPERTIES

ABSTRACT In the present study, Ba0.8Sr0.2TiO3 thin films were realized by chemical solution deposition and spin-coating on stainless steel substrates using the classical multi-layer technique. In order to influence the morphology of the films, three solutions with a different concentration, 0.1 M (BST0.1M), 0.3 M (BST0.3M) and 0.6 M (BST0.6M), were prepared. The dilution is shown to influence the grain size of the films and as consequence also its ferroelectric and dielectric properties.

Pulse polarization inversion and phase transition in ferroelectric and antiferroelectric thick films

Fast switching in thin ferroelectrics has been investigated from the respective switching current and voltage wave form signals. The polarization inversion process, established for PZT (45/55) thin films in a nanosecond time scale, takes place at the “dynamic coercive electric field” and is mainly limited by the electric circuit components. Due to the low impedance of the ferroelectric during switching, the voltage across the ferroelectric rises only when polarization inversion is getting accomplished. Hence a voltage dependence of the switching has not be found. Pulse phase transition experiments in antiferroelectric PZT (95/5) have been performed, which show that the dynamic behavior of the transition between the ferroelectric and the antiferroelectric state follows the same principle. Thus, fast phase transitions can be realized, too. Derived from the voltage and current wave form data, “pulse hysteresis loops” have been established.