Caroline Borderon

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.

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).

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.

Measurement and modeling of dielectric properties of Pb(Zr,Ti)O 3 ferroelectric thin films

In this study, the real and imaginary parts of the complex permittivity of lead zirconate titanate ferroelectric thin films are studied in the frequency range of 100 Hz to 100 MHz. The permittivity is well fitted by the Cole-Cole model. The variation of the relaxation time with the temperature is described by the Arrhenius law and an activation energy of 0.38 eV is found. Because of its nonlinear character, the dielectric response of the ferroelectric sample depends on the amplitude of the applied ac electric field. The permittivity is composed of three different contributions: the first is due to intrinsic lattice, the second is due to domain wall vibrations, and the third is due to domain wall jumps between pinning centers. This last contribution depends on the electric field, so it is important to control the field amplitude to obtain the desired values of permittivity and tunability.

A new method of dielectric characterization in the microwave range for high-k ferroelectric thin films

In this paper we propose a new method of dielectric characterization of high-k thin films based on the measurement of coplanar capacitor inserts between two coplanar waveguide transmission lines. The measurement geometry is deposed on the thin film which is elaborate on an insulating substrate. The thin film permittivity is extracted with the help of a mathematical model describing the capacitance between two conductor plates deposed on a 2-layers substrate. A simple correction is proposed in order to enhance the matching between the model and the full wave simulation. The results of the proposed measurement method are compared to those of a classical characterization technique using parallel plate capacitor geometry.

Preparation and Characterization of Barium Strontium Titanate Thin Films by Chemical Solution Deposition

In the present study, Ba 0.70 Sr 0.30 TiO 3 thin films were realized by chemical solution deposition based on the use of an alkoxide precursor. The solution is spin-coated onto stainless steel substrates and a classical multi-layer technique was used in order to obtain thicker films. The elaboration route is described and a study of the temperature dependence of the crystallization process is presented. The ferroelectric and the dielectric (permittivity and tanδ) properties were determined at different temperatures in order to determine the evolution of the tunability and the figure of merit of the films.

A simple phase-shifting cell for reflectarray using a slot loaded with a ferroelectric capacitor

A tunable reflectarray phase-shifting cell, designed for a resonance frequency of 5.6 GHz, is presented. The cell is based on a simple slot topology and loaded by a ferroelectric thin film capacitor of 60% tunability under 400 kV/cm bias electric field. The cell provides 245 degrees of phase-shifting.

Decomposition of the different contributions to permittivity, losses, and tunability in BaSrTiO3 thin films using the hyperbolic law

In this paper, the different contributions to the permittivity of a 1% manganese-doped BaSrTiO3 thin film are presented as a function of the applied DC field. The hyperbolic law has been used to discern the lattice, domain wall vibration, and pinning/unpinning contributions. This decomposition permits us to study the weight of the respective contribution in the total permittivity, the losses, and the tunability. By determining the figure of merit (FoM) of each contribution, the ratio between tun-ability and losses, it is possible to identify the phenomenon which should be limited or enhanced in order to optimize the materials dielectric properties. It is shown that the tunability of the domain wall contribution (approximately 80%) is very important compared to the lattice contribution (41%), the associated dissipation factor, however, is also much larger (0.2 instead of 0.014). Even if the domain wall contribution has been shown to be weak in the investigated thin film (less than 3% in permittivity and tunability), the weight of the losses is not negligible (around 18%). Hence, the domain contribution has to be limited in order to conserve a high FoM for the material. Moreover, it is shown that the AC field used for the materials characterization is important because it governs the weight of the domain wall losses and thus the FoM.

Domain wall motions in BST ferroelectric thin films in the microwave frequency range

The existence of domain wall motion at microwave frequencies and its contribution to the ferroelectric complex permittivity is shown by evaluating the dielectric properties of BaSrTiO 3 (BST) thin films as a function of the incident power. Even at low AC field amplitudes, the presence of the domain walls and the correlated motions (vibration and jumps) result in sensitivity of the dielectric properties to the incident field amplitude. Although the contribution of domain wall motion to the real part of the permittivity is not preponderant (less than 10 %), it represents more than 50 % of the materials global dielectric losses. This illustrates the importance to consider domain wall motion even in the microwave frequency region and the necessity to take into account the applied AC field amplitude (and thus the incident power) when characterizing ferroelectric materials. The present study has been realized on BST thin films, elaborated by pulsed laser deposition on MgO/Ir substrates.

Realization and characterization of manganese doped BST thin films for reflectarray applications

In the present work, Mn-doped Ba1-xSrxTiO3 (BST) thin films were realized by Chemical Solution Deposition (CSD) on alumina as the foreseen application for a reflect array needs integration of the ferroelectric on an insulating substrate. The optimum dopant rate to be inserted depending on the materials defect density, we have studied BST doping with a manganese content ranging from 0% to 2%. The dielectric and electrical characteristics were investigated as a function of the Mn content in the frequency range of 100 Hz to 5 GHz.