K. F. Astafiev

Can the addition of a dielectric improve the figure of merit of a tunable material

The influence of addition of a low-loss linear dielectric material to a tunable ferroelectric material has been investigated in terms of the electrostatic consideration. The calculations of the dielectric loss and dielectric non-linearity of ferroelectric-dielectric composites have been performed by using three different models. On the basis of results obtained, the figure of merit of the composite material has been evaluated. No improvement of the figure of merit of composite material compared to the pure ferroelectric has been observed for the considered models.

Quasi-Debye microwave loss as an intrinsic limitation of microwave performance of tunable components based on SrTiO3 and BaxSr1-xTiO3 ferroelectrics

A model of the quasi-Debye loss mechanism in SrTiO3 and Ba0.6Sr0.4TiO3 perovskite ferroelectrics at microwave frequencies is presented. The field dependence and the size of the field-induced quasi-Debye loss contribution in SrTiO3 and Ba0.6Sr0.4TiO3 systems have been evaluated in the simple approximation of the isotropic phonon spectra of these materials. Good agreement between experimental and simulated results for the field dependence of dielectric loss for SrTiO3 ferroelectric material is observed for the weak dc fields [tunability n=e(0)∕e(E)⪡2]. A significant feature in the field dependence of dielectric loss at microwave frequencies, a pronounced maximum, is predicted for SrTiO3 at higher values of the bias field (n≅2) whereas in Ba0.6Sr0.4TiO3 the field dependence of the loss is found to be monotonic. The influence of the quasi-Debye loss contribution on microwave performance of SrTiO3 and Ba0.6Sr0.4TiO3 ferroelectric materials is analyzed. It is concluded that the contribution of this mechanism is...

Crossover between extrinsic and intrinsic dielectric loss mechanisms in SrTiO3 thin films at microwave frequencies

The do bias field. dependence of the dielectric loss in SrTi03 thin films deposited onto MgO substrate is investigated. The experimental data obtained at different frequencies of the ac field (8 and 16 GHz) from differently processed films (as deposited and oxygen annealed) strongly suggest the occurrence of a crossover in the dominant loss mechanism. The crossover is driven by the do bias field: at weak fields the loss is governed by an extrinsic mechanism(s) whereas, at higher fields, the contribution of an intrinsic mechanism (dc field-induced quasi-Debye loss) becomes predominant

Shift of phase transition temperature in strontium titanate thin films

The results of structural and electrical characterizations of SrTiO3 thin films deposited onto MgO and LaAlO3 substrates by pulsed laser deposition technique are presented. The influence of substrate and annealing procedure on the crystalline structure and dielectric properties of these ferroelectric thin films are investigated. The obtained experimental data are analyzed in terms of the Landau theory taking into account the room-temperature lattice mismatch of the ferroelectric and substrates as well as the difference in their thermal expansion. It is shown that the behavior of the SrTiO3 thin films could not be attributed to the effect of the film/substrate mechanical coupling. As a possible nature of the observed behavior one can consider the non-stoichiometry of the film composition caused by the chemical contact of the film with the substrate and by the annealing.

Frequency conversion in coplanar waveguide based on BaxSr1−xTiO3 film

We report on both a model and experimental verification of the high frequency nonlinearity for coplanar waveguides (CPW) composed of a dielectric substrate coated with nonlinear dielectric Ba0.4Sr0.6TiO3 films. For the CPW with a propagating microwave signal (f0≈30 GHz), modulation of Ba0.4Sr0.6TiO3 film dielectric constant by a high frequency (fp≈20 MHz) pump signal results in the generation of sidebands with frequency (f0±Nfp). Experimental and simulated spectra of CPW output signals are presented as a function of the direct current bias voltage, pump voltage amplitude, CPW length, and CPW nonlinearity.

Investigation of electrical degradation effects in ferroelectric thin film based tunable microwave components

The impact of electrical stressing on the microwave performance of the SrTiO 3 films deposited onto different substrates (MgO and LaAlO 3 ) by pulsed laser deposition has been investigated. The most important characteristics of the tunable microwave components like tunability and loss tangent have been monitored. It has been found that electrical degradation effects, being sensitive to the device operating conditions and ferroelectric film properties, are large enough to affect the performance of ferroelectric-based tunable microwave devices within the expected operating time.

Quasi-Debye microwave loss in perovskite ferroelectrics

Abstract A strong dc electric field dependence of the dielectric permittivity makes perovskite ferroelectrics promising materials for tunable microwave devices like phase shifters, tunable resonators, tunable filters etc. Dielectric loss is a crucial parameter for these applications. It has been recently realized that the so-called quasi-Debye loss mechanism related to local or global violation of central symmetry of materials plays a important role- in the total balance of microwave loss. This mechanism controls the increasing dc field dependence of microwave loss in high-quality ferroelectric materials. The present paper reports on modeling of the quasi-Debye loss mechanisms in SrTiO3 and KTaO3 systems. It is concluded that the contribution of this mechanisms is large enough to affect the performance of ferroelectric-based tunable microwave devises.

Structural Investigation of Thin SrTiO3 Films Grown on MgO and LaAlO3 Substrates

Structural investigations of SrTiO3 thin films deposited by pulsed laser ablation onto MgO and LaAlO3 substrates are presented. The residual strain along the c-axis (the growth direction) was evaluated with conventional X-ray diffraction. Evaluation of the in-plane strain was accomplished with grazing-incidence X-ray diffraction. The unit cell of the STO film on LAO substrate had an orthorhombic structure with a coexistence of compressive strain along a-axis and tensile strain along b-axis. The film on an MgO substrate was tetragonal, stretched along the c-axis and compressed in-plane. The elemental composition of the samples was investigated using an INCA system for X-ray elemental analysis attached to a Hitachi S-4300 SEM. A comparison of the residual strain and stoichiometric ratio between as-deposited films and films annealed for four hours at 1100°C in flowing oxygen is presented.

Design and Investigation of Ferroelectric Digital Phase Shifter

A reflection type phase shifter based on a tunable ferroelectric capacitor is under consideration. A design and characteristics of the one-bit ferroelectric phase shifter is proposed and discussed. The minimal insertion loss of the phase shifter is limited by the commutation quality factor of the capacitor and can be achieved in the case when the losses of the phase shifter in both states have the same value. The layout of the phase shifter based on a coplanar waveguide structure is shown. Measured characteristics of the phase shifter are compared with the predicted ones.

Intrinsic and Extrinsic Loss Contributions in SrTiO3 Thin Films at Microwave Frequencies

The behavior of dielectric loss in SrTiO3 thin films at different microwave frequencies under the dc bias voltage applied is analyzed. Two types of differently processed SrTiO3 thin films (as deposited and oxygen-annealed) deposited by the pulse laser deposition technique onto MgO single crystal substrate are investigated. The half-wave microstrip resonator with the resonance frequencies being 8 and 16 GHz at the first and second resonance modes respectively was used for the microwave characterization of the investigated thin films. The experimental data obtained strongly suggest the occurrence of a crossover between different dielectric loss contributions (extrinsic and intrinsic) in the films. The crossover is driven by the dc bias field: at weak fields the loss is governed by an extrinsic mechanism(s) whereas, at higher fields, the contribution of an intrinsic mechanism (dc field-induced quasi-Debye loss) becomes predominant. The results obtained allowed us to conclude that the quality of tunable ferroelectric films can be judged from the shape of the field dependence of the loss tangent.