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Direct current (dc) bias-induced acoustic resonance phenomena is observed in micromachined tunable thin-film capacitors based on paraelectric [Ba.sub.0.3][Sr.sub.0.7]Ti[O.sub.3] thin films. The frequency of the first-order resonance has shifted down with increasing bias, whereas the antiresonance frequency might shift up or down with increasing bias.

Micromachining technology was used for the fabrication of a [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3] (BST)-based thin film resonator. Direct current (DC) bias-dependent acoustic resonance phenomena were observed in the resonators.

The epitaxial SrTi[O.sub.3] thin films were deposited on (001) LaAl[O.sub.3 single crystals by pulsed laser deposition to study the postannealing effect at 1100 degree Celsius in [O.sub.2] atmosphere for 4 h. The structural and dielectric characterizations indicate that the enhancement of permittivity is mainly due to the relaxation of the misfit strain between the film and substrate.

The effect of dilution of tunable ferroelectric with linear materials are theoretically addressed. The main impact of the dilution on the main dielectric parameters of the material is shown to be very sensitive to the composite type.

Structural and dielectric properties of strain-controlled epitaxial SrTiO3 (STO) films by two-step growth technique are investigated using in situ and ex situ observations. In the two-step grown film, the first layer accumulates defects, which leads to two effects of strain relaxation and lower loss of the film.

In this paper, the reliability of tunable ferroelectric thin film capacitors has been addressed. The long term stability of tunable capacitors based on Ba0.3Sr0.7TiO3 films deposited by chemical solution deposition onto an alumina substrate was investigated by monitoring their tunable performance as a function of the number of ON-OFF switching cycles and as a function of time. The conditions of civil aeronautic applications were chosen to plan the lifetime experiments. More than 300×106 of switching cycles with an amplitude of 150 V and frequency of 1 kHz were successfully applied to the investigated capacitors without damaging them. Moreover, no degradation in the C-V characteristic was observed after the application of 150 V dc voltage during 15 h. Additionally, the effect of illumination of the ferroelectric film by ultraviolet light on the tunable performance of the capacitors has been investigated. A positive effect of such illumination on the C-V characteristics of ferroelectric capacitor has been found. [ABSTRACT FROM AUTHOR]

Direct current (dc) bias-induced acoustic resonance phenomena have been observed in micromachined tunable thin-film capacitors based on paraelectric Ba0.3Sr0.7TiO3 thin films. Both resonance and antiresonance frequencies are dc bias dependent. The antiresonance frequency is only weakly dc bias dependent and shifts slightly down with increasing dc bias. The resonance frequency shows a much stronger dependence on the applied dc bias than the antiresonance frequency, and also shifts down to lower frequencies with increasing bias. The resonance frequency shifted by 2.0% for a frequency of about 3 GHz and an applied field of 730 KV/cm. At the same time the effective electromechanical coupling constant kt,eff2 increased up to 3.6%. The tuning of the resonance frequency depends on the tunability of the film permittivity and on the mechanical load on the piezoactive layer. The constitutive equations controlling the tuning of the resonance and antiresonance frequency were derived from the Landau theory using the P-expansion of its free energies. The theoretical predictions show that, in the considered system, the frequency of the first-order resonance (λ/2 resonance) will always shift down with increasing bias, whereas the antiresonance frequency might shift up or down with increasing bias. The sense of the shift of the antiresonance frequency depends on the sign and the magnitude of the corresponding coefficients of the tensors of linear and nonlinear electrostriction. The theoretical predictions corroborate the experimental observations very well. [ABSTRACT FROM AUTHOR]

Epitaxial SrTiO3 thin films were deposited on (001) LaAlO3 single crystals by pulsed laser deposition. The postannealing effect was studied by annealing the deposited films at 1100 °C in O2 atmosphere for 4 h. It was found that the postannealing treatment enhances the in-plane permittivity of the film. An analysis of the film lattice parameters from selected area electron diffraction and high-resolution electron microscopy shows a relaxation of the compressive strain by annealing. In addition, the density of threading dislocations is significantly reduced, and stacking faults with translation vector R=1/2a<110> resulting from the dissociation of threading dislocations are formed. Both structural and dielectric characterizations indicate that the enhancement of permittivity is mainly due to the relaxation of the misfit strain between the film and substrate. The reduction of the threading dislocation density by annealing may be beneficial to the reduction of the extrinsic dielectric loss in the film. [ABSTRACT FROM AUTHOR]

Structural and dielectric properties of epitaxial SrTiO3 (STO) thin films on LaAlO3 substrates fabricated by a two-step growth technique using pulsed laser deposition were investigated by in situ and ex situ observations in terms of strain relaxation from the lattice mismatch between the film and the substrate. In a first step, a very thin STO layer of less than 10 nm was deposited at low temperature. The main part of the film was deposited in a second step at high temperature, as commonly used for epitaxial growth. In situ reflection high-energy electron diffraction observations showed that almost full strain relaxation was realized before the deposition of the second layer, whereas the normally grown film was not fully relaxed even when the film thickness exceeded 300 nm. The relaxation process of the two-step-grown film took place through the crystallization of the first layer with substantial misfit dislocation formation, which is therefore fundamentally different from that of the normally grown film. Since the normally grown film possesses compressive strain due to less strain relaxation, the two-step-grown film showed larger permittivity and larger tuning compared to the normally grown film. The estimation of the extrinsic loss contribution from the measured loss tangent implied that the two-step growth technique could suppress the extrinsic loss due to a redistribution of defects in the film. The first layer leads to two effects: (i) strain relaxation and (ii) lower loss of the film. [ABSTRACT FROM AUTHOR]

A theory of digital phase shifters based on transition from a superconducting (S) state into a normal (N) state in high-T, superconductor (HTS) films is developed. It is shown that the insertion-loss level in the phase shifters is determined by the commutation quality of the HTS film and does not depend on the S-N switching element shape and size. The procedure for designing the S-N phase shifter with minimum insertion loss is equivalent to designing a matching circuit for the termination Z = [-square root of [R.sub.S][R.sub.N]] ([R.sub.S] and [R.sub.N] are the resistances of the switching element in the S- and N-states, respectively). Different versions of S-N phase shifters are suggested and discussed. High-quality S-N phase shifters can be successfully designed in a 1-10-GHz frequency range and find application on the receiving end of microwave systems. Index Terms - Microwave phase shifters, microwave switch, superconductor.

Epitaxial Ba0.3Sr0.7TiO3 (BST) films with significantly improved tunable performance were grown on c-plane sapphire [Al2O3(0001)] substrates using ultrathin TiN seed layers by pulsed laser deposition. A very thin (111) epitaxial TiN layer deposited at room temperature acted as a sacrificial epitaxial template for the BST film in the initial growth stage. The BST films grew with (111) orientation having BST[211]∥Al2O3[1120] and BST[211]∥Al2O3[2110] epitaxial relationships. Planar capacitors fabricated on the epitaxial BST film showed significantly higher tunability as compared to those on a polycrystalline BST film directly deposited on the substrate. [ABSTRACT FROM AUTHOR]

A Ba0.3Sr0.7TiO3 (BST) thin film composite structure was fabricated by means of a selective epitaxial growth process. The epitaxial growth of BST on SrRuO3 electrode surface was selectively achieved at 485 °C using a prepatterned ultrathin amorphous BST layer that locally prevented crystallization. This self-buildup mechanism resulted in a columnar composite structure, where epitaxial and amorphous BST columns are electrically connected in parallel. The effective permittivity of the composite capacitors decreased linearly upon increasing the amorphous BST concentration, while the tunability stayed fairly unchanged until it reached 70%. The results agree with the theoretical ferroelectric/dielectric parallel composite model. [ABSTRACT FROM AUTHOR]

Using pulsed-laser deposition, a two-step growth technique was applied to epitaxial SrTiO3 (STO) thin films on LaAlO3 substrates providing a way to obtain an effective strain relaxation in these films otherwise strained due to lattice mismatch between film and substrate. By changing the thickness of a first layer, deposited at a temperature as low as 100 °C before the deposition of the main layer at 750 °C, different strain relaxation states of the films could be systematically realized. With a 10-nm-thick first layer, an almost full strain relaxation at the deposition temperature of the main layer was achieved, suggesting a strong impact of this method on strain relaxation. The in-plane dielectric measurements displayed that the ferroelectric transition temperature increases with strain relaxation during the growth. This trend is correct and compatible with the theoretical prediction of the behavior of strained STO derived from Landau theory. [ABSTRACT FROM AUTHOR]