C. L. Canedy

Dielectric properties in heteroepitaxial Ba0.6Sr0.4TiO3 thin films: Effect of internal stresses and dislocation-type defects

A series of heteroepitaxial Ba0.6Sr0.4TiO3 were grown on 0.29(LaAlO3):0.35(Sr2TaAlO6) substrates using pulsed-laser deposition. X-ray characterization revealed compressive in-plane stresses in the thinnest films, which were relaxed in a continuous fashion with increasing thickness. A theoretical treatment of the misfit strain was in good agreement with the measured out-of-plane lattice parameter. The low-frequency dielectric constant was measured to be significantly less than the bulk value and found to decrease rapidly for films less than 100 nm. A thermodynamic model was developed to understand the reduction in dielectric constant. By observing the microstructure using plan-view and cross-section transmission electron microscopy, we identified local strain associated with a threading dislocation density on the order of 1011 cm−2 as a possible mechanism for dielectric degradation in these films.

Imaging of microwave permittivity, tunability, and damage recovery in (Ba, Sr)TiO3 thin films

We describe the use of a near-field scanning microwave microscope to quantitatively image the dielectric permittivity and tunability of thin-film dielectric samples on a length scale of 1 μm. We demonstrate this technique with permittivity images and local hysteresis loops of a 370-nm-thick Ba0.6Sr0.4TiO3 thin film at 7.2 GHz. We also observe the role of annealing in the recovery of dielectric tunability in a damaged region of the thin film. We can measure changes in relative permittivity er as small as 2 at er=500, and changes in dielectric tunability der/dV as small as 0.03 V−1.

K-band phased array antennas based on Ba/sub 0.60/Sr/sub 0.40/TiO/sub 3/ thin-film phase shifters

This paper summarizes the development of a prototype 23.675-GHz linear 16-element scanning phased array antenna based on thin ferroelectric film coupled microstripline phase shifters and microstrip patch radiators. A new type of scanning reflect array antenna is introduced.

Switching properties of Pb(Nb, Zr, Ti)O3 capacitors using SrRuO3 electrodes

We report on ferroelectric properties of polycrystalline sol-gel derived Pb(Nb, Zr, Ti)O3 (PNZT) thin films with SrRuO3 (SRO) electrodes. The processing temperature of the bottom electrode was varied between 550 and 850 °C. The polarization of the ferroelectric capacitors was dependent on the processing temperature of the bottom electrode. The capacitors exhibit low switching fields (40 kV/cm), high resistivity (1011 Ω cm at 3 V) and high remanent polarization values (19 μC/cm2 at 3 V), desirable properties for high-density ferroelectric memories. The activation field for these capacitors was measured to be ∼350 kV/cm and the polarization values exhibited a shallow dependence on the pulse width from 1 s to 1 μs. Fatigue, logic state retention, and dynamic imprint tests indicate robust capacitors from a memory viewpoint.

Structural and dielectric properties of epitaxial Ba1−xSrxTiO3/Bi4Ti3O12/ZrO2 heterostructures grown on silicon

We report on the dielectric properties of an epitaxial heterostructure comprised of Ba1−xSrxTiO3, Bi4Ti3O12, and (ZrO2)0.91(Y2O3)0.09 grown on silicon substrates for potential use in microwave devices. Careful x-ray analysis indicates crystallographic alignment of all layers and transmission electron microscopy and Auger analysis reveals high quality epitaxy with minimal interdiffusion. The viability of using such heterostructures in actual microwave devices was assessed by incorporating the films in a coupled microstripline phase shifter design. The phase shifter devices, operating in the Ku band, had losses of less than 4 dB with a maximum phase shift of nearly 40° at 40 V. We compare this performance with a (Ba, Sr)TIO3/MgO phase shifter. These results presented represent significant progress towards integrating ferroelectric films with conventional silicon technology.

Correlation of Electric Field and Critical Design Parameters for Ferroelectric Tunable Microwave Filters

Abstract The correlation of electric field and critical design parameters such as the insertion loss, frequency tunability, return loss, and bandwidth of conductor/ferroelectric/dielectric microstrip tunable K-band microwave filters is discussed in this work. This work is based primarily on barium strontium titanate (BSTO) ferroelectric thin film based tunable microstrip filters for room temperature applications. Two new parameters which we believe will simplify the evaluation of ferroelectric thin films for tunable microwave filters, are defined. The first of these, called the sensitivity parameter, is defined as the incremental change in center frequency with incremental change in maximum applied electric field (EPEAK) in the filter. The other, the loss parameter, is defined as the incremental or decremental change in insertion loss of the filter with incremental change in maximum applied electric field. At room temperature, the Au/BSTO/LAO microstrip filters exhibited a sensitivity parameter value betw...

Room temperature thin film Ba/sub x/Sr/sub 1-x/TiO/sub 3/ Ku-band coupled microstrip phase shifters: effects of film thickness, doping, annealing and substrate choice

We report on measurements taken on thirteen Ku-band coupled microstrip phase shifters (CMPS) using thin ferroelectric films of Ba/sub x/Sr/sub 1-x/TiO/sub 3/. This CMPS design is a recent innovation designed to take advantage of the high tunability and tolerate the high dielectric constant of ferroelectric films at Ku- and K-band frequencies. These devices are envisioned as a component in low-cost steerable beam phased array antennas. Comparisons are made between devices with differing film thickness, annealed vs. unannealed, Mn-doped vs. undoped, and also substrates of LaAlO/sub 3/ and MgO.

A ferroelectric tunable microstrip Lange coupler for K-band applications

We report on the modeled and experimentally verified performance of a gold/Ba/sub 0.6/Sr/sub 0.4/TiO/sub 3/ (BSTO) ferroelectric/magnesium oxide (MgO) two-layered microstrip electrically tunable Lange coupler. Tight coupling of 3 dB or higher was obtained over a frequency range of 14 to 19 GHz. The coupled power level was electrically tunable from -11.5 to -5.5 dB at 20 GHz with an applied dc electric field of 1.6 V//spl mu/m. This work demonstrates another advantageous application for ferroelectric thin-films in passive microwave components at K-band frequencies.

Performance of a K-band voltage-controlled Lange coupler using a ferroelectric tunable microstrip configuration

We report for the first time on the performance of a Au/Ba/sub 0.6/Sr/sub 0.4/TiO/sub 3/(BSTO)/MgO two-layered microstrip voltage-controlled Lange coupler (VCLC) designed for Ku- and K-band frequencies at room temperature. Tight coupling of 3 dB or higher was obtained over a frequency range of 14-19 GHz. At K-band frequencies, the coupling was voltage-controllable using the nonlinear DC electric field dependence of the relative dielectric constant of BSTO (/spl epsiv//sub rBSTO/). The coupling level was improved from -11.6 to -3.7 dB at 20 GHz with an applied DC electric field of 16 kV/cm. The introduction of the ferroelectric tuning layer enhances the bandwidth of the VCLC in comparison with a Lange coupler with no ferroelectric layer. This work demonstrates another advantageous application for ferroelectric thin films in passive microwave components.

Characterization of (Ba0.5Sr0.5)TiO3 thin films for ku-band phase shifters

Abstract The microstructural properties of (Ba0.5Sr0.5)TiO3 (BSTO) thin films (300, 700, and 1400 nm thick) deposited on LaAlO3 (LAO) substrates were characterized using high-resolution x-ray diffractometery. Film crystallinity was the parameter that most directly influenced tunability, and we observed that a) the crystalline quality was highest in the thinnest film and progressively degraded with increasing film thickness; and b) strain at the film/substrate interface was completely relieved via dislocation formation. Paraelectric films such as BSTO offer an attractive means of incorporating low-cost phase shifter circuitry into beam-steerable reflectarray antennas.