Erik Carlsson

Conformal mapping of the field and charge distributions in multilayered substrate CPWs

Conformal mapping (CM) and partial capacitance techniques are used for analytical evaluation of charge/current and electric- and magnetic-field distributions in a multilayered substrate coplanar waveguide (CPW) in the quasi-TEM approximation. The results, compared with finite-element method simulations, show that the magnetic wall assumed at the dielectric-dielectric interfaces in CM is a good approximation for many practical cases. The method is applied to a CPW with a thin ferroelectric film used in tunable microwave devices.

HTS/ferroelectric devices for microwave applications

High Temperature Superconducting (HTS, e.g. YBCO) microwave devices based on bulk or thin film ferroelectrics (e.g. Strontium Titanate-STO) are studied theoretically and experimentally. YBCO/STO/YBCO parallel-plate resonators and based on bulk single crystal STO are for electrically tunable high power filters, phase shifters and other devices in the frequency band 0.5-2.0 GHz of advanced microwave communication systems. Thin film YBCO/STO Inter-digital Capacitors (IDC), Coplanar Waveguides (CPW) phase shifters/delay lines are also designed and studied experimentally for low microwave power applications. Modelling problems of these devices and microwave losses in ferroelectrics are also discussed.

Improved SrTiO3 multilayers for microwave application: Growth and properties

Au/SrTiO3 (STO), Au/YBa2Cu3O7−x (YBCO)/STO, Au/STO/CeO2/STO, and Au/STO/MgO/STO multilayer structures on LaAlO3 substrates have been fabricated. Their properties are investigated at 1 MHz and at microwave frequencies (40 MHz–40 GHz) as a function of applied voltage, and in the temperature range between 20 and 300 K. The STO thin films showed comparatively high values of dielectric constant (up to 1800 at 50 K) with low measured film losses, tan δ<0.005, (limit of the measurement setup), at 20 GHz and 20 K, indicating their applicability for microwave devices. The dielectric constant is shown to be independent of frequency to at least 40 GHz, while the losses exhibit no or weak frequency dependence. Relaxation of the STO film in oxygen atmosphere before deposition of YBCO is shown to reduce interactions between the YBCO and STO films. After an “ex situ” high-temperature (900 °C) treatment in flowing oxygen a reduction of the STO dielectric constant as well as tan δ has been observed. Using cerium dioxide a...

Tunable microwave devices based on bulk and thin film ferroelectrics

Electrically tunable microwave resonators and filters based on bulk single crystal SrTiO3 and KTaO3 with superconducting plates are presented and discussed. Filters are designed for operation at frequencies 0.2 – 2.0 GHz. Thin film varactors and phase shifters, designed for cryogenic or room temperature operation and based on epitaxial SrTiO3 and BaSrTiO3 are also discussed. The frequency band for the thin film devices is 0.5 - 40 GHz.

Lower order modes of YBCO/STO/YBCO circular disk resonators

Lower order modes in a single crystal strontium titanate (STO) circular disk resonator are studied experimentally. Superconducting epitaxial YBCO films form the parallel-plates of the resonator. Due to the extremely high dielectric constant of STO, the electric fields are concentrated between the plates, while there is a substantial magnetic fringing field which affects both the resonant frequencies, Q-factors, and tunability of all modes, especially the TM/sub 110/ and TM/sub 210/.

Fabrication and investigation of YBa2Cu3O7−δ/Ba0.05Sr0.95TiO3 thin film structures for voltage tunable devices

High-temperature superconducting/ferroelectric thin film structures were deposited by laser ablation. Three types of voltage tunable devices were fabricated and investigated-trilayer capacitor, planar interdigital capacitor and coplanar waveguide. As ferroelectric the solid solution BaxSr1-xTiO3 was chosen with barium content x = 0.05 because its Curie temperature is close to the liquid nitrogen boiling point (77 K). Temperature and voltage dependences of the Ba0.05Sr0.95TiO3 dielectric constant epsilon(r,BSTO) were studied. High epsilon(r,BSTO) values were determined-up to 3000 at 75 K, 20 GHz and zero de bias. Efficient voltage tunability was demonstrated (up to 40%) at loss level tan delta=0.01-0.1. Compact YBa2Cu3O7-delta/Ba0.05Sr0.95TiO3 coplanar waveguide with as narrow gap as 18 mu m was tested as electrically tunable phase-shifter and field-induced phase shifts of more than 180 degrees were obtained by 35 V de bias at 20 GHz. This improvement was attained by proper choice of the ferroelectric material, accomplished epitaxial growth of the films and decrease of the specific dimensions

HTS/Ferroelectric CPW Structures for Voltage Tuneable Phase Shifters

Coplanar waveguides (CPW) based on single and multilayered ferroelectric films with high-Tc superconducting (HTS) and golden electrodes have been investigated in the frequency range between 40 MHz and 40 GHz with the goal of designing voltage tuneable phase shifters operating at cryogenic temperatures. Tuneability and losses are investigated as function of temperature and voltage. The quality factor of a tuneable component K is introduced and discussed. The results obtained are promising for design of voltage controlled analog phase shifter.

Reduction of density of subgrain boundaries and misfit dislocations in epitaxial (001) SrTiO3 thin films: Effect on dielectric tunability

The effect of a YBa2Cu3O7−x buffer layer on the quality of rf magnetron sputtered epitaxial (001) SrTiO3 thin films on a LaAlO3 substrate has been investigated using high resolution transmission electron microscopy. Magnetron sputtered SrTiO3 films generally exhibit a columnar subgrain morphology. By using a 75 nm thick YBa2Cu3O7-x buffer layer the subgrain boundary area was reduced considerably compared to single layer films. Subgrain widths around 130 nm were observed, which corresponded to an 80% reduction of the subgrain boundary area. The density of misfit dislocations was also reduced by 80%. By using a YBa2Cu3O7−x buffer layer, the dielectric tunability increased 30%, to a value of 1.43 (at U=16.5 V/μm, 2 MHz, 77 K). The influence of interfacial strain and misfit dislocations on the subgrain structure, and corresponding effect on the dielectric constant er, is elucidated.

EFFECTS OF DEGRADED EDGES IN STRIPS OF HIGH-TEMPERATURE SUPERCONDUCTING FILMS AT MICROWAVE FREQUENCIES

A simple model is proposed to analyzemicrowave properties of thin superconducting strips with degraded edge regions. A high resistance edge is assumed in the superconducting strip. It is shown that with increased temperature more current will flow inside the superconducting strip away from the damaged edge resulting in a reduction of effective surfaceresistance.

Evaluation of the Unloaded Q-factor of a Resonator from Impedance Measurements

Simple, fast and accurate methods are presented for unloaded quality factor measurement of microwave resonators. The methods are based on frequency measurements only. No magnitude measurements and thereby associated errors are involved.