Staffan Rudner

GROWTH AND FIELD DEPENDENT DIELECTRIC PROPERTIES OF EPITAXIAL NA0.5K0.5NBO3 THIN FILMS

Na0.5K0.5NbO3 thin films have been grown on LaAlO3(001) substrates using rf magnetron sputtering from a Na and K enriched target. X-ray diffraction showed that the films are epitaxial with mosaic broadening as narrow as 0.044°. Interdigital Au finger electrodes were photolithographically defined on the film surfaces. The dielectric properties of these interdigital capacitors were measured at 1 MHz from room temperature down to 50 K. The capacitor showed 35% tunability at room temperature and a loss tangent of 0.007 without dc bias applied. The loss decreased further with increasing dc bias. For lower temperatures, the capacitance exhibited a broad maximum at ∼200 K, which is possibly linked to a phase transformation of the Na0.5K0.5NbO3 film.

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.

Resputtering effects on the stoichiometry of YBa2Cu3Ox thin films

Energetic particle bombardment of the substrate during deposition will under certain process conditions adversely affect the properties of YBa2Cu3Ox thin films. Films were grown by dc magnetron sputtering from stoichiometric compound targets in a planar geometry. An increased oxygen fraction in the sputtering gas entailed a drastic decrease in the film deposition rate. Etching of the substrates was observed in pure oxygen plasmas for pressures below 2 Pa. For the whole range of sputtering gas mixtures, the film composition deteriorated if too low total pressures were used; that is, even if pure argon was used as working gas. Copper deficiency was observed under these conditions and attributed to resputtering of the growing film by energetic particles. This conclusion was supported by the copper deficiency observed in films grown at negative substrate bias, i.e., during ion bombardment from argon plasmas. The origin of various energetic particles during normal sputtering is discussed. We concluded that oxy...

Composition, structure, and dielectric tunability of epitaxial SrTiO3 thin films grown by radio frequency magnetron sputtering

Epitaxial (001) oriented SrTiO3 films have been deposited on LaAlO3(001) substrates by off-axis radio frequency magnetron sputtering in Ar:O2 gas mixtures at substrate temperatures ranging from 650 to 850 °C. For the deposition conditions used, stoichiometric targets yielded 20% Sr-deficient films, whereas Sr-enriched targets (Sr1.1Ti0.9O3.0) resulted in stoichiometric films. The Sr-deficient films had a mosaic structure and a larger lattice parameter in comparison to bulk SrTiO3. The stoichiometric films on the other hand had a much higher crystalline quality in the as-deposited condition. The mosaicity of the latter films was primarily limited by the crystalline quality of the LaAlO3 substrates. The lattice parameters of the stoichiometric films were also smaller than the Sr-deficient ones and closer to the bulk value. The dielectric properties of the stoichiometric films were superior to the Sr-deficient films. For films with a thickness of ∼300 nm, the typical dielectric constants as measured at ∼77 K...

Microstructure and microwave dielectric properties of epitaxial SrTiO3 films on LaAlO3 substrates

Sr-deficient and stoichiometric epitaxial (001) SrTiO3 films, deposited on (110)rhombohedral LaAlO3 substrates by rf magnetron sputtering, have been characterized using high-resolution transmission electron microscopy. A subsequent heat treatment in oxygen had a positive influence on the dielectric properties. Sr-deficiency had a large negative impact on the microwave dielectric constant of the films. These changes were correlated to changes in lattice parameters. In all samples, at the film/substrate interface, were misfit dislocations present. The residual elastic strain compressed the SrTiO3 unit cell in the substrate surface plane and expanded it an equal amount in the [001] direction. X-ray diffraction revealed that the tetragonal distortion, due to the mismatch strain, was concentrated to a narrow region closest to the film/substrate interface.

Target presputtering effects on stoichiometry and deposition rate of Y‐Ba‐Cu‐O thin films grown by dc magnetron sputtering

Y‐Ba‐Cu‐O thin films have been grown from stoichiometric compound targets using dc magnetron sputtering. Initially, the film composition was strongly off‐stoichiometric and the deposition rate very low. However, after extensive presputtering, stoichiometric films, with respect to the metal content, were deposited at a rate of 2–2.5 A/s. The long presputtering time (20–30 h) needed is due to a slowly changing oxygen content on the target surface. This is caused not only by preferential sputtering effects, but also by bulk diffusion in the target. The presputtering time is significantly decreased if the target is annealed in an argon atmosphere prior to use. Oxygen adsorbed on the target surface during venting of the chamber will also affect the process significantly.

Growth and characterization of Na0.5K0.5NbO3 thin films on polycrystalline Pt80Ir20 substrates

Na0.5K0.5NbO3 thin films have been deposited onto textured polycrystalline Pt80Ir20 substrates using radio frequency magnetron sputtering. Films were grown in off- and on-axis positions relative to ...

Reduction of surface particles on YBa2Cu3O7−∂ thin films through the use of nonstoichiometric sputtering targets and N2O in the sputtering gas

High quality YBa2Cu3O7−∂ (YBCO) thin films that combine good electrical properties and smooth surfaces without particles are important for many future applications. Today most films with good electrical properties have secondary phases in the form of copper‐rich surface particles and small Y2O3 inclusions in the film. This work investigates how the surface particle coverage and superconducting properties of dc‐magnetron sputtered thin films change as a function of target stoichiometry and the use of Ar:O2 or Ar:O2:N2O as sputtering gases. In an attempt to eliminate the copper‐rich surface particles, sputtering was performed using targets deficient in copper (by up to 28%). However, only a small reduction in the number of the surface particles was achieved with an Ar:O2 sputtering gas mixture and little change in the superconducting properties was found. No new secondary phases could be detected. When nitrous oxide (N2O) was added to the sputtering gas, the superconducting properties deteriorated and Ba2Cu...

Integrated receivers up to 220 GHz utilizing GaAs-mHEMT technology

The status of integrated receivers for remote sensing and communication applications from 60 GHz to higher frequencies is reviewed. Recent receiver results for silicon and III–V technologies are compared with Schottky diode receivers.

A sub-mm Wave Quasiparticle Receiver For 750 GHz - Progress Report

We have designed a sub-mm wave receiver using superconducting edge tunnel junctions (SIS/SIN). The mixer is subharmonically pumped at about 375 GHz, whereas the signal frequency is at twice this frequency. The local oscillator and the signal are both coupled to a constant width slot antenna (CWSA), and via an exponential tapering to an array of two junctions coupled in series across a slotline. Design and preliminary results will be discussed in this report.