In this paper, the potential of defects for optimizing the microwave properties of YBa2Cu3O7 (YBCO) thin films is demonstrated. On one hand, microscopic Y2O3 precipitates, which can he created in YBCO thin Films by modification of the deposition process, serve as ideal scattering centres for quasiparticles and, thus, lead to a considerable reduction of the microwave surface resistance R,. The modification R,(T) can be explained in terms of the two-fluid model. Data for the quasiparticle scattering rate can be obtained from the measurements. On the other hand, the impart of artificial defects, so called antidots, upon the microwave properties is analyzed. R-s measurements demonstrate that the ion beam etching creates a similar to 20nm broad damaged area at the edge of the antidots. First measurements of the power handling capability of YBCO thin film resonators indicate that the magnetic contribution to the nonlinear behavior can be reduced by antidots. The implementation of antidots, which have been proven to be an ideal and easy tool to improve active YBCO thin film devices, might be of use for microwave applications as well.

The impact of microscopic defects upon the microwave properties of high-Tc superconductor (HTS) films is examined. YBa2Cu3O7 films with different size and densities of Y2O3 precipitates are grown on LaAlO3 and sapphire by variation of the energy of the ions during sputter deposition. It is demonstrated, that the temperature dependence of the microwave surface resistance Rs does not depend on the type of substrate material but on the density of the defects. Films grown at low ion energy (resulting in a low density of microscopic defects) show a characteristic shoulder in the Rs(T) curve which shifts to higher temperature and decreases in size with increasing energy of the ions (i.e. increasing density of microscopic defects). Temperature dependence and reduction of the surface resistance with increasing density of defects are explained in terms of the two-fluid model with thermally excited quasiparticles characterised by a Drude-shaped conductivity spectrum. Values for the scattering rates can be derived from the measurements of the surface resistance, which agree with the classical Matthiesen rule. The impurity scattering rate increases with increasing defect density. Finally, the experimental data and the theoretical model demonstrate, that the surface resistance can be reduced by up to a factor of 2 over a wide temperature range. The reduction of the surface resistance is accompanied by an improvement of the mechanical properties of the HTS thin films which leads to an increased critical film thickness. Both properties, namely the increase of the critical thickness and the reduction of the microwave surface resistance, demonstrate the potential of microscopic defects for improvement of HTS films for applications.

Using magneto-optical visualization, individual 2 µm diameter antidots (circular holes) in YBa2Cu3O7−x thin films are clearly observed. These antidots are found to act at low temperature as very strong pinning centres. Flux penetration takes place along the lattice vectors of the antidot lattice. Both commensurate and incommensurate channels are observed as well as hopping between channels, in agreement with numerical simulations. Surprisingly, even for these large (r/λ � 7) antidots the trapped flux is in agreement with previous theoretical work for small (r � λ) antidots.

Using magneto-optical visualization, individual 2 μm diameter antidots (circular holes) in YBa2Cu3O7 − x thin films are clearly observed. These antidots are found to act at low temperature as very strong pinning centres. Flux penetration takes place along the lattice vectors of the antidot lattice. Both commensurate and incommensurate channels are observed as well as hopping between channels, in agreement with numerical simulations. Surprisingly, even for these large (r/λ ≃ 7) antidots the trapped flux is in agreement with previous theoretical work for small (r << λ) antidots.

Penetration of magnetic flux in YBa2Cu3O7 superconducting thin films in an external magnetic field is visualized using a magneto-optic technique. A variety of flux patterns due to non-linear vortex diffusion is observed: (1) Roughening of the flux front with scaling exponents identical to those observed in burning paper including two distinct regimes where respectively spatial disorder and temporal disorder dominate. In the latter regime Kardar-Parisi-Zhang behavior is found. (2) Fractal penetration of flux with Hausdorff dimension depending on the critical current anisotropy. (3) Penetration as ‘flux-rivers’. (4) The occurrence of commensurate and incommensurate channels in films with anti-dots as predicted in numerical simulations by Reichhardt, Olson and Nori. It is shown that most of the observed behavior is related to the non-linear diffusion of vortices by comparison with simulations of the non-linear diffusion equation appropriate for vortices.

The growth of Xe on the corrugated Cu(110) surface has been investigated using scanning tunneling microscopy. It is shown that the diffusion anisotropy can lead to the stabilization of energetically unfavored structures and growth modes in two dimensions. In the present case the nucleation and growth occurs at [001]-oriented steps at low surface temperature (13K) whereas at high higher temperatures (40K) an almost exclusive decoration of the [11̄0] step edges takes place. These results are explained by interaction calculations and Monte Carlo simulations.

In order to mechanically stabilise thick YBa2Cu3O7-x films on hf-suitable substrates with large lattice mismatch between substrate and high-Tc film, structural defects are introduced into the films. It is shown that the defects lead to an increase of the critical thickness of the films and a modified temperature dependence of the microwave surface reslstance, which can be explained in terms of d-wave superconductivity with a high defect density. The resulting surface resistance, hf power handling capability and intermodulation distortion are determined and seem to be improved by the presence of the defects.

Crack-free thick YBa/sub 2/Cu/sub 3/O/sub 7-x/ films are prepared on CeO/sub 2/ buffered r-cut sapphire (2 inch in diameter) with thickness up to 700 nm, smooth surfaces ('peak-to-valley' roughness <10 nm), high critical currents (J/sub c/>2 MA/cm/sup 2/ at 77 K and 0 T) and low microwave surface resistance (R/sub s/(77 K)/spl ap/1.4 m/spl Omega/ and R/sub s/(4.2 K)/spl ap/110 /spl mu//spl Omega/ at 19 GHz), comparable to the best values reported for YBCO films on structurally better matched substrates. The improvement of the critical thickness of YBa/sub 2/Cu/sub 3/O/sub 7-x/ on structurally poorly matched sapphire from typically d/sub c//spl ap/250-300 nm to d/sub c/>700 nm is achieved by introducing defects which do not hamper the superconducting properties of the films. The modified film growth was reflected by slightly broadened rocking curves and an altered temperature dependence of R/sub s/. The thick YBCO films were able to handle high microwave power corresponding to magnetic field amplitudes (B/sub HF/) of at least 54 and 37 mT at 4.2 and 50 K (limited by the available power), and 17 mT at 77 K (often limited by quenches). The excellent high-power performance was achieved without any degradation of the samples despite frequent thermal cycling.

Large-area YBa(2)Cu(3)O(7) (YBCO)-films up to 4" are grown on CeO(2) buffered sapphire substrates using an unipolar pulsed (201 kHz) de magnetron sputter process on a 6" magnetron cathode. At optimized deposition parameters, epitaxial c-axes oriented films (FWHM of rocking curve: Delta omega(005) = 0.2 degrees) of high quality are reproducibly obtained. Structure and orientation are determined by x-ray investigations. Inductive and resistive measurements exhibit transition temperatures T(c) > 86 K with Delta T(c) < 0.5 K and critical current densities J(c) > 2.10(6) A/cm(2) at LN(2). The pulsed de sputtering technique offers the possibility of a YBCO deposition on an industrial scale. The advantages and differences with respect to the standard de sputter technique are demonstrated.