F. Kober

Dry-etching processes for high-temperature superconductors

Abstract Patterning of thin films and multilevel structures of high-temperature superconductors is a key technology for microelectronic applications. We performed a comparative study of Ion Beam Etching (IBE) and Reactive Ion Etching (RIE) processes for YBa 2 Cu 3 O 7−δ thin films. The RIE process with a pure chlorine plasma yielded small etching rates, caused by chemical modifications of the sample surface which result in a passivation layer reducing the chemical etching rate. Using IBE, microstructures down to the 1 μm regime could be fabricated without reducing the critical temperature T c and the critical current density J c of the material. Etching rates up to 40 nm/min could be achieved without deteriorating the properties of the superconducting film by cooling the sample effectively during the etching process. The influence of the etching process on J c was investigated by imaging the spatial distribution of the critical current along the patterned microstructures using Low-Temperature Scanning Electron Microscopy (LTSEM).

Spatial resolution limit for the investigation of high-T/sub c/ films by low temperature scanning electron microscopy

Low-temperature scanning electron microscopy represents a promising technique for studying the local superconducting properties of high-T/sub c/ films. The spatial and temporal structure of the electron-beam-induced thermal perturbation of high-T/sub c/ films and the resulting spatial resolution limit are discussed. Typical examples illustrating the imaging technique are presented. >

Low-temperature scanning electron microscopy studies of superconducting thin films and Josephson junctions

Abstract Scanning techniques represent powerful methods for the characterization of condensed matter. Extending Scanning Electron Microscopy (SEM) to the range of low temperatures an interesting tool for studying low-temperature properties of solids with high-spatial and temporal resolution is obtained. By Low-Temperature Scanning Electron Microscopy (LTSEM) important new information on low-temperature phenomena in superconductors, semiconductors, and insulators is obtained by two-dimensional imaging. Here, we summarize the basic principles of LTSEM and show its application to the study of superconducting films and Josephson junctions.

Seebeck and Nernst effect in the mixed state of slightly oxygen deficient YBaCuO

Abstract For an epitaxial c -axis oriented slightly oxygen deficient YBaCuO film, showing a positive Seebeck coefficient around 100 K, the Nernst effect has been measured in the mixed state for magnetic fields up to 9 T. The transport entropy calculated in the usual way is about an order of magnitude smaller than for a similar but fully oxygenated film with a negative Seebeck coefficient near 100 K. This can be explained by the enhanced decoupling of the CuO 2 planes in the slightly oxygen deficient material, resulting in a strong departure of the vortices (“pancake vortices”) from the standard Abrikosov type.

Flux-flow induced Nernst effect in superconducting YBaCuO films

Abstract The thermal force due to a temperature gradient results in a flux motion from the hot to the cold end of a superconductor and in the generation of the Nernst voltage VN perpendicular to the direction of the temperature gradient and magnetic field. Measurements of VN are expected to contribute significantly to the understanding of the physics of flux flow and flux pinning. We report on the first observation of the flux-flow induced Nernst effect in a high-Tc superconductor, using polycrystalline YBaCuO films.

A new inductive method for measuring the critical current density in high-Tc superconducting films

Abstract We describe an inductive method for determining the critical current density in high- T c superconducting films. By using the sample geometry of a thin film ring, the method does not require the assumption of a specific critical state model. The values of the critical current density determined inductively have been checked by standard resistance measurements performed on the same specimens, and good agreement has been obtained. Our inductive method appears promising in particular for investigating the frequency dependence and the magnetic field dependence of the critical current density.

Spatially resolved measurements of the resistive transition in epitaxial YBa2Cu3O7-films

Abstract The spatial distribution of the resistive transition for laser deposited c-axis oriented epitaxial YBa2Cu3O7-films on &:#60;100> SrTiO3 has been imaged by low temperature scanning electron microscopy (LTSEM) with a spatial resolution of about 1 μm. The films which have critical current densities up to 5 × 106 A/cm2 at 77 K show temperature variations in the local resistive transition curves up to 200 mK. Possible reasons for the observed inhomogeneous behavior are discussed.

Thermoelectric and thermomagnetic effects in the mixed state of HTSC's

Abstract Whereas flux line diffusion in the mixed state of HTSCs driven by the Lorentz force has been studied extensively, the thermal diffusion phenomena have received much less attention. In the mixed state, thermal diffusion of the quasiparticles and of the flux lines result in the Seebeck effect and the Nernst effect, respectively. Measurements of these effects yield the transport entropy of the diffusing species. We discuss the recent results obtained mostly for YBaCuO epitaxial films or single crystals, and emphasize the points where thermomagnetic experiments can discriminate between different models for dissipation.

Characterization of superconducting YBaCuO-films by low temperature scanning electron microscopy

Low-temperature scanning electron microscopy has been performed for imaging the spatial distribution of the critical current densityjc(x,y) and of the critical temperatureTc(x,y) in polycrystalline superconducting YBaCuO films. Strongly inhomogeneous behavior has been observed, and the spatial resolution limit has been found to be 1–2 μm. The local temperature increment in the specimen film caused by the electron beam scanning has been demonstrated experimentally as the underlying mechanism of the imaging principle, and the beam-induced thermal perturbation of the high-Tc film/substrate configuration is discussed in detail. The radiation hardness of the sample films against the electron beam irradiation in our imaging experiments has been evaluated. No radiation damage could be detected up to the maximum applied dose of well above 1020 electrons/cm2 for a typical beam energy of 26 keV.

Current transport across grain boundary networks in high-Tc superconductors

Electric transport measurements across single grain boundaries in YBa2Cu3O7 films are summarized. These experiments demonstrate that YBa2Cu3O7 grain boundaries behave like SNS-type Josephson junctions. The implications of this weak link behavior for the critical currents of polycrystalline high-Tc superconductors are discussed in conjunction with spatially resolved measurements of the current transport in polycrystalline YBa2Cu3O7.