R.P.J. IJsselsteijn

High Tc thin films prepared by laser ablation: material distribution and droplet problem

The lateral material distribution of laser-deposited YBa2Cu3O7?? films and the density of droplets coming from the target were studied by varying the laser pulse energy, the laser spot size and the target-to-substrate distance. Silicon wafers at ambient temperature were used as substrates to guarantee a large sticking coefficient of the particles. The deposition rate is found to depend linearly on the laser energy density E and quadratically on the spot size S at the target, whereas the droplet density is slightly dependent on E and increases linearly with 1/S, yielding a threshold energy of 0.9 J cm?2. With a laser spot size of 7.15 mm2 and a laser energy density of 1.2 J cm?2, we were able to reduce the number of droplets to one to two per 500 ?m2 for a high quality high Tc film with a typical thickness of 100 nm.

Four layer monolithic integrated high Tc dc SQUID magnetometer

YBa2Cu3O7−x based monolithic integrated dc SQUID magnetometers, consisting of a dc SQUID integrated with a flux transformer on a single bicrystalline substrate, have been fabricated and characterized. The devices consist of four layers, including two superconducting layers, and first realizations operate up to 73 K. A maximum voltage modulation of 32 μV is observed at 40 K. A field sensitivity of 0.17 pT/√Hz is obtained above 200 Hz at 45 K and 0.49 pT/√Hz at 1 Hz and 65 K.

Subharmonic Shapiro steps in high-Tc Josephson junctions

We studied the response of high‐Tc biepitaxial grain boundaryjunctions to 100 GHz radiation in the presence of a magnetic field. Integer as well as subharmonic constant voltage steps are observed, even at one‐fifth of the voltage separation between integer (Shapiro) steps. Our results indicate that this behavior is due to the synchronized motion of Josephsonvortices along the junction. We show that this effect is directly related to the width of the junction, and not to an array of weaker regions in the barrier.

Multilayer studies and applications in template bi-epitaxial DC SQUIDS

Multilayer deposition for the creation of a well-defined grain boundary, based on different in-plane orientations of c-axis oriented thin YBa/sub 2/O/sub 2/Cu/sub 3/O/sub 7- delta / layers on a single substrate, has been performed on three different kinds of substrates: (1102)-oriented Al/sub 2/O/sub 3/,

Basic elements for photodeposited high Tc thin film devices

Flat films, high quality insulating layers and adequately superconducting via contacts are basic elements for high Tc device fabrication. We studied the influence of the process parameters of laser deposition on the occurrence of droplets and outgrowths in YBaCuO films. The droplet density is minimal when a laser fluence below about 1.0 J cm-2 is used. The outgrowth density decreases with increasing laser pulse rate or decreasing deposition temperature. High quality flat films were obtained with a rate of 10 Hz and at a temperature of 720 °C. Wet chemical etching and etching with an Argon ion source were used for structuring multilayers with SrTiO3 as an insulating layer. Smooth edges were obtained with an argon gun. Bromine and EDTA etching are not adequate techniques for fabricating controllable well-defined edges. Cross-overs, via contacts and coils were prepared.

Droplets and outgrowths on high-Tc laser ablated thin films

YBa2Cu30x thin films have been grown on silicon, SrTi03 and Zr02 substrates using the pulsed laser deposition technique. Special attention has been paid to droplets and outgrowths which appear on the thin films during the growth process. The droplet density was studied as a function of the laser spot size, the laser energy and the target density. The number of droplets could be reduced to 1 per 100 μm2 for a 100 nm thick film, by ta1cing a large laser spot size and low energy density. The droplet density does not depend on the target density in the range from 80 to 94 % . The number of outgrowths could be reduced to 1 per 100 μm2 for a 100 nm thick film by reducing the deposition temperature or increasing the laser frequency. However, the critical temperature of these layers was reduced by 5 to 10 K. Using SAM no differences in composition between an outgrowth and the rest of the film could be detected.

High T/sub c/ bi-epitaxial junctions and dc SQUIDs structured by focused ion beam etching

Focused ion beam etching has been used to pattern dc SQUIDs into previously characterised template bi-epitaxial grain boundary junctions. Using this technique the screening parameter /spl beta//sub L/ can be optimised for a chosen temperature (in our case 30 K). Electrical characteristics, including noise measurements, are presented. A minimal white noise level of 22 /spl mu//spl phi//sub 0//spl middot/Hz/sup -1/2/ (1.8/spl middot/10/sup -29/ J/spl middot/Hz/sup -1/) has been obtained at 20 K. Using bias current modulation the 1/f noise could almost completely be suppressed down to 1 Hz in the entire temperature range (10-65 K).<<ETX>>

Bi-epitaxial template grain boundaries with different in-plane angles on (100) MgO substrates

Using laser ablation CeO2 and YBa2Cu3O7−d layers have been deposited on (100) oriented MgO substrates for the creation of bi-epitaxial grain boundary weak links. X-Ray Diffraction measurements were used to measure the (in-plane) orientations of the layers. The CeO2 layer has been structured using Ar ion beam etching or CaO lift off. Using these two structuring techniques it is possible to obtain two different grain boundary weak link angles: 18 and 27 degrees. Up till now only 45 degrees weak link angles have been reported in literature, using templates for the creation of these grain boundaries.

Materials aspects of integrated high Tc dc-SQUID magnetometer fabrication

An integrated high Tc dc-SQUID magnetometer is being developed. It has in total 10 layers of five different materials. Various materials aspects of the fabrication process will be discussed, especially the smoothness of the films and the techniques to obtain smooth edges. Cross-overs and superconducting window contacts were fabricated. The critical temperature of the window-contact is 84K (jc(77K)=2·105 A/cm2) and the resistivity of the insulating SrTiO3 layer in the cross-over is 6·105 Ωcm at 77K. The complete coil often shows a small resistive component down to about 50K. Ramp type and bi-epitaxial grain boundary dc-SQUIDs show voltage modulation up to 65K and 79K, respectively. Efforts to fabricate an integrated high Tc dc-SQUID magnetometer will be discussed.

High Tc bi-epitaxial dc SQUIDs structured by focused ion beam etching from single junctions: ßL optimization

Focused ion beam etching has been used to pattern dc SQUIDs into previously characterised template bi-epitaxial grain boundary junctions. In this way, the screening parameter sL can be optimised for a chosen temperature (in our case 30 K). Electrical characteristics, including noise measurements, are presented. A minimal white noise level of 22 µ0·Hz (1.8·10-29 J·Hz-1) has been obtained at 20 K. Using bias current modulation the 1/f noise could be almost completely suppressed down to 1 Hz in the entire temperature range (10?65 K).