E.M.C.M. Reuvekamp

CeO2 as insulation layer in high Tc superconducting multilayer and crossover structures

We present a study of the electrical properties of insulating CeO2 layers in combination with superconducting (Y/Dy) Ba2Cu3O7-delta (RBCO) films over ramps and in crossover structures. CeO2 is frequently used as a buffer layer, or template layer for biepitaxial grain boundary junctions, but can also be used as an insulating layer in ramp-type junctions and other multilayer structures. Epitaxial thin films of CeO2 were deposited by pulsed laser ablation using SrTiO3 substrates. We characterized the insulating performance of CeO2 thin films in terms of breakdown field Ebd and the relative dielectric constant epsilon r. For 80 nm thick CeO2 at 77 K we found Ebd=1 × 10-6 V/cm, using a 1 nA/100 µm2 breakdown criterion, which gives us a specific resistivity of rho => 10-9 Omega cm up to breakdown. From capacitance measurements on planar RBCO/CeO2/RBCO structures we obtained for the dielectric constant: epsilon r [approximately-equal-to] 15. The texture of CeO2 in combination with RBCO on ramped surfaces, simulated by SrTiO3 (STO) (103) substrates having their normal tilted by 18° away from the STO[001] direction, has been studied by x-ray diffraction.

Microstructures of ramp-edge YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox Josephson junctions on different substrates

Ramp-edge YBa2Cu3/PrBa2Cu3Ox/YBa2Cu3Ox Josephson junctions with PrBa2Cu3Ox (PrBCO) or SrTiO3 as a separating layer on different kinds of substrate have been studied by high-resolution electron microscopy. The bottom YBa2Cu3Ox (YBCO) layer and the separating layer (PrBCO or SrTiO3) were epitaxially c oriented, irrespective of the substrate (yttria stabilized zirconia (YSZ), SrTiO3 or NdGaO3, all in (001) orientation). The use of ion milling in the manufacturing of Josephson junctions was found to yield smooth slopes with an angle of about 20°. The Josephson junction was facing away from the beam direction was found to have a dimple in the substrate near the base of the junction. The barrier layers were observed to have a homogeneous thickness. These layers were as the top YBCO layers were oriented with their c-axis perpendicular to (001) plane of the substrate for perovskite substrates and perpendicular to the surface for YSZ substrates. In the case of a YSZ substrate, the dimple in the substrate as well as the slope of the substrate close to the base of the junction were found to lead to small angle grain boundaries in the YBCO film as well as randomly oriented YBCO grains, which results in a poor ramp-edge junction. In the case of SrTiO3 or NdGaO3 substrate, all components of the device were fully epitaxial, thus resulting in good ramp-edge junctions.

An HTS quasi-one junction SQUID-based periodic threshold comparator for a 4-bit superconductive flash A/D converter

An all high-T/sub c/ periodic threshold comparator for application in a 4-bit superconductive A/D converter has been realized and tested. The theoretical threshold curve of the comparator is calculated and compared to the measured results. Furthermore, the thermal noise immunity and the influence of flux-flow are considered, resulting in practical design constraints for the comparator circuit.<<ETX>>

Design and fabrication of a HTC quasi-one junction SQUID based comparator for a 4-bit superconductive flash A/D converter

The design of a quasi-one-junction superconducting quantum interference device (SQUID)-based comparator for an all HTC superconductive 4-b flash analog-to-digital (A/D) converter is proposed and discussed. Ramp-type junctions of YBCO/PBCO/YBCO are used. The inductance can be realized by a stripline between the junctions. Simulations by the PSCAN package show correct behavior of the quasi-one-junction SQUID (QOS) up to frequencies of approximately 1 GHz. The output voltage pulses are periodic with the input current, as expected. In the dynamic simulations, output voltage pulses are observed at an applied flux of around (2m-1) Phi /sub 0//2. Tile junctions in the QOS are structured on two ramp-edges. The operating frequencies will be below 1 MHz in this first design. A first realization uses the YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) superconductor and PrBa/sub 2/Cu/sub 3/O/sub 7-x /(PBCO) as junction barrier and electrode interlayer material. On a 10-mm*10-mm SrTiO/sub 3/

YBa2Cu3O7-d α-Axis Films and Planar Junctions

In order to prepare planar junctions, multilayers were sputtered, using YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as superconductor and PrBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) as barrier material. The sandwiches are a-axis oriented. Out of these, single junctions were etched, planarized with CeO/sub 2/. Finally gold contacts were sputtered. The junctions are squares of size 20/spl times/20 to 100/spl times/100 /spl mu/m/sup 2/. A prerequisite for successful preparation of junctions from a-axis oriented multilayers is a study of such films. Films deposited by RF off-axis sputtering were characterized electrically, Their morphology was investigated by XRD, AFM and TEM. The films are very smooth and have a grain size of below 100/spl times/100 nm/sup 2/. To improve T/sub c/ and crystal quality, template layers were used. The T/sub c/ of a single film is about 62 K; by using a template layer up to 78 K can be reached. For α-axis oriented growth not only a reduction of the deposition temperature is important, but also the growth rate must be high enough. Best results were obtained at rates higher than 150 nm/h. The first planar junctions show a supercurrent, but otherwise rounded I-V curves.In order to prepare planar junctions, multilayers were sputtered, using YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as superconductor and PrBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) as barrier material. The sandwiches are a-axis oriented. Out of these, single junctions were etched, planarized with CeO/sub 2/. Finally gold contacts were sputtered. The junctions are squares of size 20/spl times/20 to 100/spl times/100 /spl mu/m/sup 2/. A prerequisite for successful preparation of junctions from a-axis oriented multilayers is a study of such films. Films deposited by RF off-axis sputtering were characterized electrically, Their morphology was investigated by XRD, AFM and TEM. The films are very smooth and have a grain size of below 100/spl times/100 nm/sup 2/. To improve T/sub c/ and crystal quality, template layers were used. The T/sub c/ of a single film is about 62 K; by using a template layer up to 78 K can be reached. For a-axis oriented growth not only a reduction of the deposition temperature is important, but also the growth rate must be high enough. Best results were obtained at rates higher than 150 nm/h. The first planar junctions show a supercurrent, but otherwise rounded I-V curves.<<ETX>>

Vortex motion and Josephson effect in superconducting microbridges in

Microbridges with dimensions smaller than the effective penetration depth have been prepared in epitaxially grown

YBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) a-axis films and planar junctions

In order to prepare planar junctions, multilayers were sputtered, using YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as superconductor and PrBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) as barrier material. The sandwiches are a-axis oriented. Out of these, single junctions were etched, planarized with CeO/sub 2/. Finally gold contacts were sputtered. The junctions are squares of size 20/spl times/20 to 100/spl times/100 /spl mu/m/sup 2/. A prerequisite for successful preparation of junctions from a-axis oriented multilayers is a study of such films. Films deposited by RF off-axis sputtering were characterized electrically, Their morphology was investigated by XRD, AFM and TEM. The films are very smooth and have a grain size of below 100/spl times/100 nm/sup 2/. To improve T/sub c/ and crystal quality, template layers were used. The T/sub c/ of a single film is about 62 K; by using a template layer up to 78 K can be reached. For α-axis oriented growth not only a reduction of the deposition temperature is important, but also the growth rate must be high enough. Best results were obtained at rates higher than 150 nm/h. The first planar junctions show a supercurrent, but otherwise rounded I-V curves.In order to prepare planar junctions, multilayers were sputtered, using YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as superconductor and PrBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) as barrier material. The sandwiches are a-axis oriented. Out of these, single junctions were etched, planarized with CeO/sub 2/. Finally gold contacts were sputtered. The junctions are squares of size 20/spl times/20 to 100/spl times/100 /spl mu/m/sup 2/. A prerequisite for successful preparation of junctions from a-axis oriented multilayers is a study of such films. Films deposited by RF off-axis sputtering were characterized electrically, Their morphology was investigated by XRD, AFM and TEM. The films are very smooth and have a grain size of below 100/spl times/100 nm/sup 2/. To improve T/sub c/ and crystal quality, template layers were used. The T/sub c/ of a single film is about 62 K; by using a template layer up to 78 K can be reached. For a-axis oriented growth not only a reduction of the deposition temperature is important, but also the growth rate must be high enough. Best results were obtained at rates higher than 150 nm/h. The first planar junctions show a supercurrent, but otherwise rounded I-V curves.<<ETX>>

YBa/sub 2/Cu/sub 3/O/sub 7/(-δ) a-axis films and planar junctions

In order to prepare planar junctions, multilayers were sputtered, using YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as superconductor and PrBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) as barrier material. The sandwiches are a-axis oriented. Out of these, single junctions were etched, planarized with CeO/sub 2/. Finally gold contacts were sputtered. The junctions are squares of size 20/spl times/20 to 100/spl times/100 /spl mu/m/sup 2/. A prerequisite for successful preparation of junctions from a-axis oriented multilayers is a study of such films. Films deposited by RF off-axis sputtering were characterized electrically, Their morphology was investigated by XRD, AFM and TEM. The films are very smooth and have a grain size of below 100/spl times/100 nm/sup 2/. To improve T/sub c/ and crystal quality, template layers were used. The T/sub c/ of a single film is about 62 K; by using a template layer up to 78 K can be reached. For α-axis oriented growth not only a reduction of the deposition temperature is important, but also the growth rate must be high enough. Best results were obtained at rates higher than 150 nm/h. The first planar junctions show a supercurrent, but otherwise rounded I-V curves.In order to prepare planar junctions, multilayers were sputtered, using YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as superconductor and PrBa/sub 2/Cu/sub 3/O/sub 7/(-/spl delta/) as barrier material. The sandwiches are a-axis oriented. Out of these, single junctions were etched, planarized with CeO/sub 2/. Finally gold contacts were sputtered. The junctions are squares of size 20/spl times/20 to 100/spl times/100 /spl mu/m/sup 2/. A prerequisite for successful preparation of junctions from a-axis oriented multilayers is a study of such films. Films deposited by RF off-axis sputtering were characterized electrically, Their morphology was investigated by XRD, AFM and TEM. The films are very smooth and have a grain size of below 100/spl times/100 nm/sup 2/. To improve T/sub c/ and crystal quality, template layers were used. The T/sub c/ of a single film is about 62 K; by using a template layer up to 78 K can be reached. For a-axis oriented growth not only a reduction of the deposition temperature is important, but also the growth rate must be high enough. Best results were obtained at rates higher than 150 nm/h. The first planar junctions show a supercurrent, but otherwise rounded I-V curves.<<ETX>>

Influence of PrBa2Cu3-xGaX07 barrier material on electrical behaviour of ramp-type Josephson junctions

The use of PrBa2Cu3-xGaxO7 barrier material with increased resistivity by Ga doping (x=0; 0.05 and 0.1) in ramp-type DyBa2Cu3O7/PrBa2Cu3-xGaxO7/DyBa2Cu3O7 Josephson junctions has been investigated. All junctions have been fabricated with very smooth sputtered films and show good RSJ-like I-V characteristics with clear Josephson behaviour. Both critical current Ic and normal state resistance Rn are influenced by the doping level as well as the barrier thickness. The temperature dependence of the normal state resistance at different Ga doping levels and barrier thicknesses will be discussed.

THE EFFECT OF THE SUBSTRATE TEMPERATURE ON THE GROWTH OF YBa 2 Cu 3 O 7-d ON LaAlO 3 USING RF MAGNETRON SPUTTERING

A series of YBa 2 Cu 3 O 7-d thin films have been in-situ RF magnetron sputtered at different substrate temperatures using a direct facing target. At the highest substrate-temperatures, which were about 700 °C, the films showed T c,zero of 85 to 87 K. Superconducting onset-temperatures higher than 93 K could be obtained. Optical- and electron-microscopy showed a very pronounced surface structure, consisting of needle-like crystallites, which were oriented perpendicularly to one another in the plane of the film. The substrate-temperature ‘window’ for obtaining a T c,zero of around 85 K was found to be 20–30 K. At a substrate-temperature of approximately 110 °C below the high-temperature window, another temperature range (width ˜ 20 K) was found in which very good oriented films could be sputtered. Although T c,zero was drastically reduced below 77 K, the onset-temperatures were almost as high as in the high substrate-temperature regime. These films were smoother than the c-axis oriented films and the pronounced needle-like crystallites at the surface were absent. The films were found to be semiconductive out of the two temperature ranges, without any sign of a superconducting transition temperature. For characterization of the films we combined a number of techniques. To determine the superconducting properties, we applied critical temperature T c measurements. X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) and scanning Auger measurements (SAM) were used as analysis techniques. Results of these techniques are presented here.