M.A.J. Verhoeven

Ramp‐type junction parameter control by Ga doping of PrBa2Cu3O7−δ barriers

We analyzed the transport of charge carriers across PrBa2Cu3−xGaxO7−δ (PBCGO) barriers as a function of barrier thickness, Ga‐doping level, temperature, and bias voltage. It was found that by Ga doping, the normal state resistance (Rn) of the junctions was systematically increased, while the critical current (Ic) remained constant. We argue that pair transport takes place by direct tunneling, whereas the quasiparticles have access to channels formed by one or more localized states inside the barrier. By Ga doping the I cRn products were increased, up to 8mV at 4.2K for junctions with 8nm thick PrBa2Cu2.6Ga0.4O7−δ barriers.

Ramp type HTS Josephson junctions with PrBaCuGaO barriers

Ramp type Josephson junctions have been fabricated using DyBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// as electrode material and PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7-/spl delta// with x=0, 0.10 and 0.40 as junction barriers. Barrier thickness lie between 6-30 nm. Several junctions without barrier were made in order to find ways to minimize the damage of the ramp interface. In total about 40 chips were fabricated each containing several junctions and their I-V characteristics measured for various temperatures down to 4.2 K. Only those junctions showing clear RSJ-like curves were selected to be analyzed. In some cases we also measured I/sub c/ as a function of a small applied field and obtained a clear Fraunhofer pattern, but there is a tendency to flux trapping as evidenced by LTSEM. It was found at 4.2 K that the critical current density J/sub c/ scales with the specific resistance R/sub n/A as J/sub c/=C/sub bar/(R/sub n/A)/sup -m/ (m=1.8/spl plusmn/0.5). The barrier material dependent constant C/sub bar/ increases with x, whereas, for a given d, J/sub c/ is constant and R/sub n/A increases.<<ETX>>

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.

Nucleation and growth of PrBa2Cu3O7−δ barrier layers on ramps in DyBa2Cu3O7−δ studied by atomic force microscopy

We studied the microstructure of Ar ion-beam etched ramps in epitaxial DyBa2Cu3O7-d films by atomic force microscopy. Generally, ramps were well aligned with one of the crystal axes of the (001) SrTiO3 substrate. In those cases we observed a surface reconstruction into a regular pattern of very long (up to 0.5 mm), about 20 nm wide facets, parallel to the ramp edge. For high misorientation angles, the reconstruction appeared much more complex and less regular. Furthermore, we investigated the nucleation and growth of very thin barrier layers of PrBa2Cu3O7-d on well aligned ramps. We found that nucleation takes place in the shallow trenches in the ramp, where facets meet. When more material is deposited, islands coalesce to form closed domains parallel to the trenches. This causes a thickness modulation close to 100% for barrier layer thickness up to 6 nm. The conclusions drawn from this research allow for an important improvement of the technology for the fabrication of ramp-type junctions.

Distribution of the critical current density and flux trapping in YBa2Cu3O7−δ ramp‐edge Josephson junctions

We have studied the spatial distribution of the critical current density in YBa2Cu3O7−δ ramp edge Josephson junctions using low‐temperature scanning electron microscopy. Applying this technique allows the imaging of the critical current density distribution with a spatial resolution of about 1 μm. Our measurements show that the geometry of the ramp‐edge junction eases the trapping of magnetic flux quanta in the YBa2Cu3O7−δ layer covering the ramp edge. These trapped flux quanta result in a spatially inhomogeneousmagnetic field parallel to the barrier layer, which in turn results in a spatially modulated supercurrent density and an unusual magnetic field dependence of the critical current.

305)SrTiO3 as substrate for coherently tilted epitaxial YBa2Cu3Ox thin films

High Tc superconducting YBa2Cu3Ox (YBCO) thin films have been prepared on (305) SrTiO3 (STO) substrates. X‐ray diffraction analysis and Rutherford backscattering experiments reveal that the c‐axis of the layers is directed along the [001] STO axis. Bragg reflection measurements from YBCO lattice planes with high h, k, and l indices confirm that the film growth is epitaxial and almost single domain. For the critical current density jc(77 K) values of 2×106 and 1×103 A/cm2 have been found in the [010] and [501] YBCO directions, respectively.

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>>

Characterization of ramp-type YBa/sub 2/Cu/sub 3/O/sub 7/ junctions by AFM

We studied the morphology of ramps in REBa/sub 2/Cu/sub 3/O/sub 7/ (REBCO) epitaxial films on SrTiO/sub 3/ substrates, fabricated by RF magnetron sputter deposition and pulsed laser deposition (PLD), by Atomic Force Microscopy (AFM) and High Resolution Electron Microscopy (HREM). The ramps were fabricated by Ar ion beam etching using different masks of standard photoresist and TiN. AFM-studies on ramps in sputter deposited films show a strong dependence, i.e., formation of facets and ridges, on the angle of incidence of the ion beam with respect to the substrate surface as well as the rotation angle with respect to the crystal axes of the substrate. Ramps in pulsed laser deposited films did not show this dependence. Furthermore, we studied the effect of an anneal step prior to the deposition of barrier layers (i.e. PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7/) on the ramp. First results show a crystallization of the ramp surface, resulting in terraces and a non-homogeneous growth of the barrier material on top of it. The thickness variations, for thin layers of barrier material, can even become much larger than expected from the amount of deposited material and are dependent on the deposition and anneal conditions. HREM studies show a well-defined interface between barrier layer and electrodes. The angle of the ramp depends on the etch rate of the mask and REBCO and on the angle of incidence of the ion beam. Hard masks, like TIN, have a much lower etch rate compared to photoresist, resulting in an angle of the ramp comparable to the angle of incidence and, subsequently, in a low etching rate on the ramp.

Ga segregation in DyBa2Cu3O7-δ/PrBa2Cu3-xGaO7-δ/DyBa2Cu3O7-δ ramp-type Josephson junctions

Ramp-type Josephson junctions with highly doped PrBa2Cu3-xGaxO7-δ barrier layers (x=0.7, 1.0) have been investigated by high-resolution electron microscopy. A Ga-rich intergrowth and Ga diffusion in the ion-milled SrTiO3 substrate are observed. The Ga segregation is responsible for the deviating electrical behavior as expected from extrapolation of low doping levels (x=0.1-0.4). The Ga diffusion in the ion-milled substrate, and possibly in the base electrode, may hamper Josephson junction fabrication using substituted REBa2Cu3O7-delta materials (RE=rare earth)

Low- or high-angle Ar ion-beam etching to create ramp-type Josephson junctions

The dependence of the ramp geometry on high- or low-angle ion-beam etching, used to structure ramp-type Josephson junctions based on high- superconductors, is investigated by cross-section transmission electron microscopy. The surface quality, interfaces and crystal defects are analysed by high-resolution electron microscopy. Technical difficulties to reproducibly obtain the desired slope angle and shape make high-angle ion-beam etching less interesting although the surface quality is comparable and no systematic differences in electrical properties are observed.