We report Josephson junctions in YBa2Cu3O7−δ thin films, fabricated by oxygen irradiation at 200 keV through a 50 nm wide slit in an implantation mask. After annealing the irradiated microbridges at 500°C in an oxygen atmosphere, the implanted region has a reduced but finite transition temperature, allowing Josephson coupling in a temperature window of <15 K. Close to the coupling temperature the critical current shows a nearly complete modulation in an applied magnetic field. This indicates a homogeneous current distribution in the junction. Over the entire temperature range of Josephson coupling, the junctions exhibit resistively shunted junction like I–V characteristics with additional excess current. Furthermore, the exponential dependence of the critical current on temperature is in good agreement with conventional superconductor-normal-superconductor proximity effect theory.

The properties of SNS Josephson junctions in YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films have been investigated, which were fabricated by oxygen irradiation at 200 keV through a 50 nm wide slit in an implantation mask. After annealing the irradiated microbridges at 500/spl deg/C in an oxygen atmosphere, the implanted region has a reduced but finite transition temperature, allowing Josephson coupling in a temperature window of <15 K. Close to the coupling temperature the critical current shows a nearly complete modulation in an applied magnetic field. This indicates a homogeneous current distribution in the junctions and therefore a homogeneous defect distribution throughout the implanted and subsequently annealed region of the superconducting microbridges. Over the entire temperature range of Josephson coupling, the junctions exhibit resistively shunted junction like I-V characteristics with additional excess current. Furthermore, the exponential dependence of the critical current on temperature is in good agreement with conventional superconductor-normal-superconductor proximity effect theory.

The differential conductance of YBa2Cu3O7-delta edge junctions with a PrBa2Cu2.9O7-delta barrier has been investigated in detail. One striking property of our edge junctions is the existence of a well pronounced, symmetric subharmonic gap structure which is observed in the differential conductance. These features can be explained by multiple Andreev reflections if we assume the existence of a second peak in the density of states. Furthermore, a zero-bias peak in the conductivity was observed in some of the junctions, which may be explained by Andreev bound states at the interface of a d-wave superconductor. [S0163-1829(99)08905-5].

We have designed, fabricated and successfully tested a first-order delta-sigma modulator using a high-temperature superconducting multilayer technology with bicrystal Josephson junctions. The circuit has been fabricated on a SrTiO3 bicrystal substrate. The YBa2Cu3O7/SrTiO3/YBa2Cu3O7 trilayer was fabricated by laser deposition. The bottom layer served as a superconducting groundplane. The Josephson junctions were formed at the bicrystal line in the upper layer. The integrator resistance has been made from a Pd/Au thin film. The circuit consists of a dc-SFQ converter, a Josephson transmission line, a comparator, an L/R integrator and an output stage. The correct operation of the modulator has been tested using dc measurements. The linearity of the modulator was studied by measuring the harmonic distortions of a 19.5 kHz sine wave input signal. From the recorded spectrum, a minimum resolution of at least 5 bits can be estimated. This accuracy was limited by the noise of the preamplifier. The correct operation of the current feedback loop was demonstrated by cutting the feedback inductance.

The determination of bit error rates in single flux quantum logic circuits operating at temperatures well above the temperature of liquid helium is essential since the question of stability against thermal noise arises. We determined experimentally the static and for the first time the dynamic error rate at temperatures of about 40 K with the help of simple test circuits. The static error rate has been investigated using two different circuits with bicrystal junctions in either single-layer or multilayer configuration. In both cases the internal state of a storage loop was observed by a dc-SQUID. A ring oscillator based on a Josephson transmission line allowed us to measure the dynamic error rate of a Josephson comparator at high frequencies. This circuit has been fabricated using junctions by focused-electron-beam irradiation. It is specially suited for the detection of seldom occurring switching errors.

The application of high temperature superconductor (HTS) Josephson junctions in digital rapid single flux quantum circuits requires a careful study of the influence of thermal noise on the bit error rate (BER). We have determined experimentally, for the first time, the BER of a HTS rapid single flux quantum circuit. A comparator, formed by two Josephson junctions, was integrated in a Josephson transmission line ring oscillator, allowing us to perform high speed testing of the comparator at GHz frequencies. For fabrication, focused-electron-beam-irradiated junctions have been used because of their small parameter spread and excellent alignment possibilities. A BER of less than 10−11 was obtained at 39 K.

We measured the static error rate of a high-Tc superconductor dc superconducting quantum interference device (SQUID), which, in the form as a storage loop for single flux quanta, is a basic element of rapid single flux quantum circuits. Using high-Tc multilayer bicrystal technology, we fabricated a stacked dc SQUID pair, one SQUID serving as the storage loop, the other one as the readout device. The escape rate of a stored flux quantum was measured as a function of the bias current at a temperature of 28 K. The measured error rates were in good agreement with a model calculation based on thermally activated barrier crossing.

The correlation between the shape and the morphology of ramps prepared by ion-beam etching of YBa2Cu3O7−δ thin films, and the properties of ramp-type junctions were investigated in detail. We examined the influence of different fabrication parameters on the YBa2Cu3O7−δ ramps by atomic force microscopy. Ramp-type junctions were fabricated using PrBa2Cu2.9Ga0.1O7−δ as barrier material. We observed a strong influence of the shape of the ramp on the homogeneity and, thus, on the transport properties of the junctions. Furthermore, we observed that the roughness of the ramps is strongly influenced by the voltage of the ion-beam during etching. Best results are achieved when an additional wet cleaning step by bromine solution in ethanol is introduced prior to the deposition of the barrier and the top electrode. As a result from our optimization, the on-chip spread of the junction critical current was reduced to 11%.

Complex high temperature superconductor digital RSFQ circuits require a multilayer technology that is compatible with freely positionable Josephson junctions. For RSFQ circuits, especially small inductances are required. This can be achieved by using superconducting striplines on a YBCO groundplane. We have produced and examined all basic elements like, e.g., superconducting cross-overs, vias, pinhole free insulation layers and on-chip resistors that are necessary to design and fabricate HTS RSFQ circuits. All structures have been successfully tested and integrated with ramp type junctions with PBCO barriers. For the superconducting lines over the groundplane an inductance of 0.8 pH/square was measured which shows the advantage of a superconducting groundplane. Based on this technology we fabricated a simple multilayer circuit consisting of two magnetically coupled d.c.-SQUIDs. Here we used bicrystal junction stacks due to the smaller spread of junction parameters. The circuit allowed essential operations like generating, erasing and detecting a single flux quantum. One d.c.-SQUID served as storage loop while the other was kept permanently in the voltage state and was used to determine the internal flux state of the storage loop.

The properties of SNS Josephson junctions in YBa2Cu3O7-δ δ thin films have been investigated, which were fa b- ricated by oxygen irradiation at 200 keV through a 50 nm wide slit in an implantation mask. After annealing the irradiated m i- crobridges at 500°C in an oxygen atmosphere, the implanted region has a reduced but finite transition temperature, allowing ...

The properties of SNS Josephson junctions in YBa2Cu3O7-δ thin films have been investigated, which were fabricated by oxygen irradiation at 200 keV through a 50 nm wide slit in an implantation mask. After annealing the irradiated microbridges at 500°C in an oxygen atmosphere, the implanted region has a reduced but finite transition temperature, allowing Josephson coupling in a temperature window of < 15 K. Close to the coupling temperature the critical current shows a nearly complete modulation in an applied magnetic field. This indicates a homogeneous current distribution in the junctions and therefore a homogeneous defect distribution throughout the implanted and subsequently annealed region of the superconducting microbridges. Over the entire temperature range of Josephson coupling, the junctions exhibit resistively shunted junction like I-V characteristics with additional excess current. Furthermore, the exponential dependence of the critical current on temperature is in good agreement with conventional superconductor-normal-superconductor proximity effect theory.

La(2)CuO(4+delta), BaTbO(3) and SrTbO(3) were tested as insulating and barrier materials for high-T(C) superconductor (HTS) thin film devices. Whereas La(2)CuO(4+delta) is a semiconductor with a layered structure, BaTbO(3) and SrTbO(3) are insulating perovskite materials with a pseudocubic structure. Thin films of these materials have been fabricated by sputtering. By exposing the La(2)CuO(4+delta) film to an oxygen microwave plasma it becomes metallic and superconducting al 32K. The microstructure of YBa(2)Cu(3)O(7-x)/BaTbO(3)/YBa(2)Cu(3)O(7-x) multilayer films was investigated by high-resolution transmission electron microscopy (HRTEM). Although the lattice mismatch in this system is about 10 %, the interface is atomically sharp and no localized misfit dislocations occur at the interface. This leads to a very small distortion of the lattice in the vicinity of the interface. We have fabricated crossover structures using BaTbO(3) as insulating layer. Field-effect devices containing BaTbO(3) as a dielectric and Josephson junctions with a BaT6O(3) barrier and a BaTbO(3) insulating layer could be produced successfully.