The deposition conditions of Y0.9Ba1.9La0.2Cu3Oy (La-YBCO) and (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (LSAT) thin films were studied with the aim of fabricating ramp-edge Josephson junctions on a superconducting ground plane. These films were deposited by a magnetron sputtering method and utilized as a base electrode and an insulating layer under the electrode, respectively. YBa2Cu3Oy thick films grown by liquid phase epitaxy (LPE-YBCO) were used for a ground plane. Insertion of a SrTiO3 buffer layer between LSAT and LPE-YBCO significantly improved the flatness of the film surface. La-YBCO films with a flat surface and Tc (zero) of 87K were reproducibly obtained by DC sputtering. We have fabricated ramp-edge Josephson junctions using these films. Resistively and capacitively shunted junction (RCSJ)-like characteristics were observed in them. An Ic spread of 10.2% (at 4.2K, average Ic = 0.5 mA) was obtained for a 1000-junction series-array.

We have developed the fabrication process, the circuit design technology, and the cryopackaging technology for high-Tc single flux quantum (SFQ) devices with the aim of application to an analog-to-digital (A/D) converter circuit for future wireless communication and a sampler system for high-speed measurements. Reproducibility of fabricating ramp-edge Josephson junctions with IcRn products above 1 mV at 40 K and small Ic spreads on a superconducting groundplane was much improved by employing smooth multilayer structures and optimizing the junction fabrication process. The separated base-electrode layout (SBL) method that suppresses the Jc spread for interface-modified junctions in circuits was developed. This method enabled low-frequency logic operations of various elementary SFQ circuits with relatively wide bias current margins and operation of a toggle-flip-flop (T-FF) above 200 GHz at 40 K. Operation of a 1:2 demultiplexer, one of main elements of a hybrid-type Σ-Δ A/D converter circuit, was also demonstrated. We developed a sampler system in which a sampler circuit with a potential bandwidth over 100 GHz was cooled by a compact stirling cooler, and waveform observation experiments confirmed the actual system bandwidth well over 50 GHz.

This paper reviews our progress on the high-Tc superconducting (HTS) sampler development, covering from the circuit design to the latest experimental data in the sinusoidal and pulse waveform measurements. A computer simulation has revealed that our sampler circuit with an improved design enables waveform measurement with the bandwidth over 100 GHz even with the thermal noise at around 40 K. Using the HTS sampler circuits fabricated employing an improved layout, we demonstrated waveform measurements for sinusoidal signals with frequencies of up to 50 GHz, the upper limit of the signal generator we used, both in the voltage-input-type system with a high-frequency input line and in the current-input-type one with a superconducting pickup coil. In the pulse measurement using an on-chip sampler, we succeeded in observing pico-second-order-wide single flux quantum (SFQ) current pulses, suggesting the potential bandwidth of our HTS sampler of more than 125 GHz.

By depositing insulating layers on oxide superconducting films, the films generally deteriorate. When an insulating multilayer of CeO2(50 )SrTiO3(200 ) was grown on 800--thick EuBa2Cu3O 7-δ (EBCO) films with Tce's (Tc endpoint) above 90 K, the films exhibited Tce's of about 40 K. Recovery of the deteriorated films was carried out by two treatment methods. A pure oxygen treatment, where the deteriorated films were annealed at a temperature (Tsa) of 550C and an oxygen pressure (PO2) of 100 kPa for 60 min, and then naturally cooled, restored the films with Tce's of about 60 K. An activated oxygen plasma (AOP) treatment, where the deteriorated films were exposed to oxygen plasma at a Tsa=550C for 40 min and subsequently oxygen gas was introduced into the chamber up to 2 kPa and then naturally cooled, restored the films with Tce's of about 84 K. The AOP-treated film was recovered with a cooling rate of less than 6.8C/min, and exhibited Tce of 90 K. The AOP-treated film took in oxygen more effectively than the pure oxygen-treated film with the cooling process at less than PO2=100 kPa.

Single flux quantum (SFQ) circuit elements have been designed and fabricated using the YBa2Cu3O7-δ ramp-edge junction technology. Logic operations of SFQ circuit elements, such as a toggle flip-flop (T-FF), a set-reset flip-flop (RS-FF), and a 96-junction Josephson transmission line (JTL), were successfully demonstrated, and dc supply current margins were confirmed up to temperatures higher than 30 K. The circuit layout was improved in order to suppress the critical current (Ic) spread that appears during the junction fabrication procedure. By employing the new circuit layout rule, correct operations at temperatures from 27 K to 34 K with dc supply current margins wider than 7% were confirmed for the T-FF with a single output. Moreover, the maximum operating frequencies of T-FFs were measured to be 360 GHz at 4.2 K and 210 GHz at 41 K, which are substantially higher than the values for the circuits with the conventional layout. According to the simulation result, the maximum operating frequency at 40 K was expected to be approximately 50% of the characteristic frequency at a bit error rate (BER) less than 10-6.

We have fabricated a multilayer structure for single flux quantum (SFQ) circuit application using a high-temperature superconductor (HTS). La0.2-Y0.9Ba1.9Cu3Ox (La-YBCO) base electrode layers were prepared by a dc or rf magnetron sputtering method. The reproducibility of film quality for dc-sputtered La-YBCO films was better than that for rf-sputtered films, and the dc sputtered films exhibited the average surface roughness Ra less than 1.0 nm and a Tc zero value of 88 K. By using the dc-sputtered La-YBCO films, a multilayer structure of SrSnO3/La-YBCO/SrSnO3/La-YBCO on MgO substrate with Ra below 2.0 nm was obtained. Interface-modified ramp-edge junctions with La0.2-Yb0.9Ba1.9Cu3Ox (La-YbBCO) counter electrodes have been fabricated by using this multilayer structure with dc-sputtered films. The fabricated junctions exhibited RSJ-type I-V characteristics with IcRn products of about 3 mV at 4.2 K. We also obtained a 1-σ Ic spread of 8% for a 1000-junction series-array. The sheet inductance values at 4.2 K for the base and counter electrodes on La-YBCO ground planes were 0.8 pH and 0.7 pH per square, respectively. Operation of several types of elementary SFQ circuits has been successfully demonstrated by using this multilayer structure.