We describe several properties of very thin stacks of 10 to 20 intrinsic Josephson junctions fabricated on the surface of Bi₂Sr₂CaCu₂O_8+δ single crystals. We show that the Joule heating is significantly reduced in these stacks and that the gap structure is clearly observable in the quasiparticle current-voltage (I-V) characteristics. The I-V curves are characterized by a large subgap conductance and a significant gap suppression due to the injection of quasiparticle current. It is found that the I_cR_n product of these intrinsic Josephson junction stacks is significantly small compared with that expected from the BCS theory. It is also found that there is a tendency that I_cR_n decreases with increasing c-axis resistivity. Both I_c and the gap voltage exhibit unsaturated temperature dependence at low temperatures. The behavior presents a sharp contrast to that of Josephson junctions made of conventional superconductors. The characteristics are discussed in relation to the d-wave symmetry of the order parameter.

We have studied the effect of Ga and Ca substitution in the PrBa₂Cu₃O_δ (PBCO) barrier on the parameters of high-temperature-superconductor ramp-edge Josephson junctions. Pr _1-XCa _XBa₂Cu₃O_δ (X=0. 15, 0. 3) had reduced bulk barrier resistivity as small as 10 mΩcm which was close to the metal-insulator transition. Also, PrBa₂Cu _3-ZGa _ZO_δ, written as GaZ-doped PBCO (Z=0. 15, 0. 3, 0. 6), had enhanced resistivity neater than 1 kΩcm at 4. 2 K. The transport mechanisms in these bulk barriers fitted well with the Mott variable hopping model. The critical current density J_c and normalized junction conductance (R _nA)^-1 decayed exponentially with almost the same decay length, as the barrier thickness increased. The decay length depended on the barrier material, and ranged from 1. 7 nm to 6. 5 nm for J_c, from 1. 9 nm to 7. 2 nm for (R_n A)^-1. Because on these experimental results, we conclude that direct tunneling is the dominant transport mechanism for both quasi particles and paired particles in our junctions, while resonant tunneling should be considered as an additional transport mechanism of these two kinds of particles in the junctions with the PBCO-based barriers reported so far. It was also found that Ga doping raised the characteristic voltage V_c while Ca doping reduced it, though the V_c values obtained here were still small compared to the theoretically predicted values. The spacewise metal insulator transition at the interfaces caused by a high density of localized states in the barriers seemed to be responsible for the reduction in V_c. The best V_c value was 0. 32 mV at 77 K and 5. 2 mV at 4. 2 K using a Ga0. 6-PBCO barrier. These V_c values are suitable for electronics applications. Furthermore, superconducting-gap-like structures were observed in the junctions with highly resistive Ga-doped PBCO barriers.

We have studied an in situ edge preparation process and the effect of a substrate rotation during the edge preparation in order to improve the uniformity and electrical characteristics of high-T_c edge-type Josephson junctions. The improved YBa₂Cu₃O_x/PrBa₂Cu₃O_x/YBa₂Cu₃O_x edge junctions showed small 1σ-critical current spreads as low as 10% for 12 junctions. We have confirmed that the spreads do not increase significantly by adding groundplane over the junctions. In this paper, we will describe these processes developed for the fabrication of high-T_c superconducting integrated circuits.

YBa₂Cu₃O_7-δ co-planar Josephson junctions by Focused Ion Beam were characterized by changing the thickness of YBa₂Cu₃O_7-δ films. The junctions had the thickness dependence of the characteristics. The characteristics were dominantly divided into two types. One had the I-V curve of a flux-flow junction and a weak magnetic response. The other had the I-V curve of RSJ, the I_cR_n product from 0. 1 mV to 0. 5 mV at 4. 2 K, and a good magnetic response. The critical current density of the junctions increased exponentially with increasing film thickness. From the observation of the junction surface, the junction length was decreased with increasing film thickness by the horizontal growth of the normally grown YBaCuO. In the thicker film (above 240 nm), the microshorts of the normally grown YBaCuO on the abnormally grown YBaCuO area were observed. It is considered that the main part of Josephson current for the junctions changes from the abnormally grown YBaCuO to microshorts when increased with the film thickness.

We investigate the basic properties of focused electron beam (FEB)-damaged Josephson junctions on silicon (Si) substrates for high-frequency device applications. YBa₂Cu₃O_7-y (YBCO) Josephson junction arrays were also fabricated by FEB irradiation to confirm the junction uniformity and to investigate their applicability. The junctions exhibit resistively shunted junction (RSJ)-like current-voltage (I-V) curves and the microwave-induced Shapiro steps for all operation temperatures. Two-junction arrays show single-junction-like behavior with the Shapiro steps in an array up to 2 mV. Microwave-induced Shapiro steps correspond to the double voltages Vn=2nV_J, where V_J=f₀h/2e in two-junction arrays. The microwave power dependence of I-V curves shows the steps corresponding to the RSJ model.

YBa₂Cu₃O_x films were grown on MgO(100) substrates by liquid phase epitaxy. Their structural and electrical properties were examined. From TEM plan-view images, it is found that the film consists of large grains whose misorientation angles are less than 1. Although the DC critical current density values decrease with increasing the film thickness, the critical current density value of 9. 310⁵ A/cm² at 77 K is obtained for a 7 µm-thick film. A microstrip resonator at 10. 8 GHz with a YBCO ground plane shows Q₀ values of 14200 at 77 K and 23300 at 40 K, which correspond to surface resistance values of 650 and 400 µΩ, respectively. By using a microstrip line resonator with a Ti/Au ground plane, the critical field of the film at 77 K and 10. 8 GHz is estimated to be 30 Oe. The third-order intercept of the resonator with the Ti/Au ground plane is the input power of +43 dBm and the output power of +30 dBm at 77 K.

Reentrant delay line memories using narrow YBa₂Cu₃O_7-δ (YBCO) coplanar transmission lines are proposed. The proposed memory is composed of a looped YBCO coplanar delay line and a 22 semiconductor crossbar switch. This type of memory is superior to semiconductor memories in operating speed, the number of logic gates, power dissipation, and so on. We have also developed narrow and low-loss YBCO coplanar transmission lines for use in these reentrant delay line memories. Etch-back planarization and a patterning process combining Ar-ion milling and wet-etching enabled us to fabricate 18-cm-long YBCO coplanar transmission lines as narrow as 5 µm, and these lines did not suffer from electrical shorts even when the spacing was only 2. 5 µm. The surface resistances calculated from the attenuation constants of 5-, 10-, and 25-µm-wide lines provide similar low values of 0. 18-0. 26 mΩ at 10 GHz and 55 K. This indicates that the process damage was sufficiently suppressed despite the narrow line widths. The 5-µm-wide line attained a low attenuation constant of 2. 7 dB/m, which is similar to that in Cu coaxial cables. Even in the 5-µm-wide line, no significant increase in transmission loss was observed up to an input power level of 16 mW at 10 GHz and 55 K. This input power is comparable to that required to propagate digital signals from semiconductor circuits. Therefore high-speed digital signals can propagate through these narrow YBCO coplanar lines without significant attenuation of the signal pulses. Thus, these narrow YBCO coplanar lines can be used in the reentrant delay line memories.

The complex conductivities of high T_c superconducting YBa₂Cu₃O_x thin films have been studied using the coplanar waveguide resonator technique. In order to evaluate the magnetic penetration depth precisely, we measured the temperature dependence of the resonant frequency and compared it with the numerical results self-consistently. The observed temperature dependence of the complex conductivities is shown to be able to distinguish the effects of the weaklink from the intrinsic property of the grain of an epitaxial thin film and demonstrate the weakly coupled grain model of YBa₂Cu₃O_x thin films.

Large-area high-temperature superconducting films and damage-free processing techniques have been developed to fabricate low insertion loss and sharp skirt filters for mobile telecommunication. An off-axis-type dc sputtering method was employed to deposit Y-Ba-Cu-O films on both sides of the substrate. The surface resistance of the films was about 0. 35 mΩ(at 70 K and 10 GHz). An 11-pole bandpass receiving filter for the IS-95 telecommunication system was designed and fabricated using 60 mm 50 mm YBCO films on a 0. 5-mm-thick MgO substrate. The passband insertion loss at 70 K was about 0. 1 dB with 0. 1 dB ripple. The third-order intercept point of the filter was 49. 5 dBm. We have assembled the filter and a low-noise amplifier in a dewar with a cryocooler. Ultralow-noise performance (noise figure: 0. 5 dB at 70 K) was presented by the cryogenic filter subsystem.

A new or future system of mobile telecommunication is built by new digital technologies to provide an improved and more consistent quality of service for the customers. These digital systems can provide greater number of transmission channel allocation for their subscribers and security. On the digital communication system, distortion of transmitted signal should be eliminated as much as possible for high communication quality. However, the need to both minimize distortion of signal amplifiers and continue to provide good filtering protection can become difficult to achieve with conventional high power amplifiers and filters. In this paper, the application of high-T_c superconducting (HTS) power filters on such digital communication systems and recent progress of filter device developments for those are discussed.

Millimeter- and submillimeter-wave low-noise superconducting receivers, such as superconductor-insulator-superconductor (SIS) mixers, hot-electron bolometer (HEB) mixers, and superconducting direct detectors, are addressed in this paper. Some general topics on the development of SIS mixers, including SIS junction and integrated tuning circuitry, mixing circuitry, and mixer-performance simulation, are extensively discussed. A tuneless waveguide SIS mixer developed at Nobeyama Radio Observatory (NRO) and its performance are presented. The fundamental mechanisms of diffusion- and phonon-cooled HEB mixers and recent advances in HEB mixers are briefly reviewed. Finally, incoherent detectors with superconducting tunnel junctions are discussed. Results for a direct detecting experiment at 500 GHz are given.

Resonant properties of resistively shunted tunnel junctions dominate the high-frequency performance of Josephson array oscillators. To improve the operating frequency, we have developed resistively shunted Nb/AlO_x/Nb tunnel junctions with a small parasitic inductance. The inductance was minimized by reducing the inductive length between the tunnel junction and the contact hole to be about 1µm. By fitting the measured I-V characteristics of the shunted tunnel junction to the simulated characteristics, we estimated the inductance to be about 105 fH. The analysis of resonant properties showed that the shunted tunnel junctions with the small parasitic inductance have a high-frequency performance up to the Nb gap frequency. Josephson array oscillators using 11 such junctions were designed and fabricated to operate at 650 GHz and 1 THz. Shapiro steps induced by Josephson oscillation were clearly observed up to 1 THz. By fitting the step heights to the simulated results, we estimated the output power of the Josephson oscillator delivered to the load resistor to be about 10 µW at 625 GHz and 50 nW at 1 THz.

We report the state of the art of superconducting network switching circuits and system technology. Mainly, we describe our switching core circuits and challenges to demonstrate superconducting prototype systems. And also, we review other approach to perform the superconducting digital communication briefly. In our switching core circuits, a ring-pipeline architecture has been proposed and the component circuits of the prototype chips have been fabricated and tested successfully. It is very important to demonstrate the prototype system in order to estimate the total performance of the system with superconducting devices. We have designed a multi-processor system with a superconducting network as a prototype system to demonstrate an interprocessor network system.

We present two types of ICF (INHIBIT Controlled by Fluxon) gates as the basic circuits of the phase-mode logic family, and fabricate an adder circuit. The experimental result demonstrates that the carry operation followed up to 99 GHz input pulses. The performance of Josephson devices is improved by the use of junctions with high current density (J_c). We may use the high-J_c junctions without external resistive shunt in the phase-mode logic circuits because of reduction of the junction hysteresis. One of the ways to overcome the large area occupancy for geometric inductance is to utilize the effective inductance of a Josephson junction itself. We investigate a circuit construction with high-J_c inductor junctions, intrinsically overdumped junctions and junction-type resistors for the compactness of circuit integration, and discuss various aspects of the circuit construction.

We have designed rapid single-flux-quantum (RSFQ) adder circuits using two different architectures: one is the conventional architecture employing globally synchronous clocking and the other is the data-driven self-timed (DDST) architecture. It has been pointed out that the timing margin of the RSFQ logic is very sensitive to the circuit parameter variations which are induced by the fabrication process and the device parameter uncertainty. Considering the physical timing in the circuits, we have shown that the DDST architecture is advantageous for realizing RSFQ circuits operating at very high frequencies. We have also calculated the theoretical circuit yield of the DDST adders and shown that a four-bit system operating at 10 GHz is feasible with sufficient operating margin, considering the present 1 kA/cm² Nb Josephson technology.

A new broadband buffer circuit technique and its analytical design method are proposed for a high-speed decision circuit featuring both a higher input sensitivity and a larger phase margin. The buffer circuit characteristics are significantly improved by employing a series peaking source follower (SPSF), where a peaking inductor is inserted between the first and second source follower stages. Optimization of the peaking inductance successfully enhances the 3-dB bandwidth of the data-input buffer and the clock buffer by 7 GHz for both, over conventional double-stage source follower SCFL buffers. The proposed circuit technique and design method are applied to a 10-Gbit/s decision circuit by the use of production-level 0. 5 µm GaAs MESFETs. The fabricated decision circuit achieves a data input sensitivity of 43 mV_p-p and a phase margin of 240 both at 10-Gbit/s: a 230 mV_p-p smaller input sensitivity and a 35 larger phase margin than those of conventional non-peaking inductor types.

The small planar packaging (SPP) system described here can be combined with card-on-board (COB) packaging in high-speed asynchronous transfer mode (ATM) switching systems with throughput of over 40-Gb/s. The SPP system provides high I/O pin count density, high packaging density and high cooling capability. Prototype SPP system with air flow control structure for switching MCMs is constructed. Each MCM contained a 35 array of low thermal resistance butt-lead pin-grid-array on a glass ceramic substrate measuring 100170 mm with a plate fin heat-sink. This allows a power dissipation of more than 125 W per MCM, and 300 W per printed circuit board (PCB). Obtained board level heat flux density of the SPP system is 0. 37 W/cm², which is six times that of conventional COB packaging. The SPP system combined with the COB packaging provides a small system foot-print and compact hardware for high-speed, large capacity ATM switching systems. This high-performance air cooling technology will be especially useful for future broadband ISDN high-speed switching systems.

An efficient finite element-integral equation method is presented for calculating scattered fields from conducting objects. By combining the integral equation solution with the finite element method, this formulation allows a finite element computational domain terminated very closely to the scatterer and thus results in the decrease of the resultant matrix size. Furthermore, we employ a new integral approach to establish the boundary condition on the finite element terminating surface. The expansion of the fields on the integration contour is not related to the fields on the terminating surface, hence we obtain an explicit expression of the boundary condition on the terminating surface. Using this explicit boundary condition with the finite element solution, our method substantially improves the computational efficiency and relaxes the computer memory requirements. Only one matrix inversion is needed through our formulation and the generation and storing of a full matrix is not necessary as compared with the conventional hybrid finite element methods. The validity and accuracy of the formulation are checked by some numerical solutions of scattering from two-dimensional metallic cylinders, which are compared with the results of other methods and/or measured data.

A correction of the physical optics approximation by accounting for the presence of specific currents concentrated near shadow boundaries on the surface of a convex non-metallic scatterer is analysed by considering a canonical problem of diffraction of a plane electromagnetic wave incident normally to the axis of an infinite circular cylinder with impedance boundary conditions. The analysis focuses on the development of Fock-type asymptotic representations for magnetic field tangent components on the surface of the scatterer. The Fock-type representation of the surface field is uniformly valid within the penumbra region, providing a continuous transition from the geometrical optics formulas on the lit portion of the surface to the creeping waves approximation in the deep shadow region. A new numerical procedure for evaluating Fock-type integrals is proposed that extracts rapidly varying factors and approximates the rest, slowly varying coefficients via interpolation. This allows us to obtain accurate and simple representations for the shadow boundary currents that can be directly inserted into the radiation integral and effectively integrated. We show that accounting for the shadow boundary currents considerably improves the traditional PO analysis of the high-frequency electromagnetic fields scattered from smooth and convex non-metallic obstacles, particularly near the forward scattering direction.

A GeSi avalanche photodetector grown on a silicon-on-insulator (SOI) passive waveguide is demonstrated. The absorption layer of the detector consisits of alternating layers of 66 Ge_{0. 44}Si_{0. 56} and 480 Si on SOI substrate. The thick SOI waveguide couples the light from an optical fiber into the GeSi/Si strain-limited thin absorption region. The detector exhibits low dark current, sharp breakdown and an external responsivity of 0. 2 A/W at 1. 3 µm wavelength.