Search

We have studied on the quasiparticles (QPs) injection effects for Nb/Al/AlOx/Al/Nb tunnel junctions at 0.4 K. A device structure was a stack of an injector Josephson junction (JJ) and a detector JJ. At a low temperature, there is small number of thermally excited QPs and we expected to observe easily the QPs injection effect of the detector JJ using the injector JJ. In this study, the structure of the Nb/Al/AlOx/Al/Nb tunnel junctions was the same for X-ray detectors using superconducting tunnel junctions. For constant-QPs current injection, we expected a small-signal current gain exceed the unity. We used pulsed-QPs current with changing pulse duration. For long duration, I-V curves for the detection junctions were sum of original current and QP current. On the other hands, for short duration time, the I-V curves showed different shapes from those for the long duration time.

Photosensitive polyimide insulation layers have been introduced to fabricate superconducting integrated circuits. It is shown to simplify the fabrication process, because the photosensitive polyimide insulation layer can be patterned by conventional photolithography process, resulting in the etching process unnecessary in the present new fabrication process. Three kinds of contact hole (junction top electrode contact, junction base electrode contact, and resistor contact) are simultaneously formed in the photolithography process of the polyimide. A minimum contact hole size is designed to be 1.5 /spl mu/m square for a 3 /spl mu/m /spl times/ 3 /spl mu/m squared junction. Superconducting current density of 2.4/spl times/10/sup 6/ A/cm/sup 2/ of the contact hole was measured. Palladium resistors were successfully made with through hole contacts of the photosensitive polyimide insulation layer. We demonstrated superconducting integrated circuits using this new fabrication process including the minimum 3 /spl mu/m /spl times/ 3 /spl mu/m squared Nb/Al-AlO/sub x//Nb Josephson tunnel junction. The circuit operation is also demonstrated in the fabricated superconducting integrated circuits with the photosensitive polyimide insulation layers.

We have studied normal-distribution-shaped Josephson tunnel junctions for STJ (superconducting tunnel junction) photon detector applications. By employing a normal-distribution function to the shape of the superconducting electrodes sidelobes of the Josephson current were entirely suppressed with a small magnetic field. This behavior was confirmed experimentally by using Nb/AlO/sub x//Nb Josephson tunnel junctions. There were no oscillatory sidelobes in the Josephson current dependence on the magnetic field. Experimental results were in good agreement with theoretical ones. The Josephson current was efficiently suppressed by an external magnetic field of about 0.5 mT at 4.2 K, which is less than one tenth that of other shaped junctions. In the experiment at 0.3 K, Fiske steps due to the resonances were not observed by suppression with a magnetic field of about 1 mT.

We have investigated subgap characteristics of Nb/AlOx/Nb tunnel junctions with high critical current density (high-J/sub c/). A good agreement of the experimental data with the solution of Werthamer's equation clearly shows that steep current steps observed at odd submultiples of the gap voltage, which cannot be explained by multiparticle tunneling and/or multiple Andreev reflection, are mainly caused by Josephson self-coupling. Our experimental results suggest that Josephson self-coupling is important to high-J/sub c/ tunnel junctions. >

It is important to develop a reliable and reproducible fabrication process of submicron Nb/AlO/sub x//Nb Josephson junctions to improve the integration level and the operating speed of the Josephson LSI circuit. For this purpose, we have developed an improved etching process by introducing a dummy etching process and using a scanning electron microscope (SEM) as an etching end-point detector. The dummy etching process improves the anisotropy of a reactive ion etching (RIE) process using CF/sub 4/. The etching residue around a junction is detected easily and correctly by the SEM observation. We have successfully fabricated Nb/AlO/sub x//Nb Josephson junctions with critical current density of 10/sup 4/ A/cm/sup 2/ using the cross-line patterning (CLIP) method and electron beam lithography, where the junction size was varied from 2 /spl mu/m to 0.5 /spl mu/m at 0.1 /spl mu/m intervals. High-quality submicron junctions for integrated circuits with suitable critical current variations were obtained. >

In this work we apply the Electrotechnical Laboratory Josephson junction fabrication technology to realize an integrated device for nuclear radiation detection. We have integrated in a single 5×5 mm2 chip 240 Josephson tunnel junctions (JTJs) having various geometrical configurations and interconnections, in order to investigate both the effects of various detection geometrical configurations and the fabrication process uniformity and reproducibility. The junction quality was extensively tested by measurements of the current‐voltage characteristics, down to T=1.2 K, showing an unexpected saturation of the Vm quality factor. Preliminary results concerning the response to α‐particle radiation of these samples have been obtained. Pulse height and rise time spectra of both ‘‘island’’ type and series connected JTJs are reported. Signals generated by nuclear radiation have been observed at temperatures up to 2 K, where a reduction of the collected charge occurs because of the enhancement of the recombination ra...

The switching mechanism of a direct-coupled Josephson logic gate, a four-junction logic gate, has been investigated. It was found that a high-speed input signal current is wasted in an input-output separation resistance (R/sub i/). A speedup method has been developed in which an inductance is connected to (R/sub i/) in series. The value of the inductance was found to be five times larger than the effective inductance of the input junction. A speedup of 40% in the gate switching was demonstrated by a logic delay experiment using submicron NbN-MgO-NbN junction technology. The minimum logic delay of 3.0 ps/gate was obtained with fan-out 1. >

A 16 kb Josephson RAM (random access memory) has been designed using a miniaturized variable threshold memory cell based on a 2 mu m Nb/Al-oxide/Nb junction technology. By using a novel strip-line structure for the inductance of the superconducting quantum interference device (SQUID) type selection gate, the miniaturized cell occupies 14 mm 28 mu m, just 20% of the area of a previous cell designed with a 3 mu m rule. OR-INVERT type address decoders are constructed with a new 4JL (four-junction logic) gate with a fan-out of four. The whole circuit occupies a 3.1 mm*5.0 mm area. The access time is evaluated to be 475 ps, with a total power dissipation of 7.3 mW/chip. Based on these performance data and data from a previous 1 kb RAM experiment, the prospects for Josephson VLSIs are discussed. >

An improved fabrication process for submicron NbN/MgO/NbN Josephson tunnel junctions has been developed. By introducing a contact layer between the junction and the wiring layer, the critical current of the wiring above the junction was considerably enhanced. A logic circuit composed of four-junction logic gates was fabricated using 0.9- mu m-square junctions. Logic delay measurement was successfully achieved with a minimum logic delay of 3.6 ps/gate and a wide operating margin of +or-17% within 50 gates. >

The specific capacitance Cs of Nb/AlOx/Nb Josephson tunnel junction has been measured by means of the superconducting quantum interference device (SQUID) resonance technique. We have investigated the junctions with critical current densities Jc in the range of 0.1–18 kA/cm2 and found that 1/Cs linearly depends on a logarithm of Jc. This suggests that the barrier thickness is moderately uniform in the junction area and increases continuously during growth. The results also show that increasing Jc reduces time constants of the junction.

Nb/Al–AlOx/Nb–NbN Josephson junctions for use as x‐ray detectors have been fabricated. The NbN overlayer has been used to realize the trapping of quasiparticles in the Nb top layer with an increase of the charge collection efficiency. The temperature dependence of quasiparticle and Josephson critical current have been investigated and compared with that without a NbN overlayer, showing low leakage currents, about 200 pA (V=0.5 mV, T=0.67 K, A=20×20 μm2), and high dynamical resistances in the subgap region. Preliminary measurements under 6 keV x‐ray irradiation have shown an increased value of the maximum collected charge.

We have fabricated anisotropic Josephson junctions consisting of all (103) oriented YBa2Cu307-δ (140nm)/PrBa2Cu307-δ’ (45nm)/YBa2Cu307-δ (70nm) trilayer films. The trilayer films were fabricated on a whole (110) SrTi03 substrate by a reactive coevaporation method using pure ozone vapor. The entire process was carried out using standard photolithography technique. In order to reduce etching damages, all etching processes were performed using the Ar gas ECR dry etching with liquid nitrogen cooling, The typical current-voltage I-V characteristics for the junctions showed resistively shunted junction (RS J) like I-V characteristics with hysteresis. The junction with a dimension of 30x30 μm2 showed both Shapiro steps and Fraunhofer interference patterns. The junctions had anisotropic Fraunhofer interference patterns depending on the direction of the external magnetic field. This anisotropic property comes from the anisotropy in the magnetic penetration depth of (103) oriented YBaCuO films.

A new fabrication process is presented to make Nb/Al/AlO x /Nb superconducting tunnel junctions with ultralow subgap leakage current at low temperatures below 4.2 K for X-ray detection. A Nb anodization process has been adopted to make electric insulation at the periphery of an ultrathin AlO x tunnel barrier through use in combination with the conventional SiO self-aligned insulation process. The temperature dependence of the subgap current of the junctions fabricated by the present process is in good agreement with that predicted by the Bardeen-Cooper-Schrieffer (BCS) theory, while junctions made by the conventional process show a weak temperature dependence of the subgap current. In addition, the Vm value of the junction fabricated by the present process was found to be more than 3000 mV at 1.2 K. It has been clarified that the anodization process introduces improvements to make the subgap leakage current extremely small in the temperature range from 4.2 K down to 1.2 K.

A new experimental method for estimation of alpha-particle induced hot spot size in Nb films is proposed. The method is based on the jump-like motion of Abrikosov vortices induced by the impact of a single alpha-particle (Am-241 5.5 MeV) on a superconducting film. Preliminary experiments were carried out on a high quality ''small'' Nb-AlOx-Nb Josephson tunnel junction with circular geometry and area 2500 mu m(2) (length smaller than lambda(j)) at T = 4.2 K. A preliminary value of d* approximate to 2.5 mu m is obtained for die dimension of the hot spot region.

For the first time the behavior of the first Fiske step (FFS) of a Josephson tunnel junction (JTJ) as a function of the concentration of trapped misaligned Abrikosov vortices (AVs) randomly distributed over the junction area is studied experimentally and theoretically.