Shinya Hasuo

Experimental investigations and analysis for high‐quality Nb/Al‐AlOx/ Nb Josephson junctions

We have made a detailed study of the fabrication process of Nb/Al‐AlOx/Nb Josephson junctions with the aim of introducing guiding principles to obtain high‐quality junctions. Junctions were fabricated by a novel process named the self‐aligned contact process. We investigated the deposition conditions of Nb and Al films and optimized them. Oxidation conditions for obtaining target critical current were also investigated. Then composition of the tunneling barrier material and the coverage of the thin Al layer on the base electrode were analyzed by x‐ray photoelectron spectroscopy, Auger electron spectroscopy, etc. We investigated aging and annealing effects on the junctions. We confirmed that the current‐voltage (I‐V) characteristics of the junctions did not change after storage for 450 days at room temperature or when subjected to temperatures of up to 150 °C for shorter periods. Applying the McMillan theory to our experimental results, we explained the characteristics of the junctions. As a result, we hav...

Fabrication of high quality Nb/AlO/sub x/-Al/Nb Josephson junctions. I. Sputtered Nb films for junction electrodes

The stress, surface morphology, superconducting characteristics, and crystal structure of sputtered Nb films were evaluated to judge their applicability to Josephson-junction electrodes. The film qualities were compared between Nb films deposited by DC and RF magnetron sputtering. The authors concluded that DC-sputtered Nb films are more suitable for junction electrodes and studied the relationship between their film quality and sputtering parameters. They observed that the Nb film characteristics were determined solely by the cathode voltage during sputtering regardless of the other parameters. The authors discuss the changes in film characteristics during Josephson integrated circuit processing.<<ETX>>

A single-chip SQUID magnetometer

A superconducting quantum interference device (SQUID) magnetometer in which all components are integrated on a single chip is discussed. The chip includes a feedback circuit, a pickup coil, and a SQUID sensor. The feedback circuit consists of a superconducting storage loop and a WRITE gate. The chip, fabricated with Nb/AlO/sub x//Nb Josephson junctions, Mo resistors and SiO/sub 2/ insulation. The sensitivity is 7*10/sup -5/ Phi /sub 0// square root Hz for the flux in the SQUID sensor and 4.5*10/sup -9/ T/m square root Hz for the magnetic field gradient at the pickup coil. This sensitivity is limited by environmental noise and can be increased with shielding. The device is useful for the construction of a magnetometer array for biomagnetic image sensing. >

A 4K Josephson memory

The authors describe the design and experimental performance of a 4 K*1-bit Josephson (RAM). For high-speed memory operation, the authors have developed a compact AND gate for the decoder, a high-voltage driver gate, and a capacitively coupled single-flux quantum memory cell. The 4 K memory was designed using these gates and cell and was fabricated with Nb/AlO/sub x//Nb junctions. The minimum access time was 590 ps, and the total power dissipation was 19 mW. >

Josephson modified variable threshold logic gates for use in ultra-high-speed LSI

A gate family called modified variable threshold logic (MVTL) is proposed. The OR gate is a two-junction interferometer with one magnetically coupled control line. Magnetic coupling and current injection are used to switch the logic state of the gate. By optimizing the gate parameters, an operating margin of +or-43% and a switching speed of 2.5 ps/gate are obtained. the gate area is 30 mu m*24 mu m with a 1.5- mu m minimum junction diameter. The gate family consists of an OR gate, a single-junction AND gate, and a timed inverter (TI) that consists of the OR gate, a junction, and resistors. The delay time of the gate operated in the actual circuit was found to be less than 10 ps. Circuits having up to 1000 gates, the critical path model of a 16-bit*16-bit multiplier, and a 16-bit arithmetic logic unit have been successfully operated. When the Josephson gate is operated with three-phase power, it is possible to construct any sequential circuit without the complex latch circuit required to prevent the race condition for one- or two-phase power supplies. >

A Josephson driver to interface Josephson junctions to semiconductor transistors

An output voltage of 150 mV was obtained with a risetime of less than 200 ps at the driver, which contained 104 junctions and 2 resistors. The driver was also used to drive a D-HEMT (high-electron mobility transistor), and an output voltage of 0.9 V with a risetime of 500 ps was obtained from the HEMT. This experiment suggests that the Josephson driver developed can be used for a Josephson computer or a SQUID (superconducting quantum interference device) system as an interface gate to drive room-temperature electronic devices.<<ETX>>

A submicrometer Nb/AlOx/Nb Josephson junction

High‐quality, submicrometer Nb/AlOx /Nb Josephson junctions were fabricated. The junction area was defined by anodizing an upper Nb electrode using a thin SiO2 mask. By utilizing the anodized Nb as an etch stop during the reactive ion etching of the contact hole in an SiO2 insulator, we could make a junction smaller than the contact hole size. Using stress‐free Nb for the junction electrodes was crucial in achieving good current‐voltage characteristics. High‐quality Nb/AlOx /Nb junctions as small as 0.7 μm square were produced. The quality parameter Vm was greater than 30 mV for a jc of 6800 A/cm2.

Josephson 8-bit shift register

A Josephson 8-bit shift register for use in Josephson computers and signal processors is described. The gates used in the circuit are modified variable threshold logic (MVTL) gates fabricated by 2.5-/spl mu/m Nb/AlO/SUB x//Nb junction technology with Mo resistors and SiO/SUB 2/ insulation. They are driven by a three-phase power supply. The shift register has 112 gates and occupies a 1.1/spl times/2.1-mm/SUP 2/ area. Correct shift, load, hold, and clear operations were experimentally confirmed for all bits. The circuit operated at a clock frequency up to 2.3 GHz. Its power consumption was 1.8 mW. This power level is two or three orders of magnitude lower than for Si or GaAs shift registers.

Self‐aligned contact process for Nb/Al‐AlOx/Nb Josephson junctions

High quality Nb/Al‐AlOx/Nb Josephson junctions have been fabricated by a novel process named the self‐aligned contact process. After the definition of the junction area by a reactive ion etching (RIE) technique, the exposed Nb layer and the junction edge are anodized to protect against electrical shorts and an Al film is deposited as the etching stopper layer. After base electrode patterning by RIE, an insulation layer is deposited. The contact hole, with the diameter of the junction, for connecting a counter electrode and a wiring layer, can be made without registration because of the deposited Al film. The junctions fabricated by this process have exhibited excellent current‐voltage characteristics (Vm =70 mV at the critical current density  jJ =1.8 kA/cm2, Vm =11 mV at  jJ =20 kA/cm2).

Electrical properties of Au/ and YBa2Cu3O7−x/SrTi1−yNbyO3 diodes

Electrical properties of Au/ and YBa2Cu3O7−x/SrTi1−yNbyO3 heterojunctions were studied by measuring their capacitance‐voltage, current‐voltage, and conductance‐voltage characteristics. The heterostructures were made by depositing Au or YBa2Cu3O7−x films on SrTi1−yNbyO3 substrates. The results of the capacitance‐voltage measurement indicated that there was an interfacial layer having a dielectric constant lower than that of bulk SrTiO3 at the Au/SrTiO3 and YBa2Cu3O7−x/SrTiO3 interfaces. The current‐voltage characteristics of the Au/SrTi1−yNbyO3 diodes with substrate Nb concentrations of 0.05 and 0.005 wt. % matched characteristics normally associated Schottky junctions and had a large ideality factor, n, consistent with the low‐dielectric‐constant interfacial layers. When the carrier concentration of the n‐SrTiO3 substrate was 2×1019 cm−3, the Au and YBa2Cu3O7−x junctions showed interfacial‐layer tunneling characteristics. The YBa2Cu3O7−x junctions exhibited two peaks in their conductance‐voltage relations...