S. Tahara

ATOMIC STRUCTURE AND COMPOSITION OF THE BARRIER IN THE MODIFIED INTERFACE HIGH-TC JOSEPHSON JUNCTION STUDIED BY TRANSMISSION ELECTRON MICROSCOPY

The atomic structure and composition of modified interface junctions which showed reproducible critical current Ic (Ic1σ<8% for 100 junctions) are investigated by transmission electron microscopy. Transmission electron microscopic observations show the existence of a thin barrier (1–2 nm) homogeneously covering the ion milled edge of the base YBa2Cu3Oy film although there is no barrier deposition and annealing process. High-resolution electron microscopy images and energy dispersive x-ray analysis with a spot size of 1 nm indicates that the barrier is a Ba-based perovskite-like structure, (Y1−xCux)BaOy with x<0.5. A thin amorphous layer whose composition deviates from YBa2Cu3Oy is formed due to the preferential sputtering of Cu. The amorphous layer recrystallizes into the nonequilibrium phase (Y1−xCux)BaOy after heating up to the deposition temperature.

Study of in-situ prepared high-temperature superconducting edge-type Josephson junctions

High-T/sub c/ edge-type Josephson junctions usually have ex-situ interfaces, that probably contain damaged layers caused by etching process and/or by exposure to air. The ex-situ interface layer may be an origin of poor reproducibility and uniformity of the device characteristics, as well as an excess interface resistance and a suppression of the critical current. We have developed an in-situ edge preparation process to improve the uniformity and electrical characteristics of the edge junctions. In our in-situ process, the base YBaCuO electrode edge is not exposed to air after the preparation of the edge and subsequently followed by the deposition of a barrier layer and a counter electrode. The in-situ YBaCuO/PrBaCuO/YBaCuO junctions showed larger critical current density (J/sub c/) and normal state conductance (G/sub n/) than the ex-situ junctions. In addition, smaller J/sub c/ and G/sub n/ spreads have been obtained for the in-situ junctions.

DIRECT-INJECTION HIGH-TC DC-SQUID WITH AN UPPER YBA2CU3O7-X GROUND PLANE

We have fabricated direct-injection high-Tc superconducting quantum interference devices (SQUIDs) with integrated YBa2Cu3O7−x (YBCO) ground planes in the upper part of structure. The YBCO/PrBa2Cu3O7−x(PBCO)/YBCO edge junctions with 10-nm-thick PBCO barriers showed RSJ-like current-voltage (I–V) characteristics before and after the ground plane formation. We measured the voltage modulations of these SQUIDs with two different hole sizes to evaluate the line inductance. At a temperature of 30 K, the estimated line inductance under the ground plane was about 1.0 pH/□, while the estimated line inductance without the ground plane was 2.8 pH/□.

INTERMODULATION DISTORTION MEASUREMENTS OF A MICROSTRIP BAND-PASS FILTER MADE FROM DOUBLE-SIDED YBA2CU3OX FILMS

A microstrip three‐pole band‐pass filter, with a midband frequency of 9.5 GHz and a fractional bandwidth of 2%, has been fabricated using double‐sided YBa2Cu3Ox (YBCO) films on MgO substrates. The nonlinear behavior of this filter was studied by intermodulation distortion measurements. The insertion loss at the midband frequency was less than 0.1 dB and the return loss was better than 15 dB throughout the passband at 55 K. This filter showed relatively high power handling capabilities, with third‐order intercept larger than +60 dBm at 55 K. The third‐order intermodulation distortion products of this filter were found to vary with frequency in the passband, being larger near each of the band edges than at the midband. These increases in the nonlinearity near the band edges are attributed to the increase in the average stored energy in the filter, which is confirmed by the increase in the group delay near the band edge.

Bit yield improvement by precise control of stray fields from SAF pinned layers for high-density MRAMs

A write-operating window with a 100% functional bit yield was successfully obtained by the control of stray fields from synthetic antiferromagnetic (SAF) pinned layers in conventional magnetic random access memories with rectangular magnetic tunneling junction bits. The stray fields were controlled by a newly developed ion-beam etching technique without causing damage and by a precise setting of the SAF pinned layer thickness, and are balanced with Neel coupling fields. As a result, it was found that symmetric switching astroid curves with no offset were obtained and switching distributions were minimized at the zero offset field.

Investigation of SFQ integrated circuits using Nb fabrication technology

In NECs standard process, the minimum junction size is 2 µm and the critical current density (JC) is 2.5 kA cm-2. In the process, i-line stepper lithography and reactive ion etching with SF6 gas are used and the standard deviation () of the critical current (IC) was 0.9% for the 2 µm junctions. This junction uniformity enables integration of more than 10M junctions if an IC variation of ±10% permits correct circuit operation. A 512-bit shift register was designed and fabricated by our standard process. Correct 512-bit delay operation was obtained. These results are promising for the large-scale integration of single flux quantum circuits.

Logic delays of 5‐μm resistor coupled Josephson logic

Logic delays of resistor coupled Josephson logic (RCJL) have been investigated. An experimental circuit with a cascade chain of ten RCJL OR gates was fabricated using Pb‐alloy Josephson IC technology with 5‐μm minimum linewidth. Logic delay was measured to be as low as 10.8 ps with power dissipation of 11.7 μW. This demonstrates a switching operation faster than those reported for other Josephson gate designs. Comparison with computer‐simulation results is also presented.

High-resolution measurement by a high-T/sub c/ superconductor sampler

We measured a signal current waveform by using a high-T/sub c/ superconductor (HTS) sampler with a 1-ps delay between every sampling point. The maximum time differential obtained in the measured waveform was 12 /spl mu/A/ps with a 2.5-/spl mu/A current sensitivity at 25 K. This result guarantees that the sampler is able to measure current waveforms correctly when their maximum time differential is less than 12 /spl mu/A/ps. The superior temporal response was achieved by using high-speed single-flux-quantum pulses generated in the HTS circuit. A unique feature of the sampler is that it directly measures the current with picosecond and microampere resolutions. Measurement of current flowing through wiring in a semiconductor large-scale integrated circuits is a promising application for the HTS sampler.

Reduction of switching field distributions by edge oxidization of submicron magnetoresistive tunneling junction cells for high-density magnetoresistive random access memories

An edge oxidization effect on magnetization reversals is investigated for submicron-patterned magnetoresistive tunneling junctions (MTJs). By the MTJ edge oxidization which causes the MTJ edge saturation magnetization (Ms) reduction, the switching field distributions (SFDs) for 0.24×0.48μm2 MTJs are reduced to less than 10%. The offset fields and the kinks in resistance-magnetic-field curves are reduced. Micromagnetic simulation results predict that the edge magnetization reversals are suppressed by the MTJ edge Ms reduction and the edge domain size at the remanent states becomes small. Consequently, the edge domain motion suppression by the edge oxidization is effective for decreasing the SFDs.

High-temperature superconducting edge junctions with modified interface barriers

This paper summarizes our recent investigations on the fabrication process, microstructure, and characteristics of high-temperature superconducting edge-type Josephson junctions with modified interface barriers. The junctions were fabricated by an in situ edge fabrication process using an epitaxial insulator layer as an etching mask. The barriers were formed not by intentional deposition but by interface modification. The modified interface barrier material was identified as a Ba-based perovskite: Ba(Y1-x Cux )Oy . The junctions showed resistively and capacitively shunted junction-like current-voltage characteristics and excellent uniformity. 100 junctions showed a spread in the critical current as low as 1 = 8% at 4.2 K and the junction characteristics remained the same after a high-temperature process at about 700 °C.