J.S. Martens

Large‐area YBa2Cu3O7−δ thin films on sapphire for microwave applications

We have deposited YBa2Cu3O7−δ(YBCO) films with low microwave surface resistance (Rs) on 5‐cm‐diam, oxide‐buffered sapphire substrates by planar magnetron sputtering. MgO buffer layers are used on M‐plane (1010) sapphire, and R‐plane (1102) sapphire is buffered by CeO2. Rs values of 450–620 μΩ at 77 K and 10 GHz were measured across an entire 5‐cm diam YBCO film on M‐plane sapphire. For YBCO on R‐plane sapphire, Rs values at 77 K and 10 GHz were 950 μΩ for a 5‐cm‐diam wafer and 700 μΩ for 1×1 cm2 samples.

Confocal resonators for measuring the surface resistance of high‐temperature superconducting films

A quasioptical technique of measuring superconductor surface resistance using a confocal resonator has been developed. The method has advantages of nondestructive analysis, high sensitivity, easy extension to higher frequencies, convenient experimental setup, and flexibility in sample size. Tl‐Ca‐Ba‐Cu‐O high‐temperature superconducting films have been measured with this technique and the measured surface resistances were less than 0.01 Ω at 36.135 GHz and 77 K. The measurements have been performed from 29 to 39 GHz, and all films showed roughly a quadratic dependence of surface resistance with frequency.

Flux flow microelectronics

Flux-flow-based devices such as the superconducting flux flow transistor and magnetically controlled long junctions have been made from thin films of TlCaBaCuO and YBaCuO. The devices are based on the magnetic control of flux flow in their respective structures: a long junction or an array of weak links. The equivalent circuits of the two devices are similar: a low-impedance input control line, an output impedance of 3-20 Omega , and an active current-controlled element. The long junctions have tended to be slower, to have lower gain, and to be somewhat less noisy than their counterparts. Circuits such as narrowband and distributed amplifiers (50-GHz bandwidths, noise figures <3 dB), phase shifters (continuous with <2-dB loss, 4-40 GHz), logic gates (2-3-ps gate delays). and memories made using these devices are compared and analyzed in terms of performance.<<ETX>>

Superconducting Josephson arrays as tunable microwave sources operating at 77 K

High‐temperature superconducting arrays of Josephson junctions have been fabricated and tested as millimeter‐wave oscillators at 77 K. Power levels in the microwatt range have been detected in the range of 90–160 GHz from arrays using from 350 to almost 60 000 YBa2Cu3O7 junctions. Monolithic log‐periodic spiral antennas as well as quasioptical resonators have been used to enable power coupling. The arrays were also found to be bias tunable over ranges limited by the receiving apparatus.

YBa2Cu3O7 nanobridges fabricated by direct‐write electron beam lithography

A direct method for nondamaging, nanometer‐scale patterning of high Tc superconductor thin films is presented. We have fabricated superconducting nanobridges in high‐quality, epitaxial thin‐film YBa2Cu3O7 (YBCO) by combining direct‐write electron beam lithography and an improved aqueous etchant. Weak links with both length and width dimensions less than 20 nm have exhibited critical currents at 77 K of 4–20 μA and IcRn products of 10–100 μV which compare favorably with results for other YBCO junction technologies. We have used this technique in the fabrication of a shock‐wave pulse former as an initial demonstration of its applicability to monolithic superconductive electronics.

S-parameter measurements and microwave applications of superconducting flux flow transistors

Microwave two-port S-parameter measurements and modeling of superconducting flux flow transistors are presented. The transistors, based on the magnetic control of flux flow in any array of high temperature superconducting weak links, exhibit significant available power gain at microwave frequencies (over 20 dB at 7-10 GHz in some devices). The input impedance is largely inductive while the output impedance is both resistive and inductive. It is shown that the characteristics of these devices are useful in numerous applications including matched amplifiers, phase shifters and active impedance convertors. >

Characteristics of superconducting flux-flow transistors

The operational characteristics and physics of three superconducting thin-film-based transistor structures are compared. The devices are based on the motion of quantized vortices, either Josephson fluxons in a long tunnel junction of Abrikosov fluxons in a superconducting film. The transistor amplification mechanism is accomplished by controlling magnetic field at the boundaries of the structures. An overview of the present understanding of device mechanisms and measured characteristics, including voltampere relations and small and large signal circuit parameters is provided. Demonstrated applications and anticipated limitations are discussed.

Improved aqueous etchant for high Tc superconductor materials

We have developed an aqueous etchant suitable for producing feature sizes of 1 μm or less in YBa2Cu3O7 and TlBaCaCuO while not degrading high‐frequency surface resistance. The 10%–300% increase in surface resistance previously seen in aqueous solutions is largely due to the presence of dissolved CO2; decarbonation of the water by bubbling N2 before and during etching can produce a better than 50% reduction in the surface‐resistance degradation. Surface‐resistance degradation can be further reduced by selecting the appropriate solution pH for ethylenediaminetetraacetic acid (EDTA)‐based etchants. We have minimized increases in surface resistance to less than 5% following etching in decarbonated disodium EDTA solutions for both YBa2Cu3O7 and TlBaCaCuO superconductors. Similar improvements can be expected from decarbonation and pH control of other aqueous processing solutions.

Tl‐Ca‐Ba‐Cu‐O step‐edge Josephson junctions

There has been considerable progress in the development of nonhysteretic Josephson junction microelectronic technologies for YBaCuO. Such a technology for Tl‐Ca‐Ba‐Cu‐O junctions would be interesting because of the higher operating temperatures and the very different grain boundary structures in the different cuprate superconductors. We have successfully made step‐edge junctions with TlCaBaCuO grown on ion‐milled LaAlO3. Nonhysteretic grain boundary‐based junctions have been demonstrated with critical current‐normal state resistance products exceeding 5 mV at 77 K, critical current densities ranging from 500 to 25 000 A/cm2, and critical current versus field profiles suggesting very uniform junctions. Yield has exceeded 70% on over 250 junctions tested and operation has been demonstrated to 100 K.

Fabrication of TlCaBaCuO step‐edge Josephson junctions with hysteretic behavior

One way to create hysteretic Josephson junctions from the currently available nonhysteretic high temperature superconducting junctions is to artificially add capacitance. We have adapted a multilayer technique for artificial capacitance addition/grain boundary modification to produce TlCaBaCuO step‐edge junctions exhibiting large amounts of hysteresis at 77 K. A gaplike structure is present at 32–36 mV that has a temperature dependence that does not contradict that predicted by the Bardeen‐Cooper‐Schrieffer theory. In addition, the junctions show fast switching times, less than a fixture limited 20 ps, and Fraunhofer‐like dependence of critical current on magnetic field.