Martin G. Forrester

Critical parameters in the single‐target sputtering of YBa2Cu3O7

The critical parameters in the single‐target magnetron sputtering of YBa2Cu3O7 have been identified and sufficiently optimized to allow the reproducible deposition of films with Tc’s of ≳90 K and Jc’s of ≫ 106 A/cm2 at 77 K. It was found that during film growth the bombardment of the YBa2Cu3O7 by energetic particles must be minimized and also a stronger oxidizing agent than molecular oxygen must be present to obtain films with these properties. Otherwise, films are deposited that, by x‐ray diffraction and energy dispersive x‐ray spectroscopy analyses, are indistinguishable from the highest‐Tc 1:2:3 stoichiometric material but which have critical temperatures of ≪90 K. Films need not have 1:2:3 overall stoichiometry to have optimum superconducting properties. In such cases the excess elements are present as second‐phase particles.The critical parameters in the single‐target magnetron sputtering of YBa2Cu3O7 have been identified and sufficiently optimized to allow the reproducible deposition of films with Tc’s of ≳90 K and Jc’s of ≫ 106 A/cm2 at 77 K. It was found that during film growth the bombardment of the YBa2Cu3O7 by energetic particles must be minimized and also a stronger oxidizing agent than molecular oxygen must be present to obtain films with these properties. Otherwise, films are deposited that, by x‐ray diffraction and energy dispersive x‐ray spectroscopy analyses, are indistinguishable from the highest‐Tc 1:2:3 stoichiometric material but which have critical temperatures of ≪90 K. Films need not have 1:2:3 overall stoichiometry to have optimum superconducting properties. In such cases the excess elements are present as second‐phase particles.

Optical response of epitaxial films of YBa2Cu3O7−δ

We present the results of measurements of optical detection in epitaxial films of YBa2Cu3O7−δ, at wavelengths of 0.63 and 10.6 μm. In contrast to the behavior observed in granular materials, these films appear to show no evidence of nonequilibrium response (breaking of Cooper pairs by photons), but only a bolometric effect (heating of the sample by radiation) in the resistive transition regime. This suggests that epitaxial films do not contain intrinsic links weak enough to be modulated by the incident radiation. For 0.63 μm radiation, mechanically chopped at 725 Hz, measurements of a 10 μm ×90 μm bridge yield a bolometric responsivity of approximately 4×103 V/W, and a detectivity D* of more than 108 cm Hz1/2/W.

High‐Tc superconductor/normal‐metal/superconductor edge junctions and SQUIDs with integrated groundplanes

Epitaxial, high‐Tc superconductor/normal‐metal/superconductor (SNS) edge‐geometry weak links and superconducting quantum interference devices (SQUIDs) have been fabricated with integrated YBa2Cu3O7 (YBCO) groundplanes and SrTiO3 insulators, using a process which incorporates six epitaxial layers. The SNS edge junctions were produced using off‐axis sputtered films and Co‐doped‐YBCO normal metal interlayers. These devices show excellent performance with typical critical current‐resistance (IcRn) products of 500–800 μV for 100–150 A thick normal metal layers at 65 K, and 1‐σ critical current density ( Jc) spreads as small as 12%. SNS SQUIDs incorporating groundplanes exhibit voltage modulation of up to 130 μV at 65 K and 40 μV at 77 K. SQUID inductance measurements indicate microstrip inductance values of 1 pH per square at 65 K.

Effect of inductance in externally shunted Josephson tunnel junctions

We report measurements on current‐voltage (I‐V) characteristics of externally shunted Josephson tunnel junctions where the external shunts have nonzero inductance. At low temperatures (T<6.6 K), we observe standard I‐V curves, but as temperature is increased above 6.6 K we observe an anomaly in the I‐V curves. We attribute this anomaly to the nonzero inductance in the shunt loop. Numerical simulations which properly take into account the effect of inductance show that the dynamics in the anomalous region of the I‐V curve are dominated by subharmonic relaxation oscillations. We use a load‐line analysis model to study the transition between the relaxation oscillations and the Josephson oscillations. The analysis is in good agreement with our data. Our analysis further predicts, and our data confirms, that in order for the relaxation oscillations to occur it is necessary to have βC=2πIcR2sC/Φ0<(4/π)2, regardless of the value of the inductance.

A single flux quantum shift register operating at 65 K

We report the fabrication and quasi-static testing of a two-stage, high-temperature superconducting, Single Flux Quantum shift register. The five-junction circuit was fabricated using a single YBCO film, with step-edge grain boundary junctions. Storage of flux, and its motion in response to LOAD and SHIFT signals, was demonstrated at 65 K.<<ETX>>

Optical response of epitaxial and granular films of YBa/sub 2/Cu/sub 3/O/sub 7- delta / at temperatures from 25 K to 100 K

The authors present the results of optical detection in epitaxial and granular films of YBa/sub 2/Cu/sub 3/O/sub 7- delta / at wavelengths of 0.63, 3.39, and 10.6 mu m and at temperatures from 25 K to 100 K. Both types of film exhibit only bolometric detection, with no evidence for nonequilibrium effects in this temperature range. For 0.63- mu m radiation, mechanically chopped at 725 Hz, measurements of a 10*90 mu m/sup 2/ epitaxial bridge yield a bolometric responsivity of approximately 4*10/sup 3/ V/W, and detectivity D* of more than 10/sup 8/ cm square root Hz/W. Granular films biased above their critical current are found to exhibit two-level switching noise, resulting in Lorentzian deviations from an otherwise 1/f noise spectrum. >

High-resistance HTS edge junctions for digital circuits

We have investigated factors affecting the resistance of edge-geometry HTS weak links, including SNS junctions with Co-doped Y-Ba-Cu-O as a normal metal interlayer. We have also studied devices with no deposited interlayer in which the weak link is produced by ion beam surface damage or by controlled disorder near the base electrode-counterelectrode interface. For each of these weak links several parameters, including the base electrode material and the deposition conditions of the normal metal and counterelectrode, are found to have strikingly large effects on device resistance. Controlling these factors has enabled the fabrication of high-quality, high-resistance (/spl ap/1 Ohm) SNS edge junctions with one-sigma I/sub c/ spreads down to 6% in 10-junction series arrays. The junctions without deposited interlayers exhibit electrical characteristics and parameter spreads approaching the best results obtained with the Co-YBCO SNS devices.

Fabrication and characterization of YBa/sub 2/Cu/sub 3/O/sub 7//Au/YBa/sub 2/Cu/sub 3/O/sub 7/ Josephson junctions

All-high-T/sub c/ Josephson junctions have been fabricated in a planar SNS geometry by bridging narrow gaps ( approximately=0.1-0.2 mu m) in epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films with Au. The resulting devices exhibit a variety of nonhysteretic I-V characteristics, with I/sub c/R/sub N/ on the order of 0.1 to 10 mV, and exhibit Shapiro steps under microwave irradiation, and weak periodic modulation of the critical current with applied magnetic field. The transport properties of the junctions appear to be dominated by the Au/YBCO interfaces rather than by the Au itself.

DEVELOPMENT OF A RELIABLE MATERIALS BASE FOR SUPERCONDUCTING ELECTRONICS

Careful studies of the corrosion, redox, galvanic and oxygen evolution/uptake reactions associated with YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and related compounds have been completed. These studies have led to an understanding of the many factors that contribute to the poor material characteristics exhibited by these popular high-T{sub c} phases. With knowledge of the structure-reactivity relationships, a powerful crystal engineering approach has been developed that is capable of producing cation substituted versions of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}; the resulting compounds therefrom produced exhibit markedly improved processability, oxygen stability, and durability characteristics. These materials have been combined in thin film structures so as to make prototype SNS junctions and SQUID sensors which exhibit promising device performance characteristics. {copyright} {ital 1997 Materials Research Society.}

Multilayer HTS SFQ analog-to-digital converters

We have fabricated and measured high T/sub c/ superconductor single flux quantum 1-bit flux-counting analog-to-digital converters (ADCs). The ADCs were made with a multilayer all-epitaxial process which incorporates 10 edge SNS (superconductor-normal-superconductor) or step-edge grain boundary (SEGB) Josephson junctions with a YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// groundplane. The ADC consists of a quantizer connected to a Toggle flip-flop through a buffer-like stage. Direct readout of the flux state of the T flip-flop was made with a Read SQUID inductively coupled through a hole in the groundplane. The circuits were operated at 65 K and low speeds. The SNS circuits outperformed the SEGB circuits because of their higher readout voltages and tighter critical current spreads.