The Naval Research Laboratory (NRL) is exploring the feasibility of deploying high temperature superconducting electrical and electronic devices and components in space. A variety of devices, primarily passive microwave components, have been procured and will integrated with a cryogenic refrigerator system and data acquisition system to form the space package. This Space Experiment will demonstrate that HTS technology is sufficiently robust to survive the space environment and that it has the potential to significantly improved space communications systems. The devices for the Initial launch have been received by NRL, their electrical characteristics verified and their packages space qualified. The devices are currently being integrated into the final space package which will be shipped from NRL late in 1991 for integration into the spacecraft payload. Once placed In earth orbit, the experimental package will be cooled to cryogenic temperatures and the characteristics of the HTS devices monitored periodically for the planned twelve months duration of the space experiment which which is schedule to start in 1993.

We present an overview of potential applications of high-Tc superconductors (HTSC) in the context of hybrid optoelectronic technology. We briefly summarize the main requirements for in-situ growth of epitaxial YBa₂Cu₃O_7-δ (YBCO) films on SrTiO₃, and discuss the properties of YBCO layers grown on Si and GaAs substrates with intermediate, conducting Indium Tin Oxide buffer layers. We compare the performance of the micro-bridge and the meander type of HTSC bolometer and conclude with a discussion of several novel concepts and results that may become relevant for future hybrid optoelectronic technology.

In this paper, we have shown that superconducting electronics can provide a complementary role to microelectronics and optoelectronics. High temperature superconductors can be used in the fabrication of ultra fast three terminal devices as well as for chip-to-chip interconnections. If the next generations of DRAMs require cooling at 77 K, high temperature superconductors offer several advantages over metals as the interconnect material. A new photoeffect reported in this paper can give rise to new devices and can be used to couple microwave and optoelectronic technology. Major challenge is to develop a low temperature processing technology that is compatible to semiconductor industry. Experimental results are presented indicating the potential of rapid isothermal processing assisted metalorganic chemical vapor deposition (MOCVD) method as a technique suitable for manufacturing.

We report recent results on the synthesis and transport properties of a-axis oriented YBa₂Cu₃0₇/ PrBa₂Cu₃0₇ (YBCO/ PBCO) superlattices. A-axis oriented YBCO is of great interest in fabricating sandwich tunnel junctions since the coherence length in this direction is much greater than that in the caxis direction. There have been reports of supercurrent in YBCO/PBCO/YBCO junctions across up to 1500Å of PBCO.^1,2 We use the YBCO/PBCO multilayer system to investigate the decoupling length of the YBCO across PBCO by changing the modulation wavelength. The resistive transitions of these superlattices in magnetic field show a crossover temperature above which they are insensitive to magnetic fields and below which broadening occurs; this may be related to the disappearance of the coupling between YBCO layers above T*.

We report on the correlations between structural quality and superconducting behavior in 1000Å thick Ba₂YCu₃O_7-δ (BYCO) films grown on LaA1O₃(100) from the coevaporation of BaF₂, Y, and Cu, followed by an optimized ex situ annealing process. Epitaxial films with smooth, laminar morphology and excellent crystallinity can be grown to have critical current density (J_c) values nearly identical to single crystals. This finding contrasts with the typical observation that J_c values in thin films of BYCO are very high compared to those of single crystals. This is attributed to a greater density of flux pinning sites due to structural defects within the films. Our most crystalline films have penetration length λ ≤2000Å. Material disorder of two types can be controlled by the high temperature stage (T_a) of the annealing process. The first type is point defects and dislocations the same size or smaller than the coherence length ξ_ab, which Rutherford backscattering/channeling suggests decrease in number with increasing T_a. The second is crevices, pinholes, and microcracks which are at least one to two orders of magnitude larger than ξab. At Ta < 850ÅC, crevices, which create areas of nonuniform thickness, occur due to incomplete epitaxial growth and correlate with the presence of weak links. Hence film resistivity is high, T_c is low, and λ is large. As T_a is increased, the film morphology becomes smoother and all electrical properties improve, except for J_c in nonzero applied magnetic fields, since the improved epitaxy correlates with reduced flux pinning. By T_a = 900° the BYCO films are similar to single crystals in both cation alignment and J_c behavior. Above this annealing temperature, pinholes and microcracks develop and increase in both size and density with increasing T_a. Although these relatively large defects do not act as weak links, they do affect magnetic screening (and hence λ).

The nature of evaporation/ablation characteristics during pulsed laser deposition strongly controls the quality of laser-deposited films. To understand the origin of particulates in laser deposited films, we have simulated the thermal history of YBa2Cu3O7 targets under intense nanosecond laser irradiation by numerically solving the heat flow equation with appropriate boundary conditions. During planar surface evaporation of the target material, the sub-surface temperatures were calculated to be higher than the surface temperatures. While the evaporating surface of the target is constantly being cooled due to the latent heat of vaporization, sub-surface superheating occurs due to the finite absorption depth of the laser beam. Sub—surface superheating was found to increase with decreasing absorption coefficient and thermal conductivity of the target, and with increasing energy density. The superheating may lead to sub—surface nucleation and growth of the gaseous phase which can expand rapidly leading to microexplosions and "volume expulsion" of material from the target. Experiments conducted by us and other research groups suggest a strong relation between degree of sub-surface superheating and particle density in laser—deposited films.

A MOCVD based atomic layer epitaxy process is being developed as a potential solution to the problems of film thickness and interface abruptness control which are encountered when fabricating superconductor-insulator-superconductor (SIS) devices using YB_a2C_u3O_7-x. In initial studies, the atomic layer MOCVD process has yielded superconducting YB_a2C_u3O_7-x films with substrate temperatures of 605°C during film growth, and no post deposition anneal. The low temperature process yields a smooth film surface and will reduce interface degradation due to diffusion.

The angular magnetoresistance technique is proving to be a highly useful tool for the characterization of texture in thin film and other conductors composed of high-T_c superconductors This qualitative technique identifies the predominant orientation of crystallites by resistance dips which occur as a sample is rotated in a magnetic field. These dips occurs when the magnetic field is aligned parallel to the copper-oxide planes. We give some illustrative examples of the technique. It is particularly useful when x-ray diffraction is difficult due to small sample size, coating on the sample, or degeneracy of x-ray diffraction peaks.

Device applications of HTSC films require a precise fine line patterning process. Pattern transfer starts with lithography, followed by a dry etch procedure and ends with the removal of the mask. UV-, X-ray, and e-beam lithography is compared with respect to the handling of the small samples of about 10x10x0.9mm³ used nowadays and with respect to the pattern fidelity. Single layer and multi layer resist structures with linewidths down to 100 nm are used to study the etching of YBa₂Cu₃O_7-x films in different etching equipments (RIE, RIBE). Physical sputtering is the dominant mechanism for the removal of these films. The electrical properties of micron and submicron structures prepared with several testmasks are examined.

This paper reviews the several families of single-flux-quantum logic that have appeared in the literature as well as a new one designed in our Laboratory. Some of these have been evaluated in full-adder circuits and found to have approximately the same latency (time between input and output) as a voltage-state circuit. It is pointed out that the throughput of single-flux-quantum circuits can be much higher than with voltage-state circuits because of a much shorter reset time. The applications for these circuits are greatest in data-independent computation such as signal processing. The prospects and problems for these kinds of circuits are discussed both for low- and high-temperature superconductors.

Superconducting digital gates promise high speed performance at exceedingly low power. Thus, if this technology meets acceptance in packing density, integration levels, margins, and ease of design, it will be extremely competitive. TRW has recently reported the first digital gates in the new high temperature superconducting material YBaCuO. The gates are embedded in a novel architecture that achieves an ease of design and resembles the circuitry employed by the CMOS community. Thus not only will the circuitry operate with convenient minature cryocoolers supporting 65K, but a large group of designers will aid the development. As well the architecture supports gate array development procedures. We report the Boolean functions of AND, EXOR, EXNOR, inversion and buffer gates developed at low speed and reduced temperature. Modelling of newer devices allready available to us predicts operation at up to a 1 GHz clock and at 50K. The additional functions of NAND, NOR, and OR are modifications of the final wiring layer on the basic gate design.

We will report advances in superconducting devices and circuits made using bi-epitaxial grain boundary Josephson junctions in YBa₂Cu₃O₇. These bi-epitaxial grain boundary junctions are created at predetermined locations by controlling the in-plane epitaxy of YBa₂Cu₃O₇ with a seed layer and a buffer layer on a base layer. SQUIDs made from these junctions show modulation up to 88 K and their white noise level is at the theoretical limit. Complete magnetometers coupled with multi-turn flux transformers made using multi-layer technology have magnetic field sensitivity at 77 K sufficient to be useful for many applications. Our recent success in fabricating integrated SQUIDs by combining the SQUID and multi-turn flux transformer on a single substrate is described. Low level integrated circuits with tens of bi-epitaxial grain boundary Josephson junctions have been fabricated. The current status of these circuits is reported.

We have investigated the influence of interface effects and CuO2 plane orientation on transport properties of high temperature superconducting (HTS) thin films in YBa₂Cu₃O₇-PrBa₂Cu₃O₇ (YBCO-PrBCO) heterostructures grown by pulsed laser deposition. Two unit cell ( »24 A) thick YBCO films, isolated in c-axis oriented PrBCO/YBCO/PrBCO trilayers, are found to superconduct at temperatures above 20 K, while films thicker than 6 unit cells display bulk superconducting properties. An epitaxial process has also been developed to grow YBCO films and YBCOIPrBCO heterostructures with the CuO₂ planes oriented normal to the film surface. These a-axis oriented films display transport properties comparable to the best c-axis oriented films, and moreover, are better suited to the fabrication of sandwich-type Josephson junction devices by virtue of the longer superconductive coherence length normal to the direction of the YBCO-PrBCO barrier interface.

High quality YBa₂Cu₃O_7-x/normal-metal/YBa₂Cu₃O_7-x edge-geometry weak links have been fabricated with high yield using nonsuperconducting Y-Ba-Cu-O barrier layers deposited by laser ablation at reduced growth temperatures. Devices incorporating 25- 100 Å thick barrier layers exhibit current-voltage characteristics consistent with the resistively-shunted-junction (RSJ) model, as well as ac Josephson steps. Magnetic field modulation of the critical currents shows Fraunhofer-like behavior with a periodicity which scales with device area. The critical currents vary exponentially with barrier thickness, ranging from ~5 x 10³ to 5 x 10⁵ A/cm², indicating an effective normal metal coherence length of 20 A. In addition, the average resistances scale linearly with Y-Ba-Cu-O interlayer thickness and device area. The scaling behavior of the device parameters combined with the well-behaved magnetic field modulation of the critical currents demonstrates good barrier layer uniformity with low pinhole densities. Near the transition temperature, the critical currents exhibit a temperature dependence proportional to (1 -T/Tc)².

A high-T_c field-effect transistor-like structure (SuFET) was made which consisted of an ultrathin YBa₂Cu₃O_7-x (YBCO) film, a dielectric SrTiO₃ layer and a gold gate electrode. The use of ultrathin films of a few unit cells thickness and an epitaxially grown dielectric layer allowed a relative change in the areal carrier density of YBCO in excess of 20%. A comparable amount of modulation was obtained in normal state resistivity and T_c. The J_c of the channel layer was changed by ~90% when a gate voltage was applied, showing the promise to build a field-effect device using high-T_c superconductors.

We report on the improvements in preparation of large area superconducting YBa2Cu3O7-d films by single target ion beam sputtering. We show that, under suitable and reproducible sputtering conditions, the '123' stoichiometry is transferred from the YBa₂Cu₃O_7-δ target to the substrates. As deposited films on <100> SrTiO₃ show excellent superconducting properties (T_c0=90±0.5 K, ΔT_c< 1 K, j_c(77 K)=1.0-1.2x10⁶ A cm^-2) over areas larger than ≈30 cm². Lower T_c0's (60-70 K) have been obtained on Si and GaAs wafers with a conducting Indium Tin Oxide (ITO) buffer layer.

The discovery of high Tc superconductors has revealed promising applications in optoelectronics. However a basic requirement for the development of successful novel optronic devices is the ability to fabricate high quality Josephson, tunnel or three terminal devices in a reproducible, controllable and reliable way.

Large area deposition of YBCO multilayers, whole wafer processing techniques are essential and in great progress. Nevertheless, the degradation of the YB_a2C_u3O_7-x surface layer close to the junction contacts is observed and induces severe limitations in the devices performances. Experimental evidence of such problems and possible

The Inverted Cylindrical Magnetron Sputtering (ICM) is a reliable and reproducible method for the production of HTSC thin films. This allows systematic studies of film growth as a function of various deposition parameters including film thickness, substrate material or buffer layers. We report the growth conditions and growth quality of ,,l-2-3" films of different orientation with special emphasis on a-axis films which may be of interest for applications if 3-dimensional patterning is attempted. Substrate orientation and substrate temperature are the essential parameters controlling the growth orientation. High quality buffer layers (FWHM mosaic spread <0.2°, x_min,<5% in channeling experiment) of Zr(Y)O₂ could be deposited on R-plane sapphire. Critical current densities near 3 X 10₆ A/cm₂ at 77 K could be achieved for the 1-2-3 films deposited on these buffer layers. Finally we report results of in situ BiSrCaCuO thin film deposition revealing first signs of channeling behaviour (x_min ≈60%) indicating towards epitaxial growth.