B. G. Bagley

Bulk and thick films of the superconducting phase YBa2Cu3O7−y made by controlled precipitation and sol‐gel processes

The synthesis of high‐temperature superconducting oxides using solution chemistry has been investigated and physical properties are compared to ceramics obtained by conventional solid‐state reactions. We report on controlled precipitation and sol‐gel processes, both of which produce materials with particle sizes smaller than 5 μm. We find that the superconducting properties of the high Tc ceramics are affected by their manner of preparation, such that the transitions are slightly lower in temperature, but sharper, for samples made by solution rather than solid‐state chemistry. The ability to prepare stable viscous gels provides an opportunity for obtaining large areas of superconducting coatings. For thick films on alumina or silicon substrates, contamination from the substrate is shown to be a problem. Finally, we observe that the sol‐gel process lowers the synthesis temperature by 100 °C.

Effect of the post-deposition processing ambient on the preparation of superconducting YBa2Cu3O7-x coevaporated thin films using a BaF2 source

We have investigated the effect of the post‐deposition processing ambient on the preparation of YBa2Cu3O7−x thin films from a BaF2 source. The role of H2O vapor during the high‐temperature anneal is understood through a thermodynamic analysis of the fluorine removal reaction. The role of a HF getter (e.g., SiO2) is understood through the same type of analysis. We have demonstrated that a zero resistance transition temperature at 77 K can be obtained for an annealing temperature as low as 690 °C for films deposited on SrTiO3 substrates by increasing the PH2O and decreasing PHF during the high‐temperature soak cycle.

Plasma oxidation of the high Tc superconducting perovskites

A near room‐temperature plasma oxidation process is shown to restore superconductivity and metalliclike behavior in oxygen deficient La2−x Srx CuO4−y and YBa2Cu3O7−x compounds. In the YBa2Cu3O7−x compound the conversion from an oxygen deficient n‐type tetragonal to the p‐type orthorhombic phase with a concomitant factor of 5×105 increase in room‐temperature conductivity is also accomplished. This process is of technological importance because oxygen can be restored in these materials at temperatures compatible with device processing. Of scientific interest, the process allows us to carefully control the oxidation state and thereby systematically study the 90 and 55 K superconducting transitions in YBa2Cu3O7−x.

As2S3/GaAs, a new amorphous/crystalline heterojunction for the III‐V semiconductors

Much of the technology of our era is based on the SiO2/Si amorphous/crystalline heterojunction interface. Now it appears that As2S3/GaAs amorphous/crystalline heterojunctions show some technological promise. We have found that properly prepared As2S3/GaAs interfaces can have reasonably good electronic quality. The interfacial recombination velocity is ≊15 000 cm/s at flat band, which results in a ∼100‐fold reduction of perimeter recombination currents in p‐n junction mesas. This can be important on heterojunction transistor emitter‐base perimeters, solar cell and light‐emitting diode perimeters, and for reducing mirror facet recombination in semiconductor lasers.

Off‐axis sputter deposition of YBa2Cu3O7 thin films for microwave applications

Thin films of superconducting YBa2Cu3O7 were grown in situ by off‐axis sputter deposition for microwave device fabrication. These ∼1 cm2 films, which are reproducible, exhibit midpoint Tc’s of 89–90.5 K as measured by ac susceptibility, ion channeling yields of 4.7%–6%, and c‐axis rocking‐curve half‐widths of 0.5°, even with a rich microstructure as seen by scanning electron microscopy. Two films were photodefined into miniature X‐band microwave bandpass filters. These narrow‐band filters (0.5% bandwidth) exhibited 4.4‐and 4.5‐dB insertion losses at 77 K and 9.25 GHz, with little temperature dependence below 80 K.

On the the crystal growth and chemistry of the new electron-type superconducting oxides

Abstract The effect on transport and superconducting properties produced by changes in x and y in the Nd2−xCexCuOy compound were studied in both polycrystalline ceramics and single crystals. Thermogravimetric analysis shows that the total oxygen content y for the as-prepared samples is always greater than 4 (i.e. presence of interstitial oxygen) whereas for the reduced sample y becomes equal to or smaller than 4 only when x is 0.15 or greater. This is the range of Ce content for which the material superconducts. For a material with Ce x=0.15 the superconducting properties can be varied reversibly by changing the oxygen content. In addition, we propose that the oxygen in these materials can be either ordered or disordered, thereby affecting the transport properties. Platelet-like crystals of Nd2−xCexCuOy with x=0 to 0.18, have been grown via a flux technique. Those having a Ce content between 0.14 and 0.17 are superconducting with the sharpest transitions (Tc = 21 ± 1K) for x = 0.14. Metallic-like behavior above Tc, with a linear temperature dependence above 150K was observed on all the crystals. Below 30K, the in-plane resistivity is independent of T. The Hall coefficient is sensitive to processing conditions but is usually negative above 100K. The upper critical field with field along the c-axis is 6T at 4.2K.

Chemical Doping and Physical Properties of the New High Temperature Superconducting Perovskites

The effects of chemical doping on the structural, magnetic, transport and superconducting properties of the new high Tc superconducting oxides are reviewed. The substitution of Sr or rare earths for La in La2CuO4−y and the replacement for Y by rare earths and for Ba by Sr in the YBa2Cu3O7−y system have been investigated, as well as the substitution of Ni for Cu in both systems. In contrast to the rare earth substitutions, a Ni substitution dramatically affects Tc. The importance of the oxygen content (y) in these oxides, which strongly depend on their processing conditions (heat treating time, temperature and ambient), is emphasized and a detailed study of changes in the physical properties as a function of y is reported. The superconducting properties of these perovskites can be destroyed by decreasing their oxygen content. However, full superconductivity can be recovered by reannealing the sample under oxygen or by a low temperature (80°C) plasma oxidation. This later method may be very useful for the technological application of these materials.

Studies of proximity-effect and tunneling in YBCO/metal layered films

Abstract The short coherence length of the high-T c superconductors, coupled with their tendency to form non-superconducting surface layers, accounts for the difficulty in achieving good tunnel junctions. A proximity layer of a longer coherence length normal metal (N) is expected to “draw out” Cooper pairs. Our goal is to fabricate reproducible, planar tunnel junctions of SNIS layered structures for proximity tunneling spectroscopy. Such structures of YBCO/N/I/Pb and SNS structures of YBCO/N/Pb indicate that the normal metal produces a low resistance contact to the YBCO surface with a supercurrent observed in the SNS. The insulating barrier in the SNIS is reproducible, insulating and continuous: A sharp Pb gap and phonons from the counter-electrode are routinely observed.

Dielectric and high Tc superconductor applications of sol-gel and modified sol-gel processing to microelectronics technology

Abstract Microelectronic applications for sol-gel processed dielectric materials (e.g. dopant sources, resist patterning, substrates, planarization layers) are briefly reviewed. The preparation and properties of dielectric thin films of a ladder siloxane, useful for some of these applications, are described. The properties of an as-spun film of this material are intermediate between those of an organic (e.g. a resist) and an inorganic (e.g. an oxide). These organosilicon polymers can also be used as precursors for the preparation of inorganic oxides; silica, phosphosilicate, borosilicate and borophosphosilicate thin films have been prepared. The amount of boron and phosphorus incorporated appears to be limited by the organosilicon functionality. Conversion to the inorganic oxide can be accomplished by either pyrolysis or plasma oxidation. Also discussed is the processing and properties of an aqueous based sol-gel process for the preparation of superconducting YBa 2 Cu 3 O 7− y , a material which could have application as a thick film chip-carrier metal interconnect. Materials prepared using this process demonstrate an excellent resistivity versus temperature superconducting transition behavior but have low critical current densities.

Growth of high Tc superconducting Bi4(Ca,Sr)6Cu4O16+x crystals

To determine intrinsic properties of the newly discovered Bi‐Ca‐Sr‐Cu‐O high Tc superconductors, single crystals are necessary. Compositions in this system have been heat treated to survey the melting temperatures and phase field in which superconductivity is detected. The nucleation and growth of the 85 K phase from the melted composition Bi4Ca3Sr3Cu4O16+x is observed to be a kinetically slow process which can be precluded by a sufficiently rapid quench, but post‐anneals produce the 85 and 110 K phases in the quenched material. The melted composition (23% Bi2O3‐46% CaO,SrO‐31% CuO), after subsequent slow cooling, results in large discrete crystals of the 85 K superconducting phase and a residual flux.To determine intrinsic properties of the newly discovered Bi-Ca-Sr-Cu-O high T/sub c/ superconductors, single crystals are necessary. Compositions in this system have been heat treated to survey the melting temperatures and phase field in which superconductivity is detected. The nucleation and growth of the 85 K phase from the melted composition Bi/sub 4/Ca/sub 3/Sr/sub 3/Cu/sub 4/O/sub 16+//sub x/ is observed to be a kinetically slow process which can be precluded by a sufficiently rapid quench, but post-anneals produce the 85 and 110 K phases in the quenched material. The melted composition (23% Bi/sub 2/O/sub 3/-46% CaO,SrO-31% CuO), after subsequent slow cooling, results in large discrete crystals of the 85 K superconducting phase and a residual flux.