D. S. Yee

Processing of La1.8Sr0.2CuO4 and YBa2Cu3O7 superconducting thin films by dual‐ion‐beam sputtering

High quality La1.8Sr0.2CuO4 and YBa2Cu3O7 superconducting thin films, with zero resistance at 88 K, have been made by dual‐ion‐beam sputtering of metal and oxide targets at elevated temperatures. The films are about 1.0 μm thick and are single phase after annealing. The substrates investigated are Nd‐YAP, MgO, SrF2, Si, CaF2, ZrO2‐9% Y2O3, BaF2, Al2O3, and SrTiO3. Characterization of the films was carried out using Rutherford backscattering spectroscopy, resistivity measurements, transmission electron microscopy, x‐ray diffraction, and secondary ion mass spectroscopy. Substrate/film interaction was observed in every case. This generally involves diffusion of the substrate into the film, which is accompanied by, for example, the replacement of Ba by Sr in the YBa2Cu2O7 structure, in the case of SrTiO3 substrate. The best substrates were those that did not significantly diffuse into the film and which did not react chemically with the film. In general, the superconducting transition temperature is found to ...

Hysteresis of submicron permalloy particulate arrays

We have prepared controlled arrays of submicron permalloy particles as part of a program to determine the magnetic properties of verifiable single‐domain particles (SDP) and the effects of interactions between SDPs. We have measured the hysteresis loops of these arrays as a function of size, aspect ratio, and spacing. The arrays are fabricated utilizing electron beam lithography and liftoff techniques. Each array consists of approximately 106 identical particles with identical spacing between the particles. Particle size and spacing [center‐to‐center distance (ccd)], vary from 5 μm×15 μm (width by length) with ccd of 25 μm to 0.07 μm×0.1 μm with ccd of 0.25 μm. The thickness of the films varied from 0.072 to 0.049 μm. Hysteresis loops are obtained utilizing a SQUID magnetometer. For well‐separated particles we observe an order of magnitude increase in Hc with decreasing particle size, and then a subsequent decrease in Hc as the ccd is reduced. However, even for the smallest particles, the ratio of the rem...

Processing of La/sub 1. 8/Sr/sub 0. 2/CuO/sub 4/ and YBa/sub 2/Cu/sub 3/O/sub 7/ superconducting thin films by dual-ion-beam sputtering

High quality La1.8Sr0.2CuO4 and YBa2Cu3O7 superconducting thin films, with zero resistance at 88 K, have been made by dual‐ion‐beam sputtering of metal and oxide targets at elevated temperatures. The films are about 1.0 μm thick and are single phase after annealing. The substrates investigated are Nd‐YAP, MgO, SrF2, Si, CaF2, ZrO2‐9% Y2O3, BaF2, Al2O3, and SrTiO3. Characterization of the films was carried out using Rutherford backscattering spectroscopy, resistivity measurements, transmission electron microscopy, x‐ray diffraction, and secondary ion mass spectroscopy. Substrate/film interaction was observed in every case. This generally involves diffusion of the substrate into the film, which is accompanied by, for example, the replacement of Ba by Sr in the YBa2Cu2O7 structure, in the case of SrTiO3 substrate. The best substrates were those that did not significantly diffuse into the film and which did not react chemically with the film. In general, the superconducting transition temperature is found to ...

Thin‐film Bi‐Sr‐Ca‐Cu‐O high‐temperature superconductors using pulsed laser evaporation from sintered disks

Films of the high‐temperature superconductor in the Bi‐Sr‐Ca‐Cu‐O system were deposited by pulsed laser evaporation from a ceramic disk. Films deposited onto (100) MgO show x‐ray diffraction patterns characteristic of c‐axis textured Bi2(Sr,Ca,Bi)3Cu2Ox phase following annealing for 10 min at 850 °C in 20% O2‐80% N2. These textured films have zero resistance at 78 K and show resistance drops near 110 K. The effects of the annealing environment on the electrical and structural properties are presented.

Substrate effect on the superconductivity of YBa2Cu3O7 thin films

The interaction between the substrate and YBa2Cu3O7 films made by ion beam sputtering and e‐beam evaporation is found to significantly influence the superconducting transition temperature, Tc. The substrates investigated are Al2O3, MgO, SrTiO3, ZrO2‐9% Y2O3 (YSZ), and BaF2. In the case of Al2O3, a spinel is formed at the interface and Al is found to be present in solution in the YBa2Cu3O7. This reaction significantly depresses Tc. For SrTiO3, both Sr and Ti are found in the film without severely depressing the Tc; this, however, depends on the annealing conditions. Although under similar conditions little interaction was observed for the MgO substrates, a moderate reduction in Tc was observed. An interaction zone of BaZrO3 perovskite is formed at the interface between the YSZ substate and the 123 structure. In addition, Zr is found in solution in the 123 film. The presence of Zr has little effect on the Tc. The films were characterized using Rutherford backscattering spectroscopy (RBS), transmission elect...

Superconducting thin films of the Bi–Sr–Ca–Cu–O system prepared by multilayer metal deposition

Superconducting thin films of the Bi--Sr--Ca--Cu--O system (2 and 10 kA) have been successfully prepared by multilayer deposition from elemental metal sources and annealed in N/sub 2//O/sub 2/ mixtures. The onset of the superconducting transition is greater than 80 K with zero resistance at 74 K. The films contain primarily a Bi/sub 2/(SrCaBi)/sub 3/Cu/sub 2/O/sub 8+//sub delta/ phase with a high degrees of (001) preferred orientation.

Carbon as a barrier for the outdiffusion of Cu

The effectiveness of carbon as a barrier for the outdiffusion of Cu is studied using C/Cu/substrate and Au/C/Cu/substrate structures. For the C/Cu structure deposited on SiO2 ‐coated Si with a 700 A C layer, heating to 700 °C for 72 h and 750 °C for 6 h shows no outdiffusion of Cu to the carbon surface in N2 ‐H2. Only reaction between Cu and the SiO2 layer underneath is observed. Application of the carbon barrier between Cu and Au in the Au/C/Cu/substrate structure shows that the dilution of the Au layer due to the Cu outdiffusion is similar to but less than that of the Au/Ni/Cu/substrate structure using Ni as a barrier. The stability of the Au/C/Cu/substrate structure is enhanced relative to that of the Au/Ni/Cu/substrate one by more than 150 °C. Both the advantage and concerns using carbon as barriers for the interconnect and contact metallurgies are discussed.

Photoemission from thin-film lanthanum hexaboride

Lanthanum hexaboride (LaB6) is investigated for its potential as a low work‐function photocathode for high brightness e‐beam applications. Thin films of LaB6 at room temperature are shown to have quantum efficiencies of 0.2×10−3 at 442 nm excitation but readily form an inhibiting oxide. This oxide is easily removed by in situ 500 eV Ar+ ion sputtering, which restores photoemission for over 30 min at an oxygen partial pressure of 10−9 Torr.

Superconductivity in Bi‐Sr‐Ca‐Cu‐O (invited)

We report on the structure and superconducting properties of a single phase polycrystalline composition given by Bi2.2Sr1.7Ca1.2Cu2O8+δ as well as measurements on single crystals. X‐ray and electron diffraction analyses of both single‐crystal and single phase materials show an incommensurately modulated orthorhombic superlattice, derivable from a pseudotetragonal substructure. The various compositions have the common feature that the major portion of the superconductivity disappears above 85 K but there is a small tail in the diamagnetic susceptibility extending to 110 K, above which the sample becomes paramagnetic. Single crystals, grown by the flux method, exhibit the same incommensurately modulated pseudotetragonal structure as the ceramic, but show a higher diamagnetic shielding and in some cases also have tails. The critical currents in the (ab) plane are only about one‐fifth of the value found for YBa2Cu3O7 crystals and the anisotropy is smaller. These reduced values may be an intrinsic property of ...

Improved ion source for use with oxygen

When Kaufman ion sources are used with pure oxygen at high discharge powers, their operational lifetime is limited to minutes. The lifetime of the source can be significantly increased by using thoria‐coated filaments for the cathodes. In addition, the lifetime can be increased further by coating the anode with a conductive oxide such as IrO2. This coating is deposited by sputter deposition. With these improvements the operational lifetime can be increased by a factor of 100.