R. B. van Dover

High critical currents in iron-clad superconducting MgB2 wires.

Technically useful bulk superconductors must have high transport critical current densities, Jc, at operating temperatures. They also require a normal metal cladding to provide parallel electrical conduction, thermal stabilization, and mechanical protection of the generally brittle superconductor cores. The recent discovery of superconductivity at 39u2009K in magnesium diboride (MgB2) presents a new possibility for significant bulk applications, but many critical issues relevant for practical wires remain unresolved. In particular, MgB2 is mechanically hard and brittle and therefore not amenable to drawing into the desired fine-wire geometry. Even the synthesis of moderately dense, bulk MgB2 attaining 39u2009K superconductivity is a challenge because of the volatility and reactivity of magnesium. Here we report the successful fabrication of dense, metal-clad superconducting MgB2 wires, and demonstrate a transport Jc in excess of 85,000u2009Au2009cm-2 at 4.2u2009K. Our iron-clad fabrication technique takes place at ambient pressure, yet produces dense MgB2 with little loss of stoichiometry. While searching for a suitable cladding material, we found that other materials dramatically reduced the critical current, showing that although MgB2 itself does not show the ‘weak-link’ effect characteristic of the high-Tc superconductors, contamination does result in weak-link-like behaviour.

High TC superconductors—composite wire fabrication

Commercially useful, bulk superconductors typically require stabilization using a normal metal cladding for reasons of electrical, thermal, and mechanical protection and, in general, need to be drawn into fine fibers and wound into a magnet configuration. The recent discovery of high‐TC superconductor materials such as Ba2YCu3O7 stimulated worldwide interest in the subject, however, with much concern about fabricability of such brittle ceramic materials into desirable fine wire geometry. In this letter, we report preliminary success in the fabrication of fine‐wire, composite superconductors consisting of a high‐conductivity normal metal shell such as Ag or Cu/Ni/Au and a superconducting core of Ba2YCu3O7 oxide. The wire is would into a coil, and then heat treated to produce the desired chemistry in a dense structure. The resistivity of the composite wire is measured to be zero at ≊90 K (in zero field) with a zero‐field critical density of ≊175 A/cm2. Microscopy and x‐ray analysis show that the superconduc...

Etching of high-k dielectric Zr1−xAlxOy films in chlorine-containing plasmas

As new, advanced high-k dielectrics are being developed to replace SiO2 in future generations of microelectronics devices, understanding their etch characteristics becomes vital for integration into the manufacturing process. We report on the etch rates and possible mechanisms for one such dielectric, Zr1−xAlxOyu2002(x≈0.2), in plasmas containing a mixture of Cl2 and BCl3, as a function of gas composition and ion impact energy. Higher concentrations of BCl3 enhance the etch rate as well as selectivity of Zr1−xAlxOy etching as compared to the etching of α-Si, whereas increasing ion energy increases the etching rates but decreases selectivity. In a high density helical resonator plasma, etching rates on the order of 700 A/min and 1:1 selectivity are typical. Angle-resolved x-ray photoelectron spectroscopy was used to study the composition of the upper ∼30 A of the film, before and at the end of the etching process. We found that the etching rate of Zr1−xAlxOy does not change with time for the range of Cl2/BCl3 ...

HIGH DIELECTRIC CONSTANT HF-SN-TI-O THIN FILMS

High dielectric constant Hf–Sn–Ti–O thin-film materials were identified using a compositional-spread approach. Thin films of composition Hf0.2Sn0.05Ti0.75O2 prepared at 250u200a°C have excellent dielectric properties: 40–70-nm-thick films with a dielectric constant of 40–60 were obtained, depending on the processing conditions, yielding a specific capacitance of 9–17 fF/μm2. Breakdown fields were measured to be about 3–4 MV/cm, yielding a figure of merit ee0Ebr∼19u200aμC/cm2. Leakage currents, measured at 1 MV/cm, were in the range 10−7−10−6u200aA/cm2.

High throughput screening of electrocatalysts for fuel cell applications

We describe methodologies for the generation and screening of combinatorial libraries of electrocatalyst materials for fuel cell applications, generated by cosputtering of three elements onto a Si substrate coated with a Ta adhesion underlayer. Screening was carried out via a fluorescence assay as well as by scanning electrochemical microscopy. Whereas the former provided rapid qualitative screening with limited spatial resolution, the latter provided high spatial resolution. The fluorescence screening method was tested on Pt, PtBi, PtPb, and PtRu nanoparticles, while both methods were tested on a film containing a Pt–Bi–Pb ternary composition spread.

Design of high frequency inductors based on magnetic films

Magnetically coated stripe inductors with conductor insulated from the magnetic film and with conductor in direct electrical contact with the magnetic film are analyzed. A simple equivalent circuit model shows that for the structure without insulation, regardless of the fact that the dc resistivity of the magnetic film is much higher than that of the conductor, most of the driving current flows in the magnetic film at frequencies where the impedance of the device is dominated by inductance, i.e., where the quality factor Q>1. This effect of current re-distribution significantly lowers the performance of the device. A design is proposed having insulation layers to avoid current flow in the magnetic film, which is broken up in sections along the conductor stripe to avoid displacement currents in the system.

Getter sputtering system for high-throughput fabrication of composition spreads

We describe a sputtering system that can deposit composition spreads in an effectively UHV environment but which does not require the high-throughput paradigm to be compromised by a long pump down each time a target is changed. The system deploys four magnetron sputter guns in a cryoshroud (getter sputtering) which allows elements such as Ti and Zr to be deposited with minimal contamination by oxygen or other reactive background gases. The system also relies on custom substrate heaters to give rapid heating and cool down. The effectiveness of the gettering technique is evaluated, and example results obtained for catalytic activity of a pseudoternary composition spread are presented.

Composition-dependent crystallization of alternative gate dielectrics.

We have investigated the crystallization of amorphous oxides that are considered likely candidates to replace amorphous SiO2 as the gate dielectric in advanced field-effect transistors. To avoid crystallization, the mole fraction of main-group oxide in the Zr–Si–O, Zr–Al–O, and Hf–Si–O systems must be greater than 83%, 65%, and 78%, respectively, leading to a maximum useful dielectric constant of only 6.9, 12.7, and 6.6, respectively. We conclude that the silicate systems are not likely to be useful as replacements for SiO2, while aluminates are more promising.

Crystallization kinetics in amorphous (Zr0.62Al0.38)O1.8 thin films

Thin films of Zr0.62Al0.38O1.8 are amorphous when deposited at room temperature by rf magnetron sputtering. Crystallization occurs during subsequent annealing in the temperature range of 700–1000u200a°C for times in the range of 10 s–100 h. The crystallite size and the fraction of the sample that had crystallized were determined using x-ray diffraction. The films were found to initially develop a low density of fairly large (∼8 nm) crystallites, while subsequent heat treatment was found to increase the density rather than the size of the crystallites. Crystallization can be described with a first-order rate equation; the rate constant is exponential in temperature with an effective activation energy of 6.6 eV. Films given a 10 s anneal at <850u200a°C develop a substantial density of 8 nm grains, making this specific composition an unsuitable candidate for replacing SiO2 as the gate oxide in hyperscaled field-effect transistors.

Structure and Magnetic Properties of Epitaxial NiFe2O4 Films

We have made NiFe 2 O 4 films by rf sputtering using the 90 °off-axis geometry. Films grown at 600°C on SrTiO 3 and Y .15 Zr .85 O 2 substrates are single crystals with (100) and (110) texture, respectively, but exhibit a very large and unexpected random anisotropy. A postdeposition air anneal at ˜1000°C has little effect on the crystallinity of the films but almost completely eliminates the random anisotropy; the remaining anisotropy is consistent with expected (bulk) values. A decrease of the saturation magnetization, indicating degradation, was observed for films on SrTiO 3 (but not Y .15 Zr .85 O 2 ) annealed at 1300°C.