B. J. Wilkens

High‐temperature superconductivity in ultrathin films of Y1Ba2Cu3O7−x

We have grown ultrathin films of Y1Ba2Cu3O7−x in situ on (001) SrTiO3 by pulsed laser deposition. The zero resistance transition temperature (Tc0) is >90 K for films >300 A thick. The critical current density (Jc at 77 K) is 0.8×106 A/cm2 for a 300 A film and 4–5×106 A/cm2 for a 1000 A film. The Tc0 and Jc deteriorate rapidly below 300 A, reaching values of 82 K and 300 A/cm2 at 77 K, respectively, for a 100 A film. Films only 50 A thick exhibit metallic behavior and possible evidence of superconductivity without showing zero resistance to 10 K. These results are understood on the basis of the defects formed at the film‐substrate interface, the density of which rapidly decreases over a thickness of 100 A. We have studied these defects by ion channeling measurements and cross‐section transmission electron microscopy. Our results suggest that the superconducting transport in these films is likely to be two dimensional in nature, consistent with the short coherence length along the c axis of the crystals.

Fabrication of thin‐film LiMn2O4 cathodes for rechargeable microbatteries

We have fabricated thin‐film cathodes of lithiated manganese oxide (LixMn2O4, where x≤1), which can reversibly intercalate nearly one Li atom per unit formula, at a steady potential of 4.1 V. This results in a theoretical energy density of about 600 Wh/kg. The polycrystalline films (thickness of 0.5–2 μm) show electrochemical performance exceeding that of bulk materials. This is attributed to the small grain size and the porosity within these LixMn2O4 three‐dimensional spinel films, reducing the diffusion path length for Li+ ions. The films are tested as the cathodes in about 70 charging/discharging cycles, in a secondary lithium cell, at current rates of 10–100 μA cm−2. No measurable self‐discharge over 12 h periods is observed. These thin‐film cathodes with above electrochemical properties are very promising for a new generation thin‐film secondary batteries.

Epitaxial growth of ferroelectric bismuth titanate thin films by pulsed laser deposition

Epitaxial thin films of ferroelectric bismuth titanate Bi4Ti3O12 have been grown by pulsed laser deposition on single‐crystal [100] SrTiO3 substrates. Bismuth titanate has a high Curie temperature (675 °C) and saturation polarization values of 4 and 50 μC/cm2 along the c and b axis, respectively. Its a,b lattice parameters allow thin‐film growth on substrates such as SrTiO3, LaAlO3, MgO, etc. These single crystalline films exhibit good quality as evidenced by x‐ray diffraction, Rutherford backscattering, and transmission electron microscopy. Applications for these films include memory devices and optical displays.

High critical currents in epitaxial YBa2Cu3O7−x thin films on silicon with buffer layers

As‐deposited superconducting thin films (∼0.1 μm) of YBa2Cu3O7−x have been prepared by pulsed laser deposition on (100) Si with buffer layers of BaTiO3/MgAl2O4. X‐ray diffraction studies reveal that the films grow epitaxially with the c axis preferentially oriented normal to the substrate surface. This is confirmed by ion channeling measurements along the (100) (normal to the surface) and (110) directions of the Si substrate showing a minimum yield of 54% along the (100), and 78% along the (110) axes using 2.8 MeV He++. Preliminary transmission electron microscopy study also supports these results. The as‐deposited films have zero resistance temperatures of 86–87 K, and critical current densities of 6×104 A/cm2 at 77 K and 1.2×105 A/cm2 at 73 K. Our results indicate that the superconducting properties of the films are limited primarily by the quality and degree of epitaxal growth of the buffer layers on the silicon substrate.

Metalorganic chemical vapor deposition of BaTiO3 thin films

We report the results of a recent study on the {ital in} {ital situ} deposition of single phase BaTiO{sub 3} thin films by using the metalorganic chemical vapor deposition technique. The effects of growth parameters on the characteristics of the thin films were investigated in the substrate temperature range of 550--800 {degree}C. Barium {beta}-diketonate and titanium isopropoxide were used as the metal sources. Growth rates ranging from submicron to several microns per hour have been deposited on various substrates in an inverted vertical warm-wall reactor. X-ray diffraction data and Rutherford backscattering spectra provided the evidence for single BaTiO{sub 3} phase. Scanning electron microscopy was used to study the surface and the cross-sectional morphology of the films. Dielectric constant of the as-deposited film was around 250 at room temperature. The resistivity of the films were greater than 10{sup 9} {Omega} cm.

Epitaxial yttria‐stabilized zirconia on (11̄02)sapphire for YBa2Cu3O7−δ thin films

Epitaxial yttria-stabilized zirconia (YSZ) films were deposited on (1{bar 1}02) sapphire by pulsed laser deposition. The films are formed in a cubic phase with the {ital a} axis normal to the substrate surface. Ion beam (2.8 MeV He{sup ++}) channeling measurements show that the YSZ films are highly crystalline with a channeling minimum yield of 8%. The epitaxial relationship between the film and substrate is further confirmed by a cross-section transmission electron microscopy study. Epitaxial YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} thin films deposited on YSZ/sapphire have {ital T}{sub {ital c}} and {ital J}{sub {ital c}} of up to 89 K and 1{times}10{sup 6} A/cm{sup 2} at 77 K, respectively.

Study of epitaxial platinum thin films grown by metalorganic chemical vapor deposition

The metalorganic chemical vapor deposition technique has been successfully applied to the growth of epitaxial platinum thin films on (100) surfaces of single‐crystal potassium tantalate (KTaO3) and strontium titanate (SrTiO3). Platinum thin films grown on KTaO3 (100) at a rate of 70 nm/h showed strong Rutherford backscattering spectroscopy (RBS)/channeling effects with a χmin of 4%. In‐plane φ and θ‐2θ scan x‐ray diffraction analysis demonstrated the three‐dimensional epitaxial alignment of the platinum film with the KTaO3 substrate. Transmission electron micrographs viewed in cross section provided additional information regarding the nature of the epitaxial Pt‐KTaO3 interface. The room‐temperature resistivity of a 60‐nm‐thick Pt film on KTaO3 (100) was 12.0 μΩ cm. X‐ray diffraction and pole‐figure analysis showed that in the case of Pt films deposited on either fused quartz or Si (100) surfaces, the resulting films were polycrystalline and were fully textured with <111≳ orientations perpendicular to the...

Surface layers on superconducting Y-Ba-Cu-O films studied with x-ray photoelectron spectroscopy

The chemical compositions of surface and bulk regions of pulsed laser deposited YBa2Cu3O7−x films on SrTiO3(001) substrates were examined. From the relative intensities of the surface and bulk components of the Ba 3d and Ba 4d spectra taken at different take‐off angles and different escape depths [using Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) excitations], we have concluded that there is a nonsuperconducting surface layer of 1 nm thickness with a composition of BaCuO2. This layer contains oxygen with a photoelectron binding energy of 531 eV, and Cu2+ ions. A superconducting film which was only 8 nm thick also had a layer of 1 nm thickness. By detecting the substrate Ti signal through this film, and ruling out a high density of pinholes, we provide evidence that the x‐ray photoelectron spectroscopy data contain information from the superconducting phase. A polycrystalline pellet scraped in vacuum had a similar surface layer, which was only 0.4 nm thick.

Epitaxial lead zirconate‐titanate thin films on sapphire

Pb(Zr1−xTix)O3 thin films covering the whole compositional range x=0 to x=1 have been grown for the first time on the sapphire (1102) plane using the metalorganic chemical vapor deposition technique. The films are three‐dimensionally epitaxial and exhibit single crystal properties. The structural transitions, examined by x‐ray diffraction, are shifted slightly from the bulk single crystal values. Ferroelectric P‐E hysteresis curves and the dielectric constant of the films were investigated using interdigitated electrodes fabricated by photolithography.

Epitaxial relations between in situ superconducting YBa2Cu3O7−x thin films and BaTiO3/MgAl2O4/Si substrates

In situ superconducting YBa2Cu3O7−x films with Tc0 up to 87 K and Jc, 77 K up to 6×104 A/cm2 were prepared on Si substrates with MgAl2O4 and BaTiO3 double‐buffer layers. The epitaxial relations between various layers were established by transmission electron microscopy. The MgAl2O4 layer is heavily faulted. The subsequent BaTiO3 layer stops most of the faults, provides a template for the YBa2Cu3O7−x growth, and partially screens off the stress due to different thermal expansion coefficients. The microstructure of the YBa2Cu3O7−x layer is very similar to that of the films deposited directly on SrTiO3, exhibiting a homogeneous heavily faulted single‐crystal‐like structure free from secondary phases and grain boundaries. The slight degradation of the transport properties is attributed to residual thermal stress.