Brian H. Moeckly

Growth of YBa2Cu3O7 thin films on MgO: the effect of substrate preparation

We discuss the results of a study of the effect of substrate preparation on the microstructure and superconductive properties of YBa2Cu3O7 thin films formed by laser ablation on (001) MgO substrates. Thermal annealing of the substrates is found to be highly effective in producing at fairly low growth temperatures (670 °C), epitaxial, c‐axis normal films with good superconductive properties. Alternative surface treatments result in the formation of large angle tilt boundaries and inferior superconductive properties.

Scaling behavior of YBa2Cu3O7−δ thin‐film weak links

The superconductive weak link properties of microbridges formed in c‐axis normal YBa2Cu3O7−δ polycrystalline thin films containing a variable amount of large angle tilt boundaries have been studied. In the low critical current density limit these weak links have current‐voltage (I‐V) characteristics that are accurately modeled by the resistively shunted junction model. The I‐V’s are found to accurately follow a simple scaling law with the product of the critical current and weak link resistance Rn varying linearly with the weak link conductance.

Transport properties of high‐angle grain boundary weak links in YBa2Cu3O7 thin films

By patterning ∼1‐μm‐wide microbridges in laser ablated YBa2Cu3O7 films containing high‐angle tilt boundaries, weak links have been isolated with critical currents low enough to avoid self‐screening effects. The current‐voltage characteristics of these high‐angle tilt‐boundary weak links are well described by the resistively shunted junction model, if noise rounding is included. The response of the supercurrent to magnetic field and temperature indicates that the weak links are spatially nonuniform, consisting of relatively small areas of high critical current density, Jc, separated by areas with very low or zero Jc. The response to rf power suggests that the current‐phase relation is nonsinusoidal.

Clean grain boundaries and weak links in high Tc superconducting YBa2Cu3O7−x thin films

It is demonstrated that polycrystalline thin films of high Tc superconducting YBa2Cu3O7−x can be grown with clean grain boundaries, i.e., without a boundary layer of segregation or different phase. In clean stoichiometric samples, angular misorientations of grains may be the origin of weak link behavior. High‐resolution scanning transmission electron microscope images of films grown on ZrO2 and MgO by reactive evaporation, reactive sputtering, and laser ablation show atomic lattice images of clean grain boundaries. X‐ray microanalysis with a 10 A spatial resolution also indicates no composition deviation at the grain boundaries. Grain sizes and epitaxial relations of samples prepared by different methods are characterized.

Strontium titanate thin films for tunable YBa/sub 2/Cu/sub 3/O/sub 7/ microwave filters

Tunable ferroelectric thin films such as SrTiO/sub 3/ (STO) are promising for a variety of rf applications requiring tunability. We describe the growth and characterization of STO thin films including their dielectric properties, and summarize attempts at reducing the loss tangent of these films. We also discuss their integration with high-temperature superconductor (HTS) microwave filters for trimming purposes.

Microwave properties of MgB/sub 2/ thin films grown by reactive evaporation

We have grown MgB/sub 2/ films using the deposition technique of reactive evaporation. This method allows high-quality, large-area, double-sided films to be grown on a large variety of substrate materials. These films are therefore well suited for applications, several of which may exploit the low-loss high-frequency properties of MgB/sub 2/, including passive microwave resonators and filters, nuclear magnetic resonance coils, and RF cavities for particle accelerators. Determining the surface resistance and nonlinear properties of our films can thus give us an idea of their suitability for these applications, as well as tell us something about their fundamental superconducting nature. We have begun measuring the microwave properties of our films using both a parallel plate technique and by patterning the films into stripline resonators and lumped-element resonators and filters. The films display very low surface resistance values.

Growth mechanism of YBa2Cu3O7−δ thin films on vicinal MgO

Abstract YBa2Cu3O7-δ thin films have been grown on vicinal MgO substrates by pulsed-laser ablation. The substrate topography has been shown to affect the microstructure of the film. The step edge act as preferential sites for island nucleation, controlling the in-plane alignment of the islands. The resulting microstructure consists of c-axis oriented grains which have a large aspect ratio. The formation of a well-defined substrate surface enables the growth of highly oriented YBa2Cu3O7-δ films, on a substrate where there is a large lattice mismatch. The films have been characterized by both X-ray diffraction techniques and transmission electron microscopy.

Growth and properties of YBa2Cu3O7 thin films on vicinal and polycrystalline MgO substrates

We discuss the results of a study on the growth by laser ablation of YBa2Cu3O7 thin films on polycrystalline and annealed vicinal (001) MgO substrates. In both instances the films were found to grow predominantly with the c axis normal to the plane of the substrate, regardless of the orientation of the MgO surface. In the case of the vicinal substrates the films were found to have superconducting properties comparable to those obtained with films grown on (001) oriented, annealed single‐crystal substrates.

Micro‐Raman spectroscopy of electromigration‐induced oxygen vacancy aggregation in YBa2Cu3O7−δ

We describe the results of micro‐Raman spectroscopy and optical microscopy studies of basal‐plane chain‐oxygen vacancy motion in YBa2Cu3O7−δ thin films under the influence of a high current bias near 300 K. Above a threshold level this bias causes vacancy aggregation and then long‐range displacement, leading to an enhancement of oxygen order in the region of highest current density and the complex accumulation of oxygen vacancies in the region where the electromigration force is near the threshold level.

Growth and properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on non-lattice-matched and polycrystalline substrates

The authors have investigated the nature of the in-situ growth of c-axis normal YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on non-lattice-matched, vicinal, and polycrystalline MgO substrates and on buffer layers of MgO. It is found that the preparation of the MgO surface determines the structural and transport properties of the films. In particular, the authors were able to reproducibly grow films exhibiting either weak link behavior or very high critical current densities. The variation of the thin-film microstructure, oxygen content, and superconductive properties with the changes in the thin-film crystal structure that result from the different growth situations is discussed. The structural and transport measurements of YBCO films grown on various MgO surfaces clearly indicate the importance of obtaining a nucleation stage which will result in high-quality YBCO on non-lattice-matched substrates. Such considerations are of prime importance when the low-temperature growth of the cuprate superconductors is required in order to obtain a morphology which facilitates the use of lithographic processing and the growth of multilayer structures.