B. Kabius

Electron microscopy image enhanced

One of the biggest obstacles in improving the resolution of the electron microscope has always been the blurring of the image caused by lens aberrations. Here we report a solution to this problem for a medium-voltage electron microscope which gives a stunning enhancement of image quality.

Microstructure of epitaxial YBa2Cu3O7 films on step-edge SrTiO3 substrates

Abstract The microstructure of YBa 2 Cu 3 O 7 films epitaxially grown on step-edge (100) SrTiO 3 substrates has been characterized by means of high resolution transmission electron microscopy. The results indicate a relationship between the microstructure of the film acreoss a step and the angle step makes with the substrate plane. On a steep, high-angle step, the film grows with its c -axis perpendicular to that of the film on the substrate surface so that two grain boundaries are formed. On a low-angle step, the film grows without any change in c -axis orientation across the step and without grain boundaries. Epitaxial second phases intergrowths across the steps have been found in some cases which may act as barrier layers when they cut through th YBa 2 Cu 3 O 7 film.

The microstructure of epitaxial YBa2Cu3O7 films on steep steps in LaAlO3 substrates

Abstract The microstructure of YBa 2 Cu 3 O 7 grown on steep steps in (001 pseudocubic LaAlO 3 substrates was studied by high-resolution electron microscopy of cross-sectional and plan-view samples. Steps with angles of about 80° were obtained by ion milling. On the substrate plane, the films grew with the c -axis parallel to [001] while on the flank of a step the c -axis was parallel to the [100] direction of the substrate. As a result, two [010] tilt axis grain boundaries were formed at which the YBa 2 Cu 3 O 7 lattice changed orientation by approximately 90°. In the upper grain boundary, a [100] tilt axis and, on the average, a (013) habit plane alternated with a [010] tilt axis and a (1 0 3) habit plane. This alternating structure was caused by twinning in the orthorhombic film structure. The lower grain boundaries were found to be rather irregular and consisted of a chain of (0 1 3)(0 1 3) and (0 1 0)(0 0 1) type segments exhibiting a tendency to tilt the whole habit plane toward the a-b plane of the flank film. Dislocations, stacking faults and misfit strains were also observed in or close to the boundaries. Grain boundary modeling indicated a good agreement with the experimental image and permitted us to determine the atomic plane of a boundary.

Electrical properties of 45° grain boundaries of epitaxial YBaCuO, dominated by crystalline microstructure and d-wave-symmetry

Abstract We show that the microstructure of 45° grain boundaries in epitaxial YBaCuO together with the d x 2 − y 2 symmetry of the superconducting wave function accounts for the measured magnetic field modulation patterns of the critical currents. Our results resolve the conflict between contradictory observations of other authors and unfold a consistent picture of d-wave-symmetry of the YBaCuO superconducting ground state. Evidence for currentless junctions with a phase difference of π is given.

Grain-boundary structure of thin films of YBa2Cu3O7 and Bi2Sr2CaCu2O8 on bicrystalline substrates

Abstract Thin films of YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8 were deposited by high-pressure oxygen sputtering and laser ablation, respectively, on bicrystalline substrates of SrTiO 3 with angles of 10,24,36 and 45 degrees. The microstructure of the grain boundary in the substrate and in the film was characterized by transmission electron microscopy. While the grain boundary in the substrate was found to be almost rectilinear with a maximum roughness of about 3 nm, the grain boundaries in the superconducting films exhibited a roughness of up to 200 nm for YBa 2 Cu 3 O 7 and up to 1 μm for Bi 2 Sr 2 CaCu 2 O 8 . The grain boundaries consist of facets, which have a low index habit plane in common with one of the adjacent grains. These facets can be found on a length scale extending from several nanometers up to one micrometer. It is concluded that the exact lateral position of the film grain boundary is not only determined by the substrate but also by the coalescence of growth islands during the film-deposition process. The microstructural results are discussed with respect to their impact on the electrical properties.

TEM investigation of grain boundaries in YBa2Cu3O7 thin films grown on SrTiO3 bicrystal substrates

Abstract Thin films of YBa 2 Cu 3 O 7 were deposited by high-oxygen-pressure dc-sputtering on [001]-tilt bicrystals of SrTiO 3 with rotation angles of 10°, 24°, 36° and 45°. The substrates and the films were investigated by transmission electron microscopy. The lateral roughness of the substrate grain boundaries was found to be about 3 nm, while the film grain boundaries exhibited a roughness of 20 nm to 200 nm. The film boundaries were dominantly asymmetrical and faceted so that the grain boundary plane corresponds to a low-index plane of one of the two adjacent grains. The majority of these low-index planes were (100) or (110) planes. Symmetrical segments were only observed for the 24° grain boundary. For the 45° boundary only (100)/(110) facets were found. The atomic structure of a (100)/(110) facet was investigated in detail by comparing the experimental high-resolution electron micrographs with calculated images. The atomic order at the grain boundary was found to be perturbed within one or two interatomic spacings from the interface.

A study of the microstructure of epitaxial heterostructures of YBa2Cu3O7 and PrBa2Cu3O7

Abstract Multilayers with a sequence of YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 /YBa 2 Cu 3 O 7 have been sputtered epitaxially on [001] SrTiO 3 substrates. The heterostructures of these samples have been characterized by high-resolution electron microscopy, Rutherford backscattering and energy-dispersive X-ray analysis. The results indicate a correlation between the substrate quality and the microstructure of the thin films. Substrates with low defect density, a very smooth surface, and good orientation favour the epitaxial growth of all three layers. The chemical composition across the interface changes within the dimension of one unit cell.

The influence of 173 MeV Xe-ion irradiation on the microstructure of YBa2Cu3O7 thin films

Abstract YBa 2 Cu 3 O 7 thin films grown epitaxially on [001] SrTiO 3 substrates have been irradiated at a temperature of 77 K with 173 MeV Xe-ions up to a dose of 3×10 16 m −2 . A study of the irradiated films by transmission electron microscopy revealed amorphous channels with diameters between 2 and 4 nm. The density of these channels depended on the irradiated dose. The twin structure of the orthorhombic phase of YBa 2 Cu 3 O 7 was found to disappear for high irradiation doses. It is concluded that the amorphous regions were introduced along the paths of Xe-ions as a results of electronic excitation. These regions can act as pinning centres for magnetic flux lines.

Investigation of YBCO step-edge Josephson junctions

The authors report on the superconducting transport properties of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) Josephson junctions fabricated by pulsed laser deposition on steep steps in epitaxial substrates. The steps were prepared by Ar-ion milling. The YBCO thin films were patterned either by Ar-ion milling or an inhibit process. The current-voltage characteristics of step-edge junctions (SEJs) fit approximately the resistively shunted-junction (Stewart-McCumber) model up to temperatures of 85 K. The temperature dependence of the characteristic voltage can be explained by a superconductor-normal-superconductor (SNS)type model. Simulations of Shapiro steps were performed. The simulations, the form of the I(V) curves, the I/sub c/(H) curves. and the DC superconducting quantum interference device (SQUID) quantization properties all suggest that a SEJ consists of two weak links in series formed by the grain boundaries at the lower and upper edges of the steps. This conclusion is in good agreement with HREM results, which show that the upper and lower grain boundaries at the step edge are different. Each of these weak links is a parallel array of Josephson junctions with different current densities.<<ETX>>

A study of antiphase boundaries and 223 planar faults in epitaxial YBa2Cu3O7 films by high resolution electron microscopy

Abstract The atomic structure of antiphase boundaries in YBa2Cu3O7 thin films has been characterized by means of high resolution transmission electron microscopy. Two kinds of antiphase boundaries were found, conservative and nonconservative. The nonconservative antiphase boundaries can be related to either intrinsic or extrinsic faults. The antiphase boundaries usually end at planar defects in the a-b plane. In addition, “223” planar faults have been observed. This type of defect includes both an extra Y and an extra O layer inserted parallel to the a-b plane at the z= 1 2 position of the YBa2Cu3O7 structure.