Debra L. Kaiser

Investigation of the structure of barium titanate thin films by Raman spectroscopy

Raman spectroscopy was used to examine the structure of barium titanium oxide thin films grown by metal‐organic chemical vapor deposition (MOCVD) and laser‐assisted deposition. The spectra were compared with the spectra of a ceramic specimen and a single crystal. Raman peaks specific to the tetragonal ferroelectric phase of BaTiO3 were seen in the spectra of several films. Other Raman peaks were ascribed to impurity (non‐BaTiO3) phases in the films or to the substrates (fused quartz, MgO). Some of the Raman peaks showed a strong polarization dependence. The MOCVD films were also characterized by x‐ray diffraction, energy‐dispersive x‐ray spectroscopy, and transmission electron microscopy. The film‐to‐film variation of the strength of BaTiO3 features in the Raman spectrum, relative to impurity‐phase features, was qualitatively consistent with the x‐ray diffraction and electron microscopy results. Spatially resolved Raman measurements showed that the structure of the laser‐deposited film varies significantl...

Thermomechanical detwinning of superconducting YBa sub 2 Cu sub 3 O sub 7 minus x single crystals

A method for the complete removal of twins from single crystals of superconducting YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} is described. The process depends on ferroelastic behavior found to exist in the phase, and should be generally applicable to the layered perovskite-type phases containing accommodation twins resulting from a tetragonal-to-orthorhombic transformation on cooling. The twin-free, superconducting single crystals will enable investigation of {ital a}-{ital b} anisotropy of properties as well as crystal structure determination without complication by the presence of microtwins.

Epitaxial growth of BaTiO3 thin films at 600 °C by metalorganic chemical vapor deposition

BaTiO3 thin films were grown epitaxially on (100) MgO substrates by metalorganic chemical vapor deposition (MOCVD) at a temperature of 600 °C. This substrate temperature is the lowest reported temperature for the growth of epitaxial BaTiO3 films by an MOCVD process. The films had a cube–cube orientation relationship with the substrate and were oriented with an a‐axis perpendicular to the substrate plane. Nanoscale energy dispersive x‐ray spectrometry measurements showed no evidence of interdiffusion between the film and substrate.

Fracture toughness of YBa2Cu3O6+δ single crystals : anisotropy and twinning effects

Indentation/crack length measurements were used to determine hardness and fracture toughness in twinned and detwinned single crystals of YBa2Cu3O6+δ (YBCO). Hardness was found to be isotropic and unaffected by twin boundaries or moisture; the average hardness value was 9.5 GPa. Fracture toughness (Kc) was determined to be anisotropic with the average value for cracks running perpendicular to the basal plane being approximately twice that of cracks parallel to the basal plane; thus, cleavage is easier along basal planes. In addition, the results suggest a small but measurable anisotropy in Kc between the a and b directions. Twin boundaries were found to toughen YBCO, enhancing Kc by about 25%. Moisture promoted crack growth, degrading Kc by about 35%.

A TEM-EELS study of hole concentrations near strongly and weakly coupled grain boundaries in electromagnetically characterized YBa2Cu3O7−δ bicrystals

Abstract High-spatial-resolution electron energy loss spectroscopy in a transmission electron microscope (TEM-EELS) was used to obtain hole-concentration profiles near grain boundaries in flux-grown YBa 2 Cu 3 O 7−δ bicrystals with misorientation relationships of 7°, 12°, 28°, and 31° about their common [001] rotation axes. A key aspect of the work is that the electromagnetic properties and coupling character of each of the bicrystals also were determined explicitly. The 28° and 31° bicrystals showed Josephson-junction-like weak coupling, and evidence for severe hole depletion at the boundary was present in the oxygen K near-edge fine structure (ELNES) of the EELS spectra obtained from them. The hole-deficient zone extended 20 to 40 nm into each crystal. In striking contrast, the 7° and 12° bicrystals were strongly coupled and dramatically less hole depleted at the grain boundary. These results show that the boundary region within which the electronic structure differs from that of the bulk is substantially wider than both the YBa 2 Cu 3 O 7−δ coherence length and the grain-boundary structural and compositional width suggested by high-resolution TEM images and cation non-stoichiometry studies. They also suggest a continuous transition from strongly to weakly coupled character.

Direct observation of anisotropic flux penetration in twinned YBa2Cu3O7−x single and polycrystals

Abstract We report microscopic maps of magnetic induction in YBa 2 Cu 3 O 7− x crystals which directly show a correlation of flux penetration with twin boundary orientation and twin density. These maps were obtained by means of a recently-improved magneto-optical imaging technique. Pinning was lowest in untwinned regions and increased with increasing twin density. Even at low temperatures (6 K), twin boundaries led to a large anisotropy in flux pinning. This anisotropy, defined as the ratio of the magnetic induction gradient across twin boundaries to that along twin boundaries, was 10 at 17 K; this ratio increased with increasing temperature. In polycrystals, twin boundaries also had a strongly anisotropic effect on flux flow into a grain from a grain boundary.

The nature of [001] tilt grain boundaries in YBa2Cu3O7−x

Fifty-two faceted grain boundary segments ([001] tilt boundaries) in clusters of bulk-scale YBa 2 Cu 3 O 7− x crystals having coincident c-axes were characterized by optical microscopy techniques. Grain boundary orientations were widely distributed. All grain boundaries, except those far from the symmetric condition and those with {110} facets, exhibited well-developed matching of the twin domains across the boundary. It is suggested that this newly reported phenomenon of twin pattern matching occurs due to a local coordination of the tetragonal → orthorhombic transformation strain across grain boundaries and may be beneficial with regard to electrical transport in highly textured polycrystalline material.

Low‐field flux pinning in twinned and detwinned single crystals of YBa2Cu3O7−x

Flux trapping in single crystals of YBa2Cu3O7−x before and after twin removal has been measured for vortices parallel and perpendicular to the c axis. The results show clearly that, for low applied magnetic fields, twins contribute significantly to flux pinning over the temperature range 60 to 85 K when the vortices are parallel to the twin boundaries (i.e., parallel to the c axis).

Anomalous second harmonic generation in BaTiO3 thin films

We present data on second harmonic generation in as‐grown BaTiO3 thin films, including the dependence of the second harmonic intensity on the incidence angle and on the polarization direction of the fundamental beam. The data indicate that the effective second harmonic generation coefficient in these films is unrelated to the crystallographic orientation of the BaTiO3 as determined by x‐ray diffraction. We suggest as an alternative explanation electric‐field‐induced second harmonic generation.

Growth and characterization of barium titanate thin films prepared by metalorganic chemical vapor deposition

Abstract Process conditions were established for the deposition of polycrystalline BaTiO 3 thin films on fused quartz substrates in a research-scale metalorganic chemical vapor deposition (MOCVD) system. Resulting films were studied by X-ray powder diffraction (XRD), Raman spectroscopy, energy dipersive X-ray spectroscopy, transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS) techniques. Films rich in Ba (molar Ba/Ti > 1) contained the second phase Ba 2 TiO 4 whereas films rich in Ti were composed mainly of an amorphous phase. Cross-sectional TEM and SIMS depth profile studies of slightly Ba-rich films (that appeared to be nearly pure BaTiO 3 by XRD analysis) revealed that the films were actually composed of several layers. The multilayer structure consisted of a thin amorphous interdiffusion layer, an orthorhombic Ba 2 TiO 4 layer, and a polycrystalline BaTiO 3 film containing some amorphous material. Raman spectroscopy confirmed the presence of tetragonal BaTiO 3 in the films.