T. Venkatesan

Application of a near coincidence site lattice theory to the orientations of YBa2Cu3O7-x grains on (001) MgO substrates

Various orientations of YBa2Cu3O7−x grains in polycrystalline films prepared on (001)MgO substrates by in situ laser deposition were determined using electron diffraction. Eight different types of in‐plane orientations have been observed. These orientations agree well with the prediction of a simplified theory of near coincidence site lattice between YBa2Cu3O7−x and MgO. The YBa2Cu3O7−x grains were found to have a high probability of forming low angle or low Σ boundaries among themselves. These grain boundaries are of low energy and should exhibit a high connectivity of Cu‐O‐Cu chains. Therefore, YBa2Cu3O7−x thin films on MgO can attain a Tc of ∼90 K and a Jc of ∼106 A/cm2 at 77 K.

Structural perfection of Y‐Ba‐Cu‐O thin films controlled by the growth mechanism

For crystalline advanced materials, such as the high transition temperature oxide superconductors, the growth of defect‐free crystals is often the most sought after desideratum because it opens the doors to fundamental studies and the development of practical applications. We report the observation of YBa2Cu3O7−x(123) thin films having unprecedented structural perfection, at temperatures near 700u2009°C on [001]u2009LaAlO3. The film’s c axis is in the surface plane, unlike films grown at higher temperatures. This orientation has important advantages for device applications and fundamental studies. The Tc,0 is only 70 K, presumably due to oxygen deficiency caused by thermal stresses; if so, it should be possible to raise the Tc,0 by relieving these stresses.

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]u2009SrTiO3 substrates. Bismuth titanate has a high Curie temperature (675u2009°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.

The atomic structure of growth interfaces in Y-Ba-Cu-O thin films

We have examined the atomic structure of growth interfaces in thin films of Y--Ba--Cu--O grown on (001) perovskite or cubic substrates. At substrate heater temperatures in the range of 780--820 {degree}C c-axis oriented growth is observed on these substrates. On SrTiO{sub 3}, the first layer appears to be either a BaO or a CuO{sub 2} plane while on LaAlO{sub 3} the first layer appears to be a CuO chain layer. The mismatch on the a-b plane is accommodated by the formation of interface dislocations. Defects on the substrate surface propagate as defects in the film. These defects are primarily translational boundaries and in some cases second phases. At lower substrate heater temperatures, i.e., 650--700 {degree}C, a,b-axis growth dominates. Defects and steps on the substrate surface are more detrimental in the growth of a,b-axis oriented films, since they tend to favor the nucleation of c-axis oriented domains. This is ascribed to the ledge mechanism of c-axis film growth, for which the surface steps are good nucleation sites.

Preparation of thin film high temperature superconductors

Fundamental issues in preparing high-quality high-T/sub c/ YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films are addressed. The techniques of inverted cylindrical magneton sputtering and pulsed laser deposition are chosen as successful examples to illustrate how the key problems can be solved. The fabrication of YBa/sub 2/Cu/sub 3/O/sub 7-x//PrBa/sub 2/Cu/sub 3/O/sub 7-x/ superlattices where superconductivity in a single unit cell layer of YBa/sub 2/Cu/sub 3/O/sub 7-x/ was observed demonstrates the state of the art of thin-film deposition of high-T/sub c/ materials. Systematic variations of the deposition parameters result in changes of superconducting and structural properties of the films that correlate with their microwave and infrared characteristics.

Performance of a narrow band microwave filter implemented in thin-film YBa2Cu3O7-δ with ohmic contacts

Using in situ deposited thin‐film Y1Ba2Cu3O7−δ on LaAlO3 substrates, we have developed a pseudo‐interdigital three‐pole stripline filter with a minimum passband loss under 3.5 dB at 9.5 GHz and 77 K. The passband is very narrow, having a bandwidth of only 50 MHz (0.5%). The filter occupies a volume of less than 0.1 cm3. An equivalent copper filter produced a loss of 18.5 and 28 dB at 77 and 300 K, respectively. The input and output coupling is achieved by ex situ sputtered Au thin‐film contacts defined on the Y1Ba2Cu3O7−δ by a lift‐off technology and ultrasonically bonded to hermetic K connectors. Some of the major concerns in the realization of narrow band filters such as air gaps between high dielectric constant substrates and uniformity of the substrate’s dielectric constant are addressed.

Microstructural evolution of laser deposited superconducting Y-Ba-Cu-O films: Effect of deposition rate

Abstract The microstructure of laser deposited, superconducting Y-Ba-Cu-O thin films grown on [001] SrTiO 3 substrates has been characterized as a function of laser pulse rate. Over a range of deposition rates from 1–50 A/s, the films consist mainly of the c -axis oriented epitaxial, YBa 2 Cu 3 O 7− x phase with a high density of polytypoidic stacking defects. High transport critical current densities, J c , are attributed to the retention of the primarily c -axis oriented microstructure along with the distribution of stacking defects that provide good flux pinning. At higher deposition rates we observe new growth directions, specifically the a,b -axis and the [103] growth directions are favored. We conclude that within the deposition rate range of 1–150 A/s, there is no significant fundamental change in microstructure except for outgrowth formation. Therefore, we are optimistic that much higher rates can be attained in the near future, although controlling the outgrowth formation may be difficult.

Polytypoidic superlattices in Y-Ba-Cu-O thin films

Abstract We report, for the first time, the observation of natural, polytypoidic superlattice structures in pulsed laser deposited superconducting Y-Ba-Cu-O thin films. These structures correspond to ordered stacking sequences of the “123”, “124” and “224” cationic ratios. Such structurally coherent, natural superlattice structures, either in the stable or metastable form, may be an attractive alternative for the artificial superlattices suggested for novel electronic applications. The results suggest the existence of a family of polytypoidic phases with different cationic compositions in the Y-Ba-Cu-O phase diagram.

Polytypoidic structures in high temperature oxide superconductors

Abstract High resolution transmission electron microscopy has been used to reveal the cationic structure of the high temperature oxide superconductors. Of the many new systems discovered recently, the Biue5f8Srue5f8Caue5f8Cuue5f8O, Tlue5f8Baue5f8Caue5f8Cuue5f8O have been extensively studied. Compositional variations are accommodated by the formation of polytypoidic stacking units whose cationic stoichiometries are in specific fixed ratios. Examples from the Bi cuprate bulk superconductors and Y-Ba-Cu-O thin films are presented. Identification of the different polytypoids, each of which has a specific transition temperature, T c , can shed light on the electrical properties of the mixed bulk or thin films. Polytypism and polytypoidism also provide a unified approach to the structural and chemical analysis of these oxide superconductors.

Recovery of original superconducting properties in ion‐irradiated Y1Ba2Cu3O7−x thin films

The changes in the superconducting properties of in situ pulsed laser deposited Y1Ba2Cu3O7−x thin films caused by irradiation with 200 keV He+ ions are due to both oxygen loss as well as oxygen and cationic displacements induced by the irradiation. This is demonstrated by a study of the recovery of these defects by plasma oxidation and relatively low temperature (∼600u2009°C) annealing in oxygen. Plasma oxidation of films irradiated to low fluences enables the replacement of oxygen atoms in the lattice, leading to a substantial recovery of Tc0, Jc, and normal state resistivity. Irradiation‐induced oxygen and cationic displacements and other microscopic defects can be further annealed out at relatively low temperatures leading to an almost full recovery of Tc0, Jc, and normal state resistivity. A transmission electron microscope study of irradiated films shows evidence that they are structurally disordered.