Current research into semiconductor clusters is focused on the properties of quantum dots-fragments of semiconductor consisting of hundreds to many thousands of atoms-with the bulk bonding geometry and with surface states eliminated by enclosure in a material that has a larger band gap. Quantum dots exhibit strongly size-dependent optical and electrical properties. The ability to join the dots into complex assemblies creates many opportunities for scientific discovery.

Semiconductor Clusters, Nanocrystals, and Quantum Dots

There are numerous potential applications for superconducting tapes based on YBa(2)Cu(3)O(7-x) (YBCO) films coated onto metallic substrates. A long-established goal of more than 15 years has been to understand the magnetic-flux pinning mechanisms that allow films to maintain high current densities out to high magnetic fields. In fact, films carry one to two orders of magnitude higher current densities than any other form of the material. For this reason, the idea of further improving pinning has received little attention. Now that commercialization of YBCO-tape conductors is much closer, an important goal for both better performance and lower fabrication costs is to achieve enhanced pinning in a practical way. In this work, we demonstrate a simple and industrially scaleable route that yields a 1.5-5-fold improvement in the in-magnetic-field current densities of conductors that are already of high quality.

Strongly enhanced current densities in superconducting coated conductors of YBa2Cu3O7-x + BaZrO3

The growth and characterization of functional oxide thin films that are ferroelectric, magnetic, or both at the same time are reviewed. The evolution of synthesis techniques and how advances in in situ characterization have enabled significant acceleration in improvements to these materials are described. Methods for enhancing the properties of functional materials or creating entirely new functionality at interfaces are covered, including strain engineering and layering control at the atomic-layer level. Emerging applications of these functional oxides such as achieving electrical control of ferromagnetism and the future of these complex functional oxides is discussed.

/pdf/advances-in-the-growth-and-characterization-of-magnetic-1r6yol3qt6.pdf

Advances in the growth and characterization of magnetic, ferroelectric, and multiferroic oxide thin films

The surface microstructures of c-axis-oriented films of YBa(2)Cu(3)O(7), deposited by off-axis magnetron sputtering on MgO and SrTiO(3) single crystal (100) substrates, have been investigated with scanning tunneling microscopy and atomic force microscopy. There is strong evidence that the films nucleate as islands and grow by adding material to the edge of a spirally rising step. This results in columnar grains, each of which contains a screw dislocation at its center. This microstructure may be of significance in determining superconducting properties such as critical current, and represents a significant difference between thin films (especially those grown in situ) and bulk materials.

http://science.sciencemag.org/content/sci/251/5001/1587.full.pdf

Growth Mechanism of Sputtered Films of YBa2Cu3O7 Studied by Scanning Tunneling Microscopy

The growth and properties of electronic oxide thin films are reviewed. In particular, the synthesis and properties of superconducting, insulating, conducting, magnetic, and semiconducting epitaxial oxide structures are discussed. Crystal structures and functional properties common to many oxide materials are briefly reviewed. A description of film-growth techniques follows. Finally, an extensive overview of the epitaxial growth for specific oxide material systems is given. This includes the epitaxial growth of electronic oxide thin films on oxide and non-oxide substrates.

Synthesis and properties of epitaxial electronic oxide thin-film materials

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 700 °C on [001] LaAlO3. 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.

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

Structural defects in thin films of nominal composition YBa2Cu3O7 (123) grown on single crystal MgO have been characterized. The main types of stacking defects correspond to the cationic stoichiometries of “248”, “247”, and “224”. Several types of edge dislocations have been observed. Due to the frequent changes in the stacking sequence, antiphase boundaries are created to accommodate the misfit across regions in which the repeat sequence is not identical. The films exhibit a mosaic microstructure due to the formation of grain boundaries in the a-b plane.

Defect structure of laser deposited Y-Ba-Cu-O thin films on single crystal MgO substrate

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 °C c-axis oriented growth is observed on these substrates. On SrTiO3, the first layer appears to be either a BaO or a CuO2 plane while on LaAlO3 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 °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.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400014126

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

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.

Preparation of thin film high temperature superconductors

Thin films of potassium tantalate niobate (KTN) were prepared by means of pulsed excimer-laser deposition and investigated with a number of analytical techniques, including electrical and electro-optical measurements. For applications in longitudinal electro-optic modulators, a transparent electrode is required between substrate and electro-optic layers. Suitable electrode materials, which at the same time permit epitaxial growth of KTN, were identified and prepared. The resulting layered samples were not only of good epitaxial and optical quality, but also exhibited the expected maximum of the longitudinal electro-optic effect at temperatures between the phase transitions from cubic to tetragonal and from tetragonal to orthorhombic. However, the maximum achievable electro-optic phase shift was found to be limited to roughly τ/100 for KTN films in the thickness range around 1 μm. Therefore, much thicker films are probably necessary for most practical applications, which requires significant improvements in the long-term stability and homogeneity of the deposition process.

T. S. Ravi

Papers

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

Defect structure of laser deposited Y-Ba-Cu-O thin films on single crystal MgO substrate

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

Preparation of thin film high temperature superconductors

Electro-optic potassium-tantalate-niobate films prepared by pulsed laser deposition from segmented pellets