K.R. Padmanabhan

Formation of thin‐film high Tc superconductors by metalorganic deposition

Metalorganic deposition (MOD) is a nonvacuum method of thin‐film deposition which allows easy alteration of chemical components and is compatible with thin‐film processing. We report the preparation of thin‐film superconductors by MOD. Rutherford backscattering spectrometry was used to determine film compositions and thicknesses. Films, approximately 500 nm thick, of YBa2Cu4Oz (z undetermined) were deposited on 〈100〉 single‐crystal SrTiO3. A superconducting onset temperature of 90 K was measured with 37 K the zero resistance temperature. Scanning electron microscopy revealed grain sizes approximately 250 nm in diameter.

Rapid thermal annealing of high Tc superconducting thin films formed by metalorganic deposition

Thin‐film superconductors of Y‐Ba‐Cu and Yb‐Ba‐Cu have been formed by the nonvacuum method of metalorganic deposition (MOD). The films produced in this manner were homogeneous and free of voids and cracks over large dimensions. A two‐step rapid thermal annealing of the MOD films, in oxygen, at 850 °C for 60 s followed by a second annealing at 920 °C for 30 s enhanced grain growth in the films and reduced the effects of substrate interaction. Preferred epitaxial grain growth, in the high Tc films, with the c axis both perpendicular and parallel to the substrate surface, occurred on 〈100〉 SrTiO3. Both the Y‐Ba‐Cu and Yb‐Ba‐Cu films showed superconducting onset temperatures above 90 K and zero resistance at 86 K.

Mass and geometry effects on the anisotropic transport in ion mixing

Experimental investigations of the effect of target species atomic mass and system geometry on the anisotropic transport in the ion mixing of metallic systems are reported. Bilayer samples with zero heats of mixing and similar cohesive energies, but different atomic mass and geometry, such as Ta on top of Nb(Ta/Nb), Nb on top of Ta(Nb/Ta), Hf on top of Zr (Hf/Zr), and Zr on top of Hf(Zr/Hf) were irradiated by 300 keV Kr2+ at a dose of 2 × 1016 Kr2+/cm2 at 77 K. The samples were investigated using embedded markers and Rutherford backscattering spectrometry. The experimental results indicate that the anisotropic transport is dominated by a preferential displacement of the top layer species into the bottom layer. This is probably due to an anisotropy in the momentum distribution within the collision cascade. In addition, there is an enhancement of the inward displacement when the lighter species is on top indicating a small preferential recoil displacement of the lighter species over the heavier one.

Effect of ion implantation on thin hard coatings

The surface mechanical properties of thin hard coatings of carbides, nitrides and borides deposited by r.f. sputtering were improved after deposition by ion implantation. The thickness and the stoichiometry of the films were measured by Rutherford backscattering spectrometry and nuclear reaction analysis before and after ion bombardment. The post ion bombardment was achieved with heavy inert ions such as Kr+ and Xe+ with an energy sufficient to penetrate the film and to reach the substrate. Both the film adhesion and the microhardness were consistently improved. In order to achieve a more detailed understanding, Rb+ and Ni+ ions were also used as projectiles, and it was found that these ions were more effective than the inert gas ions.

Ferromagnetism in CuO–ZnO multilayers

We investigated the magnetic properties of CuO–ZnO heterostructures to elucidate the origin of the ferromagnetic signature in Cu doped ZnO. The CuO and ZnO layer thickness were varied from 15 to 150 nm and from 70to350nm, respectively. Rutherford backscattering experiments showed no significant diffusion of either Cu in ZnO or Zn in CuO layers. Magnetic measurements indicate ferromagnetism at 300K, which depends on the CuO particle size, but not on the CuO–ZnO interfacial area. Polarized neutron reflectometry measurements show that the observed magnetization cannot be accounted for solely by spins localized near the CuO–ZnO interface or in the CuO layer.

A comparison between high- and low-energy ion mixing at different temperatures

Abstract High-energy ion mixing occurs when an ion beam of a few hundred keV bombards an interface under the surface. Low-energy ion mixing arises when an ion beam of a few keV bombards an interface near the surface during, e.g., sputter depth profiling and low-energy ion-assisted deposition. At low temperatures, the rate of both high- and low-energy ion mixing can be influenced by thermodynamic parameters, such as the heat of mixing and the cohesive energy of solids. These effects are demonstrated by ion mixing experiments using metallic bilayers consisting of high-atomic-number elements. A model of diffusion in thermal spikes is used to explain this similarity. Low-energy ion mixing can also be strongly affected by surface diffusion and the morphological stability of thin films. These effects are illustrated using results obtained from sputter depth profiling of Ag/Ni, Ag/Fe, and Ag/Ti bilayers at elevated temperatures. High-energy ion mixing at low temperatures can be influenced by the anisotropic momentum distribution in a collision cascade as seen from a set of marker experiments to determine the dominant moving species in high-energy ion mixing. An understanding of these similarities and differences between high- and low-energy ion mixing at different temperatures will provide useful guidelines for applications of ion mixing.

Microstructure study of Y–Ba–Cu oxide superconducting thin films

Grain growth as a function of annealing temperature in Y--Ba--Cu oxide superconducting thin films is presented. Effects of excess Cu on the temperature dependence of the resistance, grain growth, and sheet resistivity are discussed. This is the first systematic study of the microstructure of thin films formed by the nonvacuum technique of metallo organic deposition. The Cu-rich films show an anomalous behavior in the resistivity at 220--240 K. The presence of excess Cu was found to enhance grain growth for films annealed below the melting point of the 1--2--3 phase. Films deposited on SrTiO/sub 3/ and rapid thermal annealed showed partial epitaxial growth of the 1--2--3 phase. The analytical techniques of Rutherford backscattering spectrometry, ion channeling, x-ray diffraction, scanning electron microscopy, electron microprobe analysis, sheet resistivity, and temperature versus resistance measurements have been used in this study.

Cohesive energy effects on anisotropic transport in ion mixing

Abstract Experimental investigations of the effect of target species cohesive energy on anisotropic transport in ion mixing of metallic systems are reported. Bilayer samples with nearly zero heats of mixing, but different cohesive energy and geometry, such as W on top of Pd (W/Pd), Pd on top of W (Pd/W), Nb/Cu, Cu/Nb, Ag/V, and V/Ag, were irradiated with 300 keV Kr2+ at doses of 1 × 1016 Kr2+/cm2 and 2 × 1016 Kr2+/cm2 at 77 K. The samples were investigated using embedded markers and Rutherford backscattering spectrometry. The experimental results indicate a preferential transport of higher cohesive energy material into lower cohesive energy material — in some cases contrary to ballistic arguments. This phenomenon is explained in terms of thermally activated diffusion within thermal spikes.

Thermodynamic and ballistic aspects of ion mixing

Abstract Ballistic and thermal spike contributions to ion mixing are discussed using the results of recent marker experiments which determine the dominant moving species in ion mixing of several metallic bilayer systems. A greater flux of atoms from the high cohesive energy side to the low cohesive energy side is the result of the thermal spike contribution to ion mixing. A greater flux from the top layer to the bottom layer in ion mixing of bilayers consisting of elements of similar and high cohesive energy values is the result of the ballistic contribution to ion mixing. The relative importance of ballistic and thermal spike contributions is influenced by the magnitude of cohesive energy. A model based on a fractal geometry approach to spike formation is used to understand the competition between ballistic and thermal spike contributions to ion mixing.

Ion channeling through a thin Si-liquid interface

The feasibility of ion channeling through the wall of a thin Si‐liquid cell has been investigated. We have shown experimentally that it is possible to channel ions through a thin Si‐liquid interface. With water the channeling minimum yield is 0.45 at the interface. This is higher than that required for application of this technique to gain structural information at the solid‐liquid interfaces. It is also higher than that required for the determination of preferred positions of deposited impurity atoms at the interface.