Kevin M. Hubbard

Effects of beam parameters on excimer laser deposition of YBa2Cu3O7−δ

Broad angular distributions have been observed for XeCl laser ablation plumes used in the deposition of YBa2Cu3O7−δ thin films. Distributions (inferred from film thickness) and film stoichiometry were measured as a function of laser fluence, beam shape, and oxygen pressure. Parallel to the long axis of the laser spot, plumes exhibited a cos3.5(θ) spread and composition varied with angle; in the perpendicular direction, more diffuse [cos1.5(θ)] plume distributions were associated with stoichiometric deposition. The observed phenomena are consistent with formation of a Knudsen layer near the target surface.

Target modification in the excimer laser deposition of YBa2Cu3O7−x thin films

Under conditions used for pulsed laser deposition (308 nm, 20 ns, 2–8 J/cm2) of Y‐Ba‐Cu‐O superconducting thin films, we have measured a pronounced decrease in deposition rate with cumulative laser exposure of the target. This decay in rate is accompanied by evolution on the target surface of microscopic columnar structures, having yttrium‐enriched surfaces, which are aligned in the direction of the incident laser beam (45°). Neither the vapor plume direction nor film stoichiometry is affected by the presence of these oriented, chemically altered surface features.

Structure and mechanical properties of epitaxial TiN/V 0.3 Nb 0.7 N(100) superlattices

Epitaxial TiN/V 0.3 Nb 0.7 N superlattices with a 1.7% lattice mismatch between the layers were grown by reactive magnetron sputtering on MgO(001) substrates. Superlattice structure, crystalline perfection, composition modulation amplitudes, and coherency strains were studied using transmission electron microscopy and x-ray diffraction. Hardness H and elastic modulus were measured by nanoindentation. H increased rapidly with increasing Λ, peaking at H values ≍75% greater than rule-of-mixtures values at Λ ≍ 6 nm, before decreasing slightly with further increases in Λ. A comparison with previously studied lattice-matched TiN/V 0.6 Nb 0.4 N superlattices, which had nearly identical composition modulation amplitudes, showed a similar H variation, but a smaller H enhancement of ≍50%. The results suggest that coherency strains, which were larger for the mismatched TiN/V 0.3 Nb 0.7 N superlattices, were responsible for the larger hardness enhancement. The results are discussed in terms of coherency strain theories developed for spinodally decomposed materials. Nanoindenter elastic modulus results showed no significant anomalies.

Deviations from Rutherford-scattering cross sections

Abstract Recent experiments have demonstrated the utility of high-energy backscattering for thin-film analysis, often exploiting elastic-scattering resonances to increase sensitivity to low-Z elements. However, higher-Z elements may also exhibit deviations from Rutherford scattering, and it is desirable to examine the problem theoretically so that such deviations can be predicted and corrected for. We have used the optical-model computer code SCAT86 to calculate the projectile energy Enr at which the scattering cross section begins to deviate from its Rutherford value. The results agree well with data for both alpha particle and proton projectiles, but the interaction potentials are complicated, and the program does not provide an analytical predictive formula. We have therefore developed a simple classical model which treats the interaction as a sum of Coulomb and Yukawa-like potentials. For small deviations from Coulomb scattering, the problem can be solved analytically, providing a simple expression for Enr which agrees well with both experimental data and SCAT86. The classical approach is valid for all but small-angle forward scattering and can easily be extended to heavier projectiles such as Li.

Corrosion-resistant erbium oxide coatings by organometallic chemical vapor deposition

Abstract An organometallic chemical vapor deposition (OMCVD) process was developed for the preparation of protective erbium oxide coatings resistant to corrosion by liquid plutonium. The coatings were deposited from the precursor compound Er(tmhd) 3 using a standard hot-wall reactor. The substrates used for process development were 304 stainless-steel flats; coatings were subsequently deposited on the internal surfaces of high-aspect-ratio tubular parts, as well as a variety of curved samples. The coatings have the correct oxygen/erbium ratio, and small concentrations of carbon and hydrogen impurities. The impurities are present in the form of unreacted or partially-reacted precursor molecules, presumably concentrated at the grain boundaries of the material. The films are polycrystalline with a grain size of 40–50 nm, and exhibit excellent adhesion and mechanical toughness. The small grain size and the resultant favorable mechanical properties may be attributable in part to the presence of the organic fragments at the grain boundaries. Coatings deposited on stainless-steel flats are resistant to corrosion by liquid plutonium, although an erbia-thickness threshold is observed. The failure mechanism for samples with thicknesses below the threshold value is most likely related to flaws in the film morphology, for example large pinholes, rather than grain-boundary attack.

Aerosol assisted chemical vapor deposition of superconducting YBa2Cu3O7−χ

Abstract A hybrid process, aerosol-assisted chemical vapor deposotion (AACVD), is described for reproducible preparation of superconducting thin films of YBa 2 Cu 3 O 7−χ . The process consists of atomizing a toluene solution of the Y, Ba, and Cu tetramethylheptanedionato complexes using an aerosol generator. The aerosol is transported into a CVD reactor where solvent and precursor evaporation and deposition occur at atmospheric pressure on heated substrates. The process provides stable evaporation rates for all three precursors, yielding constant film stoichiometry throughout the deposition period and from film to film. Superconducting films may be deposited in-situ at substrate heater temperatures above 825°C, or may be formed at lower temperatures by deposition followed by post-deposition annealing at higher temperatures. The microstructure and quality of films are highly dependent on the conditions employed in deposition and in the case of films deposited below 825°C, the post-deposition annealing conditions. Superconducting films prepared by the AACVD/post-annealing process have a metallic normal state resistivity signature with a zero resistance temperature typically above 88K, and are highly c -axis oriented. Transport critical current densities measured at 75 K on polycrystalline films prepared by the AACVD process are 220 000 A/cm 2 and 84 000 A/cm 2 at self-field and 0.1 T, respectively.

Metalorganic chemical vapor deposition of Tl2Ba2CaCu2Ox superconducting thin films using fluorinated β‐diketonate source reagents

Superconducting Tl2Ba2CaCu2Ox films were fabricated using a three‐step process. Amorphous BaCaCuOF films were deposited on MgO(100) single‐crystal substrates by metalorganic chemical vapor deposition at 500 °C using fluorinated β‐diketonate complexes of Ba, Ca, and Cu. The fluorine was stripped in wet oxygen at 785 °C and c‐axis oriented Tl2Ba2CaCu2Ox films were then formed by annealing in dry at 870 °C in the presence of Tl2O3/Ba2Ca2CuN3Ox pellets. The best films showed onsets of superconductivity at 125 K and zero resistivity was achieved by 108 K. Critical current densities as high as 104 A/cm2 were obtained at 90 K.

Measured deviations from Rutherford backscattering cross sections using Li-ion beams

Bozoian et al. have developed an analytical theory which predicts the incident ion energy at which elastic-scattering cross sections begin to deviate from their Rutherford values as the result of nuclear perturbations. The predictions are in good agreement with experimental data for proton and helium-ion beams, and should be applicable to heavier projectiles. The theory has also been extended to account for isotopic variation of the incident beam. As a further test of the theory, we have performed a series of backscattering measurements using both 6Li and 7Li ion beams. Lithium-ion backscattering is of importance to thin-film analysis because it offers improved depth sensitivity and kinematic mass resolution as compared to helium. Elastic-backscattering cross sections for O, F, Al, and Ti were measured relative to those ofhigh-Z reference elements over an energy range of 2.5–9.25 MeV, and the energies at which they began to deviate from their Rutherford values were determined. The experimental results are compared to predictions from the analytical theory, and from optical-model calculations.

Mechanical properties of epitaxial TiN/(V0.6Nb0.4)N superlattices measured by nanoindentation

We have used nanoindentation to measure the mechanical properties of epitaxial TiN/(V0.6Nb0.4)N superlattices, grown on MgO(100), as a function of the wavelength λ. The V/Nb ratio within the VNbN layers was chosen to provide a lattice match with TiN, minimizing effects resulting from coherency strains. For λ≥4 nm, the hardness was found to be significantly enhanced relative to a homogeneous reference film of the same average composition. For λ<4 nm, the hardness decreased to a value close to that of the reference film. The elastic modulus was found to be constant for λ≥4nm, at a value close to that predicted by the law of mixtures. For samples with λ=2.3 and 2.8 nm, there was a 15% decrease in modulus. The observed variations appear not to be an effect of interfacial strain. Possible mechanisms are discussed.

Excimer laser surface processing of Ti-6Al-4V

We have examined the effect of surface processing in air, using excimer laser light at 248 nm wavelength, on the oxygen content, microstructure, and surface hardness of Ti-6Al-4V. Processing with a single pulse results in the transformation of theα +β material toα′ martensite. Multiple pulse processing results in rapid incorporation of oxygen in the material. Oxygen initially dissolves in the material in the liquid phase. As the concentration exceeds the solid solubility limit during solidification, TiO particles precipitate. In contrast to equilibrium oxidation processes in Ti, only TiO is observed as an oxidation product; further processing results in increased oxygen incorporation and an increased volume fraction of TiO but no other oxides of Ti. The TiO particle size is a function of the oxygen concentration and the number of pulses, with some grain growth occurring after many pulses. The effects of solution hardening by dissolved oxygen and precipitation hardening by the TiO are identifiable as functions of oxygen concentration and mean free path between particles, respectively. A maximum surface hardness almost twice that of electropolished Ti-6Al-4V is observed.