An assessment of synthesis effects on the high-Tc superconducting transition: With application to thin-film bolometer devices

Abstract

Abstract The characteristics of the transition to superconductivity in R Ba 2 Cu 3 O (7−δ) , where R includes: Y, Gd, Eu and Dy, have been investigated, for applications to transition-edge thin-film bolometers. Devices were fabricated from thin, epitaxial films, which were formed by pulsed-laser deposition (PLD). Bulk ceramic samples (pellets) were synthesized for use as the ablation targets. A study of the (DC) resistive transition width ( ΔT c ) of the bulk samples, reveals that the oxidation annealing cycle plays a dominant role in determining ΔT c . Peak temperatures of the oxygenation must be very carefully controlled. A slightly too high processing temperature causes changes in the grainsurface morphology and an increase in ΔT c . Rare earth ( R ) substitution has a significant effect. Residual phase impurity, off stoichiometry, lower density, pressing-induced orientation and smaller pellet sizes, were all found to be less deleterious to ΔT c . Proper synthesis improves superconductivity, narrowing ΔT c and affecting grain-boundary (GB) morphology, as revealed by XRD, SEM and AC magnetic-susceptibility analyses of the bulk ceramics. The occurrence of a substantial susceptibility-loss (χ″) peak correlates well with (undesired) larger ΔT c transitions. In bulk ceramics this may be accompanied by a toe feature in the resistive transition, i.e. additional width to the lower-half of the transition. Thin, epitaxial films, made by PLD from these same bulk ceramics, indicate that certain types of non-optimal features of the bulk transition are not present in the fabricated films. In particular, the toe feature is removed due to the complete restructuring that occurs during the epitaxial film-growth process by PLD. For films to have narrow transitions, we find that it is necessary for the ablation target to have a sharp onset to its transition. The relationship to infrared bolometer detectors is discussed and examples are reported.