AN ASSESSMENT OF SYNTHESIS EFFECTS ON THE HIGH-T-C SUPERCONDUCTING TRANSITION - WITH APPLICATION TO THIN-FILM BOLOMETER DEVICES

The characteristics of the transition to superconductivity in RBa(2)Cu (3)O((7-delta)), where R includes: Y, Gd, Eu and Dy, have been investi gated, for applications to transition-edge thin-film bolometers. Devic es were fabricated from thin, epitaxial films, which were formed by pu lsed-laser deposition (PLD). Bulk ceramic samples (pellets) were synth esized for use as the ablation targets. A study of the (DC) resistive transition width (Delta T-c) of the bulk samples, reveals that the oxi dation annealing cycle plays a dominant role in determining Delta T-c. Peak temperatures of the oxygenation must be very carefully controlle d. A slightly too high processing temperature causes changes in the gr ain-surface morphology and an increase in Delta T-c. Rare earth (R) su bstitution has a significant effect. Residual phase impurity, off stoi chiometry, lower density, pressing-induced orientation and smaller pel let sizes, were all found to be less deleterious to Delta T-c. Proper synthesis improves superconductivity, narrowing Delta T-c and affectin g grain-boundary (GB) morphology, as revealed by XRD, SEM and AC magne tic susceptibility analyses of the bulk ceramics. The occurrence of a substantial susceptibility-loss (chi '') peak correlates well with (un desired) larger Delta T-c transitions. In bulk ceramics this may be ac companied by a toe feature in the resistive transition, i.e. additiona l width to the lower-half of the transition. Thin, epitaxial films, ma de by PLD from these same bulk ceramics, indicate that certain types o f non-optimal features of the bulk transition are not present in the f abricated films. In particular, the toe feature is removed due to the complete restructuring that occurs during the epitaxial film-growth pr ocess 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 transi tion. The relationship to infrared bolometer detectors is discussed an d examples are reported.