DC characteristics of patterned YBa/sub 2/Cu/sub 3/O/sub 7-x/ superconducting thin-film bolometers: artifacts related to Joule heating, ambient pressure, and microstructure
Joule heating due to the bias current and resistance of the material in patterned YBa2Cu3O7-x superconducting films on 250-500-mu m-thick MgO, LaAlO3, and SrTiO3 crystalline substrates, results in a number of effects: 1) a temperature rise in the film with respect to the measured temperature at the bottom of the substrate; 2) a possible thermal runaway, which may be local or uniformly distributed in the film, depending upon the dimensions of the superconducting pattern relative to that of the substrate; 3) an apparently sharper normal-to-superconducting transition in the measure R versus T curve; and 4) decrease of T-c to 60 K (Delta T-x > 20 K) after being subjected to high-bias currents j similar to 10(5) A/cm(2) under vacuum, with recovery of T-c after exposure to room atmosphere. The magnitude of R at Tc-onset is found to be dependent on bias current in granular samples, with a lower R at currents higher than some on-set value. The slope of R versus T in the transition region in our granular samples is found to be lower at higher bias currents, since the widening of the transition overcomes the shift caused by the Joule heating. These various phenomena impact the responsivity of bolometers made from these films, as well as the predictions of possible attainable responsivity and speculations of mechanisms occurring in the films. In particular, misinterpretation of the Joule heating sharpening of the R versus T curve has led to predictions of responsivities over one order of magnitude higher than are justified, and shifts in properties of the films due to heating have been misinterpreted as nonequilibrium responses of the films.