Development of Low Growth-Temperature, Non-toxic High Temperature Superconducting Thin Films

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In aim of creation of a novel high temperature thin film with a low growth temperature and sufficient performances for electronics applications, we have focused on (Cu, C)Ba_2CuO_x[ (Cu, C)-1201]superconducting films grown by pulsed laser deposition of which superconductivity is realized by our group. For clarifying the lower limit of the growth temperature and the highest superconducting properties of the (Cu, C) -1201 films, i)their intrinsic crystal- and electronic-structure and ii)relationships between process parameters and transport properties have been investigated.i)In-situ measurements of photoemission spectra of the superconducting (Cu, C) -1201 thin films yielded a finite spectral weight at the Fermi level in the valence band, which proofed the intrinsic feature of the sample surface. With an increase of conductivity of the samples, a rigid-band-like shift of the major peaks of the valence band toward the Fermi level was observed. This shift positively correlated with a rise … More of Tc, which reveals hole-doping-induced superconductivity of this system. The large amount of negative shift of binding energy of Ba 4d signal in the early stage of emergence of superconductivity, and positive shift of Cu 2p signals with an increase of Tc beyond 28 K suggest excess oxygen should be introduced around Ba ions in the early stage of the emergence of superconductivity. Then, they should be preferentially introduced around Cu ions. The strong spectral weight around binding energy of 2 eV, is observed, which is characteristic feature of the cuprate-superconductors with Cu-O chains. This feature and changes of core signals with Tc and comparisons of the obtained data with infinite-layer and other cuprate superconductor compounds suggest that the (Cu, C) -O charge reservoir in this system is in the heavily hole-doped state, similar to that of the Cu-O chain in YBa_2Cu_3O_<7-δ>.ii)In order to clarify the dominant parameters for carbon incorporation and the emergence of superconductivity, films were epitaxially grown on (100) SrTiO3 at a low growth temperature of 500-600℃. It has been revealed that the CO3 content in the films increases with increasing both CO2 partial pressure during film growth and growth rate. The rise of CO3 content resulted in an enhancement of superconducting properties. The present study has also revealed that the structural and superconducting properties of the (Cu, C) -Ba-O films are seriously deteriorated by the irradiation of energetic particles during deposition. Suppression of the radiation damage is another key for a high and uniform superconducting transition. By these optimizations, a superconducting onset temperature above 50 K and a zero-resistance temperature above 40 K have been realized.These results mean that the (Cu, C) -O sandwiched with Ba-O layers in (Cu, C) -1201 is an efficient charge reservoir block. Hole-doping during deposition with the proper suppression of irradiation dameges is a key to develop a superconducting properties in this system. By utilizing the obtained conclusions, the window of the process parameters such as growth temperature and atmosphere gas conditions, becomes wide and sufficient for reproduction of Tc above 50 K even in the films grown 500 〜 54℃. These results mean that the main purpose of this project is successfully accomplished. Less