Phase-selective route to high T c superconducting Tl2Ba2Can−1CunO2n+4 films: Combined metalorganic chemical vapor deposition using an improved barium precursor and stoichiometry-controlled thallium vapor diffusion

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Abstract
Films of the Tl2Ba2Can−1CunO2n+4 high Tc superconductors (n=2 or 3) can be prepared with a high degree of phase selectivity using a combination of metalorganic chemical vapor deposition (MOCVD) and vapor diffusion. Ba-Ca-Cu-O films are first prepared by MOCVD using the volatile metalorganic precursors Ba(hexafluoroacetyl- acetonate)2(tetraglyme), Ca(dipivaloylmethanate)2, and Cu(acetylacetonate)2. The ‘‘second-generation’’ barium precursor exhibits significantly improved thermal stability and volatility over previously used compounds. Thallium is then incorporated into these films by vapor diffusion using a Tl-Ba-Ca-Cu oxide mixture of controlled composition as the source of volatile thallium oxides. Phase control is achieved by a combination of improved stoichiometry of the deposited film, a result of the new Ba source, and annealing with the appropriate oxide mixture. The resultant films consist predominantly of the Tl2Ba2Ca2Cu3Ox or Tl2Ba2CaCu2Ox phase, each having preferential orientation of the crystallite Cu-O planes parallel to the substrate surface. The films exhibit superconducting onset temperatures of ∼115 K with zero resistance by 93 and 85 K, respectively. Critical current densities for a number of films, estimated by the Bean method, are in the range 104–105 A/cm2 at 5 K.