Phase-selective route to high T_c superconducting Tl ₂Ba₂Ca_n-1Cu_nO_2n+4 films: Combined metalorganic chemical vapor deposition using an improved barium precursor and stoichiometry-controlled thallium vapor diffusion

Films of the Tl₂Ba₂Ca_n-1Cu _nO_2n+4 high T_c 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 Tl₂Ba₂Ca₂Cu₃O_x or Tl ₂Ba₂CaCu₂O_x 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.