New hybrid physical vapor deposition/organometallic chemical vapor deposition route to high T_c superconducting Tl-Ba-Ca-Cu-O thin films

High T_c superconducting Tl-Ba-Ca-Cu-O thin films have been prepared by a novel hybrid technique that combines electron beam evaporation with organometallic chemical vapor deposition (OMCVD). Multilayer thin films of Ba-Ca-Cu-O are prepared by sequential evaporation of BaF₂, CaF ₂, and Cu sources onto single-crystal MgO (100) or yttria-stabilized zirconia substrates followed by annealing in a water vapor-saturated oxygen atmosphere. Thallium is then incorporated in these films in either of two ways: (1) OMCVD using thallium(cyclopentadienide) as the source or (2) vapor diffusion using bulk Tl-Ba-Ca-Cu-O superconductor as the source. The resultant films are single phase, consisting predominantly of Tl₁Ba₂Ca ₂Cu₃O_x with crystallite Cu-O planes preferentially oriented parallel to the substrate surface. Resistivity measurements indicate superconducting onset temperatures above 120 K with zero resistance by 104 K. By eliminating the water vapor in the initial anneal, fluoride (originating from the Ba and Ca sources) may be retained in the film. Upon T1 incorporation, this promotes formation of the Tl₁Ba ₂Ca₁Cu₂O_x phase with preferential crystallite c-axis orientation perpendicular to the substrate surface.