MOCVD of high-quality YBa2Cu3O7 –δfilms: in situ preparation of fluorine-free layers from a fluorinated barium source

Abstract

An MOCVD process is reported that employs the thermally stable barium 1,1,1,5,5,5-hexafluoropentane-2,4-dionate adduct Ba(hfac)₂·(18-crown-6). This compound was used in combination with non-fluorinated copper and yttrium sources to deposit superconducting YBa₂Cu₃O_{7 –δ} films in a computer-controlled MOCVD reactor specially developed for use with solid metal sources requiring vaporisation at 120–200 °C. The characterisation of films grown on MgO and LaAlO₃ substrates by X-ray diffraction, electron microscopy, compositional analyses and electrical measurements is described. Optimised films were grown at 690 °C under O₂ partial pressures of 0.19 Torr. By depositing in the presence of an excess of water vapour, at a partial pressure of 3.5 Torr, fluorine-free layers were obtained by an in situ method not involving any post-deposition annealing. Results on films grown under other conditions suggest that hydrolysis reactions are extremely important in this deposition process. The YBa₂Cu₃O_{7 –δ} growth technique employed can produce films with crystallographic and electrical properties similar to the best films grown by any current method. This is illustrated by data on a film on LaAlO₃, characterised by the epitaxial relationships YBa₂Cu₃O_{7 –δ}(001)‖LaAlO₃(001) and YBa₂ Cu₃O_{7 –δ}[100]‖LaAlO₃[100], which showed zero resistance at 91.8 K and a critical current density in self-field at 77 K of ca. 1.5 × 10⁶ A cm^–2.