MOCVD OF HIGH-QUALITY YBA2CU3O7-DELTA FILMS - IN-SITU PREPARATION OF FLUORINE-FREE LAYERS FROM A FLUORINATED BARIUM SOURCE

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 YBa2Cu3O7-delta films in a computer-controlled MOCVD reactor specially developed for use with solid metal sources requiring vaporisation at 120-200 degrees 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 degrees C under O-2 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 YBa2Cu3O7-delta 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 YBa2Cu3O7-delta (001)parallel to LaAlO3 (001) and YBa2 Cu3O7-delta [100]parallel to 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 x 10(6) A cm(-2).