Fabrication and characterization of Bi-Sr-Ca-Cu-O MOCVD thin films

Abstract

Bi-Sr-Ca-Cu-O thin films have been grown on a MgO substrate at 850°C using MOCVD. Volatile β-diketonate complexes of Sr, Cu and Ca with the dipivaloylmethanate ligand (DPM) [Sr(DPM)2, Cu(DPM)2 and Ca(DPM)2] were employed as MO sources. For a bismuth source, triphenyl bismuth (Bi(ph)3) was employed. Cross-sectional transmission electron microscopy revealed that the film lattice was ordered from the first atomic layer on the smooth surface MgO substrate with misfit dislocations caused by the large lattice mismatch between the film and the substrate appearing every four or five lattice spacings. The first Bi2O2 layer on the surface of the substrate was found to be arranged regularly regardless of the roughness of the substrate surface. The formation of superconducting phases was investigated by X-ray diffractometry patterns as a function of deposition time. This revealed that only the low Tc superconducting phase (Bi2Sr2CaCu2Ox) was formed in the early stage of the deposition. Formation of the high Tc phase (Bi2Sr2Ca2Cu3Ox) began after 30 min growth. These results suggest that the high Tc phase is not directly grown, but forms from the low Tc phase. The large lattice mismatch between the film and the MgO substrate did not appear to cause compressive stress in films down to 8 nm thick. The 8 nm film exhibited metallic behavior at room temperature with an onset of superconductivity of around 110 K. Zero resistance was obtained at 21 K. The surface morphology of the film was observed to consist of characteristic plate-like steps and amorphous-like precipitates.