ON THE EFFECT OF PROCESSING PARAMETERS IN THE CHEMICAL-VAPOR-DEPOSITION OF YBA2CU3O7-DELTA THIN-FILMS ON POLYCRYSTALLINE SILVER

Results are reported from an investigation of the effects of selected processing parameters on the morphology and properties of YBa2Cu3O7-de lta (YBCO) superconducting thin films grown directly on polycrystallin e silver substrates by chemical-vapor deposition (CVD). These results were achieved through a set of experimental studies which examined: (i ) recrystallization mechanisms of polycrystalline silver and their eff ect on the deposition of YBCO thin films; and (ii) CVD processing cond itions leading to the growth of high-quality YBCO films. The samples w ere analyzed using dynamic impedance, four-point resistivity probe, x- ray diffraction, Rutherford backscattering, and scanning electron micr oscopy. These studies showed that substrate temperature played a criti cal role not only in the formation of YBCO films, but also in the recr ystallization of silver substrates, which in turn greatly influenced f ilm growth. The studies also led to the identification of a two-stage processing scheme for the growth of YBCO films on silver. The first pr ocessing stage consisted of a substrate conditioning cycle which invol ved a 10 min ramping from room temperature to deposition temperature w here the substrates were held for an additional 10 min in a flow of 70 sccm 02 at a reactor working pressure of 2 Torr. The second processin g stage involved actual film deposition at 760-800-degrees-C for 3-10 min (depending on desired film thickness) in a mixed flow of 70 sccm O 2 and 210 sccm N2O at a reactor working pressure of 4 Torr. Samples th us produced were highly oriented along the c axis perpendicular to the substrate with a zero resistance transition temperature of 87 K and a critical current density of 2 X 10(4) A/cm2, (77 K, B=0). The films h ad a thickness of 200-700 nm depending on the length of the growth cyc le, which corresponded to the growth rates in the range 65-130 nm/min. A growth mechanism for YBCO on polycrystalline silver, which emphasiz ed the role of silver recrystallization, was consequently proposed and discussed.