GROWTH MECHANISMS AND SUPERCONDUCTIVITY OF ULTRATHIN Y1BA2CU3O7-X EPITAXIAL-FILMS ON (001) MGO SUBSTRATES

Ultrathin YBa2Cu3O7-x films grown on (001) MgO substrates by pulsed-la ser ablation exhibit a transition from terraced-island growth to spira l growth at approximately 4-5 unit cell thickness in films grown at 72 0-degrees-C. The transition appears at greater thickness in films grow n at higher growth temperatures. Observations of the morphology of ult rathin films indicate that the film-substrate interfacial interaction plays an important role when films are only several unit-cells thick. Plastic deformation and oxygen disorder both may affect the epitaxial quality and superconducting properties. However, superconducting prope rties can be improved by achieving a relatively strain-free state. Our observations of the growth transition and resistivity measurements sh ow that this state occurs in the thicker grains of nominally 3.5 nm fi lms grown at 800-degrees-C and in nominally 7 nm films grown at both 7 20 and 800-degrees-C. These results also imply that a strain-relieving buffer layer will improve the superconductive properties of ultrathin YBa2Cu3O7-x films grown on misfit substrates.