Growth mechanisms and superconductivity of ultrathin Y1Ba2Cu3O 7-x epitaxial films on (001) MgO substrates

Ultrathin YBa2Cu3O 7-x films grown on (001) MgO substrates by pulsed-laser ablation exhibit a transition from terraced-island growth to spiral growth at ∼4--5 unit cell thickness in films grown at 720 degree C. The transition appears at greater thickness in films grown at higher growth temperatures. Observations of the morphology of ultrathin 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 properties can be improved by achieving a relatively strain-free state. Our observations of the growth transition and resistivity measurements show that this state occurs in the thicker grains of nominally 3.5 nm films grown at 800 degree C and in nominally 7 nm films grown at both 720 and 800 degree 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