Growth mechanisms and superconductivity of ultrathin Y₁Ba ₂Cu₃O _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°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°C and in nominally 7 nm films grown at both 720 and 800°C. These results also imply that a strain-relieving buffer layer will improve the superconductive properties of ultrathin YBa ₂Cu₃O_7-x films grown on misfit substrates.