The nucleation and heteroepitactic growth of YBa2Cu3O7 − δ thin films on MgO

The nucleation and growth of YBa2Cu3O7 − δ films on single-crystal (001)-oriented MgO have been studied using transmission electron microscopy (TEM). The films were formed by pulsed-laser ablation from a stoichiometric bulk sample. Observations of the early stages of film growth were made by utilizing a new specimen-preparation technique in which the films were deposited directly onto specially prepared electron-transparent substrates. The films were found to grow by an island mechanism with the nucleated islands growing to form an interconnecting film. A number of these islands are rotationally misaligned resulting in the formation of a polycrystalline, mosaic structure. The grains were predominantly oriented with their c-axis perpendicular to the film-substrate interface, although a smaller fraction of the film contained grains oriented with their c-axis in the film-substrate plane. These grains were found to grow in orthogonal directions along the [001] and [010] crystal axes of the MgO. The highly anisotropic growth of YBa2Cu3O7 − δ, fastest along the a − b plane, is an important factor in determining the microstructure of the film.