Microstructure of ultrathin films of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ on MgO

The microstructure of ultrathin films of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ on MgO has been studied for film thicknesses from 1.2 to 12 nm. For thicknesses of 1.2, 2.4, and 7.2 nm, the films appear to be in the tetragonal modification and consist of interconnected, low-aspect-ratio (thickness to diameter ratio ≤0.1) nuclei. The dominant growth mechanism is propagation in the a-b plane of one-unit-cell-high ledges by attachment onto the ledge face, resulting in a large difference between the c- and a-direction growth rates and forcing a two-dimensional appearance in the initial deposition. The 12-nm films are also tetragonal but provide complete coverage of the substrate. Their microstructure has passed the initial nucleation and growth stage and is similar to that of standard (thickness ≥100 nm) films. By analysis of the free energy of the tetragonal-to-orthorhombic transition occurring upon cooling from the deposition temperature, we note a film-thickness dependence of the energy available for the transition. We postulate this as an explanation for the lack of orthorhombicity in ultrathin films.