GROWTH AND EVOLUTION OF MICROSTRUCTURE OF EPITAXIAL YBA2CU3O7-X ULTRATHIN AND THIN-FILMS ON MGO

The nucleation and early stages of growth, and the evolution of surfac e microstructure of epitaxial c-axis YBa2Cu3O7-x thin films are studie d as a function of film thickness and growth temperature. The films we re grown in situ on polished MgO (100) substrates by unbalanced DC mag netron sputtering at a growth rate of 3 nm/min. We use a combination o f high-resolution scanning electron microscopy (SEM), scanning tunneli ng microscopy (STM), atomic force microscopy (AFM), X-ray diffraction, and transport-property measurements. The results from ultrathin films , 3 to 50 c(perpendicular-to) unit cells (1 c(perpendicular-to) unit c ell equals 1.17 nm) indicate that epitaxial growth proceeds by the cla ssical screw dislocation mediated three-dimensional island growth mode , i.e. each island grows both vertically and laterally by the incorpor ation of adatoms at a spirally expanding step emanating from one or mo re screw dislocations. The initial adatom clustering leads to a high d ensity of nuclei (almost-equal-to 2 x 10(11) cm-2). The density of scr ew dislocations, rho(screw), determined from STM images of the film su rface, is found to depend on both the thickness of films and the growt h temperature. At moderate growth temperatures (720-760-degrees-C) the early stage of coalescence of nuclei and island growth is characteris ed by a high density of screw dislocations, (3-9) X 10(9) cm-2, decrea sing to (2-3) x 10(9) cm-2 for 200 c(perpendicular-to) unit-cells film s. Higher growth temperatures result in smooth films with a low densit y of screw dislocations, e.g. less-than-or-equal-to 0.3 x 10(9) cm-2 a t 850-degrees-C for 200 c(perpendicular-to) unit-cells films. A correl ation is found between high values of rho(screw) and high values of th e critical current density. The ultrathin films exhibit suppression of superconductivity and broadening of the resistive transition with dec reasing film thickness, so that T(c0) = 55 K for a 3 c(perpendicular-t o) unit-cells film.