Growth of R1+xBa2-xCu3O7-delta epitaxial films investigated by in situ scanning tunneling microscopy

The problem of the epitaxial growth of the high temperature superconducting R1+xBa2-xCu3O7-delta (R = Y or rare earth except Ce and Tb) films has been addressed. Using in situ ultra high vacuum Scanning Tunneling Microscopy ( UHV-STM) we have studied the role of cationic substitution and substrate mi smatch on the growth mode of stoichiometric and Nd-rich Nd1+xBa2-cCu3O7-del ta thin films. The results are compared to the growth of Y1Ba2Cu3O7-delta, Dy1Ba2Cu3O7-delta and Gd1Ba2Cu3O7-delta epitaxial films. Two main phenomena are investigated: a) the first stage of the direct nucleation on the subst rate and b) the crossover between 2D and 3D growth upon increasing the film thickness. At the first stage of the growth, pseudo-cubic perovskite (Re,B a)CuO3 nuclei are formed. While they disappear after the growth of a few nm in stoichiometric films, they persist on the surface of Nd-rich films of u p to 110 nm thickness. Stoichiometric R1+xBa2-xCu3O7-delta films exhibit a rough morphology with increasing thickness due to island growth mode, where as Nd-rich films remain smooth and continue to grow layer by layer. It is p roposed that linear defects (like anti-phase boundaries), which are formed due to the misalignment of growth fronts, are the source of screw dislocati ons in stoichiometric films. In Nd-rich films, linear defects are eliminate d through the insertion of (Nd,Ba)CuO3 extra layers without introduction of any screw dislocations.