Effects and structures of the O/Cu surfactant layer in O-mediated film growth of Cu on Ru(0001)

The O-mediated Cu-film growth on O-precovered Ru(0 0 0 1) is investigated b y means of scanning tunneling microscopy for growth temperatures between 30 0 and 600 K. Cu-films on clean Ru(0 0 0 1) grow in a multilayer mode. For 0 precoverages (Theta) between 0.2 ML (monolayer) and the saturation coverag e (Theta = 0.5 ML), a layer-by-layer growth is observed at growth temperatu res between 350 and 450 K. On Cu-islands, an O/Cu surfactant layer is forme d, which floats on-top of the growing film and induces the layerwise Cu-fil m growth. The surface coverage of the O/Cu surfactant layer linearly rises with the 0 precoverage up to Theta approximate to 0.4 ML, where it complete ly covers the surface. Two different types of the surfactant layer are iden tified. inducing different surfactant mechanisms. For Theta = 0.1-04 ML, th e O/Cu surfactant structure (A-type) displays some local order and induces inhomogeneous nucleation at the misfit-induced relaxation structure of the Cu-film. The layer-wise growth is explained by the concept of two mobilitie s, implying a large attempt frequency for adatom jumps over the interlayer diffusion barrier at the steps. For Theta = 0.4-0.5 ML, a disordered O/Cu s urfactant layer is established (B-type), inducing homogeneous nucleation. T he layer-wise Cu-film growth is attributed to a reduction of the effective interlayer diffusion barrier. Cu-film growth at 400 K on the ordered (3 x 2 root3)O/Cu structure formed at temperatures around 520 K yields the conclu sion that the O/Cu surfactant structures are composed of randomly arranged O-Cu-O strings and disrupted "Cu2O(1 1 1)" fragments. (C) 2001 Elsevier Sci ence Ltd. Ali rights reserved.