O-mediated layer growth of Cu on Ru(0001)

The film growth of Cu on clean and O-precovered Ru(0001) at different growt h temperatures form 300 K to 450 K was investigated by scanning tunnelling microscopy (STM). Cu films on clean Ru(0001) grow in a multilayer mode at t hese temperatures. By using an O precoverage (Theta(o)) in the range of 0.1 monolayers (ML) up to a saturation coverage of 0.5 ML on clean Ru(0001), a t 400 K different growth regimes are obtained. For Theta(o) < 0.2 ML a mult ilayer mode is preserved which changes into an O-induced two-dimensional (2 D) growth for higher Theta(o) (0.2-0.5 ML). STM reveals the formation of an O/Cu surfactant structure on the surface due to migration of O initially l ocated at the Ru surface. Its surface coverage rises linearly with O precov erage up to Theta(o) = 0.4 ML where it covers the surface completely. By in creasing Theta(o) up to 0.5 ML, a drastic change in the morphology and dens ity of the 2D islands occurs, which is accompanied by a change of the O/Cu surfactant structure. The O/Cu surfactant structure displays some order on a local scale for low Theta(o), which changes into a disordered structure f or Theta(o) > 0.4 ML. Structural similarities to oxidized surfaces of Cu(ll l)and the structures induced by O-2 postadsorption on Cu/Ru(0001) are discu ssed. Different models of surfactant mechanisms are presented to explain th e observations. The locally ordered O/Cu surfactant structure (for Oo < 0.4 ML) together with specific Cu film defects induce a heterogeneous nucleati on of Cu with a high island density. Different mobilities of migrating Cu a datoms are established on top of the small islands and on the O/Cu structur e resulting in enhanced interlayer diffusion explaining the observed 2D gro wth. The average island density only slightly changes within the temperatur es investigated. In contrast, the saturated and disordered O/Cu surfactant structure (for Theta(o) = 0.4-0.5 ML) causes homogeneous nucleation. For th is structure, the island density strongly depends on temperature and gives rise to an Arrhenius-like behaviour. The observed 2D growth is attributed t o a reduction of the interlayer diffusion barrier. Cu growth on a formerly annealed Cu/O/Ru(0001) film system yields an almost perfect layer-by-layer growth caused by heterogeneous nucleation at periodically arranged Cu film defect sites. The relationship of the O/Cu surfactant structures to the ord ered (3 x 2 root 3) O/Cu bilayer on Rn(0001) - interpreted as a disrupted C u2O-like oxidized surface - was revealed.