O-induced surface diffusion effects of Cu/Ru(0001) thin film systems

The surface diffusion of epitaxial Cu films on Ru(0001) is drastically modified by adsorbed O. Using STM this phenomenon has been demonstrated by investigating growth and stability of Cu/Ru(0001) thin film systems at temperatures between 300 and 500 K. During the Cu film growth, O floats on top of the film and acts as a surfactant. Depending upon O precoverage of the Ru(0001) substrate, two different O/Cu surfactant structures are revealed. Both structures decrease the diffusion length of Cu adatoms and increase the interlayer diffusion of Cu adatoms over the steps downward into step sites. For temperatures around 400 K and O precoverages between 0.15 and 0.5 ML (saturation precoverage) they both induce a perfect layer-by-layer growth via two different surfactant mechanisms. Decreasing the temperature and/or the O precoverage yield three-dimensional multilayer growth. The stability of ultrathin Cu films (film thickness below 3 ML), however, is drastically reduced by O. Adsorbing O abruptly causes a layer-by-layer removal of the Cu films even at temperatures as low as 500 K.