X-ray photoelectron diffraction study of thin Cu films grown on clean Ru(0001) and O-precovered Ru(0001)

Abstract We have studied the epitaxial growth modes and near-surface interlayer relaxation of thin Cu films on Ru(0001) using X-ray photoelectron diffraction (XPD), measuring experimental Cu 2p3/2 (Ekin=556 eV) and Ru 3d (Ekin=1206 eV) intensities over one-third of the nearly full 2π solid angle above the surface for Cu coverages from submonolayer up to 40 monolayers. Reference Cu 2p3/2 data for a clean Cu(111) surface have also been obtained from Naumovic et al. and in our laboratory. These data have been compared to single scattering cluster (SSC) and more accurate multiple scattering cluster (MSC) calculations via a sum of five R-factors to derive precise structural information. MSC calculations are found to give a more accurate description for layers of ≥4 ML thickness, and comparisons of experiment and theory are also improved by allowing more accurately for the effective degree of angular averaging involved. Calculations for thicker layers are also found to converge by ∼5 ML. Our analysis indicates that the first Cu layer grows pseudomorphically on Ru(0001), in agreement with prior studies. An R-factor analysis comparing MSC and SSC calculations to experimental results further indicates that the Cu–Ru interlayer spacing at 1 monolayer (ML) is about 2.15 A, in excellent agreement with prior low-energy ion scattering (LEIS) and low-energy electron diffraction (LEED) experimental studies, as well as with prior linearized augmented plane-wave (LAPW) calculations. At higher coverages, comparison of our data to SSC and MSC calculations for various atomic clusters indicates that the short-range structure is fcc Cu(111)-like, but with significant interlayer contraction which persists up to ≥5 ML coverage. Prior STM work by Behm et al. has shown a series of misfit dislocation structures in the top layer of the Cu film at higher coverages from 2 to 4 ML. Our data indicate that these misfit dislocation structures thread to the Cu/Ru interface rather than occurring only in the top Cu layer or layers. An R-factor comparison of the more accurate MSC calculations to experiment also indicates that the ratio of the Cu–Cu interlayer distance (d⊥) to the Cu–Cu in-plane nearest-neighbor distance (d‖), d⊥/d‖=0.729±0.034 at 2 ML, and reaches 0.777±0.020 by 25 ML. For reference, the bulk value is d⊥/d‖=0.816, and the analysis of experimental data for Cu(111) yields 0.801±0.035, in good agreement with this value and prior LEED studies. This analysis shows that there is significant interlayer contraction for very thin Cu layers, and that it persists (at least in the top few layers, to which XPD is the most sensitive) for longer than would be expected on the basis of a prior theoretical analysis using the 2D Frenkel–Kontorova model by Hamilton and Foiles, as used to estimate d⊥/d‖ via either a constant atomic-volume assumption or elasticity theory. In addition, the Cu overlayer grows in two possible orientations rotated by 180° on the Ru(0001) surface, with a preference towards one of the two possible orientations at certain coverages. Finally, we have investigated the effect of oxygen preadsorbed on the Ru(0001) surface on the growth of the Cu overlayer. For this case, we find that all of the oxygen floats on top of the Cu in a highly disordered configuration, and that the oxygen promotes multilayer or island growth relative to growth on the clean Ru surface up to at least 3 ML coverage, rather than acting as a surfactant promoting smoother growth.