Struktur und Gitterdynamik ultradünner epitaktischer Silber- und Eisen- Filme auf Nickel(100)- und Kupfer(100)- Substraten

The surface phonon dispersions of ultrathin epitaxial Ag films on Ni(100) and Cu(100) substrates and epitaxial Fe films on a Cu(100) substrate have been studied using inelastic scattering of slow electrons. The experimental data are compared with lattice dynamical model calculations and conclusions about the atomic force field within the films are drawn. For the epitaxial systems investigated the relation between structure, epitaxial growth and microscopic surface stress is discussed. Ag films 1, 2, 3 and 50 monolayers (ML) thick on Ni(100) as well as 1 and 2 ML thick on Cu(100) have been prepared, characterized and investigated for surface phonondispersion in the [100]-direction of the substrates. Ag monolayers on Ni(100) and Cu(100) form close-packed structures in two orthogonal domains with c(8x2) and c(10x2) diffraction patterns, respectively. The prominent feature in the energy loss spectra is a mainly vertically polarized surface mode localized within the monolayers, with frequencies far below the substrate Rayleigh phonon at the substrate surface Brillouin zone boundary. This adlayer mode exhibits strong hybridization with the substrate Rayleigh wave for smaller wave vectors. Different maximum frequencies of this phonon for the two substrates show that the Ag-substrate bonds of the Ag/Ni system are stronger than those of the Ag/Cu system. Phonon dispersion curves calculated for a model of a coupled nearly incommensurate adlayer-substrate system are in good agreement with the experiment. As the film thickness is increased the structural and surface lattice dynamicalproperties of the films pass into those of the (111) surface of bulk Ag. The specific influence of the substrate on the surface phonon dispersion decreases rapidly withincreasing film thickness : The dispersion of the surface mode for the bilayer is essentially identical for both substrates. The transition from a two-dimensional to a threedimensional Ag crystal with the (111) surface is reflected in the surface phonon dispersion curves. The 50 ML film exhibits the Rayleigh phonon dispersion typical of a (111) surface of bulk Ag. Anomalies in the dispersion curves which are conclusive of significant surface stress within the top layers of the films are not observed.Epitaxial Fe films of good quality on a Cu(100) substrate up to a thickness of5 ML have been prepared by deposition at a substrate temperature of 350 K. At elevated temperatures ($\simeq$420 K) diffusion between Fe and Cu takes place as indicated by characteristic variations of the high resolution electron energy loss and Auger electron spectra. For the investigated temperature range (90 K - 300 K) monolayer films display a (5x1) diffraction pattern which has not been reported before. A possible structure for the monolayer is discussed. Films with thicknesses of 2 - 5 ML display (1x1)-diffraction patterns at 300 K suggesting pseudomorphic growth in agreement with previous work. For the 2 and the 3 ML films a reversible structural transition from (1 x 1) to (2x 1), which has so far not been observed, is found when the substrate is cooled from 300 K to 90 K, while the 4 and the 5 ML films undergo an irreversible order-disorder transition. In order to account for the observed diffraction pattern of the 2 and the 3 ML films at low temperatures, a structural model with the top layer Fe atoms alternatively displaced from their pseudomorphic positions is proposed. The surface phonon dispersion of the films has been investigated in the $\overline{\Gamma}$$\overline{M}$-azimuth ([100]-direction) of the substrate surface Brillouin zone. The dispersion curve of the Rayleigh phonon, which is the prominent feature in the energy loss spectra, displays a pronounced anomaly for the 2, 3 and 5 ML films. This anomaly is reproduced in lattice dynamical calculations when a lateral repulsive surface stress between the Fe atoms is included. While for the 2 ML film the stress is confined to the top layer, for the 3 and the 5 ML films stress is also required for deeper layers in order to match the measured dispersion curves. The lattice dynamical analysis also reveals the nature of the observed (1x1) to (2x1) transition. The displacement pattern of the (2x1) reconstructed surface conforms to the displacement pattern of the S$_{l}$-phonon at the $\overline{X}$-point of the 2D-Brillouin zone. This mode becomes soft when the stress is increased to a critical value. Therefore, the repulsive surface stress is considered to be the main driving force for the displacive phase transition, for the order-disorder transition in the 4 and the 5 ML films, and for the termination of the epitaxial layer-by-layer growth beyond 5 ML. The physical origin of the repulsive stress may be understood as a propensity ofthe films to realize interatomic distances of the Fe bcc structure which is the stable Fe bulk structure for the range of temperatures studied