Ch. Ammer

High-resolution LEED analysis of strained Cu layers on Ru(0001)

Scanning tunnelling microscopic observations have revealed that the relaxation of the lattice strain in the Cu film growing on Ru(0001) occurs in four different stages that are connected with different superstructures depending on the film thickness. Using high-resolution low-energy electron diffraction (HRLEED) the satellite spots of the different superstructures of Cu films with a thickness up to 7 ML (monolayers) grown at 520 K could be identified and quantitatively analysed. However, for Cu films thicker than 2 ML the diffraction patterns are very complex because satellite spots of several superstructures are incoherently superposed. Surprisingly, the structural data derived in a local scale by scanning tunnelling microscopy (STM) are highly representative for the entire surface, analysing by low-energy electron diffraction (LEED). This demonstrates the stability of the relaxation process. Corrugated Cu(111) layers formed after a deposition of 4 ML are rotated with respect to the Ru lattice within a small range of angles of only ±0.7°.

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

The film growth of Cu on clean and O-precovered Ru(0001) at different growth 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 these temperatures. By using an O precoverage in the range of 0.1 monolayers (ML) up to a saturation coverage of 0.5 ML on clean Ru(0001), at 400 K different growth regimes are obtained. For ML a multilayer mode is preserved which changes into an O-induced two-dimensional (2D) growth for higher (0.2-0.5 ML). STM reveals the formation of an O/Cu surfactant structure on the surface due to migration of O initially located at the Ru surface. Its surface coverage rises linearly with O precoverage up to ML where it covers the surface completely. By increasing up to 0.5 ML, a drastic change in the morphology and density 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 , which changes into a disordered structure for ML. Structural similarities to oxidized surfaces of Cu(111) and the structures induced by postadsorption on Cu/Ru(0001) are discussed. Different models of surfactant mechanisms are presented to explain the observations. The locally ordered O/Cu surfactant structure (for ML) together with specific Cu film defects induce a heterogeneous nucleation of Cu with a high island density. Different mobilities of migrating Cu adatoms are established on top of the small islands and on the O/Cu structure resulting in enhanced interlayer diffusion explaining the observed 2D growth. The average island density only slightly changes within the temperatures investigated. In contrast, the saturated and disordered O/Cu surfactant structure (for = 0.4 - 0.5 ML) causes homogeneous nucleation. For this structure, the island density strongly depends on temperature and gives rise to an Arrhenius-like behaviour. The observed 2D growth is attributed to 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 ordered O/Cu bilayer on Ru(0001) - interpreted as a disrupted -like oxidized surface - was revealed.

The multilayer growth mode in the epitaxy of Ag on Ag(111) analysed by SPALEED

Abstract The morphology of atomically stepped hillocks formed during homoepitaxial growth on Ag(111) is analysed by SPALEED in the deposition range of 0–20 monolaycrs (ML). At 220 K the inclination of the facets increases linearly with coverage, with the terrace widths being correspondingly reduced. Facets with (100)-like steps are preferred over those containing (111)-like steps resulting in a more triangular shape of the hillocks. At room temperature the hexagonally shaped hillocks are larger owing to enhanced terrace widths. Moreover, the beginning interlayer diffusion limits the height of the hillocks to ~ 10 layers effecting a much slower reduction of the terrace widths after deposition of 3.7 ML.

Oxygen-induced restructuring of strained Cu layers on Ru(0001)

Abstract The interaction of O with strained Cu layers on Ru(0001) has been studied by means of high-resolution low-energy electron diffraction and scanning tunneling microscopy. An O-induced (3 × 2√3) structure with glide-plane symmetry is observed after exposure in the range 50–200 Langmuir at temperatures between 500 and 700 K on thin Cu films on Ru(0001). At Cu coverages > 2 monolayers the structure coexists with different heteroepitaxial Cu Ru (0001) strain structures. Since O adsorption is accompanied by a drastic mass transport of Cu atoms, we suggest that a two-dimensional O Cu surface structure is formed containing Cu in more than one layer.

Effects and structures of the O/Cu surfactant layer in O-mediated film growth of Cu on Ru(0 0 0 1)

The O-mediated Cu-film growth on O-precovered Ru(0 0 0 1) is investigated by means of scanning tunneling microscopy for growth temperatures between 300 and 600 K. Cu-films on clean Ru(0 0 0 1) grow in a multilayer mode. For O precoverages (Θ) between 0.2 ML (monolayer) and the saturation coverage (Θ=0.5ML), a layer-by-layer growth is observed at growth temperatures between 350 and 450 K. On Cu-islands, an O/Cu surfactant layer is formed, which floats on-top of the growing film and induces the layerwise Cu-film growth. The surface coverage of the O/Cu surfactant layer linearly rises with the O precoverage up to Θ≈0.4ML, where it completely covers the surface. Two different types of the surfactant layer are identified, inducing different surfactant mechanisms. For Θ=0.1–0.4ML, the 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 mobilities, implying a large attempt frequency for adatom jumps over the interlayer diffusion barrier at the steps. For Θ = 0.4–0.5 ML, a disordered O/Cu surfactant layer is established (B-type), inducing homogeneous nucleation. The 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 (3×2√3)O/Cu structure formed at temperatures around 520 K yields the conclusion that the O/Cu surfactant structures are composed of randomly arranged O–Cu–O strings and disrupted “Cu2O(1 1 1)” fragments.

Growth of Cu films on annealed Cu/O/Ru(0001) studied by STM

Abstract We have investigated the growth of Cu at 400 K on annealed Cu films initially grown on O-precovered Ru(0001) by means of STM. Annealing the layer-like Cu films on O Ru (0001) at 700 K results in a Stranski-Krastanov film morphology. Besides three-dimensional islands a two-dimensional base layer of Cu is formed which is three monolayers thick and covered by an O Cu surfactant structure. After Cu deposition on the base layer an enhanced nucleation density is found compared to the layer-like Cu films on O Ru (0001) . The film consists of two-dimensional islands of uniform size. Compared to the growth of Cu on O Ru (0001) at 400 K a more perfect layer-by-layer growth is induced.