J. F. van der Veen

Thin epitaxial Ge−Si(111) films: Study and control of morphology

The morphology and surface structure of Ge−Si(111) films during the initial stages of heteroepitaxial growth are studied with high-resolution RBS, RHEED, SEM and TEM. It is shown that the equilibrium growth mode of Ge on Si(111) is of the Stranski-Krastanov type. After the formation of a stable film with a thickness of four monolayers, island formation takes place. Substantial indiffusion does not occur. Si on Ge(111) is found to exhibit Volmer-Weber growth. Control of the morphology was obtained by applying SPE on a Si/Ge/Si(111) multilayer system. By capping the Ge film in this way, islanding is suppressed up to annealing temperatures of ~450°C.

Surface-induced melting and freezing II. A semi-empirical Landau-type model

Thermodynamic criteria are derived for the occurrence of surface-induced melting and freezing in a semi-infinite system. Use is made of a mean-field theory of the Landau-type. The parameters which enter the Landau free energy functional are expressed in terms of known interfacial free energies. Expressions are obtained for the order parameter profile in the surface region and for the temperature-dependent thickness of the molten (frozen) surface layer. The results explain recent observations of surface melting (or nonmelting) at differently oriented crystal faces of Pb and account for the general absence of surface-induced freezing effects in liquids.

The adsorption of Ag on the Si(111) 7 × 7 surface at room temperature studied by medium energy ion scattering, leed and aes

Abstract We have studied the morphology and reactivity of ultrathin Ag films deposited on the Si(111) 7×7 surface at room temperature, employing the high depth resolution of Medium Energy Ion Scattering. In addition, LEED and AES measurements have been made. For coverages from zero to 5×1014 Ag atoms cm−2 we have observed the formation of a two-dimensional Ag layer, followed by the nucleation and growth of three-dimensional Ag islands for higher coverages (Stranski-Krastanov growth mode). Detailed analysis of the measured peak shapes in the energy spectra shows the presence of very high densities (≈ 1012 cm−2) of islands with height to width ratios less than 0.2. For the coverage range studied (zero to 1 × 1016 Ag atoms cm−2), the number of Si atoms, displaced from lattice sites does not increase above the value for the clean Si(111) 7×7 surface. This shows that no substantial mixing of Ag and Si occurs. The crystallographic relation between the Ag(111) crystallites and the Si substrate has been determined from LEED and Ion Blocking measurements, showing that part of the Ag crystallites have a preferential orientation, 180° rotated around the surface normal, with respect to that of the underlying substrate.

Melting of al surfaces

The thermal disordering of Al(110) and Al(111) surfaces was studied up to temperatures of 0.5 and 1.2 K below the bulk melting point Tm, respectively. With the use of medium energy ion scattering it was found that surface melting is present on the (110) face but not on the (111) face. The (110) disordered layer thickness increases with temperature as ln[Tm(T>m − T)]. The result in terms of a thermodynamic model that was presented earlier to explain the melting of Pb surfaces.

X-ray diffraction from rough, relaxed and reconstructed surfaces

A general formalism is presented for computing the scattered X-ray intensity from surfaces showing disorder in the form of atomic-scale roughness or in the form of reconstructed domains of finite size. Special attention is paid to the interference between bulk and surface contributions, since this is important in the determination of relaxation parameters or of the registry of a surface unit cell.

Monte carlo simulations of shadowing/blocking experiments for surface structure analysis

Abstract In the quantitative analysis of Medium Energy Ion Scattering experiments, combined with shadowing and blocking, Monte Carlo simulations play a key role. A new solution to the calculation of the backscattering intensities in single and double alignment geometries is introduced. With this new solution shadowing and blocking are no longer treated separately, which is of particular importance for accurate calculations of double alignment backscattering intensities. Results are compared with other calculations, which are briefly reviewed.

Photoemission studies of surface core‐level shifts and their applications

Surface core‐level shifts measured using photoemission with synchrotron radiation are reviewed for a variety of transition metals (Ta, W, Ir), rare earth metals (Yb, Sm, Gd, YbAl2), noble metals (Au), and semiconductors (Si, Ge, GaAs). Models of surface shifts are discussed, including the relationships of surface shifts to surface crystallography, reconstruction, valence electron configuration, surface cohesive energy, etc. Applications of surface shifts which are described include surface reconstruction on metals and semiconductors and high‐resolution chemisorption studies.

Ion-beam crystallography of clean and sulfur covered Ni(110)

Abstract Medium energy ion scattering has been used to determine the atomic structure of a Ni(110) surface covered with 0.5 monolayer of sulfur. After having confirmed that the sulfur atom resides in a fourfold-coordinated hollow site, it was found that its distance above the plane of the first Ni layer is 0.87 ± 0.03 A. We measured a 6 ± 3% outward relaxation effect for the sulfur covered Ni(110) surface layer and an inward relaxation of 4 ± 1% when this surface is clean.

Geometric structure of the NiSi2Si(111) interface: An X-ray standing-wave analysis

Abstract The atomic structure at the interface of single-crystal NiSi2 films on Si(111) has been determined with X-ray standing waves. Two types of films have been investigated, one with the same crystal orientation as the substrate (A type), and the other with its orientation 180° rotated about the surface normal (B type). For both orientations, the Ni atoms at the interface are found to be seven-fold coordinated. The bonds across the interface are contracted by 0.04 ∓ 0.05 A for the A-type film and 0.11 ± 0.03 A for the B-type film. For the first time the strain in an overlayer has been measured by the X-ray standing-wave technique, using, a moire effect between the overlayer lattice planes and the X-ray standing-wave field. The (111) interplanar distance in NiSi2 is found to be contracted by 0.4%.

Relaxation effects and thermal vibrations in a Pt(111) surface measured by medium energy ion scattering

Abstract Medium energy ion scattering, in conjunction with channeling and blocking has been applied to search for differences in the planar spacing of the first two layers of a Pt(111) crystal surface as compared to the spacing in the bulk. We find a small expansion of 1.5 ± 1%. The measurements have yielded a value for the transverse rms thermal vibration amplitude of a surface atom relative to its neighbour along the [001] direction and likewise for the [110] axis. The results are giving evidence for an enhanced amplitude for the surface atom perpendicular to the surface. A strong blocking effect has been observed along the [110] axis, which may be due to correlation effects, although the role of surface defects cannot be ruled out.