Partially dissociative adsorption of water vapor on the Si(111) (7×7) surface

High-resolution vibrational electron energy-loss spectra have been measured of the Si(111) (7×7) surface exposed to water vapor at 300 K. Direct experimental evidence is given that water adsorption on the Si(111) (7×7) surface is partially dissociative.     

Ge adsorption on the Si(111) surface

Total energy calculations, performed for one monolayer of Ge adsorbed on Si(111), indicate that 1 × 1 models such as the atop site and hollow site adsorption geometries are unstable with respect to the formation of 2 × 1 Seiwatz chains of Ge adatoms. This result indicates that, for one monolayer coverage, Ge-Ge bonds are likely to form.     

First-principle study of Mg adsorption on Si（111） surfaces

We have carried out first-principle calculations of Mg adsorption on Si(111) surfaces. Different adsorption sites and coverage effects have been considered. We found that the threefold hollow adsorption is energy-favoured in each coverage considered, while for the clean Si(111) surface of metallic feature, we found that 0.25 and 0.5 ML Mg adsorption leads to a semiconducting surface. The results for the electronic behaviour suggest a polarized covalent bonding between the Mg adatom and Si(111) surface.     

Theoretical calculations of dissociative adsorption of CH4 on an Ir(111) surface

The activation energy for chemisorption of CH(4) at an Ir(111) surface is determined using density functional theory combined with an estimate of the long range dispersion interaction. The results are found to be in good agreement with published results of bulb and beam experiments analyzed with a precursor model. A surprisingly large surface relaxation is found where an Ir surface atom is displaced outwards by as much as 0.6 A. A strongly bound molecular state at kinks and adatoms involving eta(2)-H,H bonding was also found.     

New experimental studies on the adsorption of K on Si(100) and Si(111)

We report additional experimental characterization of the adsorption of K on Si(100) and Si(111). Our AES data reveal a layer-by-layer growth of K at room temperature on both surfaces. Measurements of the work function change on K adsorption present a minimum at a K coverage of half a monolayer. Furthermore, we find evidence for an incomplete charge transfer in contradiction to recent calculations. Our data could provide a solid basis to perform calculations in this interesting model system.     

Surface temperature effects in the dissociative adsorption of D2/Cu(111) revisited

We have calculated the dissociation probability of H₂ and D₂ on low dimensional potential energy surfaces (PESs) including substrate motion by attaching the PES to a simple harmonic oscillator or to a free mass. For all potentials examined, we find that the dissociation curve has a fixed midpoint while the width increases with surface temperature as observed experimentally. This is in agreement with earlier calculations by Hand and Harris and in disagreement with more recent ones. We conclude that the explanation of Hand and Harris, which ascribes the broadening to the approaching (receding) part of the oscillator phase increasing (decreasing) the relative velocity of the incident atom, is correct. This has the further consequence that the broadening is greater for D₂ than for H₂. We speculate that yet more broadening may be caused by thermally increased corrugation of the surface.     

Atomic steps on the Si(111) surface during submonolayer gold adsorption

The effect of adsorption of submonolayer gold coatings on the Si(111) surface morphology in the temperature range 850–1260°C has been investigated by means in situ ultrahigh-vacuum reflection electron microscopy and ex situ atomic force microscopy. Reversible transformations of the silicon surface: from regular monoatomic steps to step bunches, depending on the gold coverage and direction of the electric current resistively heating the crystal, have been revealed. Stability of the regular distribution of monoatomic steps upon heating of the crystal by an alternating current is shown. The effect of an electric field applied to the sample on the diffusion of silicon and gold adatoms has been analyzed taking into account the effective adatom charge.     

An electron spectroscopic investigation of the adsorption of NO on Ni(111)

The adsorption, decomposition, and desorption of NO on the close packed Ni(111) surface have been investigated by XPS, XPS satellites, XAES, UPS, and LEED between 125 and 1000 K. At adsorption temperatures below 300 K a single molecular species (v) is formed with about unit sticking coefficient, which is interpreted as bridge-bonded; its saturation coverage is about 85% of that of CO, i.e. 0.5 relative to surface Ni atoms. Adsorption at 300 to 400 K yields dissociative adsorption (β) followed by molecular adsorption; above 400 K only dissociated species are formed. Upon heating, a full molecular layer dissociates only after some NO desorption (at 380–400 K), while dilute layers (below half coverage) dissociate already above 300 K without NO desorption. Together with quantitative findings this shows that for dissociation of one v-NO, the space of two is required. N₂ desorption from the β-layer occurs above 740 K; the oxygen staying behind diffuses into the crystal above 800 K. Readsorption of NO onto a β-layer or onto an oxygen precoverage at 125 K leads, besides to an α₁-state similar to v-NO, to another molecular state (α₂) which is interpreted as linearly bound. The resulting total coverage is considerably higher than in a virgin layer. This shows that the blocking of dissociation in a full v-layer is probably not due to β requiring the same sites, but to kinetic hindrance; an influence of β-induced surface reconstruction cannot be excluded, however. The LEED results agree with a previous report and are well compatible with the other results.     

Effect of surface reconstruction on the adsorption of Ge on clean Si(111)

The effect of ultrahigh vacuum deposition of Ge below and at monolayer (ML) coverage onto a 7 × 7 reconstructed clean Si(111) surface held at room temperature is studied by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). The results are compared to those obtained on 2 × 1 reconstructed clean Si(111) : (i) the Si dangling bond states are replaced by Ge dangling bond states at submonolayer coverages in both cases; (ii) the 7 × 7 reconstruction persists below 1 ML, it is not replaced by a ? 3 × ? 3 R30° at 1/3ML as it was on the 2 × 1; and (iii) the coverage below 1 ML is not homogeneous on the 7 × 7 reconstruction. This behaviour can be explained by the influence of the inhomogeneties associated with the 7 × 7 reconstruction.     

Xe and Kr adsorption on the Si(111) 7 × 7 surface

A study of adsorption of Xe and Kr on the Si(111) 7 × 7 surface is presented. Low energy electron diffraction indicates the substrate structure is not appreciably modified by the adsorption. The data consist primarily of isobars, i.e. the amount adsorbed as a function of temperature at fixed pressure. The stepwise adsorption of each of the first several adatoms per surface unit mesh is resolved. This indicates a sequence of distinguishable adsorption sites in the mesh each of which can be occupied by a single atom. There are appreciable differences in the measured binding energies between the various sites. Pairwise sums of 6–12 potentials have been used to estimate binding energies expected for sites presented by proposed structural models of the surface. The observed sequence of sites seems to exclude buckled models and those models which involve only arrays of identical entities. No presently published model is consistent with these data in detail. The triangular island models with some modifications might present sufficiently large binding energies and rich sequences of sites. Whatever the actual structure, it must be more complex than those which have usually been considered.     

Structure and electronic properties of cleaved Si(111) upon Ge adsorption

The effect of ultrahigh vacuum deposition of Ge below and at monolayer coverage onto clean cleaved Si(111) surface held at room temperature is studied by low energy electron diffraction, Auger electron specroscopy and photoemission yield spectroscopy. A well ordered $\text{3}\text{×}\text{3}$ R 30° structure developes at $\text{1}\text{3}$ ML, where it replaces the 2 × 1 initial pattern; it persists at $\text{2}\text{3}$ ML before transforming into a 1 × 1 diagram which fades into increasing background at 1 ML and up. Si surface dangling bonds are replaced at $\text{1}\text{3}$ ML by states associated with Ge-Si bonds and Ge dangling bonds to which states due to Ge-Ge bonds added upon increasing coverage.     

Ab initio computer simulation of adsorption of a Fe monolayer on Si(111)

Computer simulation of adsorption of an Fe monolayer on Si(111) is carried out in the generalized gradient approximation in the density functional theory. It is shown that mixing of Fe and Si atoms followed by the replacement of Si atoms and the formation of a silicide-like film containing two types of domains (with the A8 and B8 structure) takes place in the course of deposition. Analysis of the atomic structure of this compound shows that the formation of this interface makes it possible to obtain an FeSi film (with the CsCl-type structure) as well as FeSi₂ film (with a CaF₂-type structure).     

Room temperature adsorption and growth of Ga and In on cleaved Si(111)

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS) measurements have been performed on a set of ultrahigh vacuum cleaved Si(111) surfaces with different bulk dopings as a function of Ga or In coverage θ. The metal layers are obtained by evaporation on the unheated substrate and θ varies from zero to several monolayers (ML). First, the 2×1 reconstruction of the clean substrate is replaced by a $\text{3}\text{×}\text{3}$ R30° structure at $\text{1}\text{3}$ ML, meanwhile the dangling bond peak at 0.6 eV below the valence band edge E_vs is replaced by a peak at 0.1 eV for Ga or 0.3 eV for In, below E_vs. At the same time, the ionization energy decreases by 0.4 eV (Ga) or 0.6 eV (In), while the Fermi level pinning position gets closer to the valence band edge by about 0.1eV. Upon increasing θ, new LEED structures develop and the electronic properties keep on changing slightly before metallic islands start to grow beyond θ ～1 ML.