The atomic geometries of the ordered (2 × 2) and (√3 × √3)R30° phases of K, Rb, and Cs on Rh(111) in comparison with Ru(0001)

The local adsorption geometries of K, Rb and Cs in the (√3 × √3)R30° and (2 × 2) phases on a Rh(111) surface at coverages of 0.33 and 0.25 ML, respectively, are determined by analyzing LEED intensity data. For all (√3 × √3)R30° phases investigated, the three-fold hcp site is found. For the (2 × 2) overlayer, K remains in the hcp position, while Cs favors the on-top position. For the case of Rb-(2 × 2), LEED analysis suggests occupation of the unusual two-fold bridge site. Since LEED analysis of the Rb-(2 × 2) phase is not completely conclusive, additional experimental evidence is necessary to firmly establish this adsorption geometry.     

Catalytic dehydrogenation and thermal chemistry of cyclohexene and 1-methyl-1,4-cyclohexadiene on Si(111)7 × 7

Recently, we reported a thermal desorption study on the evolution of an intense mass 78 profile for the room-temperature exposure of cyclohexene to Si(111)7 × 7 surface, which was believed to give rise to the formation of benzene by a surface dehydrogenation reaction. Because mass 78 was also found to be the base ion in the gas-phase cracking patterns of both 1,3- and 1,4-cyclohexadiene, the dehydrogenation of cyclohexene on clean, sputtered and oxidized Si(111)7 × 7 surfaces has been re-examined in order to determine the origin of the intense mass 78 desorption profile; i.e. whether it was in fact due to the evolution of benzene or cyclohexadiene, or both. Moreover, a similar dehydrogenation reaction giving rise to toluene desorption between 350 and 600 K has been observed for the room-temperature exposure of 1-methyl-1,4-cyclohexadiene to clean and sputtered Si(111)7 × 7 surfaces. The effects of methyl substitution on the reactivity of these cyclic olefins towards Si(111)7 × 7 can be inferred from these studies. Furthermore, the catalytic activity of Si(111)7 × 7 was found to be enhanced significantly by extending the thermal desorption cycles to a higher temperature of 925 K. The dehydrogenation of these olefins on Si(111)7 × 7 also gave rise to a unique 7 × 1 low energy electron diffraction pattern. Possible factors that may play a role in any proposed model for the dehydrogenation reaction are discussed. Finally, evidence of other surface reactions including cyclohexene hydrogénation to cyclohexane will also be presented.     

A LEED, TPD and HREELS investigation of NO adsorption on Sn/Pt(111) surface alloys

The adsorption of NO on Pt(111), and the (2 × 2)Sn/Pt(111) and (√3 × √3)R30°Sn/Pt(111) surface alloys has been studied using LEED, TPD and HREELS. NO adsorption produces a (2 × 2) LEED pattern on Pt(111) and a (2√3 × 2√3)R30° LEED pattern on the (2 × 2)Sn/Pt(111) surface. The initial sticking coefficient of NO on the (2 × 2)Sn/Pt(111) surface alloy at 100 K is the same as that on Pt(111), S₀ = 0.9, while the initial sticking coefficient of NO on the (√3 × √3)R30°Sn/Pt(111) surface decreases to 0.6. The presence of Sn in the surface layer of Pt(111) strongly reduces the binding energy of NO in contrast to the minor effect it has on CO. The binding energy of β-state NO is reduced by 8–10 kcal/mol on the Sn/Pt(111) surface alloys compared to Pt(111). HREELS data for saturation NO coverage on both surface alloys show two vibrational frequencies at 285 and 478 cm⁻¹ in the low frequency range and only one N-O stretching frequency at 1698 cm⁻¹. We assign this NO species as atop, bent-bonded NO. At small NO coverage, a species with a loss at 1455 cm⁻¹ was also observed on the (2 × 2)Sn/ Pt(111) surface alloy, similar to that observed on the Pt(111) surface. However, the atop, bent-bonded NO is the only species observed on the (√3 × √3)R30°Sn/Pt(111) surface alloy at any NO coverage studied.     

LEED crystallographic investigation of ultrathin films formed by deposition of Sn on the Pt(111) surface

Deposition of Sn on the Pt(111) surface followed by annealing at 1000 K leads to the formation of ordered phases showing (2 × 2 and ( LEED patterns, depending on the surface coverage of Sn. Both these phases were studied by LEED dynamical analysis. The best agreement between experimental and calculated I–V curves was obtained by means of models based on the formation of mixed Pt-Sn layers on the surface where Pt and Sn atoms are nearly coplanar with a slight upward buckling of Sn atoms. The structures of these phases are similar to those already observed for the Pt₃Sn(111) surface.     

On the electronic and geometric structure of the Si(113) surface

The Si(113) surface of a p-type sample was studied by AES, LEED and ARUPS. On the clean surface, a (3 × 2) and a (3 × 1) LEED pattern coexist for a large range of annealing temperatures. Annealing to 900 K results in (3 × 1), while temperatures higher than 1050 K favour the (3 × 2) superstructure. ARUPS reveals two weakly dispersing surface resonances around 0.9 and 2.6 eV below E_F which are connected with the (3 × 2) and (3 × 1) structures, respectively. The work function was determined as φ = 4.81 eV and the photoionization threshold as ξ = 5.36 eV. The bands are bent downwards by 0.43 eV at room temperature.     

Hydrogen adsorption and surface structures of silicon

The adsorption of atomic hydrogen on silicon (111)2 × 1 cleaved, (111) 7 × 7, and (100) 2 × 1 surfaces has been studied by using electron energy loss spectrscopy (ELS) and Photoemission spectroscopy (UPS). On all surfaces the hydrogen removes the “dangling bond” surface state and a new peak in the density of states at lower energies corresponding to the SiH bond is found. The LEED pattern of the equilibrium surfaces (111) 7 × 7 and (100) 2 × 1 is not altered by hydrogen adsorption, while on the cleaved (111) 2 × 1 surface the fractional order spots are extinguished. The Haneman surface-buckling model therefore provides an explanation for the surface reconstruction of the cleaved (111) 2 × 1 surfaces. For the equilibrium surfaces, (111) 7 × 7 and the (100) 2 × 1, the data are consistent with the Lander-Phillips model.     

Substitutional adsorption of K on Fe(100)

The adsorption of potassium on Fe(100) was studied by time-of-flight forward scattering and recoiling spectroscopy (TOF-SARS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After heating to 650 K of the potassium saturated surface the formation of a p(3 × 3) potassium superstructure was observed by LEED. TOF-SARS experiments ruled out the adsorption of potassium in the on-top, bridge and four-fold hollow site. The only site which is in agreement with all experimental results is the substitutional site where K replaces an Fe atom of the topmost layer of the crystal. This is the first time a substitutional adsorption site has been found on a bcc surface. On an fcc surface such an adsorption site has been found recently for adsorption of sodium and potassium on Al(111).     

Hydrogen action in the surface space charge region of highly doped silicon

The action of atomic hydrogen on clean cleaved (1 1 1) surfaces of highly doped silicon samples, both phosphorus ([n] = 2 × 10¹⁹ cm^-3) and boron ([p] = 4 × 10¹⁹ cm^-3) doped has been compared to the case of lightly doped samples ([n] = 1 × 10¹⁴ cm^-3). Once cleaved under ultra high vacuum, the samples were exposed to increasing doses of atomic hydrogen up to saturation. Before and after each hydrogen exposure, the Si(1 1 1) 2 × 1 surface was studied by low energy electron diffraction (LEED) and photoemission yield spectroscopy (PYS). The compared PYS measurements show that H atoms adsorbed on the Si(1 1 1) surface at room temperature do totally compensate the shallow-acceptor impurities (boron) and only partially the shallow-donor impurities (phosphorus) in the space charge region. They also remove the surface dangling bond states. These effects are reversible upon heating under vacuum. Both surface stresses and space charge electric field play a role in this compensation effect.     

Electron energy loss spectroscopy studies of nitrogen adsorption on W(100)

Vibrational excitations of nitrogen on W(100) are investigated over the 100–300 K temperature range using elastic and inelastic electron scattering. New vibrational modes of nitrogen are identified that require different mode assignments from previous work. Experimental evidence for a molecular precursor to the atomic β₂ phase of adsorbed nitrogen is presented. Coverage dependent studies of vibrational modes suggests conversion between two different molecular surface phases and between atomic and molecular phases. A new ordered nitrogen phase characterized by a (4 × 1) LEED pattern is observed. The new phase appears to consist of orthogonal domains of p(4 × 1) symmetry that contain atomic nitrogen at the four fold sites (the β₂ atomic phase) with additional bridge-bonded nitrogen atoms in the unit cell.     

A multitechnique surface science examination of Sn deposition on Pt(100)

Interfaces prepared by vapor deposition of Sn onto Pt(100) surfaces have been examined using the following techniques: Auger electron and X-ray photoelectron spectroscopy (AES and XPS), low-energy electron diffraction (LEED), and low-energy ion surface scattering (LEISS) with Ne⁺ ions. Tin deposition was conducted at 320 and 600 K, and the surface composition and order was examined as a function of further annealing to 1200 K. The AES uptake plots (signal versus deposition time) indicate that the Sn growth mode can be described by a layer-by-layer process only up to one adayer at 320 K. Some evidence of 3D growth is inferred from LEED and LEISS data for higher Sn coverages. For deposition at 600 K, AES data indicate significant interdiffusion and surface alloy formation. LEED observations (recorded at a substrate temperature of 320 K) show that the characteristic hexagonal Pt(100) reconstruction disappears with Sn exposures of 4.6 × 10¹⁴ atoms cm² (θ_Sn = 0.35 monolayer (ML)). Further Sn deposition results in a c(2 × 2) LEED pattern starting at a coverage of slightly above 0.5 ML. The c(2 × 2) LEED pattern becomes progressively more diffuse with increasing Sn exposure with eventual loss of all LEED features above 2.2 ML. Annealing experiments with various precoverages of Sn on Pt(100) are also described by AES, LEED, and LEISS results. For specific Sn precoverages and annealing conditions, c(2 × 2), p(3√2 × √2)R45°, and a combination of the two LEED patterns are observed. These ordered LEED patterns are suggested to arise from ordered PtSn surface alloys. In addition, the chemisorption of CO and O₂ at the ordered annealed Sn/Pt(100) surfaces was also examined using thermal desorption mass spectroscopy (TDMS), AES, and LEED.     

Photoemission studies of the interactions of CdTe and Te with Si(100)

The interactions between CdTe, and in particular Te, and the (100) surface of Si have been probed using photoemission and low energy electron diffraction with a view to investigating the mechanisms responsible for (100) and (111) growth orientations for CdTe on Si(100). The interfacial reactions have been studied both on room temperature deposition followed by annealing and on depositions at typical epitaxial growth temperatures. In both cases the same precursor stage of an ordered submonolayer of Te on the Si(100) surface has been identified. Line shape analysis of the Si 2p core level has suggested a structural model in which Te adatoms make up an incomplete monolayer bound in bridge sites. This model is in excellent agreement both with the (1 × 1) LEED pattern and recent SEXAFS studies of this surface. The implications of the cubic symmetry of this surface in terms of the subsequent growth orientation of CdTe are discussed. Termination of the surface by Te was also seen to induce band bending suggestive of Fermi level pinning at around midgap, in contrast to the passivating behaviour of other group VI elements on this surface. The Si 2p core level line shape analysis on termination by Te has also provided evidence to support the “covalent dimer” interpretation of the clean dimerised Si(100) surface.     

The Si(111)−(5 × 1) Au structure

By combining recent results from STM, LEEM, LEED and X-ray diffraction a structure model is developed for the (5 × 1) structure observed in the Au/Si(111) system at low coverages.     

Investigation of the Si(111)-(7 × 7) surface by second-harmonic generation: Oxidation and the effects of surface phosphorus

We have studied the initial stages of oxidation, the temperature dependence of the surface electronic structure, and the effect of phosphorus on oxidation of the Si(111)-(7 × 7) surface using optical second-harmonic generation. We have also observed a (√3 × √3)R30° LEED pattern due to surface reconstruction induced by < 0.5% P on Si(111).     

Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

Dynamic observations of the formation of thin Cu layers on clean and hydrogen-terminated Si(1 1 1) surfaces

The growth of Cu on the clean and hydrogen-terminated Si(1 1 1) surfaces is studied in situ by low-energy electron microscopy (LEEM). The dependence of the growth of the “5×5” layer on the clean Si(1 1 1) 7×7 surface upon the deposition temperature is investigated by combining LEEM with LEED. After completion of the “5×5” layer not only the regular-shaped three-dimensional islands reported before are observed but also irregular shaped more two-dimensional islands. On the hydrogen-terminated Si(1 1 1) surface the formation of the “5×5” structure is suppressed and nano-scale islands form preferentially at the step edges and domain boundaries. This is attributed to the enhancement of the surface migration of Cu atoms by the elimination of the surface dangling bonds.     

Stability and surface properties of GeSi alloy films on Si(111) substrate

Several GeSi alloy films with different surface properties were prepared from a 500 Å thick Ge film that had previously been grown on a Si(111)-7×7 substrate by molecular beam epitaxy. The films were prepared by combinations of sputtering, annealing and Ge deposition from an evaporator. The surface properties were studied by Auger electron spectroscopy (AES) and by low energy electron diffraction (LEED). A novel LEED system employing position-sensitive detection was used. The Ge film surface gave a superposition of 7×7 and c(2×8) LEED patterns. A 7×7 → 1×1 phase transition was observed at 425±10°C. An irreversible 7×7 → c(2×8) transition was observed when the sample was heated above 500°C. The Ge film melted at 750±30°C and formed a Ge_xSi_1−x (x = 0.85±0.05) alloy whose surface gave a 7×7 LEED pattern. A 7×7 → 1×1 phase transition was observed at 600±0.15°C. Prolonged sputtering and annealing resulted in a Ge_xSi_1−x (x = 0.53±0.05) alloy whose surface gave a 5×5 LEED pattern. An apparent 5×5 → 1×1 phase transition was observed at 870±10°C but at that temperature the film was converted irreversibly to one with a much lower Ge atom fraction (x = 0.025±0.005) whose surface gave a 7×7 LEED pattern. A surface with a 5×5 pattern identical to that for the x = 0.53 alloy was prepared by deposition or Ge on Si. A similar 5×5 surface was prepared by deposition of Ge on a facetted GeSi alloy surface originally showing a superposition of 5×5 and 7×7 patterns. The intensity distributions in all of the 7×7 LEED pattern were found to be similar to those for Si(111)-7×7 at nearly the same electron energies. The characteristics of the 7×7 → 1×1 phase transitions were discussed in direct comparison with those of the Si(111)7×7 → 1×1 and Ge(111)-c(2×8) → 1×1 transitions observed with the same LEED system.     

Surface properties of Hafnium diboride(0 0 0 1) as determined by X-ray photoelectron spectroscopy and scanning tunneling microscopy

The surface structure and properties of the HfB₂(0 0 0 1) (Hafnium diboride, HfB₂) surface have been investigated with X-ray photoelectron spectroscopy, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Annealing temperatures above 1900°C produce a sharp (1×1) LEED pattern, which corresponds to STM images showing flat (0 0 0 1) terraces with a very low contamination level separated by steps 3.4 Å in height, corresponding to the separation of adjacent Hf planes in the HfB₂ bulk structure. For lower annealing temperatures, extra p(2×2) spots were observed with LEED, which correspond to intermediate terraces of a p(2×1) missing row structure as observed with STM.     

Hydrogen adsorption on Mo1−xRex(110) (x=0–0.25) surfaces

The effects of adsorbed H on the Mo_1−xRe_x(110), x=0, 0.05, 0.15, and 0.25, surfaces have been investigated using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). For the x=0.15 alloy only, a c(2×2) LEED pattern is observed at a coverage Θ0.25 ML. A (2×2) pattern is observed for H coverages around Θ0.5 ML from surfaces with x=0, 0.05, and 0.15. Both c(2×2) and (2×2) patterns are attributed to reconstruction of the substrate. At higher coverages, a (1×1) pattern is observed. For the alloy surface with x=0.25, only a (1×1) pattern is obtained for all H coverages. Two H vibrations are observed in HREELS spectra for all Re concentrations, which shift to higher energies at intermediate coverages. Both peaks exhibit an isotopic shift, confirming their assignment to hydrogen. For Re concentrations of x=0.15 and higher, a third HREELS peak appears at 50 meV as H (D) coverage approaches saturation. This peak does not shift in energy with isotopic substitution, yet cannot be explained by contamination. The intrinsic width of the loss peaks depends on the Re concentration in the surface region and becomes broader with increasing x. This broadening can be attributed to surface inhomogeneity, but may also reflect increased delocalization of the adsorbed hydrogen atom.     

Structural analogy between GeSi(111)-5 × 5 and Si(111)-7 × 7 surfaces

Low energy electron diffraction (LEED) patterns for the GeSi(111)-5 × 5 surface are reported and compared to those for the Si(111)-7 × 7 surface. Parallels between the observed LEED patterns are explained by a structural analogy between GeSi(111)-5 × 5 and Si(111)-7 × 7 surfaces. Both the (5 × 5) and (7 × 7) patterns are shown to be consistent with structural models of the triangle-dimer type previously proposed for Si(111)-7 × 7 surface.     

Studies on the ZrO/W(100) surface by means of low-energy electron diffraction and X-ray photoelectron spectroscopy

The low work-function ZrO/W(100) surface was examined with the aim of understanding the reducing mechanism of the work function. Low-energy electron diffraction (LEED) was employed to analyze the surface atomic arrangement, and X-ray photoelectron spectroscopy (XPS) was used to identify the surface chemical condition. The ZrO/W(100) surface was made as follows: (i) around three monolayers of Zr were deposited on a clean W(100) surface, (ii) the sample was heat treated in an oxygen ambience of 1.3x10⁻⁵ Pa for several tens of minutes at 1500 K, and (iii) the sample was flash heated at 2000 K in ultrahigh vacuum (UHV). During heat treatment in O₂, the deposited Zr was oxidized to ZrO₂, and the LEED pattern formed was p(2×1). The work function increased to 5.3 eV. Subsequent flash heating in UHV changed the p(2×1) LEED pattern into a c(4×2) pattern, and transformed ZrO₂ into the so-called Zr–O complex, the oxidized level of which is between ZrO₂ and metallic Zr. A drastic decrease in the work function to 2.7 eV ensued. The angular dependence of XPS showed that the Zr–O complex segregated within a few monolayers at the surface.