Surface structure evolution during Sb adsorption on Si(1 1 1)–In(4×1) reconstruction

The Sb adsorption process on the Si(1 1 1)–In(4×1) surface phase was studied in the temperature range 200–400 °C. The formation of a Si(1 1 1)–InSb (2×2) structure was observed between 0.5 and 0.7 ML of Sb. This reconstruction decomposes when the Sb coverage approaches 1 ML and Sb atoms rearrange to and (2×1) reconstructions; released In atoms agglomerate into islands of irregular shapes. During the phase transition process from InSb(2×2) to Sb (θ_Sb>0.7 ML), we observed the formation of a metastable (4×2) structure. Possible atomic arrangements of the InSb(2×2) and metastable (4×2) phases were discussed.     

RAS: An efficient probe to characterize Si(0 0 1)-(2 × 1) surfaces

A complete inspection of the capabilities of reflectance anisotropy spectroscopy (RAS) in studying the adsorption of molecules or atoms on the Si(0 0 1)-(2 × 1) surface is presented. First, a direct comparison between RA spectra recorded on the clean Si(0 0 1)-(2 × 1) and the corresponding topography of the surface obtained using scanning tunneling microscopy (STM) allows us to quantify the mixing of the two domains that are present on the surface. Characteristic RA spectra recorded for oxygen, hydrogen, water, ethylene, benzene are compared to try to elucidate the origin of the optical structures. Quantitative and qualitative information can be obtained with RAS on the kinetics of adsorption, by monitoring the RA signal at a given energy versus the dose of adsorbate; two examples are presented: H₂/Si(0 0 1) and C₆H₆/Si(0 0 1). Very different behaviours in the adsorption processes are observed, making of this technique a versatile tool for further investigations of kinetics.     

Covalent binding of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7

The adsorption reactions and binding configurations of cyclohexene, 1,3-cyclohexadiene and 1,4-cyclohexadiene on Si(1 1 1)-7 × 7 were studied using high-resolution electron energy loss spectroscopy (HREELS), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and DFT calculation. The covalent attachments of these unsaturated hydrocarbons to Si(1 1 1)-7 × 7 through the formation of Si–C linkages are clearly demonstrated by the observation of the Si–C stretching mode at 450–500 cm⁻¹ in their HREELS spectra. For chemisorbed cyclohexene, the involvement of π_C=C in binding is further supported by the absence of C=C stretching modes and the disappearance of the π_C=C photoemission. The chemisorption of both 1,3-cyclohexadiene and 1,4-cyclohexadiene leads to the formation of cyclohexene-like intermediates through di-σ bonding. The existence of one π_C=C bond in their chemisorbed states is confirmed by the observation of the C=C and (sp²)C---H stretching modes and the UPS and XPS results. DFT calculations show that [4 + 2]-like cycloaddition is thermodynamically preferred for 1,3-cyclohexadiene on Si(1 1 1)-7 × 7, but a [2 + 2]-like reaction mechanism is proposed for the covalent attachment of cyclohexene and 1,4-cyclohexadiene.     

Tensor LEED analysis for the electrodeposited Pt(1 1 1)-(3×3)–Ag,I surface structure

A crystallographic analysis is reported using low-energy electron diffraction (LEED) in the tensor LEED approach for the electrodeposited coadsorption (3×3) structure with 4/9 monolayer (ML) of silver and 4/9 ML of iodine on the Pt(1 1 1) surface. The structure approximates a two-layer slice of bulk AgI cut parallel to its (1 1 1) plane and superimposed on the substrate with the Ag atoms in contact with the topmost Pt(1 1 1) layer, and the I atoms forming an overlayer on the Ag atoms. There are two types of Ag atoms in the (3×3) unit mesh; one type bonds to a single Pt atom, while the other type bonds to three Pt atoms. The average Ag–Pt bond distances are close to 2.48 and 2.82 Å respectively for the one and three-coordinate Ag atoms, but both types of Ag atoms bond to three I atoms with an average Ag–I distance of 2.67 Å. No significant corrugation is observed for either the I layer or the Ag layer.     

Well-ordered (1 0 0) InAs surfaces using wet chemical treatments

Atomic ordering of HCl-isopropanol (HCl-iPA) treated and vacuum annealed (1 0 0) InAs surfaces was studied by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and reflectance anisotropy spectroscopy (RAS). On the as-treated surface, a diffused (1 × 1) pattern is observed, which successively evolves to the β₂(2 × 4)/c(2 × 8) and (4 × 2)/c(8 × 2) ones after annealing to 330 °C and 410 °C, respectively. At the intermediate temperature of 370 °C, an ₂(2 × 4)/(4 × 2) mixed reconstruction is observed. Reflectance anisotropy spectra are compared with those of the corresponding reconstructions observed after As-decapping and found to be quite similar. Therefore we conclude that high-quality (1 0 0) InAs surfaces can be obtained by wet chemical treatment in an easy, inexpensive and practical way.     

The Si(001) c(4×4) surface reconstruction: a comprehensive experimental study

We have carried out a comprehensive experimental study of the Si(001) c(4×4) surface reconstruction by scanning tunneling microscopy (STM) (at room temperature and elevated temperatures), Auger electron spectroscopy (AES), reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED). Si(001) samples were kept under ultra-high vacuum (UHV) at around 550°C until the c(4×4) reconstruction appeared. STM contrast of the c(4×4) reconstruction is strongly influenced by electronic effects and changes considerably over a range of bias voltages.

The c(4×4) surface reconstruction is a result of stress which is caused by incorporation of impurities or adsorbates in sub-surface locations. The resulting c(4×4) reconstruction in the top layer is a pure silicon structure. The main structural element is a one-dimer vacancy (1-DV). At this vacancy, second layer Si-atoms rebond and cause the adjacent top Si-dimers to brighten up in the STM image at low bias voltages. At higher bias voltage the contrast is similar to Si-dimers on the (2×1) reconstructed Si(001). Therefore, besides the 1-DV and the two adjacent Si-dimers, another Si-dimer under tensile stress may complete the 4× unit cell. This is a refinement of the missing dimer model.     

Large scale atomic ordering on uncovered GaAs(0 0 1) after InAs monolayer capping: Atomic structure of the (12 × 6) reconstruction

A well ordered c(8 × 2)-InAs monolayer is grown by molecular beam epitaxy (MBE) on a GaAs(0 0 1) substrate. After slow sublimation of this monolayer up to 560 °C, a homogeneously (n × 6) reconstructed GaAs surface is obtained. This surface is studied by scanning tunneling microscopy (STM) in UHV. This shows that it is well-ordered on a large scale with 200 nm long As dimer rows along and is also locally (12 × 6) reconstructed, the cell structure is proposed. We believe that this surface organization results from the specific As/Ga (0.7) surface atomic ratio obtained after the InAs monolayer growth and sublimation cycle.     

Coadsorption of water and CO molecules on Ru(0 0 1) at high CO coverages: comparisons with a Ru(0 0 1) electrode surface

The coadsorption of carbon monoxide (CO) and water molecules on a Ru(0 0 1) surface has been studied by infrared spectroscopy, LEED and STM. At high CO coverage phases, a 2×2-(2CO+D₂O) structure was observed on both UHV and electrode surfaces. Electrode potential dependent structures from CO and water adlayers on an electrode surface were reproduced on a UHV surface by controlling molecular orientations of the first layer and second over-layer water molecules. At lower CO coverages, a CO band center showed coverage dependent shift down to 1444 cm⁻¹ due to an electron transfer from a lone pair of a water molecule to CO 2π^*.     

Local Au coverage as driving force for Au induced faceting of vicinal Si(0 0 1): a LEEM and XPEEM study

Adsorption of Au at 850°C on a regular stepped 4° vicinal Si(0 0 1) surface results in a dramatic change of the step morphology: the surface decomposes into areas which are perfectly flat with a (0 0 1) orientation and (1 1 9) facets. Low energy electron microscopy shows the dynamics of the faceting process in real space while X-ray photoemission electron microscopy (XPEEM) allows a spatially resolved determination of the Au coverage at different stages of the faceting process. At a critical Au coverage of ≈1/3 ML (0 0 1) terraces are formed which extend anisotropically along the step edges of the surface. The steps in between the terraces bunch and form step bands in order to conserve the macroscopic miscut of the sample. Driving force for this morphological transformation is a complex (5×3.2) reconstruction formed on the (0 0 1) terraces. XPEEM shows this phase separation also for the Au coverage: on the (0 0 1) terraces the Au coverage is up to 40% higher compared to the step bands. With further increasing Au coverage the width of the Au rich terraces increases while the step bands become steeper. In a second step Au adsorbs on the step bands transforming them into well defined and smooth (1 1 9) facets.     

Decomposition mechanism of triethylindium (TEI) on a GaP(0 0 1)-(2×1) surface studied by LEED, AES, TPD and HREELS

Adsorption and decomposition of triethylindium (TEI: (C₂H₅)₃In) on a GaP(0 0 1)-(2×1) surface have been studied by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). It is found from the TPD result that ethyl radical and ethylene are evolved at about 300–400 and 450–550 K, respectively, as decomposition products of TEI on the surface. This result is quite different from that on the GaP(0 0 1)-(2×4) surface. The activation energy of desorption of ethyl radical is estimated to be about 93 kJ/mol. It is suggested that TEI is adsorbed molecularly on the surface at 100 K and that some of TEI molecules are dissociated into C₂H₅ to form P–C₂H₅ bonds at 300 K. The vibration modes related to ethyl group are decreased in intensity at about 300–400 and 450–550 K, which is consistent with the TPD result. The TEI molecules (including mono- and di-ethylindium) are not evolved from the surface. Based on the TPD and HREELS results, the decomposition mechanism of TEI on the GaP(0 0 1)-(2×1) surface is discussed and compared with that on the (2×4) surface.     

Adsorption mode change from adlayer- to restructuring-type with increasing coverage, evidenced by structural determination of c(2 × 2)→(4 × 4)→(5 × 5) sequence formed on Ni(001) by Li deposition

We determined surface structures in a structural sequence c(2 × 2)→(4 × 4)→(5 × 5) formed on Ni(001) at 370 K with increasing Li coverage by a dynamical low-energy electron diffraction analysis. The (4 × 4) and (5 × 5) are complex surface-structures involving restructuring of substrate surface atoms, and are analogous to the previously determined (3 × 3) and (4 × 4) structures formed for Li/Cu(001). The c(2 × 2) at low coverages is a Li adlayer, so a change of the adsorption mode from adlayer- to restructuring-type is evidenced in the course of increasing coverage within a monolayer range.     

Stabilization against initial oxidation using surface segregation of sulfur on Fe(1 0 0)

Surface segregation process of dissolved elements on a Fe(1 0 0) surface and the initial oxidation processes of the sulfur segregated surfaces were studied in situ by AES and RHEED. The surface enrichment of sulfur has formed a c(2×2) superstructure at elevated temperatures. The Fe(1 0 0)c(2×2)-S surface was found to have stabilization effect against initial oxidation. The retardation of oxidation was most enhanced when the sulfur surface concentration reached at the saturation coverage (1/2 ML). The possible physical origins of the stabilization effect are proposed.     

GaAs(0 0 1) surface reconstructions: geometries, chemical bonding and optical properties

We re-examine the GaAs(0 0 1) surface by means of first-principles calculations based on a real-space multigrid method. The c(4×4),(2×4) and (4×2) surface reconstructions minimize the surface energy for anion-rich, stoichiometric and cation-rich surfaces, respectively. Structural models proposed in the literature to explain the Ga-rich GaAs(0 0 1) (4×6) surface are dismissed on energetic grounds. The electronic properties of the novel ζ(4×2) structure are discussed in detail. We calculate the reflectance anisotropy of the energetically most favoured surfaces. A strong influence of the surface geometry on the optical anisotropy is found.     

Core level photoemission and STM characterization of Ta/Si(1 1 1)-7 × 7 interfaces

We present the results of scanning tunneling microscopy (STM) and photoemission spectroscopy (PES) of the Ta/Si(1 1 1)-7 × 7 system after deposition of Ta at substrate temperatures from 300 to 1250 K. The coverage of Ta varied from 0.05 up to 2.5 of a monolayer (ML). STM shows that at 300 K and coverage less than 1 ML, a disordered chemisorbed phase is formed. Deposition on a hot surface (above 500 K) produces round 3D clusters randomly distributed on the surface. Cluster height and their diameter are found to change drastically with annealing temperature and the Ta coverage. Analysis of photoemission data of the Si 2p core levels shows that at room temperature and at coverage ?1 ML core level binding energy shifts and intensity variations of Si surface related components are observed, which clearly indicate that the reaction starts already at 300 K. Shifts in the binding energy, changes of the peak shapes and intensity of the Ta 4f doublet at higher temperatures can be explained by the formation of stable silicide on the surface.     

Effect of Pb adatom flux rate on adlayer coverage for Stranski-Krastanov growth mode on Si(1 1 1)7 × 7 surface

Lead (Pb) has been a prototypical system to study diffusion and reconstruction of silicon surfaces. However, there is a discrepancy in literature regarding the critical coverage at which island formation takes place in the Stranski-Krastanov (S-K) mode. We address this issue by studying the initial stages of evolution of the Pb/Si(1 1 1)7 × 7 system by careful experiments in ultra-high vacuum with in situ characterization by auger electron spectroscopy, electron energy loss spectroscopy and low-energy electron diffraction. We have adsorbed Pb onto clean Si(1 1 1 )7 × 7 surface with sub-monolayer control at different flux rates of 0.05 ML/min, 0.14 ML/min and 0.22 ML/min, at room temperature. The results clearly show that the coverage of the Pb adlayer before the onset of 3D Pb islands in the S-K mode depends on the flux rates. LEED results show the persistence of the (7 × 7) substrate reconstruction until the onset of the island formation, while EELS results do not show any intermixing at the interface. This suggests that the flux rates influence the kinetics of growth and the passivation of dangling bonds to result in the observed rate-dependent adlayer coverages.     

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.     

A comparative infrared study of H2O reactivity on Si(1 0 0)-(2 × 1), (2 × 1)-H, (1 × 1)-H and (3 × 1)-H surfaces

The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1 × 1)-H and (3 × 1)-H surfaces show similar and less reactivity pattern compared to the (2 × 1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H–Si–Si–H + H₂O → H–S–Si–OH + H₂, which could give rise to the characteristic Si–H and Si–OH modes, respectively at 2081 and 921 cm⁻¹. A more suitable reaction mechanism involving a metastable species, H–Si–Si–H + H₂O → H₂Si  HO–Si–H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces.     

Scanned-probe topological and spectroscopic study of surface states on clean and Si-deposited GaAs (0 0 1)-c(4×4) surfaces

Microscopic topological and spectroscopic properties of MBE-grown GaAs c(4×4) surfaces without and with monolayer Si deposition were investigated by the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Empty state STM images on as-grown surface showed bright and dark cells, and they exhibited strong correlation with the spatial distribution of normal and anomalous conductance gaps of the STS spectra. Bias dependent STM images indicated presence of pinning areas with continuous space and energy distribution of surface gap states. By deposition of monolayer Si, dark areas reduced a great deal and the rate of finding normal STS spectra increased, indicating large reduction of surface states.     

Computational study of cycloaddition reactions on the SiC(1 0 0)-c(2 × 2) surface

Cycloaddition reactions between 1,3-butadiene and the C-terminated SiC(1 0 0)-c(2 × 2) surface have been addressed using quantum-chemical methods. The c(2 × 2) structure consists of ---CC--- bridges between underlayer Si atoms which themselves form Si---Si bonds. Of various possible reaction products, the one formed by a [2 + 4] reaction with the ---CC--- bridge (giving a species resembling 1,4-cyclohexadiene) is the lowest in energy. Density functional calculations for the bare c(2 × 2) surface, using a cluster model with mechanical embedding, gave good agreement with structural parameters obtained in previous fully ab initio studies. Similar calculations for the cycloaddition product and for the transition state gave a reaction energy of −50.3 kcal/mol and an activation energy of +6.1 kcal/mol to form a planar ring structure lying normal to the surface. Detailed results for the frequency and infrared polarization behavior of adsorbate vibrational modes have also been obtained.     

The structure of Na adsorbed on Ge(1 0 0) and its influence on substrate morphology

We investigated the adsorption of sodium on the (1 0 0) surface of germanium with LEED, STM and electron spectroscopy (XPS). Upon adsorption at room temperature a metastable p(4 × 1) and a p(2 × 1) superstructure have been found. Annealing of these structures, accompanied by thermal desorption, results in the formation of a commensurate p(3 × 2) phase after an incommensurate state has been passed. The formation of structures observed after annealing requires the rearrangement of substrate atoms. In addition strong evidence was found that all ordered phases discussed in this paper contain one adsorbate atom per unit mesh.