Surface-related Components Research Articles

Synchrotron-radiation photoemission study of the ultrathin Ba/3C–SiC(111) interface

Electronic structure of the Ba/3C–SiC(111) interface has been detailed studied in situ in an ultrahigh vacuum using synchrotron radiation photoemission spectroscopy with photon energies in the range of 100–450eV. The 3C–SiC(111) samples were grown by a new method of epitaxy of low-defect unstressed nanoscaled silicon carbide films on silicon substrates. Valence band photoemission and both the Si 2p, C 1s core level spectra have been investigated as a function of Ba submonolayer coverage. Under Ba adsorption two induced surface bands are found at binding energies of 2eV and 6eV. It is obtained that Ba/3C–SiC(111) interface can be characterized as metallic-like. Modification of both the Si 2p and C 1s surface-related components were ascertained and shown to be provided by redistribution effect of electron density between Ba adatoms and both the Si surface and C interface atoms.

Photoemission study of the Li/Ge(1 1 1)–3 × 1 reconstruction

In this article we report our findings on the electronic structure of the Li induced Ge(1 1 1)–3 × 1 reconstruction as determined by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and core-level spectroscopy using synchrotron radiation. The results are compared to the theoretical honeycomb-chain-channel (HCC) model for the 3 × 1 reconstruction as calculated using density functional theory (DFT). ARUPS measurements were performed in both the Γ ¯ - M ¯ and Γ ¯ - K ¯ directions of the 1 × 1 surface Brillouin zone at photon energies of 17 and 21.2 eV. Three surface related states were observed in the Γ ¯ - K ¯ direction. In the Γ ¯ - M ¯ direction, at least two surface states were seen. The calculated band structure using the single-domain HCC model for Li/Ge(1 1 1)–3 × 1 was in good agreement with experiment, allowing for the determination of the origin of the experimentally observed surface states. In the Ge 3 d core-level spectra, two surface related components were identified, both at lower binding energy with respect to the Ge 3 d bulk peak. Our DFT calculations of the surface core-level shifts were found to be in fair agreement with the experimental results. Finally, in contrast to the Li/Si(1 1 1)–3 × 1 case, no double bond between Ge atoms in the top layer was 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.

Disentangling surface and bulk photoemission using circularly polarized light

We show that in angle resolved photoemission spectroscopy (ARPES), near-surface induced fields can be useful for disentangling the surface and bulk related emission. The jump of the dielectric function at the interface results in a nonzero term $\mathrm{div}\phantom{\rule{0.2em}{0ex}}\mathbf{A}$ in the photoemission matrix element. The term happens to be significant approximately within the first unit cell and leads to circular dichroism for the states localized therein. As an example we use ARPES spectra of an $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ crystal to distinguish between the overdoped surface related component and its bulk counterparts.

Analysis of Core-Level Spectra of the Li/Ge(111)-3× 1 and Na/Ge(111)-3× 1 Surfaces

By analyzing the Ge 3 d core-level photoelectron spectra, we studied the structures of the Na/Ge(111)-3× 1 and Li/Ge(111)-3× 1 surfaces. For both surfaces, two surface related components lying on either side of the main bulk peak were identified in the spectra. The similarity of the spectra from the two surfaces indicates that the surfaces do indeed share a common structure. The core-level photoelectron spectra of the Na- and Li-induced Ge(111)-3× 1 surfaces are well consistent with the honeycomb-chain-channel model, which was proposed as the structure of the Si(111)-3× 1 induced by alkali metals.

Atomic and electronic structures of [formula omitted] surfaces

The atomic and electronic structures of 6H-SiC ( 0 0 0 1 ¯ ) - 3 × 3 reconstructions were analyzed by high-resolution medium energy ion scattering (MEIS) combined with photoelectron spectroscopy. We prepared three types of (3 × 3) surfaces by (a) annealing the Si-rich (2 × 2) surface at 1030 °C, (b) annealing the 3 × 3 silicate surface at 1050 °C, and (c) annealing the RCA-treated surface at 1050 °C in ultra-high vacuum. The present MEIS analysis reveals the fact that the (3 × 3) surfaces consist of a Si-adlayer (1.1 ML), C-adlayer (0.5 ML) and C-adatoms (1/3 ML)/C-adlayer (2/3 ML) on the 1st C–Si bilayer for samples (a), (b) and (c), respectively. Observation of the valence band spectra shows that all the surfaces are semiconducting and have dangling bond states in the band gap (1.4 eV below the Fermi level for sample (a)). The Si-rich surface (a) has one surface-related component in Si 2p and in contrast, the C-rich surfaces (b and c) have two surface-related components in C 1s spectra. It is shown that the (3 × 3) reconstructions of the SiC ( 0 0 0 1 ¯ ) are categorized into the above three types and take different atomic configurations. For the Si-rich (3 × 3) surface, the present analysis supports the structure model proposed by Hoster et al. [H.E. Hoster, M.A. Klakov, B. Bullemer, Surf. Sci. 382 (1997) L658]. The probable structures of the C-rich (3 × 3) surfaces are also discussed in detail.

Atomic geometry and electronic structure of theSi(100)2×3-Eu surface phase

The structural and electronic properties of the Eu-induced $2\ifmmode\times\else\texttimes\fi{}3$ reconstruction on Si(100) have been investigated by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and high-resolution Si $2p$ core-level spectroscopy using synchrotron radiation. STM images of this reconstruction are found to be drastically dependent on the bias voltage and tunneling current. STS measurements show that the $2\ifmmode\times\else\texttimes\fi{}3$-Eu surface is semiconducting. Two occupied states at $\ensuremath{-}0.9$ and $\ensuremath{-}1.75\phantom{\rule{0.3em}{0ex}}\mathrm{V}$ and three unoccupied states at $+0.35$, $+0.8$, and $+1.5\phantom{\rule{0.3em}{0ex}}\mathrm{V}$ are identified in the $(dI∕dV)∕(I∕V)$ spectrum of this surface. The Si $2p$ core-level spectra taken at various photon energies and emission angles are revealed to include four surface-related components $S1\ensuremath{-}S4$ with core-level shifts of $\ensuremath{-}0.54$, $\ensuremath{-}0.24$, $+0.21$, and $+0.51\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, respectively. The $S1$, $S2$, and $S4$ components are assigned to first-layer Si atoms in the $2\ifmmode\times\else\texttimes\fi{}3$-Eu structure and the $S3$ component is shown to arise from the second-layer Si atoms. The results are discussed in the context of structural models reported recently for other metal-induced $\mathrm{Si}(100)2\ifmmode\times\else\texttimes\fi{}3$ reconstructions. Finally, an atomic model is proposed for the $2\ifmmode\times\else\texttimes\fi{}3$-Eu phase.

Photoemission study on the Sb-induced reconstruction of the Si(1 1 2) surface

This study investigated the Sb-induced reconstruction of the Si(1 1 2)2 × 1 surface using low energy electron diffraction (LEED) and synchrotron radiation photoelectron spectroscopy (SRPES). When Sb was deposited on the clean Si(1 1 2) surface at 300 °C, a sharp hexagonal LEED pattern with weak “ n/5” fractional spots was observed, which is very similar to one of the Sb/Si(1 1 1)1 × 1 surfaces. The Si 2p and Sb 4d core level photoemission spectra were measured. The analysis results of the SRPES showed that the Si 2p core-level spectrum is composed of two surface-related components: a surface component at +0.2 eV and a small surface-related component at –0.2 eV with respect to the bulk component. In addition, the Sb 4d core-level spectrum was well reproduced with one doublet component. The results revealed that Sb atoms were adsorbed at unique sites on a Si(1 1 1)1 × 1 surface. On the basis of the nanofacet structure model of the Sb/Si(1 1 2) surface suggested by a previous scanning tunneling microscopy (STM) study, the structural model of the Sb/Si(1 1 2) surface was compared with the LEED pattern and the SRPES results.

Atomic structure of Si-rich6H‐SiC(0001¯)‐2×2surface

The atomic structure of the Si-rich $6\mathrm{H}\text{\penalty1000-\hskip0pt}\mathrm{SiC}(000\overline{1})\text{\penalty1000-\hskip0pt}2\ifmmode\times\else\texttimes\fi{}2$ surface has been determined by high-resolution medium energy ion scattering (MEIS) and photoelectron spectroscopy using synchrotron-radiation-light (SR-PES). The MEIS analysis reveals the fact that Si adatoms (0.2--0.3 ML) overlie a Si adlayer (0.8--0.9 ML) sitting on the bulk-truncated surface $(1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}=1.22\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{0.3em}{0ex}}\text{atoms}∕{\mathrm{cm}}^{2})$. In fact, we observed two surface-related components in the Si $2p$ core level spectra corresponding to the adatom and the adlayer Si atoms and the intensity ratio of the former to the latter was $\ensuremath{\sim}1∕3$. On the other hand, the C $1s$ core level observed has the bulk component only. We propose an adatom-adlayer model satisfying the $2\ifmmode\times\else\texttimes\fi{}2$ surface reconstruction, threefold symmetry and the results obtained by MEIS and SR-PES. Further MEIS analysis using focusing∕blocking effect clearly shows that the Si adatoms take a $H$ site. The surface structure predicted by the ab initio molecular dynamics calculation coincides with the above structure model except for slight lateral displacements of the Si adlayer and reproduces well the present experimental results of MEIS and SR-PES.

Atomic structure of theEu∕Si(111)3×2,5×1, and7×1surfaces studied by photoelectron spectroscopy

Eu-induced Si(111)3x2, 5x1, and 7x1 reconstructions have been investigated by low-energy electron diffraction (LEED) and high-resolution photoelectron spectroscopy using synchrotron radiation. According to LEED, the 3x2, 5x1, and 7x1 phases can be produced on the dominating area of the Si substrate at the 0.20, 0.40, and 0.45 monolayer (ML), respectively, and no contributions from other phases are found in the LEED patterns at these coverages. The Eu 4f spectra show that the 3x2, 5x1, and 7x1 surfaces are semiconducting, and that the Eu atoms are completely divalent in these reconstructions. Si 2p core-level spectroscopy measurements performed at various photon energies and emission angles reveal three surface-related components with core-level shifts of -0.51, -0.20, and +0.17 eV with respect to the bulk component for the 3x2 surface. In addition to these components, two other components with shifts of -0.78 eV and +0.28 eV are found for the 5x1 and 7x1 surfaces. These results are discussed in the context of previous studies and structural models reported in the literature. It is shown that the present photoemission data obtained for the 3x2 surface are well consistent with the honeycomb-chain-channel (HCC) model with an adsorbate coverage of 1/6 ML. For the 5x1 and 7x1more » surfaces, we propose atomic models which include combinations of honeycomb and {pi}-bonded Seiwatz chains, with adsorbate coverages of 2/5 and 3/7 ML, respectively. Based on the LEED and Si 2p core-level results, a local x2 periodicity is expected for the 5x1 and 7x1 reconstructions, which is in good agreement with the nonmetallic electronic structure of these phases.« less

High-resolution photoemission spectroscopy study of clean and Au-nanowire-decorated Si(5 5 12) surfaces

We have examined the Si $2p$ photoemission line shape of the clean $\mathrm{Si}(5512)2\ifmmode\times\else\texttimes\fi{}1$ surface in detail and have investigated the evolution of the Au-induced nanowires on this surface at different temperatures with low-energy-electron diffraction and photoemission spectroscopy. The Si $2p$ spectra from the clean $2\ifmmode\times\else\texttimes\fi{}1$ surface exhibit very complex line shapes composed at least of five different surface-related components. Possible origins of the most distinct surface component with a surface core-level shift of $\ensuremath{-}0.62\mathrm{eV}$ are discussed within the three different structure models proposed so far. Upon Au deposition, the intensity of that particular surface component was largely reduced indicating that the Au adsorbates interact dominantly with the specific Si surface atoms related. With an annealing above $500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ after Au deposition of 0.2 ML, the surface drastically changes into a metallic phase, as shown clearly in the valence band photoemission spectra, along with the gradual formation of the $(337)\ifmmode\times\else\texttimes\fi{}2$ facets observed in LEED. In this transition, a new Au $4f$ component appears while the Si $2p$ core-level spectra keep alike. This indicates that the surface metallization involves the rearrangement of Au adsorbates, possibly the formation of new Au chains on the surface.

Methanol adsorption on Si(1 0 0)2×1 investigated by high-resolution photoemission

The adsorption of methanol on Si(1 0 0)2×1 has been investigated at room temperature by high-resolution synchrotron radiation photoemission in the exposure range: 0.01–0.15 L. A combined evidence for a dissociative mechanism has been obtained from the analysis of the photoemission spectral features, which shows the absence of the ionization peak mainly based on the σ O−H bond in the valence band and the identification of a Si–O related component in the Si 2p core spectrum after adsorption. The line-shape analysis of the Si 2p core level shows the rising of two surface-related components upon adsorption. Their energy location allows for the assignment to Si–H and Si–OCH 3 bonds, as resulting from the interaction of the molecular fragments (H atom and methoxy group) with the surface. The growth of the Si–H and Si–OCH 3 related spectral components upon adsorption has been followed by a careful curve-fitting of the Si 2p line. The intensity increase of the new adsorption-induced components almost completely parallels the progressive intensity decrease of the surface dimer components, as found by the relative quantitative analysis of the Si 2p peak components. The analysis of the C 1s photoemission core-line suggests that no C–O bond rupture occurs at low exposure. A further fragmentation of the CH 3O– group occurs at higher exposure (>20 L), as we previously found for methanol adsorption on Si(1 1 1)7×7.

A parametric radiative transfer model for sky radiance distribution

A parametric model of sky radiance distribution is developed using radiative transfer theory. The radiation field is divided into four components: single-scattering; double-scattering; quasi-multiple-scattering; and surface related component. The first three components are associated with zero surface reflectance. The single-scattering and double-scattering radiance are exactly calculated, and quasi-multiple scattering is fitted statistically from the numerical code DISORT based on the discrete-ordinate algorithm. Surface bidirectional reflectance is incorporated into the last component. Comparisons of this simple model with a numerical code DISORT as well as another parametric sky radiance model are made under different conditions of aerosol optical depth, asymmetry parameter in the phase function and surface reflectance. Results indicate that the present model is very accurate and computationally efficient.

Core-level shifts of the Ge(100)-(2 x 1) surface and their origins.

The surface shifts of the Ge 3{ital d} core level from the clean Ge(100)-(2{times}1) surface have been investigated using Si epitaxial layers grown with Sb as a surfactant. We are able to obtain a line shape close to that of the nautral line shape of the Ge 3{ital d} and, in turn, we have resolved two surface-related components in the core-level spectrum of the clean surface, in contrast to previously observed single component. The origin of these shifts is addressed: one comes from the top layer and the other from the layer below the surface. Comparison with the Si(100)-(2{times}1) surface is also presented.

Simulated deposition rates for SO 2 on a southeastern U.S. landscape

Atmospheric transport and deposition of SO 2 are simulated over a circular area (22 km in radius) of representative landscape in East Tennessee. Data from aerial photographs were used to determine surface parameters which in turn allowed us to estimate deposition rates on the different ground cover types by taking account of both aerodynamic and surface related components of the process. Patterns of deposition rates are shown and are compared with spatial patterns of ground cover type. Area averaged values of deposition velocity are deduced for representative times of the year. We speculate on the influence of various physical factors as they bear upon the perceived sensitivity of different species to SO 2 damage.

A surface related component of iron metal in lunar samples?

SOLAR wind elements implanted into lunar soil grains come to rest within a few hundred angstroms, consequently high concentrations should build up in near surface locations. It has become common practice to ‘identify’ surface correlated species by investigating sieved soil fractions; the increased concentrations in fine grain sizes being attributed to the greater surface area. The approach used1–4, infers a surface location for finely divided iron metal (Fe(0)) and associated hydrolysable carbon (Chyd) in lunar samples. These studies, together with other evidence5, imply solar wind induced reduction processes were involved in producing metal from ferrous iron (Fe(II)) in silicates. The experiments described here performed with a reagent (CuCl2/2KCl), which should only attack exposed metal, have failed to recognise a genuine exposed surface component. We believe that some effect associated with solar wind sputtering is responsible for iron formation and we must therefore question the validity of the model on which the initial conclusions in favour of surface located metal were based.