Surface-related Features Research Articles

Comparison of the optical properties of as-grown ensembles and single ZnO nanowires

By use of micro-photoluminescence spectroscopy we analyze and compare the optical properties of individual zincoxide nanowires, with diameters 90 nm<d<620 nm, and of the as-grown ensemble. After special preparation techniques individual nanowires of different morphologies and crystalline qualities are observed which possess distinctively different near band-edge excitonic features. We show that the spectral shape of these excitonic emission lines correlates with the morphology of the nanowires. Our results clearly show that for decreasing wire diameter, distinct surface-related spectral features strongly contribute to the optical properties of individual ZnO nanowires. Finally, the temperature dependence of the near band-edge emission is analyzed. The results obtained from individual wires provide information about the homogeneity of the optical properties of the wires in the as-grown ensembles, and show that easily performed ensemble measurements indeed reflect the properties of typical individual, single nanowires.

Twofold symmetry in the surface electronic structure of ZnSe(0 0 1)-c(2 × 2): Theory and experiment

Surface electronic band structure of ZnSe(0 0 1)-c(2 × 2) calculated using ab initio methods is compared to angle-resolved photoemission data. The experimental photoelectron spectra clearly reveal the surface-induced breaking of the fourfold bulk symmetry. Predicted surface-related features are identified in the experimental data and their origin is determined by comparison with layer-resolved band-structure calculations.

Angle-resolved photoemission from stoichiometric GaN(0 0 0 1)-1 × 1

We present results from an angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) investigation using synchrotron radiation of a stoichiometric Ga-polar GaN(0001)-1×1 sample grown by metal-organic chemical vapour deposition (MOCVD). The electronic structure of the surface was investigated with ARUPS along the Γ¯–K¯–M¯, Γ¯–M¯, and Γ–A directions of the main symmetry lines of the surface and bulk Brillouin zones. We find one surface state close to the valence band maximum (VBM) at Γ, dispersing down and emerging into the projected band gap in the region around K¯. From a comparison with a density functional theory (DFT) calculation [F.-H. Wang, P. Krüger, J. Pollmann, Phys. Rev. B 64 (2001) 035305-1], three additional surface related features are proposed, but no strongly dispersing Ga dangling bond band is observed. We find the VBM at 3.45eV below the Fermi energy.

Enhanced Raman scattering from nano-SnO 2 grains

We present the Raman spectra of nano-SnO2 grains with sizes from 4nm to 80nm excited by 532nm and 1.06μm lines. The enhanced Raman scattering of the nanograins is observed for both exciting lines when the grain size is less than 8nm. The less the grain size is, the more intensely the Raman scattering is enhanced. According to our results, the enhancements of the Raman intensity are a few tenfolds and different for different exciting lines when the grain size is 4nm. It can be attributed to enhanced Raman scattering by electron–hole pair excitations in the nanograins that originate from sub-microscopic (10nm) size and other defect- and surface-related features. A critical size that divides respective predominance of bulk properties and the defect-, surface- and size-related features can be determined to be about 8nm.

Preparation and properties of clean Si 3N 4 surfaces

In situ chemical methods for preparing atomically-clean surfaces of Si 3N 4 thin films in ultra-high vacuum (UHV) have been studied using X-ray and ultraviolet photoemission, electron energy loss and Auger electron spectroscopies. Prior to the UHV studies, the films (grown ex situ on Si(1 0 0) wafers by low-pressure chemical vapor deposition) were characterized using primarily infrared reflection–absorption spectroscopy. A combination of annealing in NH 3 to remove C and deposition of Si (followed by thermal desorption) to remove O is found to be an effective cleaning procedure. Other potential cleaning methods, such as annealing in UHV without in situ chemical treatment and annealing in a flux of H atoms, were also considered and found to be only partly effective. The clean surfaces are disordered but show no evidence of SiSi bonding (which would indicate N vacancies) in the Si LVV Auger spectrum or in surface-sensitive Si 2p photoemission data. Evidence for surface-related features is seen in the N 1s photoemission and in energy loss spectra in the region of valence excitations; however, no indication of occupied surface states near the valence band maximum is seen in ultraviolet photoemission spectra. Preliminary results for O 2 chemisorption show adsorbate-induced features in the Si 3N 4 band gap and also evidence for changes in surface potential due to adsorption.

Photoemission study of clean and oxidized nonpolar4H−SiCsurfaces

The results of a photoemission study of clean and oxidized nonpolar $(101\ifmmode\bar\else\textasciimacron\fi{}0)$ and $(112\ifmmode\bar\else\textasciimacron\fi{}0)$ surfaces of $4H\ensuremath{-}\mathrm{SiC}$ crystals are reported. For the clean surfaces prominent surface shifted components are revealed in the $\mathrm{C}1s$ spectra, although with different relative strengths for the two surfaces. In the $\mathrm{Si}2p$ spectra only one weak surface related feature is observed. These observations suggest carbon enrichment on both surfaces. Valence band spectra show a semiconducting behavior for both surfaces but also distinct differences. The lower lying subband, originating mainly from $\mathrm{C}2s$ states, is for example found to be considerably weaker on the $(101\ifmmode\bar\else\textasciimacron\fi{}0)$ than on the $(112\ifmmode\bar\else\textasciimacron\fi{}0)$ surface. Observed differences are suggested to originate from differences in surface carbon arrangement and bonding. It is concluded, from the absence of a graphitelike $\mathrm{C}1s$ peak, that the clean surfaces are not graphitized. The effects induced in the core levels and valence bands upon initial oxidation were investigated. The surfaces were oxidized gradually from 1 to ${10}^{6}\mathrm{L}$ while keeping the substrate at a temperature of 800 \ifmmode^\circ\else\textdegree\fi{}C. Recorded $\mathrm{Si}2p$ spectra show three oxidation states for both surfaces, and these are interpreted to originate from ${\mathrm{Si}}^{+1},$ ${\mathrm{Si}}^{+2},$ and ${\mathrm{Si}}^{+4},$ respectively. This is quite different compared to earlier results for the polar surfaces where only ${\mathrm{Si}}^{+4}$ and one suboxide were revealed on each surface. It is concluded that the ${\mathrm{Si}}^{+4}$ oxide $({\mathrm{SiO}}_{2})$ grow as a layer on top of the ${\mathrm{Si}}^{+1}$ and ${\mathrm{Si}}^{+2}$ suboxides that are located at the interface. The surface/interface related carbon is found to decrease dramatically, but not to be totally eliminated, after the large oxygen exposures made.

Sb induced (7×7) to (1×1) surface phase transformation of the Si(111) surface

Adsorption of Sb at a very low flux rate results in an epitaxial layer-by-layer growth on Si(111) surface held at room-temperature. Band-bending is not observed for submonolayer Sb coverages while sharp changes in the photoemission features are observed for 1.0 monolayer (ML) Sb adsorption. Changes in the core level binding energy and width in X-ray photoelectron spectroscopy, surface related feature in Electron energy loss spectroscopy and spot intensity ratios in Low energy electron diffraction studies suggest a surface phase transition upon adsorption of 1.0 monolayer of Sb. A plausible model is proposed to explain the abrupt metal–semiconductor transformation at this critical coverage of 1.0 ML.

Residual thermal desorption study of the room-temperature-formed Sb/Si(111) interface

This paper addresses issues of the subtle kinetic changes on the superstructural phase formation in the technologically important Sb/Si system. The thermal stability of the room-temperature (RT) deposited Sb on a $(7\ifmmode\times\else\texttimes\fi{}7)$ reconstructed Si(111) surface by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and electron energy-loss spectroscopy (EELS) is reported. At a very low Sb flux rate of 0.03 ML/min Sb uptake shows that it grows in the Frank--van der Merwe mode yielding a $(1\ifmmode\times\else\texttimes\fi{}1)$ LEED pattern for coverages of 1.0 ML and above. On annealing, AES shows that initially Sb adatoms agglomerate into large islands on top of a stable monolayer, before the Sb islands desorb in the temperature range of $350\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}--480\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, to leave a sharp $(1\ifmmode\times\else\texttimes\fi{}1)$ stable Sb monolayer. Monolayer desorption from about $650\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ results in several surface phases such as $d(2\ifmmode\times\else\texttimes\fi{}1),$ $(5\ifmmode\times\else\texttimes\fi{}5),$ $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ensuremath{-}R30\ifmmode^\circ\else\textdegree\fi{})$ and $(5\sqrt{3}\ifmmode\times\else\texttimes\fi{}5\sqrt{3}\ensuremath{-}R30\ifmmode^\circ\else\textdegree\fi{}).$ The $(5\ifmmode\times\else\texttimes\fi{}5)$ at 0.4 ML and the $(5\sqrt{3}\ifmmode\times\else\texttimes\fi{}5\sqrt{3}\ensuremath{-}R30\ifmmode^\circ\else\textdegree\fi{})$ at 0.2 ML are novel phases observed only during this desorption route. However, the 0.5--0.7-ML $(5\sqrt{3}\ifmmode\times\else\texttimes\fi{}5\sqrt{3}\ensuremath{-}R30\ifmmode^\circ\else\textdegree\fi{})$ phase, observed while desorbing from a 1.0-ML $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ensuremath{-}R30\ifmmode^\circ\else\textdegree\fi{})$ initial phase, is not observed here. The EELS studies show the differences in the surface-related electronic features of the various superstructural phases. The results demonstrate the differences in the superstructural phase formation due to differences in the formation pathways adopted.

Surface resonances versus surface states on Fe(110)

The spin-dependent surface electronic structure of ferromagnetic Fe(110) is investigated experimentally and theoretically. Spin-resolved, inverse-photoemission results show a complex multipeak structure close to the Fermi level. Part of it is surface derived with an unexpected spin dependence and light-polarization dependence. The puzzling experimental findings are interpreted on the basis of a comprehensive theoretical analysis. Electronic slab calculations find surface-related features, caused by the crystal--vacuum interface, only well below the Fermi level. Calculations of the (inverse) photoemission intensities within the relativistic one-step model based on a bulk band structure, but with a realistic surface barrier, reveal an additional surface resonance around the Fermi level. Its nearly vanishing exchange splitting at $\overline{\ensuremath{\Gamma}}$ and abnormal energy dispersion behavior as a function of the wave vector parallel to the surface are in accordance with the experimental findings.

Surface states on GaN( [formula omitted])(1×1)––an angle-resolved photoemission study

We present the results of angle-resolved photoemission studies on the (0 0 0 1 ̄ ) (N-polar) surface of bulk gallium nitride crystals. The bulk and surface band structure was investigated along Γ– A and Γ– K– M directions on the surface with (1×1) symmetry. The sets of spectra were obtained in normal and off-normal emission for the surface prepared by Ar + ion sputtering and annealing and for the same surface enriched with Ga under molecular beam epitaxy conditions. The bulk and surface related features were identified by analysis of the acquired experimental data and by comparison with the results of band structure calculations.

Angle-resolved photoemission study of the hydrogenated 3C-SiC(0 0 1)-2×1-H surface

We report the first angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) investigation of the hydrogenated 3C-SiC(0 0 1)-2×1-H surface using synchrotron radiation. The 2×1-H surface was formed by exposing a clean Si-terminated 3C-SiC(0 0 1)-c(4×2) surface to excited hydrogen gas. Desorption of the hydrogen at 900°C led to the restoration of the c(4×2) periodicity. The electronic structure of the 2×1-H surface was investigated with ARUPS along the main symmetry directions of the surface Brillouin zone. A strong hydrogen-induced state was observed in the Γ – J ̄ ′ direction about 4.3 eV below the Fermi energy ( E F), dispersing slightly downwards. This binding energy (2.4 eV below the valence band maximum ( E V)) is considerably lower than hydrogen-induced states on Si and Si-rich SiC surfaces, but can be explained by the existence of Si–C backbonds. A second, possibly surface related feature was observed at an energy of 1.9 eV below E F, in the same direction. These results have significant implications for the question about the atomic structure of the c(4×2) surface. The reversible c(4×2)–2×1 transition and the low binding energy of the H-induced state suggest a model with a single Si monolayer termination.

Valence-band photoemission fromGaAs(100)−c(4×4)

The energy distributions of photoelectrons emitted from the $c(4\ifmmode\times\else\texttimes\fi{}4)$ reconstructed GaAs(100) surface are carefully analyzed within the one-step model of photoemission, thus demonstrating that such calculations work for large unit cells. It is used for detailed interpretation of published and new angular resolved $\mathrm{He}I$ experimental data. Surface-related features are found and their localization and symmetry are determined. Backfolding of the electron energy bands and its splitting at the reduced Brillouin zone's boundaries lead to smaller energy dispersion of electron surface states and resonances. The experimentally observed longer period of a surface resonance than that corresponding to the translation symmetry of the reconstructed surface is confirmed and explained. The existence of two surface states near the upper valence-band edge is verified. One major difference between the theory and the experiment is found that cannot be explained by means of the one-step model for a perfectly reconstructed surface.

Electronic surface structure of CoSi 2(111)/Si(111): implications for ballistic electron-emission microscopy currents

Using a decimation technique, and imposing electrostatic self-consistency, we compute the surface electronic structure of various CoSi 2(111)/Si(111) phases. The projected band structures and LDOS indicate a richness of surface related features. For the (1×1)Co-rich termination excellent agreement with experimental data and a recent DFT investigation is obtained. Strongly localized surface states on the high chain atoms of the (2×1) Pandey-chain like reconstruction are identified as causing the experimentally observed surface topography induced contrast in Ballistic Electron Emission Microscopy (BEEM) images of such films.

Surface related features of laser welding of aluminium alloys

A program of research was carried out to investigate the dependence of laser welds in aluminium alloys upon proces parameters. Four different laser systems were used: carbon dioxide (CO2), carbon monoxide (CO), pulsed neodymium yttrium aluminium garnet (Nd:YAG) and high powered continuous Nd:YAG lasers.This paper describes phenomenon that occur at the surface of the weld pool and keyhole.The energy transfer from the laser beam to the workpiece, termed the coupling is a crucial factor in determining the heat input to the weld which is one of the major process variables. The physical mechanisms involved in this multi-part transfer are complex, hence an experimental route was taken to determine the coupling and its dependence upon laser and material properties. Coupling was measured for the combinations of five aluminium alloys and four laser systems.The high surface temperatures found in laser welding result in preferential evaporation of certain alloying elements. By studying welds made with a range of laser systems, the dependence of this effect upon process parameters could be observed. The high evaporation rates were also found to be linked to another problem that arises in laser welding-that of undercutting in autogenous welds.Porosity was observed and is discussed here in terms of its mechanism of formation.

3C SiC(001) surface structure studied by angular resolved photoelectron spectroscopy and reflectance anisotropy spectroscopy

Abstract We present a study of the (3×2) and c(2×2) reconstructed (001) surfaces of cubic (3C) silicon carbide (SiC) using angular resolved photoelectron spectroscopy (ARUPS) and reflectance anisotropy/difference spectroscopy (RAS/RDS). The carbon terminated c(2×2) structure and the (3×2) structure which is characterized by additional silicon (Si) dimers on the Si terminated surface were prepared by annealing the sample in a Si flux. Afterwards the geometric structures were confirmed by low energy electron diffraction. Comparing the ARUPS data of both reconstructions surface related features were identified and their dispersion in the reciprocal space was determined. The RAS data of the (3×2) surface also show a strong feature which is attributed to the sample surface. The position of the known V2 photoemission feature of the SiC(3×2) surface suggests the location of the corresponding electronic states in the ad-layer surface-dimers. This assignment is supported by comparing the RAS data of the SiC(3×2) surface to results obtained on silicon and gallium arsenide.

Effective-mass superlattice: surface states and zero energy gaps

The electronic structure of a terminated effective-mass superlattice (EMSL) is investigated taking into account the effect of the EMSL surface (i.e. the EMSL/substrate interface). Special attention is paid to the conditions under which two neighbouring miniband edges cross each other, which results in the vanishing of a corresponding mini-gap for certain EMSL bulk parameters (the so-called zero energy gap conditions). This is shown to influence considerably the surface electronic structure of EMSL and, in particular, to affect the existence of surface states. It is found that usually a surface state appears within one of the mini-gaps opening at the bulk band crossing point, so this mini-gap does not effectively exist, while the other mini-gap exhibits no surfacerelated features and, therefore, is well defined. The importance of the obtained results for the device applications of EMSL is pointed out.

The electronic structure of the InP(110) surface

As part of an extensive project on the surface electronic structure of semiconductor compound we report here on an experimental and theoretical study of the InP(110) surface. By comparison of angle-resolved photoemission data with results from empirical tight-binding calculations it has been possible to identify electronic surface related features and specify their origin

Spin-polarized surface states of Fe(100).

Spin- and angle-resolved photoemission has been used to reinvestigate the origin of some prominent surface-related features of the Fe(100) spectra. In agreement with earlier studies the spin-integrated spectra show intense surface emission near the Fermi level from states at the center of the surface Brillouin zone. The spin-polarization measurements show that this surface emission is of majority-spin character, in contrast with the previous interpretations. By exposing the clean surface to 0.1--0.4 langmuir ${\mathrm{O}}_{2}$ the intensity of the majority-spin surface-related feature is strongly reduced, whereas an oxygen-induced minority-spin state develops with increasing exposures at a binding energy of 1.4 eV.

Spin polarization of secondary electrons from Fe(110) excited by unpolarized primary electrons

We report on angle- and energy-resolved spectroscopy of secondary electrons from Fe(110). Intensity and polarization distributions in normal emission as a function of energy of the primary electrons are studied. For the first time angle-resolved spectra as a function of emission angle are measured and compared to theoretical calculations of the polarization fine structure. The influence of oxygen exposure on the polarization spectra is studied and surface-related features are identified. We conclude that surface resonances as well as the bulk band structure both contribute to the polarization fine structure. Consequences for the analysis of magnetic microstructures in a scanning electron microscope with polarization analysis are pointed out.

Experimental and theoretical surface component of the PL 2,3VV Auger lineshape in GaP(110): the use of ordered (1 × 1) Sb overlayers

An analysis of the theoretical and experimental surface contribution of the PL 2,3VV Auger lineshape in GaP(110) is performed. From the theoretical point of view the lineshapes are calculated within a single-particle framework from a slab-by-slab tight-binding calculation of the ideal and real relaxed surfaces. The experimental information is extracted by comparing the Auger feature of as-cleaved GaP(110) and of the GaP(110) interfaced with one ordered monolayer of Sb. This allows the LVV surface component to be isolated. A comparison with theory, showing strong similarities in the position of the surface-related features is discussed.