Crystalline structure of the Au(1 1 1) surface revealed by stereographic intensity DEPES maps

The intensity of elastically backscattered electrons at the primary electron beam energy 1.9 keV was used to obtain a stereographic map of Au(1 1 1) by means of the directional elastic peak electron spectroscopy (DEPES). An experimental result is compared with the theoretical data obtained by using multiple scattering calculations (MS) performed for both not-reconstructed and model-reconstructed clusters. The lateral lattice misfit of the first layer leads to quantitative changes of theoretical intensities showing a sensitivity of DEPES to the short atomic chain axial order. This comparison proves that a main contribution of the experimental contrast originates from a higher background level. Moreover an anisotropy of the inelastic mean free path is discussed in the paper.     

Growth and crystalline structure of Co layers on Cu(0 0 1)

The crystalline structure of Co layers deposited on the Cu(0 0 1) surface was investigated with the use of the directional elastic peak electron spectroscopy (DEPES). For clean Cu(0 0 1) the experimental DEPES profiles obtained for different energies of the primary electron beam exhibit intensity maxima corresponding to the close packed rows of atoms. The Auger peak kinetics recorded during continuous Co deposition suggest the layer-by-layer growth mode. The DEPES profiles recorded for 10 monolayers (ML) of Co on Cu(0 0 1) reflect a short-range order in the adsorbate. Intensity maxima observed in the DEPES profiles for Co along [1 0 0], [0 1 0], and [1 1 0] azimuths of Cu(0 0 1) are characteristic of the face centered cubic (fcc) Co(0 0 1) layers. Low-intensity reflections and considerable background intensities were found in the low energy electron diffraction (LEED) patterns recorded from 10 ML of Co, which indicates a weak long-range order in the adsorbate. The adsorption of about 20 ML of Co results in considerable background contribution to DEPES. No reflections but a large background were observed with the use of LEED for this layer. The heating of the Co/Cu(0 0 1) system at T = 770 K leads to an increase of the short- and long-range order in the overlayer, observed in the DEPES profiles and LEED patterns, respectively. The theoretical DEPES profiles were obtained with the use of a multiple scattering approximation. A very good agreement between experimental and theoretical scans was found for the clean and covered copper substrate. The latter proves the epitaxial growth of Co layers on Cu(0 0 1).     

Influence of the ultra-thin Ag layer on Si(1 1 1)7 × 7 on directional elastic peak electron spectroscopy profiles

The directional elastic peak electron spectroscopy (DEPES) polar profiles for the clean Si(1 1 1)7 × 7 surface and the Si(1 1 1)√ 3 × √3R30°-Ag system are presented. The results were obtained for the and azimuths of the substrate for primary electron energies from the range 0.5-2 keV. A simple qualitative analysis of the observed profiles revealed the influence of the ultra-thin silver layer on the shape of the measured DEPES polar profiles, i.e. both on their background level and on the height of some intensity maxima. Thus, the information on the position of silver atoms in the investigated structure and other ultra-thin layers on crystalline substrates seems to be obtainable by the analysis of the DEPES profiles. The presence of numerous maxima in the measured profiles imply the application of a more advanced method in qualitative and quantitative interpretation of the DEPES profiles.     

Ultrafast electron dynamics in amorphous and crystalline D2O layers on Ru(0 0 1)

Femtosecond dynamics of excess electrons photo-injected into amorphous and crystalline D₂O layers on Ru(0 0 1) have been investigated by time-resolved two-photon photoelectron spectroscopy. In the crystalline case, excited electrons are transferred into delocalized states considered as image potential states in the conduction band of ice and relax back to the metal on an ultrafast time scale. The life time of the n = 1 image potential state is <5 fs. In the amorphous case, spectral features arise from delocalized and localized electronic states. Relaxation of delocalized electrons back to the metal is as fast as in the crystalline case. The binding energy of localized electrons, however, is found to increase as a function of time delay by 1 eV/ps, which is attributed to the formation of solvated electrons. Such energetic stabilization starting at the bottom of the conduction band is clearly absent in crystalline layers. This pronounced correlation of electronic structure and electron dynamics with molecular structure is associated with the presence of localized states near the bottom of the conduction band in amorphous ice. Such localized states are absent for perfect periodic crystalline structures but prevail in amorphous systems where they serve as precursor sites for electron solvation.     

Spin polarization and structure of molecular oxygen adsorbed on Fe(1 1 0)

We investigated the structural and magnetic properties of Fe(1 1 0) for molecular oxygen adsorption at room temperature. For the c(2 × 2) and c(3 × 1) superstructures, spin-polarized secondary electron emission (SPSEE) induced by protons and electrons reveals a nearly unchanged polarization compared to clean Fe(1 1 0). An appreciable decrease in polarization is found for the disordered layer of oxygen at a coverage Θ ≈ 1. This decrease is more pronounced for the spin polarization determined by electron capture (EC) to excited atomic He levels after grazing scattering. From a comparison of data obtained by proton-induced SPSEE and spin-polarized EC we conclude that the polarization at the vacuum boundary vanishes for an oxygen coverage Θ ≈ 1 while the polarization in the underlying Fe substrate layers is hardly changed.     

Surface morphology of epitaxial lattice-matched Ba0.7Sr0.3O on Si(0 0 1) and vicinal Si(0 0 1)-4°[1 1 0] substrates

Exploring ways for self-organized structuring of insulating thin films, we investigated the possibility to produce replicas of step trains, given by a vicinal Si(0 0 1)-4°[1 1 0] surface, in layers of crystalline and perfectly lattice-matched Ba_0.7Sr_0.3O. For this purpose, we carried out high-resolution spot profile analyses in low-energy electron diffraction (SPA-LEED) both on flat Si(0 0 1) and on Si(0 0 1)-4°[1 1 0]. Oxide layers were generated by evaporating the metals in oxygen ambient pressure with the sample at room temperature. Our G(S) analysis of these mixed oxide layers reveals a strong influence of local compositional fluctuations of Sr and Ba ions and their respective scattering phases, which appears as an unphysically large variation of layer distances. Nevertheless, we are able to show that quite smooth and closed oxide films are obtained with an rms roughness of about 1 ML. These Ba_0.7Sr_0.3O films directly follow the step train of Sr-modified vicinal Si surfaces that form (1 1 3) oriented facets after adsorption of a monolayer of Sr. This proves that self-organized structuring of insulating films can indeed be an effective method.     

Kinetic length and step permeability on the Si(1 1 1) (1 × 1) surface

Information on the kinetic regime of step motion and step permeability on the Si(1 1 1) (1 × 1) surface has been obtained from observations of island decay that were made with low energy electron microscopy. Island area during decay exhibits the expected power law dependence on time, with exponent, α, that is a qualitative indicator of the kinetic regime. A new method is presented for determining the kinetic length quantitatively from measurements of the decay exponent in the symmetric island decay geometry on top of a larger concentric circular island. Using this approach, we determine the kinetic length on the Si(1 1 1) (1 × 1) surface at 1163 K to be d ∼ 75a, where a is the lattice constant. It is shown that this result locates step motion firmly in the diffusion limited regime. Mass conservation of decaying island stacks is also observed at this temperature, which indicates that steps are effectively impermeable in the context of diffusion limited step kinetics.     

Scattering effects of primary electrons in Co/Cu(1 1 1) measured by Auger; elastic and loss electrons

Electron energy losses were measured as a function of the incidence angle of the primary electron beam for the Co/Cu(1 1 1) adsorption system. The measurements performed for the clean and covered substrate reveal characteristic intensity maxima associated with the close packed rows of atoms, as it was observed in the so called directional Auger and directional elastic peak electron spectroscopy profiles. The incidence angle dependent signal of electron energy losses measured for the clean (Cu 3p_3/2) and covered (Co 3p_3/2) substrate gives the so called directional electron energy loss spectroscopy (DEELS) profiles which contain structural as well as chemical information. The scattering of primaries and different emission processes associated with electron energy losses, Auger, and elastically backscattered electrons are discussed. A change in the h_Cu (Cu M_2,3VV transition) Auger signal recorded during the continuous cobalt deposition shows that the growth mode is not a pure layer by layer type. The complete covering of the substrate by Co at higher coverages is confirmed by the comparison between experimental and theoretical ratios of the Auger peak heights.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

Structure and magnetism of bcc(1 1 0)-like Fe films grown on Cu(0 0 1) via ion beam triangulation and spin-polarized electron capture

The structure and magnetism of thin epitaxial Fe layers grown on Cu(0 0 1) is investigated by grazing scattering of fast H and He atoms. Information on the atomic structure of the film and substrate surfaces is obtained by making use of ion beam triangulation with protons. The magnetic behavior is studied via the polarization of light emitted after capture of spin-polarized electrons into excited atomic terms during scattering of He atoms. For the formation of bcc(1 1 0)-like Fe films at higher coverages, we detect differences in structural and magnetic properties for room and low temperature growth. We suggest that the crystalline structure depends on the film morphology and that Cu impurities affect the magnetic properties.     

The Fe/ZnO(0 0 0 1) interface: Formation and thermal stability

The room temperature growth mode and the interface reaction of Fe films on single crystalline ZnO(0 0 0 1) substrates prepared in ultra high vacuum (UHV) has been investigated by means of X-ray photoelectron and Auger electron spectroscopy (XPS, AES), low energy electron diffraction (LEED) and low energy ion scattering spectroscopy (LEIS). The results show that Fe grows in the pseudo layer-by-layer mode. At ambient temperature the deposited Fe film reduces the underlying ZnO single crystal resulting in FeO at the interface and metallic Zn, which partially diffuses into the remaining Fe overlayer. Annealing leads to a stepwise oxidation of the Fe to FeO (670 K) and Fe₂O₃ (820 K). The Fe₂O₃ mixes with the substrate resulting in two (1 1 1) oriented textures of a spinel phase found by electron backscatter diffraction analysis (EBSD). Fe-based spin-injection may play a vital role for ZnO-based spintronic devices.     

Growth mode and electronic structure of Au nano-clusters on NiO(0 0 1) and TiO2(1 1 0)

The growth mode and electronic structure of Au nano-clusters grown on NiO and TiO₂ were analyzed by reflection high-energy electron diffraction, a field-emission type scanning electron microscope, medium energy ion scattering and photoelectron spectroscopy. Au was deposited on clean NiO(0 0 1)-1 × 1 and TiO₂(1 1 0)-1 × 1 surfaces at room temperature with a Knudsen cell at a rate of 0.25-0.35 ML/min (1 ML = 1.39 × 10¹⁵ atoms/cm²:Au(1 1 1)). Initially two-dimensional (2D) islands with thickness of one Au-atom layer grow epitaxially on NiO(0 0 1) and then neighboring 2D-islands link each other to form three-dimensional (3D)-islands with the c-axis oriented to the [1 1 1] direction. The critical size to form 3D-islands is estimated to be about 5 nm². The shape of the 3D-islands is well approximated by a partial sphere with a diameter d and height h ranging from 2.0 to 11.8 nm and from 0.95 to 4.2 nm, respectively for Au coverage from 0.13 to 4.6 ML. The valence band spectra show that the Au/NiO and Au/TiO₂ surfaces have metallic characters for Au coverage above 0.9 ML. We observed Au 4f spectra and found no binding energy shift for Au/NiO but significant higher binding energy shifts for Au/TiO₂ due to an electron charge transfer from Au to TiO₂. The work function of Au/NiO(0 0 1) gradually increases with increase in Au coverage from 4.4 eV (NiO(0 0 1)) to 5.36 eV (Au(1 1 1)). In contrast, a small Au deposition(0.15 to 1.5 ML) on TiO₂(1 1 0) leads to reduction of the work function, which is correlated with an electron charge transfer from Au to TiO₂ substrate.     

Crystalline structure of copper-gold alloys investigated by DEPES: Theoretical approach

We present results of calculations concerning the angle integrated electron emission from crystalline samples as a function of the incidence angle of the primary electron beam. Diffraction processes of primary electrons in directional elastic peak electron spectroscopy (DEPES) were described by the single scattering cluster (SSC) approximation applied to clusters composed of sublattices formed by different atoms. Theoretical polar profiles were calculated for Cu(0 0 1), Au(0 0 1), AuCu(0 0 1), and AuCu₃(0 0 1) in terms of phase shifts, inelastic mean free paths and scattering cross-sections. In calculations a real experimental geometry was considered concerning the acceptance angle of the analyser and polar rotation of the sample. Profiles calculated for particular incident electron beam energies depend on the structure and stoichiometry of crystalline samples. Application of above investigations is discussed in the context of superficial alloy formation and order-disorder transitions associated with the change of the concentration and order of alloy components in first few atomic layers.     

MeV-ion axial surface-channeling from an electron irradiated KCl(0 0 1) surface

Surface-channeling of protons incident with 5 mrad on an electron-irradiated surface is investigated using a 0.55 MeV beam of protons. The target surface is KCl(0 0 1), which is damaged by electron-stimulated desorption with 5 keV electron irradiation. The direction of the incident beam is adjusted along the 〈1 0 0〉 and 〈1 1 0〉 channeling conditions and the channeling-dips of the scattering yields are observed. The irradiation dose-dependence of the minimum yields and widths of the dips is measured. Two dips are compared, i.e., for the 〈1 0 0〉 and 〈1 1 0〉 channelings. By increasing the irradiation dose up to 2 × 10¹⁶ cm⁻², the dip around the 〈1 0 0〉 axis becomes opened, but that of the 〈1 1 0〉 axis becomes shallow. The irradiated surfaces are observed to have many overlapped terraces of sub-micron with monolayer steps. The surface morphology deformed by the irradiation effects to truncate trajectories of the 〈1 0 0〉 channeling protons. This roughness of the surface is more effective for the trajectories of the 〈1 1 0〉 channeling protons. The protons incident on the rough surface along the 〈1 1 0〉 axis are not reflected from the atomic row but reflected by a potential of the surface with steps. Results by a simple computer simulation of the trajectories of protons at stepped surfaces also indicate the scattering processes.     

The growth and structure of titanium dioxide films on a Re(1 0 −1 0) surface: Rutile(0 1 1)-(2 × 1)

Titanium dioxide films were grown on Re(1 0 −1 0) by Ti vapor deposition in oxygen at T = 830 K and studied by means of low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS) and X-ray diffraction (XRD). The Ti oxide stoichiometry was determined by XPS as Ti:O = 1:2, with the Ti oxidation state (4+). The TiO₂ growth was monitored by means of LEED as a function of film thickness. Extending the coverage from the submonolayer into the multilayer regime gives rise to a p(2 × 2) pattern, a (poorly ordered) (1 × 1), and, finally, a stable (2 × 2) structure, the latter being associated with a homogeneous TiO₂ phase. For normal electron incidence, the (2 × 2) LEED pattern exhibits systematically extinguished beams at (n ± 1/2, 0) positions, indicating a glide mirror plane. The pg(2 × 2) structure could be explained by both a rutile(0 1 1)-(2 × 1) reconstructed surface and a bulk truncated brookite(0 0 1) surface. Faceting phenomena, i.e. running LEED spots, observed with thin TiO₂ films point to the formation of a rutile(0 1 1)-(2 × 1) surface with two domains and {0 1 1}-(2 × 1) facets and rule out the brookite alternative. Confirmation of this assignment was obtained by an XRD analysis performed at the Berlin synchrotron facility BESSY.     

Methylthiolate-induced reconstruction of Ag(1 1 1): A medium energy ion scattering study

Medium energy ion scattering (MEIS), using 100 keV H⁺ incident ions, has been used to investigate the structure of the Ag(1 1 1)(√7 × √7)R19° -CH₃S surface phase. The results provide the first direct evidence that this structure does involve substantial reconstruction of the Ag surface layer. The measured absolute scattered ion yields and blocking curves are in generally good agreement with a specific structural model of the surface based on a reconstructed layer containing 3/7 ML Ag atoms, previously suggested on the basis of scanning tunnelling microscopy (STM) and normal incidence X-ray standing wave (NIXSW) studies. However, the MEIS data indicate that any rumpling of the thiolate layer, is small, and probably ?0.2 Å. This value is smaller than the amplitude suggested in the STM and NIXSW studies, but could be entirely consistent with the earlier experimental data.     

Structural and magnetic properties of evaporated Fe thin films on Si(1 1 1), Si(1 0 0) and glass substrates

We present experimental results on the structural and magnetic properties of series of Fe thin films evaporated onto Si(1 1 1), Si(1 0 0) and glass substrates. The Fe thickness, t, ranges from 6 to110 nm. X-ray diffraction (XRD) and atomic force microscopy (AFM) have been used to study the structure and surface morphology of these films. The magnetic properties were investigated by means of the Brillouin light scattering (BLS) and magnetic force microscopy (MFM) techniques. The Fe films grow with (1 1 0) texture; as t increases, this (1 1 0) texture becomes weaker for Fe/Si, while for Fe/glass, the texture changes from (1 1 0) to (2 1 1). Grains are larger in Fe/Si than in Fe/glass. The effective magnetization, 4πM_eff, inferred from BLS was found to be lower than the 4πM_S bulk value. Stress induced anisotropy might be in part responsible for this difference. MFM images reveal stripe domain structure for the 110 nm thick Fe/Si(1 0 0) only.     

High resolution analysis of the rotational levels of the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states of SO2

A high resolution (0.0018 cm⁻¹) Fourier transform instrument has been used to record the spectrum of an enriched ³⁴S (95.3%) sample of sulfur dioxide. A thorough analysis of the ν₂, 2ν₂ − ν₂, ν₁, ν₁ + ν₂ − ν₂, ν₃, ν₂ + ν₃ − ν₂, ν₁ + ν₂ and ν₂ + ν₃ bands has been carried out leading to a large set of assigned lines. From these lines ground state combination differences were obtained and fit together with the existing microwave, millimeter, and terahertz rotational lines. An improved set of ground state rotational constants were obtained. Next, the upper state rotational levels were fit. For the (0 1 0), (1 1 0) and (0 1 1) states, a simple Watson-type Hamiltonian sufficed. However, it was necessary to include explicitly interacting terms in the Hamiltonian matrix in order to fit the rotational levels of the (0 2 0), (1 0 0) and (1 0 1) states to within their experimental accuracy. More explicitly, it was necessary to use a ΔK = 2 term to model the Fermi interaction between the (0 2 0) and (1 0 0) levels and a ΔK = 3 term to model the Coriolis interaction between the (1 0 0) and (0 0 1) levels. Precise Hamiltonian constants were derived for the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states.     

High resolution scanning tunneling spectroscopy of ultrathin Pb on Si(1 1 1)-(6 × 6) substrate

The electronic structure of Si(1 1 1)-(6 × 6)Au surface covered with submonolayer amount of Pb is investigated using scanning tunneling spectroscopy. Already in small islands of Pb with thickness of 1 ML Pb_(1 1 1) and with the diameter of only about 2 nm we detected the quantized electronic state with energy 0.55 eV below the Fermi level. Similarly, the I(V) characteristics made for the Si(1 1 1)-(6 × 6)Au surface reveal a localized energy state 0.3 eV below the Fermi level. These energies result from fitting of the theoretical curves to the experimental data. The calculations are based on tight binding Hubbard model. The theoretical calculations clearly show prominent modification of the I(V) curve due to variation of electronic and topographic properties of the STM tip apex.