Low-energy Electron Diffraction Data Research Articles

STM atomic-scale characterization of the γ′-Al 2O 3 film on Ni 3Al(111)

We report the first atomic-resolution study by scanning tunneling microscopy (STM) of the γ′-Al 2O 3 film formed on the Ni 3Al(111) surface at ∼1000–1100 K. STM images of this well-ordered oxide film reveal a hexagonal array of corrugations with an average interatomic spacing of 3.0±0.1 Å. The low-energy electron diffraction (LEED) pattern of the aluminum oxide film reveals the ordering of O 2− ions in the surface. On the basis of LEED data and theoretical considerations, the corrugations in the STM atomic-resolution images are tentatively assigned to oxygen anions. Annealing the oxide prepared at ∼300 K to ∼1100 K results in the appearance of a metallic aluminum peak in the Auger electron spectra and a substantial increase in the Al (1396)/Ni (848) ratio, indicating aluminum segregation close to the surface.

Substitution sites of Pb and Y in Bi2Sr2Ca1Cu2O8+δ: X-ray photoelectron diffraction as fingerprinting tool

The substitution site of Y and Pb in the cuprate-type high temperature superconductor Bi2Sr2Ca1Cu2O8+δ is determined in a very direct and unambiguous way by means of angle-scanned x-ray photoelectron diffraction (XPD). Using XPD as a fingerprinting tool, we conclude that Y occupies the Ca sites and Pb the Bi sites, respectively. Furthermore, low-energy electron diffraction data unequivocally show the presence of the incommensurate lattice modulation which is known for pure Bi2212, but not for sufficiently Pb doped Bi2212. We can, therefore, attribute the reappearance of the modulation directly to the Y doping.

Structure of Al(100)-c(2×2)-Li: A binary surface alloy

The atomic geometry of the Al(100)-$c(2\ifmmode\times\else\texttimes\fi{}2)$-Li phase formed by adsorption of $\frac{1}{2}$-ML Li on Al(100) at room temperature has been determined by analysis of extensive low energy electron-diffraction (LEED) measurements. The structure is found to be a binary surface alloy, in which Li atoms occupy fourfold-coordinated substitutional sites formed by displacing every second Al atom in the first layer of the substrate. The surface structure is very similar to the (100) plane of the metastable, bulk ${\mathrm{Al}}_{3}\mathrm{Li}$ alloy. An analysis of experimental LEED data for clean Al(100) leads to the conclusions that the first interlayer spacing is expanded by about 2% with respect to the bulk value, and that the vibrations of Al atoms in the first layer are about twice as large as for atoms in the bulk. Adsorption of Li leads to a contraction of nearly 6% of the first Al-Al interlayer spacing, and further enhances the vibrations of Al atoms in the first, mixed Al/Li layer

Low sputter damage of metal single crystalline surfaces investigated with medium energy ion scattering spectroscopy

It was observed clearly that the sputter damage due to Ar + ion bombardment on metal single crystalline surfaces is extremely low and the local surface atomic structure is preserved, which is totally different from semiconductor single crystalline surfaces. Medium energy ion scattering spectroscopy (MEIS) shows that there is little irradiation damage on the metal single crystalline surfaces such as Pt(111), Pt(100), and Cu(111), in contrast to the semiconductor Si(100) surfaces, for the ion energy of 3–7 keV even above 10 16–10 17 ions/cm 2 ion doses at room temperature. However, low energy electron diffraction (LEED) spots became blurred after bombardment. Transmission Electron Microscopy (TEM) studies of a Pt polycrystalline thin film showed formation of dislocations after sputtering. Complementary MEIS, LEED and TEM data show that on sputtered single-crystal metal surfaces, metal atoms recrystallize at room temperature after each ion impact. After repeated ion impacts, local defects accumulate to degrade long range orders.

Initial stages of Al(111) oxidation by oxygen: Temperature and surface morphology effects

The initial stages of Al(111) oxidation by oxygen were investigated by using x-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy. It was found that surfaces exhibiting no impurities from the Auger electron spectroscopy and low-energy electron diffraction data may show different characteristic adsorption parameters (sticking coefficient, adsorbate vibrational modes, etc.). These parameters change rather dramatically with extended time of Ar+ bombardment followed by annealing prior to O2 adsorption. It was shown that extensive sputtering, followed by 700 K annealing, was necessary to prepare a surface with reproducible behavior. The integral reactive sticking coefficient, S(T), for the carefully prepared surface was evaluated by measuring the O(1s) XPS peak area for O2 exposures below 50 L. At surface temperatures from 95 to 773 K nonmonotonic behavior of S(T) was observed. A molecular precursor state preceding dissociative adsorption is proposed. It was found that the balance between oxidic and chemisorbed oxygen is strongly temperature dependent and influences the oxygen sticking coefficient on Al(111). A step-like formation of the oxidic phase in the temperature range 473–573 K was found to influence the adsorption kinetics. Oxidation causes the transformation of the O2 adsorption process from an activated into a nonactivated one.

An STM study of the potassium-induced removal of the Ni(100)(2×2)p4g-N reconstruction

In previous work we have shown using surface X-ray diffraction (SXRD) and low-energy electron diffraction (LEED) that the Ni(100)p4g(2×2)-N reconstruction is removed by coadsorption of ≥0.16 ML of potassium. Here we report scanning tunnelling microscopy (STM) observations of this coadsorbate system at 300 K and 120 K. At low coverages ( θ K≤0.10 ML) the potassium atoms decorate step edges where they are imaged as large protrusions. Once the steps are saturated, further adsorption takes place randomly on the terraces. The potassium atoms on the terraces interact strongly with nitrogen atoms, their diffusion is inhibited, and bias-dependent imaging reveals the disruption of the nitrogen overlayer underneath the potassium-induced protrusions. At higher coverages ( θ K≥0.16 ML), where it is known that the p4g reconstruction is removed, the protrusions appear to coalesce and the surface [although exhibiting c(2×2) periodicity] looks disordered. Lowering the temperature of the system after adsorption does not reveal any new behaviour. The results indicate that a local chemical interaction produces the removal of the reconstruction, and provide a plausible explanation for the coverage dependence of the LEED data.

Structure of the H-induced vacancy reconstruction of the (0001) surface of beryllium

A unique chemisorption structure has been determined for the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\mathrm{H}\ensuremath{-}\mathrm{B}\mathrm{e}(0001)$ phase formed at a saturation coverage of 1 ML and $Tl~270\mathrm{K}.$ The analysis of low-energy electron diffraction data shows that $\frac{1}{3}$ of the Be top layer atoms are removed to form a honeycomb structure of Be vacancies. Each vacancy is decorated by three H adatoms bonded in tilted bridge sites with a H-Be bond length and angle of 1.53 \AA{} (\ifmmode\pm\else\textpm\fi{}0.2) and 42\ifmmode^\circ\else\textdegree\fi{}(\ifmmode\pm\else\textpm\fi{}10), respectively.

Inversion of low-energy electron diffraction data with no pre-knowledge factor beyond optical holography

In this review, I describe how low-energy (typically below 500eV) electron diffraction spectra can be inverted to produce three-dimensional coordinates of atoms neighbouring a reference atom with no prior knowledge of what type or types of atom are present. The reference atom may be one of many equivalent near-surface atoms from which a core-level photoelectron is excited or, in the case of diffuse low-energy electron diffraction, one of many equivalent adsorbate atoms (lacking in long-range order) on the surface of a crystalline substrate. Other variants apply to low-energy electron diffraction, Kikuchi electron diffraction and time-reversed versions in which the wavenumber (energy) and direction of the incident beam are varied. I show that, for such low-energy electron diffraction spectra, the relative phases between the reference wave and scattered waves have a known geometric form if the spectra are taken from within a small angular cone in the near-back-scattering directions. By using the back-scattering small cone at each direction of interest, a simple algorithm is developed to invert the spectra and extract object atomic positions with no input of calculated dynamical factors. The article also reviews key ideas and works which led to the current understanding of this field.

Low Energy Electron Diffraction Analysis of Ultrathin Ag Films on W(110)

We have measured low energy electron diffraction data for clean W(110), ultrathin and thick Ag films on W(110). The data are analyzed by full dynamical multiple scattering calculations to determine the structure of the Ag-film/W(110) system. The multiple scattering calculation takes into account the incommensurate scattering between the non-pseudomorphic Ag films and the W(110) substrate. We have examined the effect of dynamical inputs used in the calculation. We find that for normally incident electrons, the surface barrier at the vacuum-film interface and the inelastic damping modify mainly relative intensities of the diffraction peaks while the energy-dependent inner potential at low energies influences peak positions. After the dynamical inputs are independently determined, we use the data below 25 eV where the electron's mean free path is long, to determine the layer spacing at the Ag film – W substrate interface. A major trend we find is that the layer spacing at the interface decreases as the Ag film's thickness increases.

Structure of the (100) surface of the icosahedral borideYB66

The composition and structure of the (100) surface of ${\mathrm{YB}}_{66}$ has been investigated with the techniques of angle-resolved x-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). The LEED and STM results reveal that the surface possesses a structure with the same periodicity as a bulk (100) plane. Angle-resolved XPS data shows that the topmost surface layer has a higher B to Y ratio than the bulk, while the ratio in the near surface region is approximately the same as the bulk crystal. STM images under a variety of tunneling conditions are compared with structural images drawn for various assumed bulklike surface terminations. A specific surface termination is proposed that is consistent with all of the STM, XPS, and LEED data.

Structural studies of sulfur-passivated GaAs (100) surfaces with LEED and AFM

We present the results of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and atomic force microscopy (AFM) analysis of sulfur-passivating layers on the GaAs (100) surface. The GaAs surfaces were passivated with both inorganic [(NH 4) 2S x ] and organic [octadecylthiol (ODT)] S-based compounds. We prepared the inorganic sulfur-passivated GaAs(100) surfaces with a wet chemical treatment using (NH 4) 2S solution. This was followed by thermal annealing of the treated sample in an ultra-high vacuum (UHV). After ex-situ and in-situ treatments, the surface resulted in a (2×1) LEED pattern. The LEED data ( I– E curves) were recorded and compared with dynamical LEED calculations for different structural models for the sulfur-passivated GaAs (100) surface. The results showed that the sulfur-passivated (2×1) surface structure is an arsenic–sulfur dimer on a Ga-terminated substrate. The ex-situ AFM results also revealed a (2×1) structure for the inorganic passivation and a very smooth surface for the organic ODT in the ethanol-treated sample.

Comparison of reflection high-energy electron diffraction and low-energy electron diffraction using high-resolution instrumentation

Reflection high-energy electron diffraction (RHEED) is a standard diffraction method in surface science, but contrary to low-energy electron diffraction (LEED) the analysis of morphology and defect structure is not as reliable due to inelastic scattering and a more complicated scattering geometry. These difficulties are mastered by adequate instrumentation and measuring procedures as done with a novel high-resolution, energy-filtered RHEED instrument. The elastically scattered intensity close to the specular beam is measured for many angles of incidence in both directions — parallel and perpendicular to the shadow edge. The series of profiles obtained perpendicular to the shadow edge have been used to produce profiles with constant momentum transfer perpendicular to the surface corresponding to LEED profiles with ultra-high resolution. By means of these profiles, the surface morphology is characterized on the basis of kinematic approximation as done in LEED with much success. The validity of that procedure is demonstrated for homoepitaxy on Si(111) by comparison of the results with LEED and STM data.

Hydrogen on W(110): an adsorption structure revisited

We present a quantitative study of the atomic structure of the clean and H-covered W(110) surface employing an analysis of lowenergy electron diffraction (LEED) as well as density functional theory (DFT) calculations. Our results give no evidence of a noteworthy reconstruction of the W(110) surface upon H-adsorption, and thus discard the widely accepted model of a H-induced lateral shift of the top layer based on earlier LEED data. Moreover, we offer a reinterpretation of the latter which goes along without such a surface reconstruction. In detail, we find good agreement between the LEED analysis and the DFT calculations on a small contraction of the first interlayer distance by about 3% for the clean surface, which is reduced to half this size at full H coverage. Hydrogen itself is found to be adsorbed in quasi-threefold coordinated hollow sites at a height of about 1.20 Å above the first substrate layer.

Electronic properties of Cu clusters and islands and their reaction with O2 on SnO2(110) surfaces

The deposition of Cu on SnO2(110) surfaces, and its oxidation to CuxO, have been studied by low-energy electron diffraction (LEED) and angle-integrated photoemission using synchrotron radiation photoemission spectroscopy (SRPES). With the growth of copper on SnO2(110), which was found to follow the Volmer-Weber (“islanding”) growth mode, a small amount of metal-phase Sn segregates to the surface, and even when the copper thickness reaches several tens of A, Sn metal still is seen at the surface. But when this surface is annealed at 800 K in 5 × 10−6 mbar O2 for 20 min, the Sn atoms are totally converted to SnO2. Simultaneously, the deposited Cu atoms become oxidized. The surface charges up both during LEED and SRPES data acquisition. The clean SnO2(110) surface shows a 1 × 1 structure. With Cu deposition, the substrate LEED pattern gradually becomes weaker. With even more copper deposited, a Cu(111)-1 × 1-oriented particle structure appears, indicating coalescence of the Cu islands to 3-dimensional Cu(111) epitaxy. After subsequent heating to 500 K, the substrate signal appears again, and we see the SnO2 1 × 1 pattern. In conclusion, Cu atoms quite easily form clusters on the SnO2(110) surface already after a slight heat treatment. The results show that this system is quite active towards O2 gas exposure, and that the surface conductivity changes during O2 exposure.

Surface atomic geometry of Si(001)-(2×1): A low-energy electron-diffraction structure analysis

The reconstruction of the Si(001)-2×1 surface consists of asymmetric and buckled Si dimers. The vertical separation between the up and the down atom within the dimer is about 0.72±0.05 A and the dimer bond length of 2.24±0.08 A has been found to be slightly smaller than the Si-Si distance in the bulk. The tilt of the dimer is 19±2°. The formation of Si dimers induces pronounced distortions in the substrate that were detectable down to the fifth Si layer. The structure determination is based on two independent low-energy electron-diffraction data sets taken in two different laboratories. The structural results agree well within the error limits, though noticeable differences occur between the experimental data sets. These differences in the experimental data can possibly be attributed to different preparation procedures.

STM, LEED and AES studies on the oxidation of a CrN overlayer on Fe 0.8Cr 0.2(100)

A combination of Scanning Tunneling Microscopy (STM), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED) was used to study the CrN surface phase grown on a Fe 0.8Cr 0.2(100) substrate. The co-segregation of Cr and N was achieved by annealing the Fe 0.8Cr 0.2(100) crystal to 833 K. LEED data of the annealed surface showed a sharp (1 × 1) pattern corresponding to the segregated CrN phase. STM images showed that the CrN overlayer is rough and inhomogeneous, exhibiting many deviations from ideal regularity. Steps were observed with terrace widths ranging from 5 to 65 nm. The step heights are in the range 0.2–3.5 nm. Most of the steps are oriented preferentially in the 〈100〉 direction. Oxidation of the surface results in disordering of the CrN layer (1 × 1) LEED pattern. STM measurements revealed oxide island formation at saturation coverage. Our Auger data suggest that a Cr oxide overlayer forms over the surface nitrogen. Auger spectra indicated that exposure to O 2 results only in the oxidation of Cr while the underlying Fe is still metallic. Tunneling current-voltage measurements and Auger spectra at different O coverages can be correlated with the surface oxidation behavior of the CrN overlayer. Comparison of the CrN(1 × 1) layer with a N-depleted surface indicates that surface N retards oxidation at 300 K.

Comparison of the interaction of Cl 2 and Br 2 with Cu(100)

The adsorption, reaction and etching of Cu(100) by Cl 2 was studied using temperature programmed desorption (TPD) and low energy electron diffraction (LEED), and the results were compared with recent results for Br 2. Although the general etching mechanism was the same for both gases (adsorption rate limited Cu halide formation followed by halide sublimation), significant differences between the behavior of Cl 2 and Br 2 were observed. The desorption of CuCl was characterized by a single zero order sublimation peak, independent of CuCl coverage, while limiting the CuBr coverage resulted in a desorption peak at temperatures lower than a prediction based on vapor pressure data of all known phases of CuBr. In addition, Cl 2 was found to be at least an order of magnitude less reactive than Br 2 towards halide formation. For both Cl 2 and Br 2, the halide formation rate reversibly decreased with increasing reaction temperature. However, for Br 2, but not Cl 2, annealing a chemisorbed halogen layer prior to further reaction irreversibly increased the halide formation rate. Structural differences between CuCl and CuBr on Cu(100) were also observed. For CuCl, LEED data suggested that highly faceted crystallites form at 325 K and remain stable until desorption, while LEED data for CuBr reveal a compressed epitaxial (111) layer that disorders below 400 K and then desorbs. The implications of these differences on etching and oxidation processes are discussed.

Pi -bonded-trimer formation on the clean diamond C(111) surface.

Using the local-orbital density-functional molecular-dynamics method, we have found a (2\ifmmode\times\else\texttimes\fi{}2) $\ensuremath{\pi}$-bonded-trimer structure, which has a slightly lower total energy than that of Pandey's (2\ifmmode\times\else\texttimes\fi{}1) $\ensuremath{\pi}$-bonded-chain structure, has occupied surface states/resonances that lie about 1 eV below the valence-band maximum in excellent agreement with angle-resolved photoemission data, and forms a (2\ifmmode\times\else\texttimes\fi{}2) pattern that may explain the low-energy electron-diffraction data. This surface structure has mixed-bond arrangements of a graphite structure and a diamond structure.

Ag films on [formula omitted]: from clean surfaces to atomic Ga structures

We prepare 150 nm thick epitaxial Ag buffer layers on Fe-precovered GaAs(001) at 380 K. Post-annealing at 570 K improves the morphology of the films, as observed by scanning tunneling microscopy (STM), without changing the chemical composition at the surface. The main type of defects are screw dislocations. Nevertheless we achieve a mean terrace width of 36 nm. At temperatures above 620 K, Ga atoms diffuse through the Ag buffer layer to the surface and form an overlayer. Depth profiles and quantitative analysis of X-ray photoemission data yield surface coverages of up to 0.3 ML and Ga concentrations within the Ag bulk of a few at%. The GaAs substrate acts as a source of Ga atoms even at temperatures much lower than the GaAs vacuum decomposition temperature of 890 K. STM images show that in some areas the overlayer consists of Ga tetramers. Their size and orientation are determined by the separation vector between next-nearest neighbour atoms of the Ag(001) substrate. In other areas the tetramers line up to form parallel rows of Ga pairs. This row structure is reflected in the low-energy electron diffraction (LEED) patterns as a (4√2 × √2)R45° superstructure. Based on STM and LEED data we propose a structure model with two Ga atoms per surface unit cell. Finally we predict a long-range interaction between rows and a relaxation of the GaGa separation within the rows.

The interaction of oxygen and hydrogen on a diamond C(111) surface: a synchrotron radiation photoemission, LEED and AES study

We report an investigation on the adsorption of atomic O and coadsorption with H on a diamond C(111) single crystal surface employing the core and valence level synchrotron radiation photoemission (SRPES) technique, Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Additional emission bands in the valence spectrum and a shoulder peak to the higher binding energy of the C 1s core level are found after oxygen exposure in the annealed C(111)-(2 × 1) reconstructed surface, indicating the formation of CO bonds on the surface. The CO reaction, however, does not convert the (2 × 1) reconstructed phase to the bulk-truncated (1 × 1) structure. Even at a saturation coverage of O, the LEED pattern shows the (2 × 1) structure, and the Auger and the photoemission spectra exhibit the characteristics of the reconstructed surface. This suggests a bonding of the O atoms to the (2 × 1) Π-bonded chain. The O-covered surface exposed to H atoms can drive the O from the surface, where it is replaced by H. Only a small amount of O is left on the surface. AES, LEED and SRPES data show the typical features of an H-terminated (1 × 1) state for the H/O/C(111) coadsorbed surface.