Elastic Low-energy Electron Diffraction Research Articles

CALCULATION OF THE ATOMIC GEOMETRIES AND ELECTRONIC PROPERTIES OF ZnS (110) SURFACE

The total-energy minimization method is applied to the determination of the surface structure of the ZnS(110) surface. The predicted surface atomic geometry agrees well with that obtained from elastic low-energy electron diffraction intensity analysis. The electronic structure of ZnS(110) surface has been studied by using the method of Green's function scattering theory. Our results are eonsis-tent with the first principle calculations, and the influence of lattice relaxation on surface electronic properties are discussed.

Large-angle bond-rotation relaxation for CdTe(110).

A combination of x-ray standing-wave measurements and first-principles total-energy calculations have determined the surface relaxation of the clean CdTe(110) surface. These two methodologies directly determine the local atomic geometry and therefore provide a model-independent means of verifying the results obtained indirectly from elastic low-energy electron-diffraction (ELEED) studies. By comparing our results for this strongly ionic material (Phillips ionicity {ital f}=0.72) with the results of numerous earlier studies of less ionic III-V compounds, we are able to assess the influence of ionicity on the degree of relaxation for the cleavage faces of zinc-blende semiconductors. Our measurements and calculations indicate that, contrary to a previous theoretical study but in agreement with ELEED, the relaxation is approximately independent of the semiconductor ionicity. Our model-independent measurements yield a vertical buckling {Delta}{sub 1,{perpendicular}}=0.120{plus_minus}0.023 (in units of the lattice constant {ital a}=6.482 A), which agrees well with our calulated buckling of 0.114.

Spin polarization of low energy electrons scattered from magnetic surfaces

A full analytic approach to the elastic low energy electron diffraction (LEED) from magnetic surfaces is presented. In the approach the dynamical surface scattering LEED and exchange potentials are derived from the effective scattering potentials of a free atom. The LEED and exchange scattering amplitudes obtained in the paper are given in their analytical form and thus polarization effects are analyzed. The formula obtained for the polarization of electrons scattered from magnetic surfaces leads in a special kinematic case to that obtained by Saldana and Helman [7].

Surface states on GaAs(110)

The electronic structure of the (110) surface of GaAs is recalculated using the relaxation geometry recently obtained from analyzing elastic low-energy electron diffraction intensity data and a self-consistent ab initio pseudopotential approach. Better agreement is found for the occupied surface states compared with photoemission data, giving support for the new structural model. The influence of convergence of the plane-wave expansion and relativistic effects on the surface states is also examined.

Analysis of low-energy electron diffraction and angle-resolved photoemission from InP(110)-p(1 × 1)-Sb (1 ML)

The deposition of Sb on InP(110) at room temperature produces a stable, ordered, saturated adsorbate structure at a coverage of approximately one monolayer (1 ML, i.e., one Sb for each In and P surface species). An analysis of the atomic geometry of this InP(110)-p(1 × 1)-Sb(1 ML) overlayer system was performed by the comparison of dynamical calculations of elastic low-energy electron diffraction (ELEED) intensities with those measured at T = 110 K. A relativistic model embodying energy-dependent Hara exchange was utilized to calculate the electron scattering from the In, Sb and P ion cores because both relativistic effects and energy-dependent exchange have been found previously to be important in the analysis of ELEED intensity data from In and Sb containing compounds. On the basis of prior results for GaAs(110)-p(1 × 1)-Sb and of total-energy minimization calculations the structural search was confined to models in which the Sb adsorbates occupy inequivalent sites corresponding to the top layer In and P species on nearly unreconstructed InP(110). The quality of the description of the measured ELEED intensities by the model geometries was evaluated using the X-ray ( R X) and integrated intensity ( R I ) R-factors. Two comparable minimal R X structures are found corresponding to perpendicular displacements of Δ 1,⊥ = 0.25 A ̊ and 0.75 A ̊ , respectively, of the two inequivalent surface Sb species. In both structures the SbSb bond lengths are at nearly the value characteristic of bulk Sb (i.e., 2.87 Å) but the SbP bond length is anomalously small (i.e., 2.1 Å) for the Δ 1,⊥ = 0.75 A ̊ structure. Utilization of these structures to calculate the surface state eigenvalue spectrum reveals that both of them provide a satisfactory description of the surface states extracted from angle-resolved valence-electron photoemission.

Summary Abstract: Atomic geometries of the (110) surfaces of III–V compound semiconductors: Determination by total-energy minimization and elastic low-energy electron diffraction

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Sb overlayers on (110) surfaces of III–V semiconductors: A new type of chemical bond

An elastic low-energy electron diffraction (ELEED) structure analysis of ordered p(1×1) monolayers of Sb on GaAs(110) reveals a puzzle: the Sb atoms form chains within which the bonding is purely p2 but which exhibit tetrahedral (sp3) coordination with the substrate. No bulk or molecular compounds with such properties are known. Using a tight-binding total-energy minimization model we predict the atomic geometries of saturated Sb monolayers on the (110) surfaces of GaP, GaAs, GaSb, InP, InAs, and InSb. The known structure for Sb on GaAs(110) is obtained and the prediction for InP(110) is verified by a preliminary ELEED intensity analysis. The same model is used to predict the surface state eigenvalue spectra and to analyze the nature of the Sb-substrate bonds. The bonds are hybrids between pi states in the Sb chain and sp3 substrate dangling bond orbitals. Their existence is a unique surface phenomenon caused by the structural constraint of epitaxial growth of the Sb monolayer.

Calculation of the atomic geometries of the (110) surfaces of III–V compound semiconductors

A total-energy minimization method, developed earlier, is applied to predict the minimum-energy atomic geometries of the (110) surfaces of GaP, GaAs, GaSb, InP, InAs and InSb. These predicted geometries are shown to be in excellent correspondence with those determined by elastic low-energy electron diffraction intensity analyses. The magnitude of the thermal rms displacements of the surface atoms are predicted to be comparable to those of bulk atoms in agreement with ion channeling experiments in GaAs(110).

Displacements parallel to the surface of reconstructed GaAs(110)

Analysis of measured elastic low energy electron diffraction (ELEED) intensities from GaAs(110) reveals that the relaxations parallel to the surface associated with a ω1=29° bond-length-conserving rotated surface structure can be reduced by 75% without substantially affecting the quality of the model description of these intensities. Optimizing the surface geometry while constraining the atomic displacements parallel to the surface to be zero, however, leads to a significantly inferior description of these data. A model embodying greatly reduced but nonzero parallel displacements is compatible both with the ELEED intensities and with recent ion channeling data.

The ZnSe(110) puzzle: Comparison with GaAs(110)

New low-temperature (T∼200 K) elastic low-energy electron diffraction (ELEED) intensity data obtained for ZnSe films grown epitaxially on GaAs(110) are analyzed using an R-factor methodology. The differences between the measured ELEED intensities for ZnSe(110) and those for GaAs(110) reveal the possibility that these two surfaces may not exhibit comparable atomic geometries. A bond-length-conserving top-layer rotation of the Se species outward and the Zn inward corresponding to ω1=4° provides a description of the measured intensities (Rx =0.22, RI =0.21) comparable to that afforded by the ZnSe(110) analog of the GaAs(110) structure (a relaxed version of an ω1=29° structure, Rx =0.24, RI =0.16). Since GaAs and ZnSe exhibit essentially identical bulk lattice constants, the possibility that their (110) surfaces exhibit different atomic geometries poses a puzzle within the framework of current understanding of this topic.

The atomic geometry of Si(100)-(2×1) revisited

An R-factor analysis of published elastic low-energy electron diffraction (ELEED) intensity data from the (2×1) structure on Si(100) is presented. This analysis reveals the existence of a new asymmetric dimer structure which is compatible with ion-scattering measurements yet provides a description of the measured ELEED intensities which is improved relative to that afforded by structures proposed on the basis of prior ELEED intensity analyses. The structure is similar to but not identical with those predicted by several energy-minimization calculations. Consequently, our analysis resolves an extant incompatibility between ELEED and ion-scattering results as well as reveals the qualitative correctness of modern total-energy-minimization methods for surface-structure determinations

The atomic geometry of Si(100)-(2×1) revisited

An R-factor analysis of published elastic low-energy electron diffraction (ELEED) intensity data from the (2×1) structure on Si(100) is presented. This analysis reveals the existence of a new asymmetric dimer structure which is compatible with ion-scattering measurements yet provides a description of the measured ELEED intensities which is improved relative to that afforded by structures proposed on the basis of prior ELEED intensity analyses. The structure is similar to but not identical with those predicted by several energy-minimization calculations. Consequently, our analysis resolves an extant incompatibility between ELEED and ion-scattering results as well as reveals the qualitative correctness of modern total-energy-minimization methods for surface-structure determinations

The atomic geometries of GaP(110) and ZnS(110) revisited: A structural ambiguity and its resolution

The atomic geometries of GaP(110) and ZnS(110) are reexamined using our R-factor minimization procedure, developed for GaAs(110) and previously applied to GaSb(110), ZnTe(110), InAs(110), and AlP(110), to analyze experimental elastic low-energy electron diffraction intensities. Unlike most of the earlier cases, both GaP(110) and ZnS(110) exhibit two distinct minimum-Rx structures which cannot be distinguished by analysis of the shapes of the intensity profiles alone. One region of best-fit structures exhibits top-layer displacements normal to the surface characterized by a small bond-length-conserving, top-layer rotation (ω∼2–3°), a small relaxation of the top layer away from the surface, and a 10% expansion of the top-layer bond length. The other region of best-fit structures is the conventional one: nearly bond-length-conserving rotations of ω=26–28° in the top layer and a small (∼0.1 Å) contraction of the uppermost layer spacing. This ambiguity may be removed, however, by consideration of the integrated beam intensities. The conventional region of structural parameters provides a decisively better description of the relative magnitudes of the integrated beam intensities and hence is the preferred structure.

Comparison of the atomic geometries of GaSb(110) and ZnTe(110): Failure of ionicity-structure correlations

The atomic geometries of the (110) surfaces of GaSb and ZnTe are determined by comparing calculated elastic low energy electron diffraction (ELEED) intensities with those measured at T=125 K. The quality of the model description of the observed intensities is measured by the x ray R factor Rx. Using the minimization of Rx as the structure determination criterion, the atomic geometries of these two surfaces are shown to be very similar, in fact, nearly identical to within the uncertainties inherent in the analysis. GaSb(110) is reconstructed via a bond-length conserving rotation in its uppermost atomic layer characterized by ω1=30±2°, and the Sb relaxed outward from the substrate. ZnTe(110) exhibits an analogous bond-length conserving rotation of ω1=28±2° in its uppermost layer. In addition, however, this layer is relaxed by 0.05±0.05 Å toward the substrate, and the second layer exhibits a counter relaxation of the Zn outward by 0.025±0.05 Å and the Te inward by 0.025±0.05 Å. The spectroscopic ionicity of GaSb is fi=0.26, whereas the difference in Pauling electronegativities of Sb and Ga is ΔX=0.3. For ZnTe the values are fi=0.55 and ΔX=0.5. The absence of second-layer relaxations for GaSb(110) reveals that such relaxations are not characteristic of all highly covalent compound semiconductors (i.e., fi<0.5, ΔX<0.7). Moreover, their suggested presence for ZnTe(110) reveals that their absence is not characteristic of materials with fi>0.5. Thus, this characteristic feature of the surface atomic geometries of the (110) surfaces of zinc-blende-structure compound semiconductors does not correlate simply with either the spectroscopic or Pauling ionicities.

Electronic structures of the monohydride (2×1):H and the dihydride(1×1)∷2HSi(001) surfaces studied by angle-resolved electron-energy-loss spectroscopy

We have identified the monohydride Si(001)-(2\ifmmode\times\else\texttimes\fi{}1):H surface and the dihydride $\mathrm{Si}(001)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)\ensuremath{\mathbin{::}}2\mathrm{H}$ surface by angle-resolved electron-energy-loss spectroscopy and elastic low-energy electron diffraction. On the monohydride (2\ifmmode\times\else\texttimes\fi{}1):H surface the ${S}_{3}$ transition from the back-bond surface state was distinctly observed although the ${S}_{1}$ transition from the dangling-bond surface state disappeared, while on the dihydride $(1\ifmmode\times\else\texttimes\fi{}1)\ensuremath{\mathbin{::}}2\mathrm{H}$ surface both the ${S}_{3}$ and the ${S}_{1}$ transitions completely disappeared. These facts show that on the monohydride surface the subsurface strain due to dimerization remains; on the other hand, on the dihydride surface the strain is healed out. The hydrogen-induced transitions for the two surfaces were clearly distinguished; the transition energy for the (2\ifmmode\times\else\texttimes\fi{}1):H surface (S${\mathrm{H}}_{1}$) was 8.0 eV and that for the $(1\ifmmode\times\else\texttimes\fi{}1)\ensuremath{\mathbin{::}}2\mathrm{H}$ surface (S${\mathrm{H}}_{2}$) increased from 7.0 to 7.5 eV with increasing $|{k}_{\ensuremath{\parallel}}|$ due to dispersion. The present results support and develop the models for the monohydride and the dihydride surfaces proposed by Sakurai and Hagstrum.

Atomic geometry of GaSb(110): Determination via elastic low-energy electron diffraction intensity analysis

Elastic low-energy electron diffraction (ELEED) intensities from GaSb(110) of normally incident electrons with energies $30\ensuremath{\le}E\ensuremath{\le}210$ eV were measured at $T=125$ K. Intensity versus incident-energy profiles were recorded for 14 diffracted beams. The surfaces were prepared by a chemical-polish---ion-bombard---anneal cycle. The stoichiometry of the surfaces and reproducibility of the data from one sample to another were verified explicitly. Comparison of these measured intensities with dynamical ELEED intensity calculations indicates that the dimensions of the surface unit cell are identical to those of truncated bulk GaSb, but that the atomic geometry within that cell is reconstructed. The best-fit structure consists of a bond-length---conserving rotation by ${\ensuremath{\omega}}_{1}={(30\ifmmode\pm\else\textpm\fi{}2)}^{\ensuremath{\circ}}$ of species in the uppermost atomic layer with the Sb relaxing outward and the Ga inward. No displacements of the second-layer species are indicated by the analysis. The structure resembles those of ZnTe(110) and GaP(110), but is quite distinct from those of GaAs(110), InSb(110), and CdTe(110). This result reveals that ionicity alone is an inadequate index of the surface atomic geometries of compound semiconductors, independent of the definition chosen for the ionicity.

Room temperature hydrogenation of the Si(001)2 × 1 surface studied by angle-resolved electron energy loss spectroscopy

We observed the hydrogen adsorption on the Si(001)2 × 1 surface achieved at room temperature by angle-resolved electron energy loss spectroscopy (AR-ELS) and elastic low-energy electron diffraction. From measurements of the intensities of elastically diffracted beams, we found a characteristic hydrogen covered surface (called Si(001)2 × 1H(RT) surface in this paper), where all the diffracted beam intensities were enhanced drastically and a sharp 2 × 1 LEED pattern was observed. The angular dependence of the elastically diffracted beams on the 2 × 1H(RT) surface was different from that on the monohydride 2 × 1:H surface. On the 2 × 1H(RT) surface the S 3, transition from the back bond surface state disappeared in contrary to the 2 × 1:H surface and two hydrogen induced transitions were observed at 7.0 and 8.0 eV in AR-ELS spectra. We revealed that the 2 × 1H(RT) surface consisted of the monohydride and the dihydride phases with comparable weights. Additionally, we found the new transition S' 1, ascribed to the newly produced dangling bond surface state due to the rupture of the dimerization bond with hydrogen adsorption.

Determination and application of the atomic geometries of solid surfaces

In order to calculate the electronic charge density and excitation spectra associated with a surface, the positions of the atomic constituents in the vicinity of the surface must be known. This paper is an assessment of the accuracy with which these positions can be determined by state-of-the-art analyses of experimental measurements, especially elastic low-energy electron diffraction, ion scattering spectroscopy (at both medium and high energies), and photoemission spectroscopy. The methodology is that of comparing the results of various techniques as applied to specific systems. Our major finding is that when carefully and accurately applied, the various methods provide structural results which are consistent to within about 0.1 Å. Perusal of the literature reveals larger discrepancies which, however, have exhibited a tendency to disappear as a function of increasing time once conflicting results became sufficiently precisely specified that the origin of the discrepancies could be identified and resolved. The examples of Pt(111), GaAs(110) and CO on Ni(100) provide explicit examples of the convergence of the various spectroscopies to common results with the passage of time. Finally, two applications of spectroscopically-determined surface structures are illustrated: the determination of surface states at the GaAs(110) surface and the description of the nature and consequences of Al replacement reactions at Al-semiconductor interfaces.

The influence of electron exchange and relativistic corrections on elastic low energy electron diffraction from compound semiconductors

The influence of electron exchange and relativistic corrections on elastic low energy electron diffraction from compound semiconductors

The influence of electron exchange and relativistic corrections on elastic low energy electron diffraction from compound semiconductors

The model of the electron-solid interaction used for dynamical low energy electron diffraction (LEED) calculations is extended to include both an energy dependent local exchange interaction and relativistically computed ion-core charge densities. These extensions of earlier work based on non-relativistic, local exchange models are tested for the (110) surfaces of InP, ZnTe and InSb. Calculations reveal discernible differences between LEED intensities computed using the energy dependent exchange force and those obtained using energy independent local exchange forces. The replacement of the non-relativistic ion-core potential with a relativistic one produces smaller changes which are most apparent when the energy dependent exchange force is used for compound semiconductors containing one or more components from the fifth row or lower in the periodic table (e.g., Cd, In, Sb or Te).