Evidence for subsurface atomic displacements of the GaAs(110) surface from LEED/CMTA analysis

The low-energy electron diffraction constant momentum transfer average (LEED/CMTA) technique is applied to the GaAs(110) surface. In addition to verifying our earlier model of the top layer atomic displacements, atomic displacements in the first subsurface layer, opposite in direction to those in the top layer, are suggested. Best estimates for atomic positions in the surface and first subsurface layers are given.     

Epitaxial growth of Xe on the Si(111) 7×7 surface observed by LEED

The epitaxial growth up to 20 monolayers of Xe on the Si(111) 7×7 surface at several temperatures between 25 and 50 K has been studied by LEED. A variety of different LEED patterns demonstrate that there is a significant influence of the silicon structure on the Xe growth mode. The 7×7 superstructure has a very peculiar influence depending on temperature, so that a periodic sequence of stacking faults is observed in the xenon even in thicker layers.     

Temperature dependence of surface electronic structure of Si(111) surface

Temperature dependence of angle-resolved ultraviolet photoelectron spectra has been obtained for Si(111) surfaces starting with a thermally quenched “1 × 1” surface and ending with a high temperature “1 × 1” surface. it has been found that the surface state at 0.8 eV below the Fermi level exhibits degradation with the increase in temperature, which explains the difference of surface electronic structures between a quenched “1 × 1” surface and a high temperature “1 × 1” surface. Electron correlation effect in a dangling-bond derived surface state is postulated as a cause for the phenomenon.     

LEED and Auger investigations of Cu (111) surface

After ultrahigh vacuum bake-out, electropolished Cu (111) surfaces were shown by Auger analysis to be contaminated by C, N, O, S and Cl. Other than C and S, which were contained in the bulk, the impurities were introduced by surface preparation; but all were easily removed by light Ar ion bombardment. Heating to ≈ 750°C caused diffusion of C and S from the bulk to the extent that a clear diffraction pattern corresponding to a √7 × √7 structure was produced by S on the surface. At ≈ b 900°C evaporation of Cu occurred to an observable degree, and S and C could no longer be detected on the surface. Auger analysis of clean Cu surfaces showed many details of the LMM and MMM types of transitions. Kinetic energies of all observed Auger electrons were in excellent agreement with calculated values. Also, the ≈ 62 eV MMM peak was resolved into two components related to the small differences in the M₂ and M₃ energy levels. The LMM transitions were classified according to their intensities, which could be rationalized on the basis of Coster-Kronig transitions and transition probabilities, as L₃MM > L₂MM > L₁MM.     

LEED crystallographic analysis for the Rh(111)-(2 × 1)---O surface structure

A tensor LEED analysis is reported for the Rh(111)-(2 × 1)---O surface structure in which atoms in the O overlayer chemisorb close to the regular (fcc type) three-fold hollow sites for half-monolayer coverage. The structure shows significant relaxations: for example, a buckling of about 0.07 Å is indicated in the first metal layer and O appears to displace laterally by about 0.05 Å. The individual O---Rh bond lengths are around 2.01 and 1.92 Å to top layer Rh atoms, which bond to two and one O atoms, respectively, but the average value (1.98 Å) is close to that in bulk RhO₂ (1.96 Å). Comparison is also made with the previously determined O---Rh bond lengths in the Rh(110)-p2mg(2 × 1) surface structure.     

LEED analysis of the surface structure of Mo(001)

A detailed comparison between theoretical and experimental LEED data for the Mo(001) surface, using the quantitative r-factor method of Zanazzi and Jona, yields a value for the surface layer contraction of 9.5 ± 2.0%, consistent with the 11.5% found previously by Ignatiev et al. Non-structural parameters are also in agreement, although the surface Debye temperature, 230 K, is slightly higher than found before. r-Factors were used to investigate the effects of all the parameters, and show that an energy-dependent inner potential gives better agreement than a fixed value and that the second layer is contracted by about 1% of the bulk layer spacing. The effect of using different ion core potentials is examined and discussed. A superposition potential which models the surface ion environment is found to have a significant effect on the layer displacement concluded.     

Growth and ordering of Ni(II) diphenylporphyrin monolayers on Ag(111) and Ag/Si(111) studied by STM and LEED

The room temperature self-assembly and ordering of (5,15-diphenylporphyrinato)nickel(II) (NiDPP) on the Ag(111) and Ag/Si(111)-(√3 × √3)R30° surfaces have been investigated using scanning tunnelling microscopy and low-energy electron diffraction. The self-assembled structures and lattice parameters of the NiDPP monolayer are shown to be extremely dependent on the reactivity of the substrate, and probable molecular binding sites are proposed. The NiDPP overlayer on Ag(111) grows from the substrate step edges, which results in a single-domain structure. This close-packed structure has an oblique unit cell and consists of molecular rows. The molecules in adjacent rows are rotated by approximately 17° with respect to each other. In turn, the NiDPP molecules form three equivalent domains on the Ag/Si(111)-(√3 × √3)R30° surface, which follow the three-fold symmetry of the substrate. The molecules adopt one of three equivalent orientations on the surface, acting as nucleation sites for these domains, due to the stronger molecule-substrate interaction compared to the case of the Ag(111). The results are explained in terms of the substrate reactivity and the lattice mismatch between the substrate and the molecular overlayer.     

A LEED/UPS study on the interaction of oxygen with a Ni(111) surface

Exposure of a Ni(111) surface to oxygen leads at first to the formation of a chemisorbed overlayer which is characterized by a 2 × 2-superstructure and a maximum in the photoemission spectrum (hv = 40.8 eV) centered at 5.6 eV below the Fermi level E_F. The emission from the Ni d-states is nearly unaffected at this stage of interaction. After high oxygen exposures the epitaxial growth of NiO can be identified from the LEED pattern. The corresponding photoelectron spectrum is strongly altered and exhibits close agreement with the transition energies as calculated by Messmer et al. for a NiO₆^10- -cluster.     

Surface structure of MgO(001) surface studied by LEED

MgO(001) surfaces prepared by three different heat treatments were studied by LEED. The first was the surface just after the vacuum cleavage; the second was that after the annealing of the first one at 300°C in UHV. The third was prepared by cleaving in air and heating in oxygen until the carbon Auger peak disappear. LEED I-V curves of the diffraction spots, (10), (11) and (20), were recorded and compared carefully. No remarkable difference corresponding to the preparing methods was found. Therefore, the atomic structures of the surfaces seem to be similar to each other. These experimental I-V curves were compared with the theoretical curves obtained by dynamical calculations for several grades of surface relaxation. The theoretical curves with no relaxation or with the smallest one in the calculation (2.5% expand) fitted best with the experimental curves. The experimental curves were also compared with the theoretical ones calculated for the rumpled surface. But no effect induced by the rumpling was found in the experimental curves.     

The surface structure of Si(100) surfaces using averaged LEED: II. The (2×1) clean surface structure

Constant momentum transfer averaged LEED data from a Si(100) (2 × 1) clean surface structure have been produced for the $\text{(00), (10), (11), (}\text{1}\text{2}\text{0)}$ and $\text{(1}\text{1}\text{2}\text{)}$ beams. All averages show strong features in addition to those attributable to single scattering from the bulk. If this structure is assumed to also originate from single scattering, the surface reconstruction must be deep (>^～4 layers). Alternatively this structure can be ascribed to multiple scattering. As data from the Si(100) (1 × 1)H surface structure produced “good” averaging over an identical range of data, this latter conclusion has considerable bearing on the future usefulness of this averaging approach to surface structure analysis by LEED.     

A bond orbital model investigation of the surface electronic energy structure of Si(111)

Intrinsic surface states for the Si(111) surface are investigated using the Bond Orbital Model. The semi-infinite crystal is simulated by gradually increasing the number of layers until the convergence is achieved. Total density of states are presented for unrelaxed, relaxed and the hydrogen chemisorbed Si(111) surfaces. The effect of the (2 × 1) reconstruction on the dangling bond surface state is also investigated. The results are in excellent agreement with photoemission experiments.     

A LEED study of the Si{100}(1 × 1)H surface structure

Extensive LEED intensity-energy data have been collected for a Si{100}(1 × 1)H surface and dynamical theory LEED calculations have been performed using an ideal unreconstructed Si(100) surface to model this structure. Agreement between experiment and theory is good indicating that the probable structure for this surface does involve (weakly scattering) H atoms on the “dangling bonds” of an unreconstructed Si(100) surface, and that difficulties in achieving good agreement between experiment and theory for the clean Si{100}(2 × 1) surface is more probably due to deficiencies in the model structure than to deficiencies in the non-structural aspects of the LEED theory.     

Electronic structure of Si(111) surfaces

We report on new angle-resolved photoemission studies of Si(111) 2 × 1 and 7 × 7 surfaces. The emission from the 2 × 1 surface shows much structure. For normal emission the energy positions are insensitive to the photon energy in the range 19–27 eV. The emission has been interpreted as a probe of the surface density of states, SDOS, including both surface states, resonances and bulk-like states. The SDOS was also calculated as a function of parallel momentum k_∥ for a model of the Si(111) 2 × 1 surface obtained from energy minimization considerations. We identify emission from the dangling bond band, which has a positive dispersion of 0.6 eV, and also emission from surface resonances which have some character of the compressed and stretched back bonds. There are also other predicted surface resonances that correspond to experimental peaks which have not been identified in previous work. Except for the dangling bond band, the surface resonances are limited in k_∥ space, so that it is not possible to follow these resonance bands over all angles. Maximum intensity for the normal emission from the dangling bond is obtained at 23 eV, while the emission from the lowest s-like states monotonically increases towards 30 eV photon energy. When annealing the cleaved 2 × 1 surface to the 7 × 7 reconstructed surface, the spectra broaden significantly. The intensity of the dangling bond decreases and we see a very small metallic edge.     

Study of the Si(111) 7 × 7 surface structure by alkali- metal adsorption

When alkali-metals such as Li, Na, K, Rb and Cs were adsorbed on clean Si(111) 7 × 7 surface at room temperature, the intensity distribution of the original 7 × 7 RHEED pattern changes gradually with the increase of alkali-metal adsorption, and at last a new superstructure with 7 × 7 periodicity, here named δ-7 × 7 structure, has been observed. This change at room temperature can be explained if the 7 × 7 structure is mostly made of displacement. The structure model estimated from the intensity distribution of the δ-7 × 7 pattern is the one that has one vacancy at the corners of the 7 × 7 unit mesh and relaxed surrounding atoms. The change of the 7 × 7 structure by alkali-metal adsorption to this model is naturally understood with our new model (1984). For all alkali metals, by adsorption at high temperature 3 × 1 superstructure has also been observed for the first time.