Electronic states and electron-phonon coupling at the 2×1 reconstructed Si(111) surface

The electronic structure of Si(111)-2x1 is studied selfconsistently for several values of the reconstruction parameters within Haneman's model. The derivatives of the dangling-bond bands with respect to the surface atom displacements are calculated and the surface electron-phonon coupling is estimated.     

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.     

Electronic structure of the Cu(111) surface

Self-consistent electronic structure calculations are reported on bulk Cu, and 3- and 5-layer Cu films. These yield a size insensitive work function, φ = 5.0±.1 eV, and a surface energy of 0.75 eV, in agreement with experiment. Good size convergence of the film potential permits the construction of a self-consistent potential for an 11-layer Cu(111) film, whose spectral properties we studied. A prominent p-like surface band was found within 0.1 eV of experiment, serving as a check on the surface potential.     

Electronic structure of H chemisorbed on Si(111) surfaces

We study the electronic density of states of hydrogen chemisorbed (111) Si surface. We analyse two situations: one single chemisorbed hydrogen atom in an otherwise clean surface and a complete monolayer of hydrogen chemisorbed at the surface. The method of calculation is based on an extension of the cluster-Bethe lattice approximation developed by the authors to study surfaces. Our results for the monolayer are in good agreement with UPS data, as well as with other theoretical calculations.     

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.     

Electronic structure of chlorine covered Si (111) 7 × 7 surface by the cluster method

Electronic structure of Cl covered Si (111) 7 × 7 surface is calculated on the basis of the first principle DV-Xα cluster method. Two typical reconstruction models of the Si (111) 7 × 7 surface, i.e. the buckled and the vacancy models, are investigated in detail. The vacancy model, where two chemisorption sites of onefold and threefold coordinated geometries coexist, can explain the polarization dependences of the recent ultravioler photoemission spectroscopy (UPS) results.     

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.     

Electronic structure of the Au-intercalated graphene/Ni(111) surface

We report the electronic structure of the Au-intercalated graphene/Ni(111) surface using angle-resolved photoemission spectroscopy and low energy electron diffraction. The graphene/Ni(111) shows no Dirac cone near the Fermi level and a relatively broad C 1s core level spectrum probably due to the broken sublattice symmetry in the graphene on the Ni(111) substrate. When Au atoms are intercalated between them, the characteristic Dirac cone is completely recovered near the Fermi level and the C 1s spectrum becomes sharper with the appearance of a 10?×?10 superstructure. The fully Au-intercalated graphene/Ni(111) surface shows a p-type character with a hole pocket of ～0.034?Å^?1 diameter at the Fermi level. When the surface is doped with Na and K, a clear energy gap of ～0.4?eV is visible irrespective of alkali metal.     

Model band structure of reconstructed (111) 2 × 1 surface of silicon

We have computed the electronic band structure for a model of the 2 × 1-reconstructed (111) surface of Si, based on 2 two-dimensional net of dangling bonds. A pairing of surface atoms is assumed, involving a displacement and a tilting of the dangling bonds towards each other. The energy gap that separates the two bands of surface states obtained increases with the degree of reconstruction, which is taken as a parameter. Experimental data, particularly surface optical absorption, find a satisfactory explanation in terms of this calculation, which indicates in Si (111) 2 × 1 a shift of the surface atoms by about 8% of their ideal distance.     

Au钝化Si(100)表面的电子特性

用TB-LMTO方法研究单层的Au原子在理想的Si(100)表面的化学吸附.计算了Au原子在不同位置的吸附能,吸附体系与清洁Si(100)表面的层投影态密度, 以及电子转移情况.结果表明, Au原子在吸附面上方的A位(顶位)吸附最稳定, Au钝化Si(100)表面可以取得明显的钝化效果, 这一结论与实验事实相符合.     

Theoretical study on diffusion mechanism of fluorine atom adsorbed on Si(111) reconstructed surface

We study diffusion mechanisms of fluorine atom adsorbed on Si(111) reconstructed surface by means of the first-principles density functional calculation. Recently, it was revealed experimentally that the fluorine diffusion is assisted by extra silicon atoms diffusing freely on the surface, whereas the detailed mechanism of the Si assist has not been understood. We first investigate F atom adsorption structures with and without a Si atom and find that extra Si atoms do not affect them. Next, we investigate the diffusion mechanism, considering four models in which extra silicon atoms act a different role, and find that SiF-complex diffusion model with activation energy of 1.34 eV is appropriate to explain the experiment. In this model, the Si–F bond is kept during diffusion, while in the others, it repeats breaking and re-bonding. This diffusion mechanism is also understood as the Si diffusion with carrying an F atom. We analyze why this diffusion mechanism is preferable to the others and find that two important features play roles. One is the strong Si-F bond that favors to keep the complex form, and the other is the existence of an extra silicon atom, which takes over the adatom position to passivate dangling bonds left behind after the SiF-complex removal.     

Atomic and electron structure of reconstructed (111) surface in ZnSe and CdSe crystals

The atomic and electron structure of four variants of polar (111)-(2 × 2) surfaces in ZnSe and CdSe terminated by a cation, namely, the ideal, relaxed, reconstructed, and relaxed after reconstruction surfaces, are calculated for the first time from the first principles. The surface is simulated by a film with a thickness of 12 atomic layers and a vacuum gap of ~16 Å in the layered superlattice approximation. Four fictitious hydrogen atoms with a charge of 0.5 electrons each are added for closing dangling Se bonds on the opposite side of the film. Ab initio calculations are performed using the QUANTUM ESPRESSO software based on the density functional theory. It is shown that relaxation results in splitting of atomic layers. We calculate and analyze the band structures and total and layer-wise densities of electron states for four variants of the surface.

     

Polarization dependence of the absorption spectrum of the reconstructed surface of Si(111) 2×1

It is explicitly shown that the ionic (buckling) model of the Si(111) 2×1 reconstructed surface should exhibit specific polarization dependences in its optical absorption spectrum between the split dangling bond bands. This can be a definite method to distinguish the ionic and covalent (pairing) models for this reconstructed surface.     

Electronic structure of Si (111) interface between diamond and hexagonal phases

The electronic structure of the Si (111) interface between the diamond and hexagonal wurtzite structure is studied by first-principles LCAO method. No gap interfacial states or resonances are found; instead, the calculated density of states curve shows an oscillatory redistribution of electron states and a reduction of band gap of about 10%. It is concluded that the electronic structure of this interface can be approximated as the average of that of the bulk diamond and the hexagonal Si.     

Lead growth on Si(111) surfaces reconstructed by indium

We study the Pb growth on both √3 × √3-In and 4 × 1-In reconstructed Si(111) surfaces at room and low temperature (160 K). The study takes place with complementary techniques, to investigate the role of the substrate reconstruction and temperature in determining the growth mode of Pb. Specifically, we focus on the correlation between the growth morphology and the electronic structure of the Pb films. The information is obtained by using Auger electron spectroscopy, low energy electron diffraction, soft x-ray photoelectron spectroscopy, scanning tunneling microscopy and spot profile analysis-low energy electron diffraction. The results show that, at low temperature and coverage ≤12 ML on the Si(111)√3 × √3-In surface, Pb does not alter the initial semiconducting character of the substrate and three-dimensional Pb islands with poor crystallinity are grown on a wetting layer. On the other hand, for the same coverage range, Pb growth on the Si(111)4 × 1-In surface results in metallic Pb(111) crystalline islands after the completion of a double incomplete wetting layer. In addition, the bond arrangement of the adatoms is studied, confirming that In adatoms interact more strongly with the silicon substrate than the Pb ones. This promotes a stronger Pb-Pb interaction and enhances metallization. The onset of the metallization is correlated with the amount of pre-deposited In on the Si(111) surface. The decoupling of the Pb film from the 4 × 1-In interface can also explain the unusual thermal stability of the uniform height islands observed on this interface. The formation of these Pb islands is driven by quantum size effects. Finally, the different results of Pb growth on the two reconstructed surfaces confirm the importance of the interface, and also that the growth morphology, as well as the electronic structure of the Pb film can be tuned with the initial substrate reconstruction.