Many-body effects in the Si(111)7×7 reconstructed surface

A two-dimensional Hubbard Hamiltonian is introduced in order to analyse the many-body effects associated with a dangling bond Si(111) 7×7 surface state located at the Fermi energy. Our results indicate that many-body effects can offer an explanation of recently reported experimental data.     

Diffusion of Au atoms and (5×2) phase formation on the Si(111) (7×7) surface

In this article we investigate the complex 1D mesoscopic model of adatom diffusion and the evolution of an ordered phase on the substrate surface. The analysis of the theoretical model is compared with the experimental results of the spreading of Au adatoms on Si(111)-(7×7) surface. The steady state solutions and their stability conditions are determined within the concept of the traveling-wave solution. It is shown that the formation of the ordered phase (5×2) and the difference in the diffusion of Au on (7×7) and on (5×2) structure results in a sharp edge of diffusion front which corresponds to the coverage of a saturated (5×2) phase. This edge moves linearly in time and α can be determined by experiment. The system of model equations enables the damped waves solution or temporary evolution of two steps.     

Kinetic Monte Carlo simulations of Si/Si(111)7×7 homoepitaxy

Kinetic processes that influence epitaxial growth of thin films are being more and more explored. Recent observations of growth on reconstructed surfaces suggested that material attachment to existing islands may be hindered by a barrier to attachment. We present a kinetic Monte Carlo model of Si/Si(111)7×7 homoepitaxy with the barrier to attachment implemented. We show that this model successfully reproduces available experimental results obtained during initial stages of growth and during surface relaxation. We show that the high scaling exponents experimentally observed are due to specific growth kinetics of the Si/Si(111)7×7 system. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported by the Grant Agency of the Czech Republic, project GAČR 202/97/1109 and by the Volkswagen Stiftung.     

Oscillatory diffusion in gold submonolayers over the Si(111)7 × 7 surface

A gold-deposited area of 0.1 mm diameter and several monolayers thick on the Si(111)7 × 7 surface is annealed at 700° C. In the diffusion process, the concentric oscillatory rings of the √3 × √3 and 5 × 1 structures are formed in an area of 0.4 mm diameter and observed by the dark field image in scanning LEED microscopy in an ultra-high vacuum. The diffusion is apparently independent of surface steps and crystallographic orientation from a macroscopic viewpoint. The formation of the periodic rings with the different surface structures and coverages is interpreted by using the surface diffusion equation and regarded as an intermediate phase prior to achieving the equilibrium state.     

Adsorption of Perylene on Si(111)(7×7)

We investigate the adsorption of organic molecular semiconductor perylene on(7 × 7) reconstructed Si(111)surface by ultraviolet photoemission spectroscopy.It is observed that seven features that derive from the organic material are located at 0.71,2.24,4.0,5.9,7.46,8.65 and 9.95 eV in binding energy.The theoretical calculation results reveal the most stable adsorption geometry of organic molecule perylene on Si(111)(7 × 7) substrates is at the beginning of deposition.     

Formation of ultrathin magnetic cobalt films on the Si(111)7 × 7 surface

The phase composition, electronic structure, and magnetic properties of ultrathin cobalt films (no thicker than 20 ?) applied on a Si(111)7 × 7 surface at room temperature are studied by high-resolution photoelectron spectroscopy using synchrotron radiation and magnetic linear dichroism. It is shown that, as the cobalt thickness increases, first interface cobalt silicide and then an island (discontinuous) film of silicon-in-cobalt solid solution form on the silicon surface. A metal cobalt film starts growing after the deposition of a ∼7-?-thick Co layer. It is found that the ferromagnetic ordering of the system, which is characterized by surface magnetization, sets in after the deposition of a ∼6-?-Co layer at the stage of Co-Si solid solution formation.     

Spectroscopic investigation of the early formation stage of the Si(111)(2 × 1)-Mo interface

The overgrowth of Mo onto Si(111)(2 × 1) at room temperature up to four monolayers is studied with valence band photoemission (hv = 21.2 eV) and with Auger spectroscopy. The interface is reactive and the results suggest an overgrowth which takes place via the formation of islands having electron states typical of silicides.     

Influence of the Si(111)-7×7 surface reconstruction on the diffusion of strontium atoms

The diffusion of strontium atoms on the Si(111) surface at room temperature has been investigated using scanning tunnel microscopy and simulation carried out in terms of the density functional theory and the Monte Carlo method. It has been found that the reconstruction of a clean silicon surface with a 7 × 7 structure has a profound effect on the diffusion process. The average velocity of motion of a strontium atom in a unit cell of the 7 × 7 structure has been calculated. The main diffusion paths of a strontium atom and the corresponding activation energies have been determined. It has been demonstrated that the formation of scanning tunnel microscope images of the Si(111)-7 × 7 surface with adsorbed strontium atoms is significantly affected by the shift of the electron density from the strontium atom to the nearest neighbor silicon adatoms in the 7 × 7 structure.     

Formation of Br-terminated Si6 rings during etching of Si(111)-7 × 7

Scanning tunneling microscopy was used to study the morphological evolution of Si(111)-7 × 7 exposed to molecular bromine. Etching at 900 K results in the removal of adatoms and conversion from the 7 × 7 surface to a 1 × 1 Br-terminated structure. Removal is dominated by bilayer step retreat with edge profiles that reflect etching anisotropies. Unique structures attributed to regrowth of Si released during step etching were consistent with six-membered Si rings terminated with Br. The distribution of such rings near the steps reflects the local bonding and etching anisotropies for the steps. Theoretical analyses of these rings terminated with Cl determine bond lengths, bond angles, and charge transfer within the ring.     

Second harmonic response of the Si(111)7 × 7 surface

Optical second harmonic generation spectra have been experimentally obtained from a clean Si(111) 7 × 7 in two different polarization configurations isolating the rotational anisotropic and isotropic contributions. The energy of the fundamental photon is varied from 0.8 eV to 2.5 eV. For comparison, we also use a microscopic formulation based on the semi-empirical tight binding method to evaluate the nonlinear surface susceptibility tensor χ(2ω). Good agreement between theory and experiment is obtained with respect to the number of resonances, their position in energy, and surface or bulk character.     

STM study of nucleation of Ag on Si(111)−(7 × 7) surface

Initial stages of Ag on Si(111)−(7 × 7) surface nucleation were studied at submonolayer coverage. Samples were prepared by thermal evaporation of Ag from tungsten wire under UHV conditions (p<2.5 × 10⁻⁸ Pa). Various deposition rates (0.002–0.1 ML s⁻¹) were used to prepare Ag island films with coverages (0.002–2) ML (1 ML ≈ 7.58 × 10¹⁴ atoms cm⁻²) at room temperature. We observed preferential growth on faulted half unit cells (F cells). At constant coverage both the island density and ratio of occupied F and U (unfaulted) cells are independent of the deposition rate, which is an evidence for dominant influence of substrate structure. The preference of nucleation in the F cells against U cells decreases with the coverage until the ratio is 1:1 for 1 ML Ag film. We have observed that presence of an Ag island in any type of the half unit cell (F or U) considerably reduces nucleation probability in neighbouring cells. This results in forming of structural patterns observed among randomly grown Ag-islands which is a new feature found for Ag/Si(111)−(7 × 7) system. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported by the Grant Agency of Charles University — projects GAUK 34/97 and 147/99, by the Grant Agency of Czech Republic — project GAČR 202/97/1109 and by the Ministry of Education grant VS 97116.     

Si(111)7×7结构模型的稳定性研究

本文基于有相互作用的结构件的概念,对Si(111)7×7结构模型的稳定性进行研究。利用紧束缚法,计算了各种结构件的形成能及其相互作用能。由这些结构件可灵活地构成各种大单胞的表面结构并对其进行总能计算,而避免了大工作量的计算。对Si(111)5×5和7×7DAS模型,我们得到其表面能分别为-0.467eV和-0.477eV,与Qian和Chadi的结果很接近。并指出,Harrison提出的7×7吸附原子模型和Binnig等人提出的7×7吸附原子-空位模型都是不稳定的。     

Atomic geometry of Si(111) 7×7 by dynamical low-energy electron diffraction

A complete determination of individual atomic coordinates for the Si(111) 7×7 surface has been carried out by dynamical low-energy electron diffraction. The dimer-adatom-stacking fault (DAS) model, after large vertical and lateral relaxations, was found to produce calculated spectra in remarkably good agreement with the data. Atoms in the faulted half of the unit cell are raised from those of the unfaulted half. Adatoms at corner sites are higher than those at center sites.     

Continuous-time observation of pseudo-vacancy diffusion at Si(111)-7 × 7 surfaces

Scanning tunneling microscopy (STM) is used to study surface diffusion of a special type of point defects at Si(111)-7 × 7 surfaces. These defects survive even after annealing up to 1250°C. They appear darker than Si adatoms at the tunneling biases ranging from −3 to +3 V, but they are not true vacancies. We found that these vacancy-like defects (hereafter, we refer to them as pseudo-vacancies) are not caused by adsorption of major contaminants in the vacuum chamber, nor by dopants. We also observed migration of pseudo-vacancies between nearest neighboring Si adatom sites at temperatures above 500°C. Most of the jumps are within a half of the 7 × 7 unit cell. Thousands of STM images were recorded from 520 to 610°C and the activation energies and frequency factors were determined. Varying the tunneling current produces almost no effect on the diffusion, but varying the scanning speed produces a small effect.     

Site discrimination of adatoms in Si(111)-7 × 7 by noncontact atomic force microscopy

Site discrimination of adatoms in Si(111)-7 × 7 by dynamic mode noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum has been demonstrated. At a fixed frequency shift, NC-AFM images were acquired at various amplitudes. At amplitudes less than 30 nm_p-p, no atomic image was observed. At 33 nm_p-p, clearly resolved atomic images showed 6 bright atoms in one half of each 7 × 7 unit cell and 6 less bright atoms in the other half of each unit cell. Furthermore, the corner adatoms were observed to be higher. At 36 nm_p-p, such differences were not observed. The observed site differences are not derived by the differences in the actual positions of adatoms, but by the differences in the interactions between the atom at the AFM tip apex and the adatoms on the Si surface.     

Peculiarities of the initial stage of growth of niobium-based nanostructures on a Si(111)-7 × 7 surface

The initial stage of growth of nanoislands prepared by thermal deposition of niobium on the reconstructed surface of Si(111)-7 × 7 in ultrahigh vacuum is experimentally investigated. The morphological and electrophysical properties of niobium-based nanostructures are studied by means of low-temperature scanning tunneling microscopy and spectroscopy. It is found that upon the deposition of niobium on a substrate at room temperature, clusters and nanoislands are formed on the silicon surface, having a characteristic lateral size of 10 nm with the metallic type of tunneling conductivity at low temperatures. Upon the deposition of niobium on a heated substrate, quasi-one-dimensional (1D) and quasi-two-dimensional (2D) structures with typical lateral dimensions of up to 200 nm and three-dimensional pyramidal islands with semiconducting type of tunneling conductivity at low temperatures are formed.     

Size dependences in the adsorptive properties of the surface of ytterbium nanofilms deposited on silicon: The CO-Yb-Si(111)7 × 7 system

This paper reports on a study of the adsorption of CO molecules on the surface of ytterbium nanofilms of different thicknesses, which were sublimed on Si(111)7 × 7 at room temperature. Dependences of two types were investigated: the surface concentration of adsorbed molecules vs. CO dose expressed in langmuirs and the work function of films vs. CO dose. It was shown that the behavior of these dependences is mediated by size effects and Friedel oscillations generated by the ytterbium-silicon interface. Both effects exert an influence on the binding of CO molecules to the surface. At low molecule concentrations, this binding is effected through lone electron pairs localized at the carbon ends of the molecules. These electrons form a donor-acceptor bond to the vacant 5d level of the metal, with the level dropping below the Fermi level. At high CO molecule concentrations, the pattern becomes more complex; indeed, the enhanced Coulomb interaction gives rise to a partial transfer of electrons from the 5d level to the vacant 2π* orbital of CO molecules.