Electronic stabilization of the Si(111)5 × 2-Au surface: Pb and Si adatoms

Using scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES) and first-principles density functional theory (DFT), we study the structural and the electronic properties of the Si(111)5 × 2-Au surface decorated with Pb adatoms. The STM topography data reveal that Pb adatoms form a similar superstructure to that observed in the case of Si adatoms on a bare Si(111)5 × 2-Au surface. The DFT calculations show that preferential adsorption sites of Pb atoms are located near the double Au chain. Bias dependent STM topography and spectroscopy together with the DFT calculations allow us to distinguish Pb from Si adatoms. Both the Si and Pb adatoms modify the electronic properties in the same way, which confirms the electronic origin of the stabilization of the surface.     

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.     

Effect of NH3 adsorption on the atomic structure of Si(111) √3 × √3 - Al and Si(111) √3 × √3 - Ag surfaces

The room temperature (RT) adsorption of ammonia (NH₃) on Si(111)√3 × √3-Al and Si(111)√3 × √3-Ag surfaces has been studied using LEED and AES. The transformation from Si(111)√3 × √3-Al surface structure to Si(111)1 × 1-(Al, H) upon NH₃ exposure has been found to be similar to the previously observed structural transformation induced by exposure in the atomic hydrogen. It has been demonstrated that the transformation is caused by hydrogen atoms which are generated by NH₃ dissociation on the Si(111)√3 × √3-Al surface. It has been estimated that about 0.1 ML of ammonia molecules is needed to complete the structural transformation. No interaction of NH₃ with the Si(111)√3 × √3-Ag surface has been found. The dissociation of NH₃ molecules is believed to be impossible on this surface     

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.     

Systematic analysis of the structural,elastic, and electronic properties of Ti–Cu–Me (Me=Al,Ga and In) ternary intermetallics

The structural, elastic, and electronic properties of eight intermetallics in Ti–Cu–Me (Me=Al, Ga and In) systems were investigated with the first-principles method. The calculations were performed within the generalized gradient approximation (GGA) with the density functional theory (DFT) using the supercell (SC) method. Calculation results show that the SC approximation is accurate at zero pressure. These intermetallics are classified as stable and metastable in our investigation. The stable phases are those presented in the equilibrium phase diagram at room temperature. The metastable phases are those that usually appear in the equilibrium phase diagram at higher temperatures. The values of calculations show that the TiCuAl, Ti₂CuAl₅, and TiCu₂In compounds are brittle, and the Ti_0.5CuAl_0.5, TiCu₂Al, TiCuGa, Ti₂CuGa₅, and Ti_0.5CuIn_0.5 compounds are ductile at zero pressure.     

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 Au clusters on Si(111)-7 × 7 surface

Self-assembled growth of Au nanoclusters on the Si(111)-7 × 7 surface has been studied using kinetic Monte Carlo simulations. A model considering various atomic processes of deposition, adsorption, diffusion, nucleation, and aggregation is introduced, and the main energetic parameters are optimized based on the experimental results. The evolution of surface morphology during Au growth is simulated in real time, from which the atomic behaviors of Au could be really captured. Most of Au atoms diffuse on the substrate in the very early stage of growth, and Au clusters nucleate and grow with the increasing coverage. The competition among various atomic processes results in the distinct distribution of Au clusters under different coverages. The growth conditions are further optimized, showing that the higher uniformity of Au clusters would be obtained at a low deposition rate and an optimal substrate temperature of about 380 K.     

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.     

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.     

Investigation of the Si(111)-(7 × 7) surface by second-harmonic generation: Oxidation and the effects of surface phosphorus

We have studied the initial stages of oxidation, the temperature dependence of the surface electronic structure, and the effect of phosphorus on oxidation of the Si(111)-(7 × 7) surface using optical second-harmonic generation. We have also observed a (√3 × √3)R30° LEED pattern due to surface reconstruction induced by < 0.5% P on Si(111).     

Neutralization of low energy helium ions scattered from Au adatoms on Si(111)-√3 × √3-Au

The neutralization of 5 keV He⁺ ions scattered from Au adatoms on the Si(111)-√3 × √3-Au surface was studied by impact-collision ion-scattering spectroscopy (ICISS). The He⁺ ICISS data contained false shadowing features that were actually the result of local neutralization effects. The radially dependent ion-atom neutralization theory of Woodruff, when used in our simulations of the ion scattering results, was reasonably successful in describing the neutralization of the He⁺ ions by the Au atoms. Good agreement for both the [112&#x0304;] and [1&#x0304;10] azimuths was obtained for a neutralization rate R = A exp(− ar), where A and a are 15.5 fs⁻¹ and 1.94 Å⁻¹ , respectively. An Auger neutralization model assuming a planar ion-solid interaction surface was also tested, yielding much poorer agreement.     

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.     

Surface diffusion of Si,Ge and C adatoms on Si （001） substrate studied the molecular dynamics simulation

Depositions of Si, Ge and C atoms onto a preliminary Si (001) substrate at different temperatures are investigated by using the molecular dynamics method. The mechanism of atomic self-assembling occurring locally on the flat terraces between steps is suggested. Diffusion and arrangement patterns of adatoms at different temperatures are observed. At 900 K, the deposited atoms are more likely to form dimers in the perpendicular [110] direction due to the more favourable movement along the perpendicular [110] direction. C adatoms are more likely to break or reconstruct the dimers on the substrate surface and have larger diffusion distances than Ge and Si adatoms. Exchange between C adatoms and substrate atoms are obvious and the epitaxial thickness is small. Total potential energies of adatoms and substrate atoms involved in the simulation cell are computed. When a newly arrived adatom reaches the stable position, the potential energy of the system will decrease and the curves turns into a ladder-like shape. It is found that C adatoms can lead to more reduction of the system energy and the potential energy of the system will increase as temperature increases.     

One-dimensional diffusion of vacancies on Sr/Si(100)-c(2×4) surface

An Sr/Si(100)-c(2×4) surface is investigated by high-resolution scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The semiconductor property of this surface is confirmed by STS. The STM images of this surface shows that it is bias-voltage dependent and an atomic resolution image can be obtained at an empty state under a bias voltage of 1.5 V. Furthermore, one-dimensional (1D) diffusion of vacancies can be found in the room-temperature STM images. Sr vacancies diffuse along the valley channels, which are constructed by silicon dimers in the surface. Weak interaction between Sr and silicon dimers, low metal coverage, surface vacancy, and energy of thermal fluctuation at room temperature all contribute to this 1D diffusion.     

Stability and surface properties of GeSi alloy films on Si(111) substrate

Several GeSi alloy films with different surface properties were prepared from a 500 Å thick Ge film that had previously been grown on a Si(111)-7×7 substrate by molecular beam epitaxy. The films were prepared by combinations of sputtering, annealing and Ge deposition from an evaporator. The surface properties were studied by Auger electron spectroscopy (AES) and by low energy electron diffraction (LEED). A novel LEED system employing position-sensitive detection was used. The Ge film surface gave a superposition of 7×7 and c(2×8) LEED patterns. A 7×7 → 1×1 phase transition was observed at 425±10°C. An irreversible 7×7 → c(2×8) transition was observed when the sample was heated above 500°C. The Ge film melted at 750±30°C and formed a Ge_xSi_1−x (x = 0.85±0.05) alloy whose surface gave a 7×7 LEED pattern. A 7×7 → 1×1 phase transition was observed at 600±0.15°C. Prolonged sputtering and annealing resulted in a Ge_xSi_1−x (x = 0.53±0.05) alloy whose surface gave a 5×5 LEED pattern. An apparent 5×5 → 1×1 phase transition was observed at 870±10°C but at that temperature the film was converted irreversibly to one with a much lower Ge atom fraction (x = 0.025±0.005) whose surface gave a 7×7 LEED pattern. A surface with a 5×5 pattern identical to that for the x = 0.53 alloy was prepared by deposition or Ge on Si. A similar 5×5 surface was prepared by deposition of Ge on a facetted GeSi alloy surface originally showing a superposition of 5×5 and 7×7 patterns. The intensity distributions in all of the 7×7 LEED pattern were found to be similar to those for Si(111)-7×7 at nearly the same electron energies. The characteristics of the 7×7 → 1×1 phase transitions were discussed in direct comparison with those of the Si(111)7×7 → 1×1 and Ge(111)-c(2×8) → 1×1 transitions observed with the same LEED system.