Formation of nanosize silicides films on the Si(111) and Si(100) surfaces by low-energy ion implantation

The formation of nanosize silicides films by implantation of B, P, Ba, and alkali metal atoms in Si(111) and Si(100) followed by thermal annealing is studied by electron spectroscopy and slow-electron diffraction methods. It is shown that implantation of ions with a large dose D > 10¹⁶ cm^?2 and short-term heating lead to the formation of thin silicides films with new surface superstructures: \(Si(111) - (\sqrt 3 \times \sqrt 3 )R30^ \circ - B\) , Si(100)-2 × 2Ba, Si(111)-1 × 1P, etc.     

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.     

Anisotropic photoemission and chemisorption-bond geometry on Ge(111) and Si(111) surfaces

In order to study the effects of photon-polarization selection rules on chemisorptionbond geometry, we have measured photoelectron spectra as a function of angle of incidence, θ_i, in the range 28° ? θ ? 80°. A noble-gas UV resonance lamp and cylindrical mirror analyzer were used to measure both bonding and non-bonding surface Orbitals. A large enhancement (200–400%) of the photocmission is found when photon electric field intensity is near the maximum normal to the surface. This indicates a spatial variation of microscopic fields which is approximately independent of adsorbate bonding since it is determined by the optical properties of the substrate. In addition, we observe some effects on adsorbate photoelectron peaks due to different orbital symmetry. The case of atomic hydrogen chemisorption is discussed as an example of this latter effect.     

Correlations between the scanning tunneling microscopy imaged configurations and the electronic structure on stepped Si(111)-( 7x7) surfaces

We have observed the dependence of the scanning tunneling microscopy (STM) imaged atom intensity within the (7x7) unit cell on stepped Si(111) as a function of the tunneling voltage. Pronounced differences from the corresponding atom intensity on the flat surface are observed for the contrast of atoms on the low versus the high side of the step and for the contrast between the faulted versus unfaulted subcells of the (7x7) structure. These differences can be accounted for by changes in the electronic structure within the (7x7) subcells adjacent to the step. Calculations of the local density of states and the STM images using a tight-binding method are in excellent agreement with the experimental results.     

The decomposition of ammonia on the flat (111) and stepped (557) platinum crystal surfaces

Ammonia adsorption, desorption and decomposition to H₂ and N₂ has been studied on the flat (111) and stepped (557) single crystal faces of platinum using molecular beam surface scattering techniques. Both surfaces show significant adsorption with sticking coefficients on the order of unity. The stepped (557) surface is 16 times more reactive for decomposition of ammonia to N₂ and H₂ than the flat (111) surface. Kinetic parameters have been determined for the ammonia desorption process from the Pt(111) surface. The mechanism of ammonia decomposition on the (557) face of platinum has been investigated.     

Nucleation and growth of Si on Pb monolayer covered Si(111) surfaces

With a scanning tunneling microscope (STM), we study the initial stage of nucleation and growth of Si on Pb monolayer covered Si(111) surfaces. The Pb monolayer can work as a good surfactant for growth of smooth Si thin films on the Si(111) substrate. We have found that nucleation of two-dimensional (2D) Pb-covered Si islands occurs only when the substrate temperature is high enough and the Si deposition coverage is above a certain coverage. At low deposition coverages or low substrate temperatures, deposited Si atoms tend to self-assemble into a certain type of Si atomic wires, which are immobile and stable against annealing to ~ 200 °C. The Si atomic wires always appear as a double bright-line structure with a separation of ~ 9 Å between the two lines. After annealing to ~ 200 °C for a period of time, some sections of Si atomic wires may decompose, meanwhile the existing 2D Pb-covered Si islands grow laterally in size. The self-assembly of Si atomic wires indicate that single Si adatoms are mobile at the Pb-covered Si(111) surface even at room temperature. Further study of this system may reveal the detailed atomic mechanism in surfactant-mediated epitaxy.     

Observation of Si(111) and gold-deposited Si(111) surfaces using micro-probe reflection high-energy electron diffraction

Observations of clean Si(111) and gold-deposited Si(111) surfaces have been performed using micro-probe reflection high-energy electron diffraction. It was found that many atomic steps on a Si(111) surface run in nearly the same direction, about 9° off the [1̄1̄2] direction. When gold was deposited on this surface at a substrate temperature of about 800°C, 5 × 1, diffuse √3 × √3R30°, sharp √3 × √3 R30° structures and Au clusters appeared on the surface with continuation of the deposition. During the deposition process, it was found that one kind of Si(111) 5 × 1 Au domain grew selectively along these atomic steps and nearly covered the entire surface. A phenomenon of gold clusters moving during the deposition was also observed. These clusters all moved in nearly the same direction so as to climb the atomic steps.     

Atomic structure, energetics, and dynamics of topological solitons in indium chains on Si(111) surfaces

Based on scanning tunneling microscopy and first-principles theoretical studies, we characterize the precise atomic structure of a topological soliton in In chains grown on Si(111) surfaces. Variable-temperature measurements of the soliton population allow us to determine the soliton formation energy to be ～60 meV, smaller than one-half of the band gap of ～200 meV. Once created, these solitons have very low mobility, even though the activation energy is only about 20 meV; the sluggish nature is attributed to the exceptionally low attempt frequency for soliton migration. We further demonstrate local electric field-enhanced soliton dynamics.     

First-principle study of Mg adsorption on Si（111） surfaces

We have carried out first-principle calculations of Mg adsorption on Si(111) surfaces. Different adsorption sites and coverage effects have been considered. We found that the threefold hollow adsorption is energy-favoured in each coverage considered, while for the clean Si(111) surface of metallic feature, we found that 0.25 and 0.5 ML Mg adsorption leads to a semiconducting surface. The results for the electronic behaviour suggest a polarized covalent bonding between the Mg adatom and Si(111) surface.     

Lattice vibrations at ideal Si(111) surfaces

Surface vibrations at an ideal (111) silicon surface with vanishing wavevector components parallel to the surface are calculated within the shell-model for a simple model of surface force constants.     

Ultraviolet photoemission spectroscopy of HCl on Si(111) surfaces

Chemisorption of HCl on thermally cleaned Si(111) surfaces has been studied by ultraviolet photoemission spectroscopy. HCl chemisorption, both at 300K and 850 K, induces two main peaks in the photoemission spectrum which are attributed to Cl lone pair p_x, p_y orbitals and Si: sp³?Cl:p_z bonding orbital, respectively. This strongly suggests that HCl molecules dissociate on Si(111) surfaces already at room temperature.     

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.