Surface Segregation Of Si Research Articles

Comments on ‘surface segregation of Si on Fe single crystal surfaces and interaction with carbon’

The experimental results of carbon and silicon segregation at a (100) surface of a Fe-3wt%Si alloy obtained by de Rugy and Viefhaus [Surface Sci. 173 (1986) 418] are interpreted using the ternary regular solution model developed by Guttmann [Surface Sci. 53 (1975) 213]. It is assumed that there is a competition and repulsion between carbon and silicon at the alloy's surface. The segregation free enthalpies of carbon and silicon obtained by fitting the calculated curves to the experimental data are: ΔG Si = −48+0.015 T+25 θ C in kJ/mol, and ΔG C = −102.7+16.8 θ C+25 θ Si in kJ/mol. It is shown that these results describe the segregation phenomena in the FeSiC system better than those given by de Rugy and Viefhaus.

Ion beam effects on surface and subsurface composition of near-noble silicides

Modifications of Cr–silicide (Cr3Si and CrSi2) and Ni–silicide (NiSi2) surface and subsurface regions induced by an Ar ion beam were investigated by angle integrated Auger electron spectroscopy. The dependence of the surface modification on the ion beam energy and density was investigated between −80 and 400 °C. At room temperature (RT) Cr–silicides are metal enriched in the surface and subsurface region. Ion bombardment of a CrSi2 surface at high temperature (HT) produces Si enrichment with respect to RT, resulting in the formation of a CrSi layer. The Si enrichment occurs at a temperature lower than the temperature characteristic of purely thermal segregation. On the contrary, the Cr3Si surface composition is independent of the temperature. The NiSi2 surface is stoichiometric at RT; ion bombardment at HT produces a Si surface segregation in two distinct steps. On the basis of a careful line-shape analysis of the Si core–valence–valence Auger transitions, we attribute the first step to the formation of an ordered, ∼1.4 Å thick, Si overlayer, the second one to the growth of a thicker (∼3 Å) amorphous Si layer.

Surface segregation of Si on Fe single crystal surfaces and interaction with carbon

AES and LEED are applied to the study of silicon surface segregation on Fe−3wt%Si single crystals between 400 and 900°C. The variations of the Si coverage with time at a given temperature or with the temperature at equilibrium show a strong orientation dependence. For the (100) orientation, the diffusivity and segregation enthalpy of silicon on iron could be derived from the measurements. Lateral interactions between segregated atoms are better observed in the presence of a third element. The study of the Fe-Si-C ternary system thus confirmed the weakness of the Si-Si lateral interactions. It also showed that the repelling of Si by C on the surface is mainly due to a site competition reaction, whereby the strong Si-C repulsive interaction plays a minor role.

Surface segregation of Si on Fe single crystal surfaces and interaction with carbon

Surface segregation of Si on Fe single crystal surfaces and interaction with carbon

Chemical reaction and silicide formation at the Pt/Si interface

Surface spectroscopy (UPS and AES) and transmission electron microscopy (TEM) techniques have been used to study the chemical behavior of interfaces formed by Pt deposition (∼0.1–100 Å) on atomically clean Si(111) and Si(100) surfaces prepared by sputter/annealing. As shown by the annealing behavior of a thicker (∼100 Å) overlayer, growth of an intermixed phase begins only at ∼200–300 °C and leads to formation of a product which is stable to 500 °C. TEM observations show this product to be single-phase PtSi. The UPS spectrum for PtSi is dominated by two distinct peaks at −3.7 and −6.2 eV, while the Si L2,3VV AES spectrum also show features characteristic of silicidelike bonding. With annealing above ∼400 °C, excess Si segregates to the top surface of the PtSi overlayer. Upon 25 °C deposition, both UPS and AES show clear indications of strong Pt/Si intermixing across the interface from the submonolayer range to ∼40 Å. In the low (submonolayer to few monolayer) coverage range at temperatures ≤300 °C, the new chemical bonds formed are similar but not identical to those for the PtSi. With increasing coverage the UPS peaks shift toward the Fermi energy as expected for a more Pt-rich environment. For these thin layers (∼5–30 Å), TEM shows the presence of both Pt and silicide. Upon annealing to ∼300–400 °C, a mixture of PtSi with some Pt2Si is observed and the UPS and AES features become typical of PtSi; these features remain unchanged with higher temperature annealing until the Si surface segregation becomes appreciable.

Non-equilibrium surface segregation of silicon in iron-silicon single crystals

The non-equilibrium surface segregation of Si to the 110 surface in an Fe-Si 10 at% single crystal was determined in the temperature range 467–528°C. The segregation properties showed a significant deviation from those previously determined in an Fe-Si 6 at% sample in both the form of the segregation profile and the rate of the segregation. The reason for this deviation is discussed in terms of a higher total silicon-carbon repulsive interaction in the bulk of the crystal.

Stability and reactivity of (001) and (111) NiO: a RHEED-AES investigation of Si surface segregation and Ni formation by gas reduction

Abstract In this contribution are reported recent results regarding the preparation and the reactivity of different nickel oxide (NiO) monocrystalline surfaces. At first, it concerns a structural investigation of metal formation by hydrogen reduction of (001) NiO. In most cases, Ni nuclei form three-dimensional islands as observed by RHEED and corroborated by AES. At low temperatures, metal stacking, previously described as pure hcp stacking from (1120) planes, is shown to develop, in reality, with stacking faults from which fcc stacking is often formed. At high temperatures, metal stacking is retained as within the oxide and, from a comparative transmission electron microscopy investigation, a model for the topotactic nucleus formation is put forward. Secondly, the preparation and the stability of (111) NiO are investigated. Apparent (111) NiO stability is found to be due to the thermal surface segregation of silicon. Even in very low amount, this very mobile impurity segregates at the (111) NiO surface leading to a two-dimensional O-Si-Ni compound.

Stability and reactivity of (001) and (111) NiO: A RHEED-AES investigation of Si surface segregation and Ni formation by gas reduction

In this contribution are reported recent results regarding the preparation and the reactivity of different nickel oxide (NiO) monocrystalline surfaces. At first, it concerns a structural investigation of metal formation by hydrogen reduction of (001) NiO. In most cases, Ni nuclei form three-dimensional islands as observed by RHEED and corroborated by AES. At low temperatures, metal stacking, previously described as pure hcp stacking from (112̄0) planes, is shown to develop, in reality, with stacking faults from which fcc stacking is often formed. At high temperatures, metal stacking is retained as within the oxide and, from a comparative transmission electron microscopy investigation, a model for the topotactic nucleus formation is put forward. Secondly, the preparation and the stability of (111) NiO are investigated. Apparent (111) NiO stability is found to be due to the thermal surface segregation of silicon. Even in very low amount, this very mobile impurity segregates at the (111) NiO surface leading to a two-dimensional O-Si-Ni compound.

The thermal stability of very thin Pd 2Si films on Si

The changes in structure of very thin (∼ 20 A) Pd 2 Si layers on Si(111) and Si(100) surfaces have been studied by Medium Energy Ion Scattering combined with channeling and blocking. Annealing at temperatures ≲ 300°C results in surface segregation of Si. Heating above 300°C causes formation of Pd 2 Si islands, which on the Si(111) surface grow epitaxially to the substrate. The Pd 2 Si lattice in the epitaxial islands is found to be stretched parallel to the interface to match the difference in lattice constants between Si and Pd 2 Si (1.8%).

Fly ash surface formation and segregation during heating

The possible importance of various physical and chemical processes relevant to fly ash surface formation during coal combustion have been studied. Chemical segregation phenomena and crystal formation processes occurring on fly ash glass surfaces during heating in air and vacuum have been studied for extended periods and temperatures up to 1100°C. XPS, Auger and SIMS methods were used to obtain both relative surface elemental concentrations and depth profiles for major and minor components. Major differences were noted between samples heated in air and in vacuum environments. For the fly ash glass heated in air Fe, Ti and Mg become enriched on surfaces while heating in a vacuum leads to Si surface segregation. The results indicate two levels of surface enrichment; a thin (< 300 Å) layer and a thicker (1–2 μm) layer most evident for heating in air where an Fe-rich layer is formed. The formation of a crystalline granular structure concurrent with Fe enrichment is observed using scanning electron microscopy for heating in air at > 1000°C. The results indicate that the rates of surface segregation may not be sufficiently fast on the time scale of fly ash formation to result in equilibrium levels of surface segregation and that condensation processes during fly ash formation probably play a major role in the observed fly ash surface enrichments.

Surface structure of epitaxial Pd2Si thin films

The surface structure of Pd2Si thin films epitaxially grown on clean Si (111) 7×7 surfaces has been studied by low-energy electron diffracton (LEED) and Auger electron spectroscopy (AES). Heat treatment in the range of 200–600 °C causes surface segregation of elementary Si over a Pd2Si (0001) surface and produces a reconstructed 3×3 superstructure of Pd2Si. A nonreconstructed Pd2Si-1×1 surface is produced after removing the segregated Si layer by Ar ion beams and its ordering is found to be not damaged by ion sputtering. A possible cause for the thermally induced Si segregation is also examined experimentally and discussed.