Solid-phase synthesis of manganese silicides on the Si(100)2 × 1 surface

The solid-phase synthesis of manganese silicides on the Si(100)2 × 1 surface coated at room temperature by a 2-nm-thick manganese film has been investigated using high-energy-resolution photoelectron spectroscopy with synchrotron radiation. The dynamics of variation of the phase composition and electronic structure of the near-surface region with increasing sample annealing temperature to 600°C, has been revealed. It has been shown that, under these conditions, a solid solution of silicon in manganese, metallic manganese monosilicide MnSi, and semiconductor silicide MnSi_1.7 are successively formed on the silicon surface. The films of both silicides are not continuous, with the fraction of the substrate surface occupied by them decreasing with increasing annealing temperature. The binding energies of the Si 2p and Mn 3p electrons in the compounds synthesized have been determined.     

Formation and magnetic properties of the silicon-cobalt interface

Formation of the Si/Co interface and its magnetic properties have been studied by high-resolution photoelectron spectroscopy with synchrotron radiation. The experiments have been performed in situ in superhigh vacuum (5 × 10^?10 Torr) with coating thicknesses up to 2 nm. It has been found that, in the initial stage of silicon deposition on the surface of polycrystalline cobalt maintained at room temperature, ultrathin layers of the Co₃Si, Co₂Si, CoSi, and CoSi₂ silicides are formed. The three last phases are nonmagnetic, and their formation gives rise to fast decay of magnetic linear dichroism in photoemission of Co 3p electrons. At deposition doses in excess of ～0.4 nm Si, a film of amorphous silicon grows on the sample surface. It has been established that the Si/Co interphase boundary is stable at temperatures up to ～250°C and that further heating of the sample brings about escape of amorphous silicon from the sample surface and initiates processes involving silicide formation.     

Formation of the Co/Si(110) interface: Phase composition and magnetic properties

The formation of the Co/Si(110)16 × 2 interface and its magnetic properties are studied by high-energy-resolution photoelectron spectroscopy using synchrotron radiation and magnetic linear dichroism in the photoemission of core electrons. It is shown that a cobalt coating less than 7 Å thick deposited on the silicon surface at room temperature results in the formation of an ultrathin (1.7 Å) interfacial cobalt silicide layer and a layer of silicon-cobalt solid solution. The ferromagnetic ordering of the interface is observed at an evaporation dose corresponding to 6–7 Å in which case a cobalt metal film begins to grow on the solid solution layer. During 300°C-annealing of the sample covered by a nanometer-thick cobalt layer, the metal film gradually disappears and four silicide phases arise: metastable ferromagnetic silicide Co₃Si and three stable nonmagnetic silicides (Co₂Si, CoSi, and CoSi₂).     

Processes of silicide formation in the Fe/Si(111)7 × 7 system

The initial stages of the formation of iron silicides in the Fe/Si(111)7 × 7 system in the course of solid-phase epitaxy are investigated using high-resolution photoelectron spectroscopy (～100 meV) with synchrotron radiation. The spectra of the Si 2p core and valence-band electrons obtained after deposition of iron coverages of up to 28 monolayers on the surface of the sample and subsequent isochronous annealings at 650°C are measured and analyzed. It is shown that the first to form under Fe deposition is an ultrathin film of the metastable silicide FeSi with a CsCl-type structure, on which a layer of the Fe-Si solid solution with segregated silicon grows. At coverages in excess of 10 monolayers, an iron film grows on the surface of the sample. Annealing of a silicon crystal coated with a Fe layer leads to the sequential formation of two stable silicide phases, namely, the ?-FeSi and β-FeSi₂ phases, in the near-surface region of the sample. It is found that the process of solid-phase synthesis of the ?-FeSi phase passes through the stage of transformation of the iron film into the Fe-Si solid solution.     

Formation of ultrathin iron silicide layers on the single-crystal silicon surface

The solid-phase synthesis of iron silicides on the Si(100)2 × 1 surface with a 5-ML-thick iron film deposited at room temperature was studied by high-resolution photoelectron spectroscopy with the use of synchrotron radiation. Computer simulation of the measured Si 2p spectra revealed the formation of silicides in this system already under annealing at a temperature of 60°C. The process of formation consists in successive syntheses of three iron silicide phases, more specifically, monosilicide ε-FeSi, metastable disilicide γ-FeSi₂, and disilicide β-FeSi₂. The temperature ranges of existence of these phases were determined. Silicon was found to segregate on the γ-FeSi₂ surface.     

Silicide formation in bilayer ultrathin iron and cobalt films on silicon

The processes that occur in ultrathin (up to 1 nm) Fe and Co layers during deposition onto the Si(100)2 × 1 surface in various sequences and during annealing of the formed structures to a temperature of 400°C are studied. The elemental and chemical compositions of the films are analyzed by in situ high-resolution X-ray photoelectron spectroscopy using synchrotron radiation, and their magnetic properties are determined using the magnetic linear dichroism effect in the angular distribution of Fe 3p and Co 3p electrons. It is shown that, when iron is first deposited, the formed structure consists of the layers of FeSi, Fe₃Si, Co-Si solid solution, and metallic cobalt with segregated silicon. The structure formed in the alternative case consists of the layers of CoSi, Co-Si solid solution, Co, Fe-Si solid solution, and Fe partly covered by silicon. All layers (apart from FeSi, CoSi) form general magnetic systems characterized by ferromagnetic ordering. Annealing of the structures at temperatures above 130dgC (for the Co/Fe/Si system) and ～200°C (for Fe/Co/Si) leads to the formation of nonmagnetic binary and ternary silicides (Fe _x Co_{1 ? x} Si, Fe _x Co_{2 ? x} Si).     

Interpretation of auger energy shifts of silicon in solid silicon compounds

Chemical shifts of Auger transitions and photoelectron binding energies of silicon have been measured and interpreted using the quasi-atomic approach. The Si KL_2,3L_2,3 and L_2,3V₁V₁ Auger transitions and the binding energies of Si 2p and of the valence electrons at the maximum of the density of states V₁ have been investigated in solid silicon and in the compounds SiC, Si₃N₄, SiO₂, Na₂SiF₆ and T₃Si (T = V, Cr, Mn, Fe, Co, Ni). The relaxation-energy shift ΔR^ea_S(2p, 2p) describing the polarization effect (final-state effect) has been evaluated by AES and XPS measurements. Furthermore, the extra-atomic relaxation energy R^ea_D(2p) of the 2p electrons has been determined experimentally for silicon atoms in differing environments. This allows estimation of the potential parameter V(2p) describing the potential effect (initial-state effect). In general R^ea_D(2p) was found to be more sensitive to changes in chemical bonding than V2p). The behaviour of the quasi-atomic Si V₁ electrons seems to be the converse.     

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.     

Characterization of plasma polymer films of phenylsilane by X-ray photoelectron spectroscopy

Carbon 1s and silicon 2p X-ray photoelectron spectra of phenylsilane plasma polymer films prepared at substrate temperatures,T_s, between 50 and 450°C were recorded. The binding energies, lineshapes, and shake-up satellite intensities are in accordance with a structure consisting of a silicon network with pendant phenyl groups, and the minor dependence on T_s is consistent with the main effect of increasing preparation temperature being the loss of hydrogen and some pendant phenyl, with a concurrent increase in the interconnectivity of the network. The observed C 1s binding energy and linewidth specifically rule out the presence of any significant amount of carbon in silicon carbide form. A simultaneous shift of about 0.6 eV in the binding energies of both the C 1s and Si 2p lines is tentatively interpreted as a shift in the Fermi level with respect to the valence band edge.     

Interaction of thin silicon layers with the (0001) surface of rare-earth metals

The electronic and crystalline structures of the systems formed upon deposition of silicon layers onto the Gd(0001) and Dy(0001) surfaces of single-crystal films annealed subsequently at T=450–500°C have been studied by low-energy electron diffraction (LEED) and also by the Auger electron and angle-resolved photoelectron spectroscopy of the valence band and the Si(2p) core level. It is shown that the systems thus produced can be described as starting single-crystal films of Gd and Dy, with 3D islands of the silicides of these metals on the surface of the corresponding metalfillms.     

Topmost-surface-sensitive Si-2p photoelectron spectra of clean Si(1 0 0)-2 × 1 measured with photoelectron Auger coincidence spectroscopy

Topmost-surface-sensitive Si-2p photoelectron spectra of a clean Si(1 0 0)-2 × 1 surface have been measured using Si-2p photoelectron Si-L₂₃VV Auger coincidence spectroscopy (Si-2p–Si-L₂₃VV PEACS). The escape depth of the PEACS electrons is estimated to be ～1.2 Å. The results support the assignments of the Si up-atoms, the Si down-atoms, the Si 2nd-layer, and the Si bulk proposed in previous researches. The Si-2p component with a binding energy of ?0.23 eV relative to the bulk Si-2p_3/2 peak, is shown to originate mainly from the topmost surface. Site selectivity of PEACS is indicated to be achieved to some degree by carefully selecting the kinetic energy of the Auger electrons. Since PEACS can be applied to any surface, the present study opens a new approach to identify PES components.     

Photoelectron Si 2<Emphasis Type="Italic">p</Emphasis> spectra of ultrathin CoSi<Subscript>2</Subscript> layers formed on Si(100)2×1

Solid-phase formation of ultrathin CoSi₂ layers on Si(100)2×1 was studied using high-resolution (～140 meV) photoelectron spectroscopy with synchrotron radiation (hν=130 eV). The evolution of Si 2p spectra was recorded both under deposition of cobalt on the surface of samples maintained at room temperature and in the course of their subsequent annealing. It was shown that Co adsorption on Si(100)2×1 is accompanied by a loss of reconstruction of the original silicon surface while not bringing about the formation of a stable CoSi₂-like phase. As the amount of deposited cobalt continues to increase (up to six monolayers), a discontinuous film of the Co-Si solid solution begins to grow on the silicon surface coated by chemisorbed cobalt. The solid-phase reaction of CoSi₂ formation starts at a temperature close to 250°C and ends after the samples have been annealed to ～350°C.     

Magnetic linear dichroism in photoemission from an ultrathin iron silicide film

The effect of magnetic linear dichroism in photoemission of Fe 3p electrons was used to investigate the magnetic properties of the Si(100)2 × 1 surface on which iron films up to 10 monolayers thick were deposited at room temperature under ultrahigh vacuum. The experiments were performed with linearly polarized light (at a photon energy of 135 eV) incident at an angle of 30° to the surface. The photoelectron spectra were measured in a narrow solid angle oriented along the normal to the sample surface for two opposite magnetization directions which were parallel to the surface plane and perpendicular to the polarization vector of the light wave. An analysis of the data obtained showed that the effect has a threshold character and appears after deposition of eight Fe monolayers, when the ferromagnetic silicide Fe₃Si is formed on the surface.     

Angle-resolved ultraviolet photoemission study of Si(111) 7 × 7 and 1 × 1 surfaces

Angle-resolved ultraviolet photoelectron spectra of Si(111) 7 × 7 and 1 × 1 surfaces have been measured as a function of temperature from ambient temperature to ≈ 1120°C. Both the Si(111) 7 × 7 and 1 × 1 surfaces show obvious surface metallic edge at all the temperatures. A middle peak among three surface-state peaks observed for ambient-temperature 7 × 7 surface has been found to disappear for high-temperature 7 × 7 surface. In going from high-T 7 × 7 surface to high-T 1 × 1 surface, no essential changes in the surface-state peaks have been found to occur.     

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.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

Photoelectron spectroscopy of atomic core levels on the silicon surface: A review

Recent studies of the atomic structure of the single-crystal silicon surface (both clean and covered by adsorbates) that are performed by high-resolution core-level photoelectron spectroscopy using synchrotron radiation are reviewed. The physical principles of the method, experimental techniques, the spectrum processing procedure, and the procedure of determining the energy shifts of the core levels in the subsurface layer are outlined. Emphasis is placed on the surface modes of silicon 2p spectra, which are observed for the main types of silicon surface reconstruction (Si(111)-7×7 and Si(100)-2×1), and on a correlation between these modes and the atomic structure of the (111) and (100) surfaces. Also, particular attention is given to the studies of the Ge/Si system, which is viewed as a promising material of nanoelectronics, as well as to those concerned with metal and gas adsorption on basic (low-index) silicon faces. These studies clearly demonstrate that core-level photoelectron spectroscopy provides extremely detailed information on the structure of adsorbed layers and on the adsorption-stimulated reconstruction of the substrate surface.     

Surface electronic structure of single-domain Si( 001) 2 × 2-Al: an angle-resolved photoelectron spectroscopy study using synchrotron radiation

The electronic structure of a single-domain Si(001)2 × 2-Al surface has been studied by angle-resolved photoelectron spectroscopy (ARPES) using synchrotron radiation. Through detailed ARPES measurements along various symmetry axes of the surface Brillouin zone, the existence and dispersions of five surface states are identified, one at binding energies a little less than 1 eV and the others between 1 and 2 eV. The origin of the surface states are discussed in terms of the Al-dimer structures on Si(001).     

Interaction of iron atoms with the oxidized silicon surface

The room-temperature interaction of iron atoms with the oxidized Si(100)2×1 surface at a coverage from a submonolayer to four monolayers is studied by core-level photoelectron spectroscopy using synchrotron radiation. Computer simulation of the Si 2p core electron spectra demonstrates that iron atoms penetrate beneath the silicon oxide even at room temperature. This process causes the initial silicon phases at the SiO_x/Si interface to disappear; gives rise to a complex ternary phase involving Fe, O, and Si atoms; and favors the formation of a Fe-Si solid solution at the interface.     

Relation between orbital binding energies and ionicities in alkali and alkaline earth flourides

The differences between the ionization potentials and binding energies for M^q+ (np⁶), F^?(1 s²), andF^?(2p⁶) orbital electrons, adjusted for the electrostatic self-potentials, in alkali and alkaline earth fluorides have been correlated with ionicites derived from the indices of refraction through optical dispersion theory of Phillips and van Vechten. The differences are linear in ionicities and are related to the covalent energies and the polarizabilities. The gap between the valence band and the Fermi level, determined with X-ray photoelectron spectroscopy, has been compared with the energy in the Phillips-van Vechten model. Whereas the former is a measure of the thermal activation energy for conduction, the latter is determined essentially by the valence electron density, the molar volume, and the polarizability.