TiSi mixing at room temperature: A high resolution ion backscattering study

Very thin Ti films (0–20 Å) have been deposited in ultra-high vacuum on atomically clean Si(111) surfaces. The films have been analyzed with Medium Energy Ion Scattering using the high depth resolution of this technique in combination with ion shadowing and blocking. The measurements show that mixing of Ti and Si already occurs at room temperature, for coverages below 2.8 × 10¹⁵ Ti atoms/cm². The composition of the uniformly mixed film is close to TiSi. For higher coverages no further mixing takes place and pure Ti is present on top of the ultrathin mixed film. The implications of these results for current silicide formation models are discussed.     

Valence variations in the monolayer regime of Sm on the Nb(110) surface

Sm overlayers in the monolayer regime, deposited on a Nb(110) single-crystal surface at room temperature, have been studied by means of high-resolution photoelectron spectroscopy of the Nb 3d_5/2 core level and the Sm valence band. In the submonolayer regime, the Sm valence varies from mainly divalent for very low coverages to essentially trivalent close to a complete monolayer. Above 1 ML, a new divalent component appears in the Sm 4f spectra, corresonding to divalent Sm in the second layer. The mixed valence in this overlayer system is concluded to be heterogeneous (all Sm atoms have integer but site-dependent valence). Sm forms ordered overlayers on Nb(110) and the Sm growth is consistent with a layer-by-layer growth mode (Frank–van der Merwe growth).     

Els investigation of the Si-Pd interface

Energy loss and Auger spectra of Pd deposited onto Si(111) at increasing coverages are presented, together with those of pure Pd and of bulk Pd₂Si obtained in situ. The loss evolution with coverage is interpreted with the aid of a simple model calculation of the surface loss function of an ideal Si-Pd system. The results indicate that for coverages between 1 and 7 Å an intermixed phase forms at the interface, while for higher coverages pure Pd grows on the surface. In the first stage some losses seem to shift in energy with coverage due to interface mode coupling.     

Subsurface As-layer formation in Au films deposited on GaAs(001)

Au deposited at room temperature on MBE grown GaAs (001) has been studied by UPS, XPS and AES. At Au coverages below ～2Å the adsorption interaction spreads the Au (6s) states density over the width of the GaAs valence band, while at higher coverages the Au overlayer has metallic properties. For the thick Au overlayers a subsurface As-rich region is established. It is suggested that this has a stabilizing influence on the continuous Au overlayer.     

Mechanisms of formation and the electronic properties of the Yb-Si(100) thin-film system

The formation and properties of the Yb-Si(100) thin-film system were studied by high-resolution photoelectron spectroscopy with synchrotron radiation, Auger electron spectroscopy, the contact potential difference technique, and low-energy electron diffraction measurements over a wide range of coverages and at different temperatures. It was established that the formation of the Yb-Si(100) system prepared by solid-phase epitaxy occurs through a mechanism close to the Stranski-Krastanow mechanism. It was shown that, at submonolayer coverages, it is primarily these two-dimensional (2D) 2 × 3 and 2 × 6 structures that are formed, while at higher coverages a three-dimensional Yb silicide film grows. Data on the morphology and phase composition of the silicide film and on the electronic state of the Si atoms and the valence state of the Yb atoms in the silicide and the 2D structures, as well as on the atomic structure of these films, were obtained. The components of the Si 2p spectra were studied for different coverages. The relation between the shape of these spectra obtained for multilayer Yb silicide films and their phase composition was revealed.     

Coverage- and temperature-dependent vibrational spectra of hydrogen chemisorbed on Si(100)2 × 1

The adsorption of atomic hydrogen at Si(100)2 × 1 has been studied for coverages at and below one monolayer at temperatures between 300 and 1200 K using high-resolution Electron Energy Loss Spectroscopy (EELS) and Low Energy Electron Diffraction (LEED). Measurements of EELS frequencies, linewidths and intensities are discussed for different coverages and temperatures during exposure as well as subsequent annealing. Formation of a monohydride Si(100)2 × 1 : H adsorption phase is observed at room temperature in the sub-monolayer range, at 650 K for all coverages up to the saturation, and during thermal decomposition of the low temperature dihydride Si(100)1 × 1 : : 2H adsorption phase. The latter is formed by saturating Si(100) at 300 K with atomic hydrogen.     

Valence variations of Sm on polycrystalline Ag

Thin deposited films of Sm on a polycrystalline Ag are investigated by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and photoemission electron microscopy (PEEM). The Sm valence is mainly divalent for low Sm coverage, while the trivalent contribution to the XPS intensity increases considerably for higher coverage. For an Sm overlayer thicker than 4 Å, the average valence is estimated to be 2.65. The mixed valence in this system is concluded to be heterogeneous (all Sm atoms have integer and site-dependent valence). Alloy formation between Sm and Ag is observed upon annealing to temperatures between 400 and 550 °C. For these temperatures the change in average Sm valence is dependent on the initial Sm coverage deposited onto the Ag-foil. Systems with low initial coverage exhibit an increase in the average valence, while a decrease is observed for systems with coverage above 6 Å. For intermediate coverages around 3 Å an initial decrease in average valence is followed by a rapid increase for temperatures above 400 °C due to morphological changes in the surface layer.     

Scanning tunneling microscopy observation of ultrathin epitaxial CoSi<Subscript>2</Subscript>(111) films grown at a high temperature

Scanning tunneling microscopy (STM) is used to study the basic laws of growth of ultrathin epitaxial CoSi₂(111) films with Co coverages up to 4 ML formed upon sequential deposition of Co and Si atoms taken in a stoichiometric ratio onto the Co–Si(111) surface at room temperature and subsequent annealing at 600–700°C. When the coverage of Co atoms is lower than ~2.7 ML, flat CoSi₂ islands up to ~3 nm high with surface structure 2 × 2 or 1 × 1 grow. It is shown that continuous epitaxial CoSi₂ films containing 3–4 triple Si–Co–Si layers grow provided precise control of deposition. CoSi₂ films can contain inclusions of the local regions with (2 × 1)Si reconstruction. At a temperature above 700°C, a multilevel CoSi₂ film with pinholes grows because of vertical growth caused by the difference between the free energies of the CoSi₂(111) and Si(111) surfaces. According to theoretical calculations, structures of A or B type with a coordination number of 8 of Co atoms are most favorable for the CoSi₂(111)2 × 2 interface.     

Photo-Absorption Studies on Formaldehyde Using Synchrotron Radiation at Indus I

Photo-absorption spectra of formaldehyde (HCHO) is recorded in the range of 6–11.5 eV at various pressures (<0.001–2 mbar) at an average resolution of 1.2 Å using Photophysics beam line at the 450 MeV Indus-1 synchrotron radiation facilities at RRCAT Indore, India. The spectrum is found to consist exclusively of n → Rydberg transitions converging to the ground state of HCHO⁺. The highest identified Rydberg states, observed up to the first ionization limit of HCHO, correspond to 7s, 11p, 9d, and 12f orbitals. Analyzed electronic spectrum along with the intensities and quantum defects are presented. To interpret the observed weak valence transitions instead of strong valence transitions, a theoretical study of Rydberg and valence electronic states of HCHO is performed in the framework of single configuration interaction (CIS) and time-dependent density functional theory (TDDFT) using different basis sets. Electronic transition energies of high-lying singlet and triplet valence states as calculated using TDDFT (B3LYP) level of theory are found to give fairly-good agreement with the experimental data.     

Formation and properties of a binary adsorbed layer in a two-component adsorption system (Sm + Yb)-Si(111)

Low-energy electron diffraction, thermodesorption spectroscopy, and contact potential difference techniques were used in a first study on the coadsorption of Sm and Yb atoms on the Si(111) surface. At comparatively low coverages, in both one-component adsorption systems of the rare-earth metal-Si(111) type and the two-component system (Sm + Yb)-Si(111), the same sequence of diffraction patterns of the (n×1) type, where n=3, 5, and 7, was observed. This indicates that Sm and Yb atoms occupy the same adsorption centers in a mixed film. At higher coverages, at which the \((\sqrt 3 \times \sqrt 3 )R30^\circ \) reconstruction forms in the case of the Sm-Si(111) system and the surface undergoes the 2×1 reconstruction in the Yb-Si(111) system, the structure of the mixed film is governed by the ytterbium coverage θ(Yb). At low ytterbium coverages, θ(Yb)<0.15, superposition of the \((\sqrt 3 \times \sqrt 3 )R30^\circ \) and (2×1) diffraction patterns is observed. For θ (Yb)>0.15, however, the former pattern disappears, whereas the latter persists. A comparison of this evolution of a binary adsorbed layer with the properties of the Sm-Si(111) and Yb-Si(111) systems indicates its anomalous character.     

Magnetic-dichroism study of iron silicides formed at the Fe/Si(100) interface

The interplay between the phase composition, electronic structure, and magnetic properties of the Fe/Si(100)2×1 interface has been studied at the initial stages of its formation (at Fe doses up to 8 Å). The experiments were carried out in ultra high vacuum by using high-resolution photoelectron spectroscopy with synchrotron radiation. The interface magnetic properties were examined in terms of magnetic linear dichroism in angle-resolved Fe 3p core-level photoemission. It was found that at room temperature a disordered Fe–Si solid solution is formed at the first stage of Fe deposition (≤3.4 Å). In the coverage range of 3.4–4.3 Å the solid solution transforms into Fe₃Si. However, the in-plane ferromagnetic ordering of the silicide occurs only at 6.8 Å Fe that demonstrates the thickness dependence of the magnetic properties of Fe₃Si. The subsequent sample annealing to 150°C transforms Fe₃Si to ε-FeSi, leading to the disappearance of ferromagnetic behavior.     

Surface crystallography of W(110) and W(110) p (2×1)-O: The position of W atoms

The position of W atoms in the surface layers of clean W (110) and W (110) p (2 × 1)-O is studied using Constant-Momentum-Transfer Averaging of LEED intensities. It is shown that the clean surface is not relaxed to an uncertainty of < 0.06 Å. Analysis of superstructure beam intensity averages from W(110) p (2 × 1)-O indicates that oxygen does not reconstruct W(110) at these coverages and below 1000 ° K. An upper limit of 0.05 Å can be put on the out-of-plane displacement of W atoms by the oxygen. Substrate beam averages from W (110) p (2 × 1)-O verify the non-reconstruction. The use of CMTA for adsorbed-layer crystallography in general is briefly discussed.     

Synchrotron radiation photoemission studies of the pentacene—Ag/Si(1 1 1) √3 × √3 interface

The interaction of monolayer coverages of pentacene with the √3 × √3 silver terminated Si(1 1 1) surface has been studies by high resolution core level and valence band photoemission spectroscopies. Core level Si 2p spectra reveal that there is only a very weak interaction between the pentacene and the underlying silicon, however, there is evidence of Fermi level movement. Valence band spectra acquired with both s and p polarised light indicate that for the surface coverages investigated, the molecular layers are oriented parallel to the plane of the surface. These results are in agreement with recent scanning tunneling microscopy (STM) studies which indicated that the pentacene molecules form highly ordered layers with the plane of the molecule parallel to the surface. Changes in the workfunction and Fermi level movements have been used to determine the energy level alignment at the interface. A 0.35 eV interface dipole forms between the pentacene and the silver terminated Si(1 1 1) surface within a two monolayer deposition. Photoemission measurements of the energy level alignment at the interface reveal that there is almost no barrier to charge injection from the conduction band of the semiconductor to the lowest unoccupied molecular orbital (LUMO) of the pentacene molecule.     

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.     

Calculation of shuffle 60° dislocation width and Peierls barrier and stress for semiconductors silicon and germanium

The dislocation width for shuffle 60° dislocation in semiconductors Si and Ge have been calculated by the improved P-N theory in which the discrete effect has been taken into account. Peierls barrier and stress have been evaluated with considering the contribution of strain energy. The discrete effect make dislocation width wider, and Peierls barrier and stress lower. The dislocation width of 60° dislocation in Si and Ge is respectively about 3.84 Å and 4.00 Å (～1b, b is the Burgers vector). In the case of 60° dislocation, after considering the contribution of strain energy, Peierls barrier and stress are increased. The Peierls barrier for 60° dislocation in Si and Ge is respectively about 15 meV/Å and 12–14 meV/Å, Peierls stress is about 3.8 meV/ų (0.6 GPa) and 2.7–3.3 meV/ų (0.4–0.5 GPa). The Peierls stress for Si agrees well with the numerical results and the critical stress at 0 K extrapolated from experimental data. Ge behaves similarly to Si.     

An exploratory study of the reactive Ni-GaAs (1 1 0) interface

Soft X-ray photoemission spectroscopy measurements have been carried out on cleaved n-type GaAs (1 1 0) surfaces covered with Ni overlayers ranging in thickness from 0.05 to 53 Å. The results of these room temperature measurements show that we have band bending effects occurring in conjunction with strong interfacial chemical reactions. Deconvolution of the Ga 3d core line into substrate and metallic components shows dissolution of the substrate at the interface with Ga diffusing into the surface of the metal overlayer for the intermediate coverages (1–15 Å). Observation of the As 3d core level shows out-diffusion of As to the surface over the entire Ni coverage range. Using this deconvolution scheme we are able to follow the band bending of the Schottky barrier formed here up to the 8 Å coverage. The Schottky barrier height is 1.0 ± 0.1 eV for this overlayer thickness.     

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).     

Fe在Ru(1010)表面上沉积的LEED，ARUPS研究

Fe在Ru(1010)表面上的沉积,经过LEED和ARUPS的测量研究.结果表明,经过退火Fe在Ru(1010)表面根据覆盖度大小分别形成(1×1)和P(2×1)结构.当覆盖度大于10个单层时,表面结构不再保持与衬底相关的晶格常数与对称性,形成fcc铁的(111)外延结构.铁覆盖度较小时,费米能级附近的价电子能级与清洁时相似,外延fcc铁表面则形成了与表面有关的价电子能级. 关键词：     

High resolution studies of NiSi2 ultrathin film formation by ion scattering and cross-section tem

Ultrathin epitaxial NiSi₂ films (0–40 Å) have been grown on Si(111) surfaces. Medium energy ion shadowing and blocking has been used to determine the orientation, morphology and interfacial order of these films. For the whole coverage range studied ((0–1) × 10¹⁶ Ni $\text{atoms}\text{cm}{}^2$) the films are found to be rotated 180° about the surface normal with respect to the Si(111) substrate. Using the high depth resolution of the technique the annealed films are shown to consist initially of islands, which coalesce into continuous films for coverages above ≈ 5 × 10¹⁵ Ni $\text{atoms}\text{cm}{}^2$. High resolution cross-section transmission electron microscopy shows the NiSi₂Si interface to be atomically abrupt. This interface has been probed directly using the ion channeling technique, and the number of disordered Ni atoms at the interface is found to be less than 7.5 × 10¹³$\text{atoms}\text{cm}{}^2$.