Adsorption of molecular SiO2 on a clean Si(1 0 0) surface

We have investigated the adsorption of molecular (gaseous) SiO₂ on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. The SiO₂ molecule is found to be chemisorbed on various sites on the Si surface and the most energetically favourable structure is on top of the dimers. The minimum energy pathways for the various adsorption channels indicate that the reaction is barrierless in all cases. The corresponding vibrational spectrum is also calculated and the adsorbed molecules are, as expected, found to have red-shifted vibrational frequencies. The energetically favourable adsorption sites and adsorption energies are comparable to the results found for SiO.     

Growth and characterization of CoSi2 films on Si (1 0 0) substrates

In the present work, a special solid phase epitaxy method has been adapted for the preparation of CoSi₂ film. This method includes an epitaxial growth of Co films on Si (1 0 0) substrate, and in situ annealing of the Co/Si films in vacuum. It has been found that at the substrate temperature of 360°C, fcc cobalt film grows epitaxially on the Si (1 0 0) surface. The crystallographic orientation relations between fcc Co film and Si substrate determined from the electron diffraction result are: (0 0 1) Co//(0 0 1) Si, [1 0 0] Co//[1 1 0]Si. Upon annealing at temperatures range from 500 to 600°C, Co film reacts with Si substrate and transforms into CoSi₂. The CoSi₂ films prepared by this way are characterized by XTEM, XPS and AFM.     

Observation of hydrogen adsorption on 6H-SiC(0 0 0 1) surface

We have used coaxial impact-collision ion scattering spectroscopy (CAICISS) and time-of-flight elastic recoil detection analysis (TOF-ERDA) to investigate the adsorption of atomic hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface. It has been found that the saturation coverage of hydrogen on the 6H-SiC(0 0 0 1)√3×√3 surface is about 1.7 ML. Upon saturated adsorption of atomic hydrogen, the √3×√3 surface structure changes to the 1×1 structure. The data of the CAICISS measurements have indicated that as a result of the hydrogen adsorption, Si adatoms on the √3×√3 surface move from T₄ to on-top sites.     

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.     

Nitrogen nanobubbles and butane nanodroplets at Si(1 0 0)

Nanobubbles and nanodroplets were spontaneously formed at Si(1 0 0) in contact with nitrogen and butane saturated water, respectively. The topographic images obtained by tapping mode AFM were similar truncated nanospheres, but the phase images suggested that the nanobubbles were harder than the nanodroplets. The tip–sphere interactions showed the nanodroplets were much viscoelastic than the nanobubbles. The surface and three-phase contact line energies were estimated by analysis of the topographic images. The nanodroplet was stable, but the nanobubble was unstable in spite of the experimental long life. The two-dimensional spatial distribution indicated an attractive interaction between the nanodroplets, but no interaction was observed between the nanobubbles.     

Initial stage of CdTe on Si(1 0 0) grown by MBE

The initial stage of CdTe growth on silicon has been investigated using angle-resolved photoemission and scanning tunneling microscopy (STM). In order to study initial stage of CdTe on Si, we have desorbed CdTe by annealing at 600 °C so that only one monolayer of Te remains on the Si(1 0 0) substrate. Te/Si(1 0 0)2×1 superstructure has been observed by LEED. Photoemission spectra indicate that Te atoms bond with the Si dangling bond. Atomically resolved STM images reveal that the Te atoms form dimers. It is observed that buckling direction of Te-dimer changes and the dimmers are broken in the site of some dimmer rows. It can be explained that the large lattice mismatch cause the switching of the buckling direction and the breaking of Te-dimer resulted surface relaxation.     

Room-temperature deposition of group III metals on Si(1 0 0): A comparative study of nucleation behavior

In this paper, we compare and contrast the processes of nucleation and subsequent growth of single-atom wide metal chains formed when group III metals (Al, Ga, In) are deposited onto Si(1 0 0) at room-temperature (RT). Employing Density Functional Theory (DFT) calculations, diffusion pathways on Si(1 0 0) surface are identified and their associated activation barriers are calculated. Then, the relative stabilities of various C-defect-pinned chains are examined by comparing the relevant adsorption energies. We also account for the observation that defect-nucleated chains grow on only one side of a C-defect by showing that the latter’s presence breaks the symmetry between the two previously equivalent binding sites on either side and rendering one much more stable than the other. Next, a growth model tailored for each group III metal/Si(1 0 0) system and incorporating the above results was simulated using Kinetic Monte Carlo (KMC) techniques to show that the surface morphologies generated by this model accurately reflect the observed ratio of homogeneously to heterogeneously nucleated chains. Finally, we examine through KMC simulations the consequences of the contrasting roles of a defect on In/Si(1 0 0) and Al/Si(1 0 0) – it captures adatoms in the former while it merely blocks direct adatom diffusion in the latter – on key quantities such as the mean island density.     

Magnetic anisotropy of epitaxial Fe layers grown on Si(0 0 1)

The magnetic properties of epitaxial iron films up to 80 monolayers (ML) thickness grown on Si(0 0 1) by using a template technique were investigated by means of superconducting quantum interference device and magneto-optic Kerr effect techniques. The thinnest films investigated (∼3 ML) exhibit a composition close to Fe₃Si with a Curie temperature below room temperature (RT) and strong out-of-plane remanent magnetization that reflects the presence of a dominant second order surface anisotropy term. Thicker films (⩾4 ML) are ferromagnetic at RT with remanent magnetization in film-plane and a composition closer to pure Fe with typically 8–10% silicon content. When deposited at normal incidence such films show simple in-plane fourfold anisotropy without uniaxial contribution. The relevant fourth-order effective anisotropy constant K₄^eff was measured versus film thickness and found to change its sign near 18 ML. The origin of this remarkable behavior is investigated by means of a Néel model and mainly traced back to fourth-order surface anisotropy and magneto-elastic effects related to the large biaxial in-plane compressive strain up to 3.5% in the thinnest (⩽25 ML) films.     

Peculiarities of Al magic cluster self-assembly on Si(1 0 0) surface

Using scanning tunneling microscopy observations and density functional theory calculations, regularities of the Al magic cluster array self-assembly on Si(1 0 0) surface has been elucidated. While a single Al cluster occupies an area of 4a × 3a, an ordered Al-cluster array exhibits a 4 × 5 periodicity, as the clusters in the array are separated by the 4a × 2a “spacers”. The plausible structural model for the “spacer” was proposed in which the “spacer” is arranged as an ordinary 4a × 3a-Al cluster in which the central atomic row with the topmost Si atom is missing. Appearance of the “spacers” in the Al-cluster array was found to reduce formation energy of the array. Ability to incorporate the rows of Al-“spacers” into the completed 4 × 3 In-cluster array was demonstrated.     

Phase transition of Sr on Si (0 0 1): First principles prediction and experiment

The ability to understand and predict the phase diagrams of surface phases from first principles can be valuable for developing processes for growth of epitaxial structures. In the growth of epitaxial oxides on Si (0 0 1), a submonolayer phase of Sr plays a key role. The physical structure for this phase, which has 2 × 3 symmetry and occurs at 1/6 monolayer Sr coverage, was recently elucidated using both first principles theory and diffraction experiments [J.W. Reiner, K.F. Garrity, F.J. Walker, S. Ismail-Beigi, C.H. Ahn, Role of strontium in oxide epitaxy on silicon (0 0 1), Phys. Rev. Lett. 101 (10) (2008) 105503.]. Our approach to understanding the broader Sr/Si phase diagram combines density functional theory with a thermodynamic analysis of the phase equilibrium between a Sr lattice gas and the 2 × 3 structure. We use reflection high energy electron diffraction (RHEED) to experimentally determine the phase diagram, finding good agreement with theoretical predictions.     

Photoelectron spectroscopy (PES) and photoelectron diffraction (XPD) studies on the local adsorption of cyclopentene on Si(100)

We investigated the chemical and geometrical characteristics of the system cyclopentene on Si(100) in a combined photoelectron spectroscopy (PES) and photoelectron diffraction (XPD) investigation. In this study synchrotron radiation was applied to achieve a high resolution with high surface sensitivity. Our PES and XPD results show that the cyclopentene reacts with the silicon dimers of the (2 × 1)-reconstructed surface, changing the planar molecule to a diverse tilted molecule after the adsorption.     

In-plane uniaxial magnetic anisotropy of thin Fe layers on Si(1 1 1) induced upon grazing deposition of a Si capping layer

We demonstrate that an in-plane uniaxial magnetic anisotropy can be induced in Fe films grown on Si(1 1 1) by means of an appropriate Si capping layer. It is shown that grazing-incidence Si deposition reduces the three-fold symmetry of the body-centered-cubic (BCC) Fe(1 1 1) surface to two-fold, thus generating a surface-type uniaxial in-plane magnetic anisotropy contribution. The relevant magnetic easy axis direction depends in a critical way on the azimuth of the Si evaporation flux with respect to the Fe crystal surface directions. A simple qualitative model based on the change of the Fe surface by inhomogeneous chemical reaction with Si is proposed to interpret the magnetic anisotropy evolution.     

Structural and electronic characterization of the MgO/Mo(0 0 1) interface using STM

The nucleation and growth of ultrathin MgO films on Mo(0 0 1) have been investigated with scanning tunneling microscopy and spectroscopy. In the initial growth stage, the MgO forms rather uniform islands with rectangular shapes and defined orientation. This behavior reflects a preferential binding of the oxide O ions to the top positions in the Mo support, which can be realized only in confined areas due to the MgO/Mo lattice mismatch. At monolayer coverage, a characteristic square pattern becomes visible in the STM, indicating the formation of an MgO/Mo coincidence lattice. In the coincidence cell, the interface registry alternates between O and Mg ions being in Mo top positions. The resulting imaging contrast in the STM is dominated by a work-function modulation and not by a topographic effect, as demonstrated with STM-conductance and light-emission spectroscopy. The modulated work function in the coincidence cell is assigned to a small buckling of the oxide film with either O or Mg ions being closer to the Mo surface.     

A model for the dissociative adsorption of N2O on Cu(1 0 0) using a continuous potential energy surface

For the first time, a continuous ab initio potential energy surface (PES) taking into account all molecular degrees of freedom is built and used to model the reaction of a polyatomic molecule with a surface. DFT slab calculations are used to sample the configuration space of the system N₂O/Cu(1 0 0), and the PES function is built with a method of Manzhos and Carrington [J. Chem. Phys. 127 (2007) 014103] using dimensionality reduction from only 4300 single-point energies. Molecular dynamics simulations are performed on the PES to calculate the probability of dissociative adsorption.     

Near-surface stresses in silicon(0 0 1)

We report here on a detailed atomistic stress analysis of the near-surface stresses on the 2 × 1 surface reconstruction on Si(0 0 1). We find that the surface stresses are spatially periodic with fairly large amplitude. We additionally investigate the stresses in the neighborhood of an S_A-type step.     

AFM study of a SiC film grown on Si(1 0 0) surface using a C2H4 beam

A 3C-silicon carbide (SiC) thin film grown on a Si(1 0 0) surface using an ethylene (C₂H₄) molecular beam has been studied by atomic force microscopy. At the center of the irradiation area of the ethylene beam, the shape of the SiC islands was rectangular, the average length of which was 74.5 nm and the average height was 13.1 nm. Each SiC island consists of the SiC particles with the average diameter of 17 nm. Just inside of the boundary region of the beam irradiation, the average size and height of the islands decreased to 50.1 and 8.2 nm, respectively. Just outside of the boundary region, the average size and height decreased to 17.7 and 5.1 nm, respectively. The average reaction probabilities at the above three points were estimated to be 0.14, 0.27 and 2.7%, respectively. New growth mode of the crystal growth is proposed (particles gathering island mode).     

Competition between relaxation and decay of a resonantly excited core-hole state

By a many-body theory it is shown that in general when the time scale of the relaxation from the resonantly excited core-hole state to the fully relaxed core-hole state is much shorter than that of core-hole decay, the autoionization (deexcitation) spectrum is more or less identical to the Auger-electron spectrum by core-hole decay of the fully relaxed core-hole state. Here, the fully relaxed core-hole state is the lowest core-hole state in the X-ray photoelectron spectroscopy spectrum. The present theory explains why the C 1s autoionization spectrum of CO molecule adsorbed on Ni(1 0 0) surface measured at the resonant core-level electron excitation energy is more or less indentical to the Auger-electron spectrum measured at far above the core-level electron ionization limit.     

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO (α₁ peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites (α₂ peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H₂ in the same temperature regime, but again no reaction between the two species is observed.     

Structural and dynamical properties of Ru(0 0 0 1) surface

I performed density functional theory (DFT) calculations combined with MD simulations to study the structural relaxation of Ru(0001) surface. The surface relaxation of the topmost layer is found to be about ?4% at absolute zero temperature. Using MD simulations in the temperatures range of 50 K and 900 K, the effect is found to be minor on the surface relaxation as compared to Pd (1 1 1) clean surface. The effect of surface vibration is also investigated using a LEED code and shows no effect of the vibrational level on the IV curves, which rules out any disagreement between proper theory and LEED results of well prepared surfaces.     

Surface electromigration of In on vicinal Si(0 0 1)

Surface mass transport of In film on vicinal Si(0 0 1) has been systematically investigated by a scanning Auger electron microscopy (SAM), low energy electron diffraction (LEED) and atomic force microscopy (AFM). It was observed that the temperature dependence of the mass transport shows the critical phenomenon. Above a critical temperature T_c, surface electromigration of the In film toward the cathode side dominated the surface mass transport on the vicinal Si(0 0 1) surface. The LEED and AFM observations revealed that the In film surface on the vicinal Si(0 0 1) consists of 3×4 terraces and (3 1 0) facets. The area ratio of the facet to the terrace exhibited abrupt an increase at T_c. It is believed that the change of the mass transport is related to the abrupt change of the area ratio of the facet to the terrace. Both the critical temperature T_c and the spread due to the surface electromigration of the In film depended on the configuration of the DC current direction and the step edge.