Growth process and surface structure of MnSi on Si(1 1 1)

The solid-phase epitaxial growth process and surface structure of MnSi on Si(1 1 1) were investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). For the Si(1 1 1) sample deposited with 30 monolayers (ML) of Mn at room temperature, the intermixing of Mn and Si gradually started at 100 °C and reached equilibrium at approximately 400 °C. At this equilibrium state, the Mn atoms were transformed into crystalline MnSi film. Further annealing caused the desorption of Mn atoms. We identified the structure of MnSi as cubic B20 and the crystallographic orientation relationships as Si(1 1 1)//MnSi(1 1 1) and Si[]//MnSi[]. The MnSi(1 1 1) surface was found to have a dense Si terminating layer on its topmost surface. On the other hand, 3 ML of Mn deposited on Si(1 1 1) reacted with Si even at room temperature and formed a pseudomorphic structure. This structure was transformed into MnSi after annealing. A filmlike morphology with protrusions was observed for the sample with 30 ML of Mn, while island growth occurred for the sample with 3 ML of Mn.     

Step formation on hydrogen-etched 6H-SiC{0 0 0 1} surfaces

The formation of step bunches and/or facets on hydrogen-etched 6H-SiC(0 0 0 1) and () surfaces has been studied, using both nominally on-axis and intentionally miscut (i.e. vicinal) substrates. It is found that small miscuts on the (0 0 0 1) surface produce full unit-cell high steps, while half unit-cell high steps are observed on the () surface. The observed step normal direction is found to be for both surfaces. Hence, for intentionally miscut material, a miscut oriented towards this direction produces much better order in the step array compared to a miscut oriented towards a direction. For (0 0 0 1) vicinal surfaces that are miscut towards the direction, the formation of surface ripples is observed for 3° miscut and the development of small facets (nanofacets) is found for higher miscut angles. Much less faceting is observed on miscut () surfaces. Additionally, the (0 0 01) surface is found to have a much larger spatial anisotropy in step energies than the () surface.     

Tetra-σ bonding adsorption of pyridine on Si(1 0 0)-2 × 1

We studied adsorption of pyridine on Si(1 0 0) at room temperature using high resolution photoemission spectroscopy (PES) and near edge X-ray adsorption fine structure (NEXAFS) in the partial electron yield (PEY) mode. The Si 2p, C 1s, N 1s spectra of pyridine on Si(1 0 0) showed that pyridine is chemisorbed on Si(1 0 0)-2 × 1 through the formation of the tetra-σ-bonded structure with the N atom and three C atoms. NEXAFS was conducted to characterize the adsorption geometry of pyridine on Si(1 0 0). The π* orbital of CC bond showed a good angle dependence in C K-edge NEXAFS spectra, and we were able to estimate the adsorption angle between chemisorbed pyridine of CC bond and the Si(1 0 0) surface using an analytical solution of NEXAFS intensity. We find the coexistence of two different tight bridges with the adsorption angles 42 ± 2° and 45 ± 2° with almost equal abundance.     

ARPES measurements on Si(1 1 1) hole subband induced by Pb and Ga adsorption

The subband dispersions in the Si(1 1 1) p-type inversion layers induced by Pb and Ga adsorbed surface structures were measured by angle-resolved photoemission spectroscopy (ARPES). The surface structures used here were and Si(1 1 1)6.3 × 6.3-Ga. is a new surface phase found in this study. Because it is significant in our study to investigate potential effects of surface superstructures on the hole subband dispersion, we investigated the subband energy levels quantitatively comparing them with those calculated using the triangular approximation. It was found that the energy separation of the adjacent subband quantum levels in the inversion layers induced by gallium adsorption does not follow the triangular approximation. The possible band bending shape was proposed to explain the quantum level spacing of the subbands in Ga-induced inversion layers.     

Substantially low desorption barriers in recombinative desorption of deuterium from a Si(1 0 0) surface

We have studied desorption kinetics of deuterium molecules from a Si(1 0 0) surface by means of temperature-programmed desorption (TPD) spectra and isothermal desorptions.Three desorption components, denoted as β_1,A,β_1,B, and C, can be distinguished in semi-logarithmic plots of the TPD spectra.Their peak positions and intensities are strongly affected by the surface preparation methods employed, either with or without annealing to control the initial D coverage .Peak C appears at the leading edge of the TPD peak.It accounts for only about 5% of the TPD peak at and it diminishes rapidly with decreasing , vanishing at .In contrast, together the β_1,A and β_1,B peaks account for the whole TPD peak at any less than 1.0 ML. The maximum of the β_1,A peak is nearly constant at around the maximum temperature of the TPD peak.On the other hand, the β_1,B peak appears on the high-temperature side of the TPD peak and it systematically shifts to higher temperatures with decreasing .These results imply that first- and second-order kinetics are operating for the β_1,A and β_1,B desorptions, respectively.Isothermal desorption experiments confirm the above predicted kinetics for a limited region, namely .From the results for the rate curve analysis, the desorption barriers are evaluated to be 1.6 ± 0.1 eV and 1.8 ± 0.1 eV for the β_1,A and β_1,B desorptions, respectively.These values are substantially lower than the widely accepted value of ∼2.5 eV. To reproduce the measured TPD spectra we take the Arrhenius-type rate equation containing the first- and second-order rate terms for the β_1,A and β_1,B desorptions.The TPD spectra measured for can be reasonably fit with the proposed rate equation when the values given above for E_d,A and E_d,B are used. For , however, the TPD curves are not fit with the same values; rather, the best-fit curves require values for E_d,A and E_d,B larger than those given above. Combining the present kinetics results with those obtained by STM along with the studies, the β_1,A and β_1,B peaks may be attributed to desorption along the 2H path, while peak C may be attributed to desorption along the 4H path. The atomistic desorption mechanism as well as the energy relationship between the desorption barrier and isosteric heat of adsorption are discussed.     

Electronic structure of dysprosium silicide films grown on a Si(1 1 1) surface

The thickness-dependent electronic structures of Dy silicide films grown on a Si(1 1 1) surface have been investigated by angle-resolved photoelectron spectroscopy. Two (1×1) periodic bands, both of them cross the Fermi level, have been observed in the silicide films formed by Dy coverages of 1.0 monolayer and below, and more than five () periodic bands have been observed in thicker films. Taking the () periodic structure of Dy atoms in the submonolayer silicide film into account, the periodicity of the two metallic bands indicate that they mainly originate from the orbitals of Si atoms, which form a (1×1) structure. Of the () periodic bands observed in thick films, four of them are well explained by the folding of the (1×1) bands into a () periodicity. Regarding the other band, the three () periodic bands would originate from the electronic states related to the inner Si layers that form a () structure, and the one observed in the 3.0 ML film only might originate from the electron located at the interface between bulk Si and the Dy silicide film.     

A novel approach to measure the step line tension and the step dipole moment on vicinal Au(0 0 1) electrodes

In this paper we introduce a new experimental approach to determine the potential dependence of the step line tension on metal electrodes in contact with an electrolyte: (0 0 1) and (1 1 n) surfaces of single crystal gold electrodes were investigated by impedance spectroscopy in solutions containing weakly adsorbing anions, such as , F⁻ and . Within the limits of error the shift in the potential of zero charge is proportional to the step density of the vicinal surfaces indicative of a well-defined dipole moment per step length. The dipole moments per step atom are 6.8 ± 0.8, 5.2 ± 0.4, 5.8 ± 0.5 × 10⁻³ eÅ for , , and F⁻ containing electrolytes, respectively. Using the values for the pzc and the potential dependence of the capacitance curves, the potential dependence of the surface tension of the vicinal surfaces is determined. The line tension of the steps is then calculated from the difference between the surface tension of the stepped and the step free surface. Our results represent the first experimental confirmation of a recent theoretical model proposing that in absence of specifically adsorbed ions the step line tension should decrease (roughly linear) with potential.     

High-resolution angle-resolved photoemission study of Ni(1 1 1) surface state

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been conducted to study the Shockley state (SS) in ferromagnetic Ni(1 1 1) located at the point of the surface Brillouin zone. We have determined the Fermi wave vector and Fermi energy of the state with excitation photon energies of hν = 6.9-27.5 eV. On the basis of ARPES spectral shape analyses, we have found significant electron-electron interaction in the SS.     

STM study of l-serine adsorption on Cu(0 0 1)

The adsorption of l-serine on Cu(0 0 1) surface at 310 K was studied by scanning tunneling microscope (STM). l-serine molecules on the Cu(0 0 1) initially formed a domain of thick lines with a order structure along the direction on the terraces regardless of the coverage of serine. The thick lines were partly replaced by thin line along the direction, and completely disappeared in 2 h. It is considered that in these structures hydrogen bonds involved in hydroxymethyl group between adsorbates play some role in addition to intermolecular hydrogen bond between a hydrogen atom of amino group and an oxygen atom of carboxy group for alanine adsorption.     

Co-induced nano-structures on Si(1 1 1) surface

The interaction of cobalt atoms with silicon (1 1 1) surface has been investigated by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Besides the Co silicide islands, we have successfully distinguished two inequivalent Co-induced reconstructions on Si(1 1 1) surface. Our high-resolution STM images provide some structural properties of the two different derived phases. Both of the two phases seem to form islands with single domain. The new findings will help us to understand the early stage of Co silicide formations.     

Half-metallicity characteristic at zincblende CrSb(0 0 1) surfaces and its interfaces with GaSb(0 0 1) and InAs(0 0 1)

Electronic and magnetic properties of the zincblende CrSb(0 0 1) surfaces and its interfaces with GaSb(0 0 1) and InAs(0 0 1) semiconductors are studied within the framework of the density-functional theory using the FPLAPW+lo approach. We found that the Cr-terminated surfaces retain the half-metallic character, while the half-metallicity is destroyed for the Sb-terminated surfaces due to surface states, which originate from p electrons. The phase diagram obtained through the ab-initio atomistic thermodynamics shows that at phase transition has occurred. Also the half-metallicity character is preserved at both CrSb/GaSb and CrSb/InAs interfaces. The conduction band minimum (CBM) of CrSb in the minority spin case lies about 0.63 eV above that of InAs, suggesting that the majority spin can be injected into InAs without being flipped to the conduction bands of the minority spin. On the other hand the CrSb/GaSb interface has a greater valence band offset (VBO) compared with the CrSb/InAs interface and the minority electrons have lower contribution in the injected currents and hence more efficient spin injection into the GaSb semiconductor. Thus the CrSb/GaSb and CrSb/InAs heterojunctions can be useful in the field of spintronics.     

Photoelectron spectroscopy study of Pb/Ag(1 1 1) in the submonolayer range

The vacuum deposition of Pb onto Ag(1 1 1) gives rise to two different surface structures depending on coverage and deposition temperature. At room temperature (RT), low energy electron diffraction (LEED) reveals a sharp reconstruction completed at 1/3 Pb monolayer (ML). Beyond, a close-packed Pb(1 1 1) incommensurate overlayer develops. At low temperature (LT, ∼100 K) the incommensurate structure is directly observed whatever the coverage, corresponding to the growth of close-packed two-dimensional Pb(1 1 1) islands. Synchrotron radiation Pb 5d core-level spectra clearly demonstrate that in each surface structure all Pb atoms have essentially a unique, but different, environment. This reflects the surface alloy formation between the two immiscible metals in the reconstruction and a clear signature of the de-alloying process at RT beyond 1/3 ML coverage.     

Electronic structure of the Sb-induced Si(1 1 3)2 × 5 surface

The surface electronic structure of Sb/Si(1 1 3)2 × 5 was investigated by angle-resolved photoemission spectroscopy experiments. This reveals Sb/Si(1 1 3)2 × 5 to have three surface bands with anisotropic two-dimensional characteristics. The band widths of the surface bands along is larger than along . The number of surface bands of Sb/Si(1 1 3)2 × 5 and their band dispersions along and are quite analogous with those of Sb/Si(1 1 3)2 × 2 composed of Sb adatom and Si tetramer chains. The electronic structure analogy suggests that Sb/Si(1 1 3)2 × 5 and Sb/Si(1 1 3)2 × 2 have common building blocks such as Sb adatom and Si tetramer chains.     

Growth of well-aligned Bi nanowire on Ag(1 1 1)

We report the fabrication of one-dimensional (1D) Bi nanowires grown on Ag(1 1 1) with average lateral width from 9 to 20 nm by physical vapor deposition in ultra high vacuum conditions. In situ low-temperature scanning tunneling microscopy analyses reveal that the preferred growth of 1D Bi nanostructures is driven by the highly anisotropic bonding in the crystallographic structure of the Bi(1 1 0) plane. The Bi nanowires grow along direction and align with the directions on Ag(1 1 1). The growth of the Bi nanowires proceeds in a bilayer growth mode resulting from the layer pairing in Bi(1 1 0) which saturates the dangling bonds and lowers the total energy.     

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S_A and rebonded S_B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and directions.     

Low temperature adsorption of oxygen on reduced V2O3(0 0 0 1) surfaces

Well ordered V₂O₃(0 0 0 1) films were prepared on Au(1 1 1) and W(1 1 0) substrates. These films are terminated by a layer of vanadyl groups under typical UHV conditions. Reduction by electron bombardment may remove the oxygen atoms of the vanadyl layer, leading to a surface terminated by vanadium atoms. The interaction of oxygen with the reduced V₂O₃(0 0 0 1) surface has been studied in the temperature range from 80 to 610 K. Thermal desorption spectroscopy (TDS), infrared reflection absorption spectroscopy (IRAS), high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) were used to study the adsorbed oxygen species. Low temperature adsorption of oxygen on reduced V₂O₃(0 0 0 1) occurs both dissociatively and molecularly. At 90 K a negatively charged molecular oxygen species is observed. Upon annealing the adsorbed oxygen species dissociates, re-oxidizing the reduced surface by the formation of vanadyl species. Density functional theory was employed to calculate the structure and the vibrational frequencies of the O₂ species on the surface. Using both cluster and periodic models, the surface species could be identified as η²-peroxo () lying flat on surface, bonded to the surface vanadium atoms. Although the O-O vibrational normal mode involves motions almost parallel to the surface, it can be detected by infrared spectroscopy because it is connected with a change of the dipole moment perpendicular to the surface.     

Surface alloys, overlayer and incommensurate structures of Bi on Cu(1 1 1)

Using surface X-ray diffraction, we have determined the structure of three different sub-monolayer phases of Bi on Cu(1 1 1). In contrast to an early report, we find that at a coverage of 1/3 monolayer a substitutional surface alloy is formed with a (√3 × √3)R30° unit cell. For increasing coverage, de-alloying occurs, leading to an overlayer structure at a coverage of 0.5 ML in which the Bi atoms form zigzag chains. The surface contains three domains of this phase. Finally, at a slightly higher coverage of 0.53 ML, the unit cell is compressed in one direction, leading to a uniaxial-incommensurate phase with three rotational domains.The structure determination includes relaxations in the topmost layers and therefore allows a detailed comparison of the most important bond distances. This shows that an increased charge density of the Cu(1 1 1) surface is the main driving force for the different phases.     

Polarity inversion of GaN(0 0 0 1) surfaces induced by Si adsorption

We employ density-functional theory within the local-density approximation to study the structural and electronic properties of Si monolayers adsorbed at GaN(0 0 0 1) surfaces. We find that when the N atoms reside in the outermost layer of the surface, the (0 0 0 1) surface converts into a surface, giving rise to a polarity inversion. We explain this inversion as a charge compensation effect between the N dangling bonds states at the outermost surface layer and the Si donor state in the subsurface layer.     

Temperature dependent low energy electron microscopy study of Ge growth on Si(1 1 3)

We investigated the initial Ge nucleation and Ge island growth on a Si(1 1 3) surface using low energy electron microscopy and low energy electron diffraction. The sample temperature was varied systematically between 380 °C and 590 °C. In this range, a strong temperature dependence of the island shape is observed. With increasing temperature the Ge islands are elongated in the direction. Simultaneously, the average island size increases while their density decreases. From the Arrhenius-like behaviour of the island density, a Ge adatom diffusion barrier height of about 0.53 eV is deduced.     

X-ray initiated molecular photochemistry of Cl-containing adsorbates on a Si(1 0 0) surface using synchrotron radiation

X-ray initiated molecular photochemistry for SiCl₄ and CCl₄ adsorbed on Si(1 0 0) at ∼90 K following Cl 2p core-level excitation is investigated by photon stimulated ion desorption and ion kinetic energy distribution measurements. The Cl excitation of solid SiCl₄ induces the significant enhancement (∼900%) of the Cl⁺ yield, while the Cl excitation of condensed CCl₄ leads to a moderate enhancement (∼500%) of the Cl⁺ yield. The enhancement of Cl⁺ yield at the specific core-excited states is strongly correlated to the ion escaped energy. Upon X-ray exposure for CCl₄ adsorbed on Si(1 0 0) (20-L exposure), the Cl⁺ yields at resonances decrease and new structures at higher photon energies are observed. Cl⁺ yields at these new resonances are significantly enhanced compared to those at other resonances. These changes are the results of desorption and surface reaction of the CCl₄-Si surface complex due to X-ray irradiation. We have demonstrated that state-specific enhancement of ion desorption can be successfully applied to characterize the reaction dynamics of adsorbates adsorbed on surfaces by X-ray irradiation.