Angle resolved photoemission measurements on AgSi(111) 7 × 7 interfaces

The AgSi(111) interface is investigated by LEED, AES and angle resolved photoemission spectroscopy using 50 eV synchrotron radiation in p-polarization. Results on room temperature (RT) silver growth on Si(111) 7 × 7 are characterized by an evolution of the LEED pattern and of the d band shape which is consistent with 2D island formation in the submonolayer range. When the Ag coverage (Θ) is increased, a progressive build-up of Ag layers occurs with a possible interdiffusion of the atomic constituents. The ordered $\text{Si(111)}\text{3}\text{×}\text{3}\text{R(30°)Ag}$ structure (R₃) obtained by annealing a 1 ML RT deposit gives rise to new interface states near E_F. In contrast to the RT deposit at the same Θ, two well defined d band peaks are present while the bulk Si emission near 3.4 eV is clearly seen. The R₃ data would favour recent crystallographic models which conclude to an embedment of the Ag atoms in a threefold hollow adsorption site.     

High-resolution photoemission study of adatom bonding effects: Cl (√2 × √2) R 45° on Cu (100)

We have investigated photoemission from an ordered $\text{(}\text{2}\text{×}\text{2}\text{) R 45°}$ Cl-overlayer on Cu (100) at good angular (Δθ ? 1°) and energy (ΔE = 100 meV) resolution. We observe two prominent adsorbate- induced features at 1.95 eV (FWHM ≈ 100 meV) and 5.4 eV (0.65 eV) below E_F. By determination of their angular momentum character, their spatial orientation and their energy dispersion, they are identified as “antibonding” and “bonding” resonances, respectively. Significant modification of the Cu d-band emission is observed and interpreted in terms of substrate orbitals which are involved in bonding.     

Room temperature adsorption and growth of Ga and In on cleaved Si(111)

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS) measurements have been performed on a set of ultrahigh vacuum cleaved Si(111) surfaces with different bulk dopings as a function of Ga or In coverage θ. The metal layers are obtained by evaporation on the unheated substrate and θ varies from zero to several monolayers (ML). First, the 2×1 reconstruction of the clean substrate is replaced by a $\text{3}\text{×}\text{3}$ R30° structure at $\text{1}\text{3}$ ML, meanwhile the dangling bond peak at 0.6 eV below the valence band edge E_vs is replaced by a peak at 0.1 eV for Ga or 0.3 eV for In, below E_vs. At the same time, the ionization energy decreases by 0.4 eV (Ga) or 0.6 eV (In), while the Fermi level pinning position gets closer to the valence band edge by about 0.1eV. Upon increasing θ, new LEED structures develop and the electronic properties keep on changing slightly before metallic islands start to grow beyond θ ～1 ML.     

Electronic states of (2 × 1) and (1 × 1) (111) surfaces of Ge,Si, diamond,GaAs and Ge on Si

The “dangling-bond” surface state dispersion curves, E(k), have been calculated for the (2 × 1) and (1× 1) (111) surfaces of Ge, Si, and diamond, for (1 × 1) GaAs, and for (2 × 1) Ge on Si. The calculations employ the sp³s1 empirical tight-binding model of Vogl et al. and the atomic relaxation of Feder et al. The surface state band gaps are in good agreement with optical-absorption and electron-energy-loss measurements for Ge and Si. For the assumed epitaxial geometry, Ge on Si is predicted to shift the dangling-bond states downward by ≈0.1 to 0.4 eV.     

Inverse photoemission of pyridine on silver (111)

Inverse photoemission, together with UPS and EELS, is used to obtain information on the position of the affinity level of pyridine adsorbed on Ag(111). The unresolved a₂ and b₁ affinity levels (AL)(1.20 and 0.62 eV above the vacuum level for free pyridine) are found at E_F+(2.9±0.2) eV (indicating a Coulomb relaxation of about 2.1 eV). The reported phase transition in the first adsorbed monolayer of pyridine had no influence on the position and halfwidth of the AL nor on the electronic excitation intensities. The assignment of UPS structure is discussed. Relaxation is different for unoccupied and occupied states. The second layer “initial state” model of photoionization is not confirmed.     

A study of the (00) LEED beam intensity at normal incidence from CdS(0001), Cu(001), Cu(111), and Ni(111)

A Faraday cage apparatus is used for the measurement of the (00) LEED beam intensity, I₍₀₀₎, and the total secondary emission coefficient, δ(E_k), for angles of incidence from 0° ± 2° to 8° ± 2°, with an energy resolution of ± 0.037 of the incident beam energy, in the energy range 1 to 200 eV. The data are normalized and expressed as a fraction of the incident beam intensity. The basic principle of operation is the separation of the incident and specularly diffracted beams in a uniform magnetic field. Monolayer, or in-plane, resonances associated with the emergence of nonspecular beams, as well as beam threshold minima, are observed in I₍₀₀₎ at normal incidence from clean CdS(0001), Cu(111), and Ni(111). Some major differences are observed in the I₍₀₀₎ profiles for the clean (111) surfaces of nickel and copper. All secondary Bragg peaks, except the 2$\text{2}\text{3}$ order, have greater intensities for Ni(111) in the energy range 50–150 eV, thus indicating that the atomic scattering cross-section for electrons in this energy range is larger for nickel than for copper. For the (111) surface of nickel, the (11) resonance is missing, but the (10) resonance and all $\text{1}\text{3}$ order secondary Bragg peaks between the second and fifth orders are observed. For Cu(111) both the (10) and (11) resonances are observed, but the $\text{1}\text{3}$, $\text{2}\text{3}$, 1$\text{2}\text{3}$, and 3$\text{1}\text{3}$ order secondary Bragg peaks are missing in this energy range. These data indicate that multiple scattering with evanescent intermediate waves, or “shadowing”, is predominate on the (111) surfaces on nickel and copper for energies above 30 eV, and that below 30 eV multiple scattering with propagating intermediate waves is predominate on Cu(111). Correlation of the (00) beam intensity profiles from clean Ni(111) at 0°, 2°, and 6° with the intensity profiles of the (10). (1̄0), and (11) non-specular beams is nearly one-to-one from 30 eV to 100 eV, thus supporting the dynamical theories of LEED in which peaks in the (00) beam are expected to occur at nearly the same energies as peaks in the non-specular beams.     

Interaction of NO and O2 with Pd(111) surfaces. II

At least three different types of oxygen atoms may be present in the surface region of Pd(111) which may be distinguished by their thermal, chemical, structural and electronic properties. Exposure to O₂ at low temperatures causes the formation of 2 × 2 and $\text{3}\text{×}\text{3}\text{R30°}$ structures from chemisorbed oxygen, the latter being probably stabilized by small amounts of H_ab or CO_ab on the surface. The initial sticking coefficient was estimated to be about s₀ ≈ 0.3, the adsorption energy ~55 kcal/mole. The photoelectron spectrum exhibits an additional maximum at 5 eV below E_F. During thermal desorption dissolution of oxygen in the bulk strongly competes; on the other hand absorbed oxygen may diffuse to the surface giving rise to high temperature peaks in the flash desorption spectra. High temperature (~1000 K) treatment of the sample with O₂ causes the formation of a more tightly bound surface species also characterized by a 2 × 2 LEED pattern which is chemically rather stable and which is considered to be a transition state to PdO. The latter compound is only formed by interaction with NO at about 1000 K via the reaction $\text{Pd}\text{+}\text{NO}\text{→}\text{PdO}\text{+}\text{1}\text{2N}{}_2$ which offers a rather high “virtual” oxygen pressure. This reaction leads to drastic changes of the photoelectron spectrum and is also identified within the LEED pattern.     

Hydrogen chemisorption on W {110}: Angle-resolved photoemission

Normal photoemission spectra (k_∥ = 0) from a W {110} surface reveal 2 major resonances during hydrogen chemisorption, at 2.0 and 4 eV below the Fermi level, E_F. The former appears at 300 K during filling of the low coverage β₂ state, and the latter grows as the β₁ state is filled. Detailed spectra obtained along the $\text{}\text{?}$gS and $\text{}\text{?}$gD azimuths are reported, showing additional resonances at ~0.5, 1,3,6 and ~7 eV below E_F. A structural model is proposed, based on a consideration of the present results in relation to RHEED and HRELS data, for the H-saturated surface in which a p(1 × 2) structure is formed, at a H: surface W ratio of unity, with β₁ adatoms occupying a close bridge position and β₂ adatoms in a 3-fold hollow site.     

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

CO chemisorption on PtCu surfaces I. CO on (1 × 3) Pt0.98Cu0.02(110)

Thermal desorption and photoemission spectroscopy (PES) have been used to investigate the chemisorption of CO on an annealed Pt_0.98Cu_0.02(110) surface. The clean surface shows 9.1 ± 2.6% Cu within the top 4 Å, and is (1 × 3) reconstructed. Thermal desorption of CO has revealed the existence of various adsorption states with these respective heats of adsorption: (α) 35.2 to 37.8 kcal/mol and (β) 24.5 to 26.3 kcal/mol on Pt sites, (γ) 16.0 to 17.2 kcal/mol on PtCu “mixed” sited, and (δ) 12.9 to 13.9 kcal/mol on Cu sites. PES observation of Cu 3d-derived states (using hv = 150 eV) and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels (using Mg Kα radiation) shows that the electronic structure of the Cu constituent is changed only when CO adsorbs on the Pt-Cu “mixed” sites or the Cu sites. Furthermore, the CO states associated with Pt sites reflect the structural difference between the (1 × 3) alloy surface and the (1 × 2) pure Pt(110) surface: α-CO on the alloy surface desorbs at a temperature 17 to 21 K. higher than the maximum desorption temperature of CO from pure Pt(110), and the ratio of β-CO to α-CO desorption from the alloy surface is larger than the ratio of low temperature to high temperature peaks in the desorption of CO from pure Pt(110).     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

Angle-resolved photoemission of the c(2 × 2) and c(3 × 1) oxygen overlayers on Fe(110)

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the band structure of the c(2×2) and (3×1) oxygen overlayers on Fe(110). The symmetries of the O-2p-derived states at the center of the surface Brillouin zone $\text{(}\text{Γ}\text{)}$ were identified using polarized light. At $\text{Γ}$ the p_xp_y- and p_z-derived levels are at about 5.5 and 7.0 eV below the Fermi level, respectively, for both ordered overlayers. The p-states of the c(2×2)-O structure show very little dispersion (?0.1 eV) with $\text{k}{}_{\mathrm{}}$. On the other hand, the c(3×1)-O overlayer exhibits considerable dispersion of ～1.6 eV. The essential features of the measured dispersion are reproduced well by the dispersion predicted in a qualitative way for an isolated c(3×1) oxygen monolayer.     

Adsorbate band structure of bromine on Pd(111) studied by angle resolved ultraviolet photoemission

The photoemission spectra from Pd(111) and from the Pd(111)-Br(√3 × √3)R30° system are reported; some new features for the clean surface are detected and assigned. The principal effect of the Br overlayer on the direct transitions is a general intensity reduction. Three adsorbate derived features are detected; one at 4 eV with no dispersion is probably an adsorbate-induced feature of the metal, and the other two which disperse are assigned as Br 4p_x, (4.5 eV) and Br 4p_z (6 eV) at $\text{}\text{?}$gG.     

Angular resolved photoemission from intrinsic surface states on Al (100)

Using photon energies of 11.7, 16.8 and 21.2 eV, we have recorded angular resolved photoemission spectra from clean Al (100). A dominant, surface sensitive peak is interpreted as emission from a two-dimensional band of surface states. The band mass is $\text{m}{}^1\text{=(1.03±0.10)m}$, and the band minimum, (2.8±0.2) eV below E_F for surface momentum k₆=0, is located within the bulk band gap. The observed existence of the peak for large values of k₆, indicates a transition from a true surface state to a surface resonance.     

A field emission study of silver on rhenium

Field emission measurements of the change in average work function ?f of rhenium with adsorbed silver indicate that a rhenium-silver dipole forms with silver positive, of moment μ₀=5.2±1.5 ×10^?30 C m and polarizability $\text{α=29±12}\text{A}\text{?}{}^3$. Measurement of the rate of thermal desorption yields a mean binding energy of 2.31 ± 0.04 eV for sub-monolayer silver and 2.69±0.04 eV for a 2.5 monolayer deposit. Changes in work function induced by adsorption of silver on low-index rhenium plane surfaces are characterised by the formation of well-defined states and in this, silver resembles gold. These states are thought to result from a relatively large difference between the binding energy of adatoms on the low-index planes and on the surrounding surfaces, and this differnce is maintained when the surfaces are covered with silver. At the lowest coverage, silver is believed to be absent from all four observed planes and the measured rise in work function is thought to be apparent and to result from a decrease in field strength on the plane due to extension of the plane area by surrounding adsorbed silver. The structures adopted by silver overlayers are not known, but it is argued that on (101?0) and (101?1?) the final state at high coverage has the Ag(111) surface structure. On (112?0) and (112?2?) the silver layer at high coverage is thought to have either Ag(110) or Ag(100) surface structure. The structures of intermediate states found on all four low-index planes remain unkown. Field emission spectroscopy shows that emission from clean (101?0) is free-electron like and confirms earlier observations that emission from (202?1) is not. Spectroscopy also reveals a feature in the spectrum from silver on (101?0) which may be identified with a known surface state on Ag(111), thus providing some support for the assignment of Ag(111) to the surface structure of thick silver layers (> 3 monolayers) on (101?0).     

Relation between interface states and temperature behavior of the barrier height of silver contacts on clean cleaved n-type silicon

The properties of silver-silicon interfaces formed by cleaving n-type silicon in ultra high vacuum (UHV) in a stream of evaporating silver atoms were studied. The barrier heights of these contacts were measured at different temperatures by using C-V techniques. All measurements were performed in UHV. The dependence of the barrier height upon temperature did not follow the temperature dependence of the Si band gap as it is usually found. The measured temperature behavior depended on the roughness of the Si surface. The temperature behavior can be explained by assuming a specific band structure of the interface states. For Ag contacts on atomically smooth n-type Si, the interface states were found to be arranged in two bands, one band 4 × 10^?3 eV wide with acceptor type states 0.18 eV below the intrinsic level E_i and a density of 10¹⁷ states/cm² eV, and the other 1 eV wide with donor type states with its upper edge 0.28 eV below E_i, and a density of 4 × 10¹⁴ states/cm²eV.     

Structure of the Ag on Si(111) 7 × 7 interface by means of surface exafs

Polarization dependent surface extended X-ray absorption fine structure (SEXAFS) measurements are used to determine the structure of the Ag on Si(111)7 × 7 system at the early stages (< 3 monolayers (ML)) of interface formation. At room temperature (RT) Ag is found to initially (< 0.5 ML) chemisorb in the threefold hollow site, approximately 0.7 Å above the outermost Si layer with an average Ag-Si distance of 2.48±0.05 Å. Above monolayer coverage the SEXAFS spectrum is dominated by the Ag-Ag distance indicating Ag island formation on the surface. Upon heating (200 ?T? 600°C) a (√3 ×√3)R30° LEED pattern is observed. At the lowest coverage ( < 0.7 ML) this pattern is determined to arise from Ag atoms which are embedded in the threefold hollows, ~ 0.7 Å below the first and above the second Si layer, with a Ag-Si distance of 2.48 ± 0.04 Å. At higher coverage ($\text{\$}\text{?}$1 ML) Ag clusters are found to grow on this interface with the same Ag-Ag distance as in Ag metal. Our results are discussed in the context of previous experimental and theoretical results.     

Structure and electronic properties of cleaved Si(111) upon Ge adsorption

The effect of ultrahigh vacuum deposition of Ge below and at monolayer coverage onto clean cleaved Si(111) surface held at room temperature is studied by low energy electron diffraction, Auger electron specroscopy and photoemission yield spectroscopy. A well ordered $\text{3}\text{×}\text{3}$ R 30° structure developes at $\text{1}\text{3}$ ML, where it replaces the 2 × 1 initial pattern; it persists at $\text{2}\text{3}$ ML before transforming into a 1 × 1 diagram which fades into increasing background at 1 ML and up. Si surface dangling bonds are replaced at $\text{1}\text{3}$ ML by states associated with Ge-Si bonds and Ge dangling bonds to which states due to Ge-Ge bonds added upon increasing coverage.