K-resolved Inverse-photoemission Spectroscopy Research Articles

Unoccupied electronic band structure of pentagonal Si nanoribbons on Ag(110).

Silicon nanoribbons - one dimensional silicon structures with a pentagonal atomic structure and mixed sp2- and sp3-hybridisation - grow on Ag(110) upon deposition of silicon. These nanostructures are viewed as promising candidates for modern day electronics as they are comprised of the same element as today's semiconductor devices. Even though they have been studied extensively over the last decade, only little is known about their unoccupied band structure which is important for possible future optoelectronics, semiconductor, and spintronics applications. In order to elucidate the unoccupied band structure of the nanoribbons, k-resolved inverse photoemission spectroscopy (KRIPES) studies were performed on both nanoribbon structures reported in the literature as well as on the bare Ag(110) substrate within the energy range of E-EF = 0-6.5 eV. The obtained experimental results are compared to density functional theory (DFT) calculated band structures to assign individual spectral features to specific bands. Since even small changes in the structural model of the nanoribbons lead to a change in the calculated band structure, this comparison allows us to assess the validity of the proposed structural models.

Photoemission and inverse photoemission spectroscopy of V(100)

The electronic structure of the V(100) surface has been studied by means of angularly resolved photoelectron spectroscopy (ARUPS) and momentum- ( k-) resolved inverse photoemission spectroscopy (KRIPES). A narrow peak in the photoemission spectrum which is seen at the Fermi level is attributed to a transition from a surface resonance. The existence of this surface resonance at the Fermi level of the V(100) surface is potentially very important for the occurrence of the expected magnetic ordering of this surface. The energy of the critical point , which defines the top of the unoccupied part of the 3d band, has been determined to be within 3.0 - 3.2 eV above from the normal incidence KRIPES spectra.

Inverse photoemission study of the Cu(100)c(2*2)-Mn phase

Momentum- (k-) resolved inverse photoemission spectroscopy (KRIPES) has been used to investigate the electronic structure of Mn deposited on Cu(100), including the substitutional surface alloy phase Cu(100)c(2*2)-Mn. A well-resolved unoccupied state 2.0 eV above the Fermi level is found to be associated with the Mn adsorbate; this state shows no evidence of dispersion in either perpendicular or parallel electron momentum and is associated with a localized Mn 3d state. This result is consistent with previous evidence that the Mn remains in an atomic-like high-spin ground state in this surface, and contrasts with its behaviour in thicker Mn films. Measurements following exposure of the surface to oxygen show a greatly enhanced oxidation rate, with evidence of bulk oxide formation not possible on the clean Cu(100) surface at room temperature under high-vacuum conditions.

Unoccupied electronic structure of Al(111).

The unoccupied electronic states of the Al(111) surface have been studied using k-resolved inverse-photoemission spectroscopy (KRIPES). In addition, a first-principles calculation of the bulk Al electronic structure has been performed to facilitate interpretation of the experimental data. The KRIPES spectra obtained along the [1\ifmmode\bar\else\textasciimacron\fi{}10], [112\ifmmode\bar\else\textasciimacron\fi{}], and [1\ifmmode\bar\else\textasciimacron\fi{} 1\ifmmode\bar\else\textasciimacron\fi{}2] azimuths of the surface Brillouin zone are characterized by well-defined features within 5 eV of the Fermi level, and broad, weak features at higher energies. In general, surface states and resonances appeared as strong spectral features while bulk transitions were weak for this surface. First-principles electronic-structure calculations were necessary to obtain a qualitative account of the bulk features, and semiquantitative agreement was obtained when excitation effects were considered. Dispersion of an unoccupied surface resonance along the [112\ifmmode\bar\else\textasciimacron\fi{}] azimuth is consistent both with an occupied surface resonance found by an earlier photoemission study and with the predictions of surface electronic-structure calculations in the literature. A strong feature observed in the [1\ifmmode\bar\else\textasciimacron\fi{}12] direction is identified as an odd surface state occurring in a symmetry gap and may account for earlier electron-energy-loss data.

Inverse-photoemission spectroscopy of GaSe and InSe

The lamellar semiconductors GaSe and InSe have been studied with k-resolved inverse-photoemission spectroscopy along two major symmetry directions (\ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} K\ifmmode\bar\else\textasciimacron\fi{} and \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} M\ifmmode\bar\else\textasciimacron\fi{}) of the surface Brillouin zone. Three bands with well-resolved features are observed from which the dispersion of the conduction bands can be determined with good precision. The minimum of the conduction band is found at M\ifmmode\bar\else\textasciimacron\fi{} in GaSe and at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} in InSe. These results are compared with theoretical studies using pseudopotential and tight-binding calculations.

Inverse-photoemission spectroscopy of electron irradiated epitaxial CaF 2 on Si(111)

We have investigated the electronic properties of the empty states of epitaxial layers of CaF 2 on Si(111) substrate, for film thicknesses of a few monolayers. The changes induced in the inverse photoemission spectrum by the electron beam irradiation were measured at a fixed angle, and are discussed in terms of fluorine desorption. After stabilisation of the surface, k-resolved inverse photoemission spectroscopy (KRIPES) was performed and the dispersion of the four lowest empty states was measured. The lowest band has a metallic character and is attributed to a Ca + plane at the surface. Other features are tentatively assigned to interface states.

Inverse-photoemission spectroscopy of the unreconstructed, ideally H-terminated Si(111) surface.

The chemically prepared H/Si(111)-(1\ifmmode\times\else\texttimes\fi{}1) surface has been studied by k-resolved inverse-photoemission spectroscopy (KRIPES) along the two main directions (\ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} K\ifmmode\bar\else\textasciimacron\fi{} and \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} M\ifmmode\bar\else\textasciimacron\fi{}) of the surface Brillouin zone. Two conduction bands are evident at 2.6 and 4.0 eV at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} and display some dispersion. A marked asymmetry is observed in their dispersion along the M\ifmmode\bar\else\textasciimacron\fi{} '\ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} M\ifmmode\bar\else\textasciimacron\fi{} direction. This asymmetry is a manifestation of the different contributions of the eigenstates associated with ${\mathbf{k}}_{\mathrm{\ensuremath{\parallel}}}$ and -${\mathbf{k}}_{\mathrm{\ensuremath{\parallel}}}$ to the KRIPES cross sections.

The formation of the epitaxial CaF 2/Si(111) interface investigated by k-resolved inverse photoemission spectroscopy

k-Resolved inverse photoemission spectroscopy (KRIPES) measurements are reported for the epitaxial system CaF 2/Si(111) at low coverage, in the submonolayer range.CaF 2 was deposited on H-passivated Si(111) surfaces at room temperature and subsequently annealed at 800° C. A 3×3R30° reconstructed is observed and Ca is assumed to be bonded to three Si atoms in the threefold coordinated T 4 or H 3 sites. KRIPES spectra show a well-resolved peak at about 1.3 eV above the Fermi level. This peak is attributed to emission from antibonding states resulting from the interaction between the Ca 4s and Si 3p orbitals. The energy dispersion of this interface state was recorded along the ΓM′ and ΓK′ directions of the reconstructed surface Brillouin zone. These results are interpreted in the framework of the general scheme proposed in the literature for describing the CaF 2/Si(111) interface formation.

Electronic structure of the InP(110) surface: an inverse photoemission study

The unoccupied electronic structure of the InP(110) surface has been investigated with k-resolved inverse photoemission spectroscopy. Two surface states were detected, partly dispersing in the projected bandgap, with energy positions that agree rather well with optical spectroscopy data. The measured band dispersions, however, are in poor agreement with existing band calculations for InP(110), whereas some recent self-consistent, quasiparticle calculations for GaAs(110) and GaP(110) present surface state bands with shapes similar to that measured.

Electronic structure of the InP(110) surface: an inverse photoemission study

The unoccupied electronic structure of the InP(110) surface has been investigated with k-resolved inverse photoemission spectroscopy. Two surface states were detected, partly dispersing in the projected bandgap, with energy positions that agree rather well with optical spectroscopy data. The measured band dispersions, however, are in poor agreement with existing band calculations for InP(110), whereas some recent self-consistent, quasiparticle calculations for GaAs(110) and GaP(110) present surface state bands with shapes similar to that measured.

Position of the empty surface-state band on Si(111)2 x 1.

The cleaved silicon surface of a highly n-type doped crystal has been studied with k-resolved inverse-photoemission spectroscopy (KRIPES). The dispersion of the unoccupied electronic surface-state band of a single-domain 2\ifmmode\times\else\texttimes\fi{}1 reconstruction was found to be very similar to p-type doped samples. The band minimum at point J\ifmmode\bar\else\textasciimacron\fi{} occurs at the same energy value with similar intensity as for p-type doped samples. Comparison of the present results with scanning tunneling microscopy (STM) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), on similar samples, indicates that the empty band observed in KRIPES differs from that seen at the Fermi level by STM and ARUPS. An explanation in terms of either many-body effects in KRIPES or of strong electron-phonon interactions in the different techniques is suggested.

Antibonding surface state band of the Ge(111)2 x 1 surface

Abstract The unoccupied surface electronic structure of cleaved Ge(111)2 × 1 surfaces has been investigated with k-resolved inverse photoemission spectroscopy. A prominent empty surface state is detected, which exhibits a large dispersion in good agreement with the antibonding band calculated for the π-bonded chain model. Identical dispersions are obtained for undoped and highly n-doped crystals, which together with photoemission results indicate that no band gap narrowing or shift of the energy levels occurs for highly n-doped crystals as suggested recently from a theoretical model.

Antibonding surface state band of the Ge(111)2 × 1 surface

The unoccupied surface electronic structure of cleaved Ge(111)2 × 1 surfaces has been investigated with k-resolved inverse photoemission spectroscopy. A prominent empty surface state is detected, which exhibits a large dispersion in good agreement with the antibonding band calculated for the π-bonded chain model. Identical dispersions are obtained for undoped and highly n-doped crystals, which together with photoemission results indicate that no band gap narrowing or shift of the energy levels occurs for highly n-doped crystals as suggested recently from a theoretical model.

Inverse-Photoemission Studies of Si and III-V Compounds

k-resolved inverse-photoemission spectroscopy is used to measure the energy dispersion E(k||) of the unoccupied electronic surface-state bands of Si(111)2 × 1, Si(111)1 × 1-Ge, GaP(110), and GaAs(110). Comparison with optical transitions between occupied and empty surface states is made by measuring the bulk valence-band states in-situ with ultraviolet-photoemission spectroscopy. This procedure gives larger surface band gaps than the optical results and the discrepancy increases in going from Si(111)2 × 1 to GaAs(110).

Inverse photoemission of Pd grown on Al(111)

Pd layers of thickness from 0.5 to 10 monolayers have been evaporated onto Al(111). The evolution of the empty Pd states with layer thickness has been studied by k-resolved inverse photoemission spectroscopy. Empty Pd4d states at 0.25 eV above E F and a dispersing sp band are found only at coverages well above the monolayer. This corroborates the ARUPS result of an atomic-like 4d 105s 0 configuration of the Pd monolayer.

Unoccupied surface states revealing the Si(111) sqrt 3 sqrt 3 -Al, -Ga, and -In adatom geometries.

k-resolved inverse-photoemission spectroscopy has been used to determine the empty surface-state band structures of Si(111)..sqrt..3 x ..sqrt..3 -Al, -Ga, and -In surfaces. The results are compared with first-principles pseudopotential total-energy and electronic-structure calculations for energy-minimized geometries of the filled- (T/sub 4/) and the hollow- (H/sub 3/) site adatom models. Good agreement between experiment and theory for the unoccupied surface-state band structures is found for the T/sub 4/ model, which also has the lowest total energy.

Inverse-photoemission study of ordered Cu, Ag, and Au overlayers on Si(111)

Si(111)-(7 × 7) surfaces with deposits of noble metals form ordered overlayers after annealing. Coverages in the monolayer range of Ag and Au result in ( 3 × 3 )−R30° overlayer structures and for Cu a pseudo “(5 × 5)” structure is produced, while at higher coverage of Au a (6 × 6) overlayer structure is formed. We have studied all four overlayer structures with k-resolved inverse-photoemission spectroscopy and observed strong surface-state emission at ∼ 2.0 eV above the Fermi level for all these surfaces. This emission can hardly be quenched by exposure to oxygen or hydrogen. Hence we attribute it to the local bonding between the metal atoms and the Si(111) surface. In addition we observe an increased spectral intensity near the Fermi level for the two larger unit-cell surfaces, corresponding to the long-range order and a possible onset of metallization.

Unoccupied bulk and surface states on Ag(111) studied by inverse photoemission.

We present k-resolved inverse photoemission spectroscopy (KRIPES) results for Ag(111) in the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} K\ifmmode\bar\else\textasciimacron\fi{} direction of the surface Brillouin zone. The spectra were taken as a function of electron energy and angle of incidence at a fixed photon energy (9.7 eV) of the emitted radiation. The crystal-induced surface state and the image-potential-induced surface state are observed with final-state electron energies at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} in good agreement with a recently proposed multiple reflection model and with dispersion characterized by effective masses ${m}^{\mathrm{*}}$=0.7m and ${m}^{\mathrm{*}}$\ensuremath{\approxeq}m, respectively. The measured intensity of the surface state roughly follows a ${\mathrm{cos}}^{2}$\ensuremath{\alpha} dependence, where \ensuremath{\alpha} is the angle between the sample normal and the polarization vector of the detected photons. Transitions into bulk-derived final states are observed, with final-state electron energies ranging from ${E}_{F}$ to ${E}_{F}$+20 eV. The dispersions of these bulk-derived final states agree with very simple ``empty-lattice isochromats'' derived from free-electron bands of proper symmetry, as well as with results of a band-structure calculation based upon a combined interpolation scheme.

Unoccupied electronic surface states on Cu(110).

Using k-resolved inverse photoemission spectroscopy, we have found an unoccupied Shockley-type surface state on the Cu(110) surface and measured its E(k${\ensuremath{\rightarrow}}_{\mathrm{para}}$) dispersion along the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} X\ifmmode\bar\else\textasciimacron\fi{} symmetry line. Its minimum energy, ${E}_{F}$+(2.0\ifmmode\pm\else\textpm\fi{}0.4) eV, is found at the surface Brillouin-zone boundary X\ifmmode\bar\else\textasciimacron\fi{} in accordance with thin-film calculations of Dempsey, Kleinman, and co-workers. In addition, we interpret a weak emission feature near the vacuum level in the near-normal-incidence spectra of Cu(110) as arising from image-potential surface states.

Inverse photoemission

Inverse photoemission (or ultraviolet bremsstrahlung spectroscopy) is an emerging new technique for the study of the electronic structure of solid surfaces. It has the important characteristic of being able to probe states between the Fermi level and vacuum level. Here we survey current experimental practice and future prospects, including discussion of band-pass and low-pass photon filters, grating spectrographs, and electron sources. Examples of recent work are presented, drawing on studies of band mapping, energy-loss satellites, surface states, adsorbates, and spin polarization effects. Emphasis is on k-resolved inverse photoemission spectroscopy (KRIPES).