K-resolved Inverse Photoemission 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.

K-resolved inverse photoemission of four different 6H-SiC (0001) surfaces

We have investigated the unoccupied electronic states of the Si-terminated (0001) surface of hexagonal 6H-SiC. The main problem with these surfaces is the reliable preparation of well defined surface reconstructions. We give reproducible methods to prepare the $(1\ifmmode\times\else\texttimes\fi{}1),$ $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{},$ $(3\ifmmode\times\else\texttimes\fi{}3),$ and the $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ surface by controlled heating of the SiC sample in a Si flux. These surface reconstructions show a characteristic LEED pattern and a characteristic Si/C peak ratio in Auger electron spectroscopy. We present k-resolved inverse photoemission spectra for the $(1\ifmmode\times\else\texttimes\fi{}1),$ $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{},$ and $(3\ifmmode\times\else\texttimes\fi{}3)$ surface. We compare the measured dispersion relations with ab initio local density approximation surface band structure calculations of the $(1\ifmmode\times\else\texttimes\fi{}1)$- and the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$-reconstructed 6H-SiC(0001) surface and with a Mott-Hubbard model of the electronic ground state of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ and $(3\ifmmode\times\else\texttimes\fi{}3)$ reconstruction. The comparison between experiment and theory supports the Hubbard model: The experiment determines a value of $U=2.0\mathrm{eV}$ for the Mott-Hubbard Coulomb interaction parameter for the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction and $U=1.25\mathrm{eV}$ for the $(3\ifmmode\times\else\texttimes\fi{}3)$-reconstructed surface, respectively.

Band Discontinuity and Band Gap of MBE Grown HgTe/CdTe(001) Heterointerfaces Studied by k-Resolved Photoemission and Inverse Photoemission

Band Discontinuity and Band Gap of MBE Grown HgTe/CdTe(001) Heterointerfaces Studied by k-Resolved Photoemission and Inverse Photoemission

Band Discontinuity and Band Gap of MBE Grown HgTe/CdTe(001) Heterointerfaces Studied by k-Resolved Photoemission and Inverse Photoemission

Band Discontinuity and Band Gap of MBE Grown HgTe/CdTe(001) Heterointerfaces Studied by k-Resolved Photoemission and Inverse Photoemission

Band Discontinuity and Band Gap of MBE Grown HgTe/CdTe(001) Heterointerfaces Studied by k-Resolved Photoemission and Inverse Photoemission

We have investigated the valence band offset (ΔE VBO ) of the molecular beam epitaxially grown heterostructure HgTe/CdTe(001) by k-resolved ultraviolet (UPS) and X-ray (XPS) photoemission. Our angle-dependent UPS measurements at two different photon energies demonstrate that the dispersion of the valence band (VB) must be checked carefully in order to find the true position of the VB maximum in the Brillouin zone. With this information the valence band discontinuity was determined as (0.53 ± 0.03) eV, which is different from previously found photoemission values but now agrees well with magneto-optical investigations. In addition, we have determined the energy gap of CdTe(001) and HgTe(001) by a combination of UV and inverse photoemission. The energy gap of CdTe is determined to (1.57 ± 0.06) eV and that for HgTe as (0.0 ± 0.06) eV. We therefore conclude that HgTe is a semi-metal with inverted band structure in agreement with other results.

Unoccupied surface states of Cr(001) detected by K-resolved inverse photoemission

The energies of unoccupied surface states of Cr(001) have been measured using angle-resolved inverse photoemission spectroscopy. Spectra of the clean surface were recorded varying the momentum parallel to the surface along the \\ ̄ gG- \\ ̄ gD-X̄ and \\ ̄ gG- \\ ̄ gS-M̄ high-symmetry directions. The surface states were distinguished from the bulk d bands by contaminating the surface and comparing the difference between the clean and contaminated spectra. This showed surface states near E F and 1.7 eV above it. In the spectra of the clean surface, the surface states could be distinguished from the bulk d bands at some angles of incidence and appeared to disperse very little. The observed surface states are identified with the unoccupied majority and minority spin surface states calculated by Klebanoff et al. [1]. The energies of the bulk bands were in agreement with the ASA-LMTO calculation of Skriver [2].

Inverse photoemission and Auger electron spectroscopy of Rh thin films on Cu(100)

The growth of Rh films on Cu(100) has been investigated using Auger electron spectroscopy and momentum- (k-) resolved inverse photoemission (KRIPES) with a view to probing the electronic structure of near-monolayer films and any possible magnetic ordering within these films. A pronounced change in the line-shape of the Rh M4,5VV Auger electron peak as a function of average layer thickness indicates significant d-band narrowing in the films of monolayer or submonolayer thickness relative to thicker films. KRIPES data indicate that in films grown at room temperature the spectra are dominated by density-of-states effects implying poor order, and showing the expected unoccupied d-band feature at the Fermi level. Annealing at 750-900 K leads to significant loss of Rh into the substrate, and KRIPES spectra dominated by surface-localized states showing significant dispersion in parallel momentum more characteristic of s-p-band features.

Normal incidence grating spectrometer for inverse photoemission

An ultraviolet k-resolved inverse photoemission spectrometer (KRIPES) is constructed for studies of the unoccupied electronic states of solids and surfaces. The spectrometer is based on the spherical grating in the off-Rowland geometry. The sample is bombarded by low-energy electrons from the electron source with the minimum energy distribution width of 0.25 eV. The UV photons emitted from the sample are dispersed by the grating onto the position-sensitive detector. The spectra can be measured in the photon energy range from 10 eV to 40 eV with parallel detection of the wavelength region 560 Å and with an optical resolution of about 8 Å. The overall resolution of the spectrometer is determined to be from 0.26 eV at 10.0 eV to 0.6 eV at 30 eV by measuring the Fermi edge of the polycrystalline gold sample. The typical collection time per spectrum is about an hour with total amount of 3000 c/ch at the Fermi edge of gold when the beam current and energy are 25 μA and 23 eV, respectively. The resolution of the optical system is obtained from the emission line of a uncleaned tantalum sample at the photon energy of 14.8 eV. In the k-resolved measurement of Cu(110) surface the spectra show the typical band dispersion.

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.

Si(100)2×1+Sb surfaces studied with photoemission and optical spectroscopy

Applying in situ combination of angle-resolved photoelectron spectroscopy and k-resolved inverse photoemission to Si(100):Sb-1×1 and Si(100):Sb-2×1 surfaces, we have determined the surface band gaps between the Sb-derived filled and empty surface states at several points of the surface Brillouin zone. The values of the surface band gaps are compared with optical surface excitations obtained by means of surface differential reflectivity.

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.

K-resolved inverse photoemission study of half-monolayer structures of O, C and N on Ni(100): a fingerprint of adsorbate-induced restructuring?

K-resolved inverse photoemission (KRIPES) spectra have been measured in the isochromat mode at fixed photon energies of 10.0 and 11.5 eV from the half-monolayer chemisorption phases formed by O, N and C on Ni(100). In all cases adsorbate-induced features in the energy range 1–4 eV above the Fermi level are observed which are assigned to adsorbate 2p-related anti-bonding states. In the case of the simple overlayer chemisorption phase of Ni(100)− c(2 × 2)−O the O 2p state has an energy very similar to the bulk s-p band transition seen in 9.7 eV isochromats, consistent with previous speculation concerning this state. The dispersion of the O-induced surface band in parallel momentum is typical of s-p bonding. By contrast, the C- and N-induced bands are essentially dispersionless, indicating that the Ni d-bands may contribute more strongly to the adsorbate bonding of these species. The possible relevance of this d-band interaction in producing, or being a consequence of, the adsorbate-induced reconstruction in the C and N phases, is discussed.

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.

Occupied and unoccupied surface states on the single-domain Si(100):Sb-2 x 1 surface.

Angle-resolved photoelectron spectroscopy and k-resolved inverse photoemission have been used to study the electronic structure of the Si(100):Sb-2\ifmmode\times\else\texttimes\fi{}1 surface. For both techniques, one occupied and one unoccupied surface-state band has been mapped along the [010] and [011] directions. The surface shows semiconducting behavior with an estimated minimum band gap of 1.45 eV along the [010] direction.

Occupied and unoccupied surface states on the Si(111) sqrt 3 x sqrt 3 :B surface.

The Si(111)√3×√3:BR30 o surface has been studied with k-resolved inverse photoemission and angle-resolved photoemission. In the unoccupied region of the band structure, one strongly dispersing surface state is observed and in the occupied region, three surface states are observed. The unoccupied surface state is identified as an empty dangling-bond state and two of the occupied surface states are identified as backbond states of the adatom

K-resolved inverse photoemission of InP(100) surfaces

k-resolved inverse photoemission (h̵ω = 9.9 eV) spectra were taken from (4 × 2) reconstructed non-ideal InP(100) surfaces prepared by sputter annealing at very low ion energies. A surface sensitive peak at Ef + 1.5 eV shows the periodicity of the (4 × 2) reconstructed surface Brillouin zone (SBZ) along Γ-J. Together with low-energy photoemission data this peak is in accordance with an empty surface state found with electron energy-loss spectroscopy (EELS) as well as with a simple surface density-of-states (SDOS) calculation based on extended Hückel theory. Two other features are assigned to bulk direct transitions in agreement with an EPM band structure calculation. A broad feature at EF + 5.2 eV coincides with a density-of-states maximum.

Inverse-photoemission study of the Pd(001) surface.

Empty surface states of both crystal-derived and image-potential-derived types have been found on Pd(001) using k-resolved inverse photoemission. At \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}, we observed a surface resonance at 1.0\ifmmode\pm\else\textpm\fi{}0.2 eV above the Fermi level ${\mathrm{E}}_{\mathrm{F}}$ and an image-potential state at 4.9\ifmmode\pm\else\textpm\fi{}0.2 eV above ${\mathrm{E}}_{\mathrm{F}}$ (-0.9 eV relative to the vacuum level). A surface resonance was found at X\ifmmode\bar\else\textasciimacron\fi{}, very close to the ${\mathrm{L}}_{2}^{\ensuremath{'}}$ band edge at 1.0\ifmmode\pm\else\textpm\fi{}0.2 eV above ${\mathrm{E}}_{\mathrm{F}}$. We observed two surface states in the bulk band gap at the M\ifmmode\bar\else\textasciimacron\fi{} point for this metal surface. One is located near the bottom of the gap at 0.8\ifmmode\pm\else\textpm\fi{}0.2 eV above ${\mathrm{E}}_{\mathrm{F}}$ and the other is below the top of the gap at 2.5\ifmmode\pm\else\textpm\fi{}0.2 eV above ${\mathrm{E}}_{\mathrm{F}}$.

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.

Unoccupied electronic band structure of Na on Cu(111) as studied by inverse photoemission.

We have determined the unoccupied part of the electronic structure of sodium on Cu(111), employing k-resolved inverse photoemission in the isochromat mode. For Na coverages above 0.15 monolayer, an image-potential surface state is observed 2.3 eV above ${\mathit{E}}_{\mathit{F}}$. For the completed p(2\ifmmode\times\else\texttimes\fi{}2) Na phase at 0.25-monolayer coverage, a pronounced Na ${3\mathrm{p}}_{\mathrm{z}}$\char21{}derived feature occurs 0.4 eV above ${\mathit{E}}_{\mathit{F}}$, which exhibits a nearly-free-electron-like energy dispersion. For wave vectors ${\mathit{k}}_{\mathrm{?}}$\ensuremath{\ge}0.6 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$, the unoccupied part of the Na 3s band appears close to the Fermi level. We compare our results with recent band-structure calculations for free and adsorbed Na monolayers.