Showing papers on "Photoemission spectroscopy published in 1995"

Photoelectron spectroscopy : principles and applications

Abstract: 1. Introduction and Basic Principles.- 2. Core Levels and Final States.- 3. Charge-Excitation Final States: Satellites.- 4. Continuous Satellites and Plasmon Satellites: XPS Photoemission in Nearly Free Electron Systems.- 5. Valence Orbitals in Simple Molecules and Insulating Solids.- 6. Photoemission of Valence Electrons from Metallic Solids in the One-Electron Approximation.- 7. Band Structure and Angular-Resolved Photoelectron Spectra.- 8. Surface States, Surface Effects.- 9. Inverse Photoelectron Spectroscopy.- 10. Spin-Polarized Photoelectron Spectroscopy.- 11. Photoelectron Diffraction.- A.1 Table of Binding Energies.- A.2 Surface and Bulk Brillouin Zones of the Three Low-Index Faces of a Face-Centered Cubic (fcc) Crystal Face.- A.3 Compilation of Work Functions.- References.

Photoemission studies of high-tc superconductors: the superconducting gap.

Abstract: Over the last several years there have been great improvements in the energy resolution and detection efficiency of angle-resolved photoemission spectroscopy. These improvements have made it possible to discover a number of fascinating features in the electronic structure of the high transition temperature (T(c)) superconductors: apparently bandlike Fermi surfaces, flat-band saddle points, and nested Fermi surface sections. Recent work suggests that these features, previously thought explainable only by one-electron band theory, may be better understood with a many-body approach. Furthermore, other properties of the high-T(c) superconductors, which are difficult to understand with band theory, are well described using a many-body picture. Angle-resolved photoemission spectroscopy has also been used to investigate the nature of the superconducting pairing state, revealing an anisotropic gap consistent with a d-wave order parameter and fueling the current debate over s-wave versus d-wave superconductivity.

The effect of rapid thermal N2O nitridation on the oxide/Si(100) interface structure

Abstract: High‐resolution x‐ray photoelectron spectroscopy (XPS) was used to study the chemical nature and physical distribution of N in oxynitride films formed by rapid thermal N2O processes (RTPs). High‐resolution synchrotron Si 2p core level photoemission spectroscopy (PES) was used to study the oxide/Si(100) interface suboxide structures with and without the presence of N. XPS N 1s studies indicated that there are two types of N in the RTP oxynitride films. The chemical bond configuration of the first type of N is similar to that N in Si3N4 and is mainly distributed within the first 1 nm from the interface. The second type of N is distributed mainly outside of the first 1 nm region, and the N is likely bonded to two Si and one oxygen atom. PES studies showed that Si formed suboxides with oxygen at the interface for all oxynitride films. It is found that there is no change in the Si+1 structure while there is a dramatic intensity decrease in the Si+2 and Si+3 peaks with the inclusion of N in the oxide. Both the ...

A photoemission determination of the band diagram of the Te/CdTe interface

Abstract: Two experiments designed to assist in understanding the physics of certain back contacts on p‐type CdTe solar‐cell devices are described. In the first experiment, x‐ray photoelectron and Auger electron spectroscopies are used to show that etching CdTe in HNO3:H3PO4 results in a Te layer on the CdTe surface. In the second experiment, photoemission spectroscopy is used to explore the electronic properties of evaporated Te deposited on thin‐film, polycrystalline p‐CdTe in an effort to develop a band diagram for the Te/p‐CdTe interface. The motivation for developing the band diagram derives from previous observations that chemically etching polycrystalline p‐CdTe solar‐cell device material before application of the back contact reduces the series resistance of the device. The key results are that the evaporated Te overlayer is p type and that the valence‐band offset between Te and p‐CdTe is favorable for low‐series‐resistance contact, ΔEv=0.26±0.1 eV.

Two-photon photoemission spectroscopy of image states

Abstract: In two-photon photoemission a photon from a pulsed laser excites an electron from a state below the Fermi level to an unoccupied intermediate state below the vacuum level. A second photon ionizes the intermediate state. The energy distribution of the photoelectrons yields information on the energetic position and the lifetime of the intermediate state. Image states are bound states of electrons in front of metal surfaces in the potential well, built by the image potential and the surface. They have been studied extensively by two-photon photoemission. Experimental results on the binding energies of image states on (111) and (100) surfaces of Ag, Au, Cu, and Pd agree with calculations by a one-dimensional scattering model. The results for Fe, Co, and Ni are not yet understood. Lifetimes of image states between 4 and 180 fs have been measured. Particular emphasis is placed on image states on metal overlayers. Information on film growth and surface morphology has been obtained. Lateral localization and coupling to quantum well states has been observed. Dielectric overlayers have also been investigated.

Time‐delayed two‐color photoelectron spectra of aniline, 2‐aminopyridine, and 3‐aminopyridine: Snapshots of the nonadiabatic curve crossings

Abstract: We present time‐delayed two‐color photoionization photoelectron spectra of aniline, 2‐aminopyridine, and 3‐aminopyridine seeded in a cold molecular beam. The molecules are prepared in their S1 electronic states by a picosecond UV laser pulse and ionized by a time‐delayed 200 nm probe pulse. The photoelectron spectrum is observed with a time‐of‐flight spectrometer. All time‐delayed spectra reveal only one product of the nonradiative relaxation process. Careful considerations of electronic and vibrational overlap propensity rules for the ionization step lead to the conclusion that the dominant nonradiative decay mechanism in these molecules is the intersystem crossing to a bath of vibrationally excited levels of the T1 electronic state. Our observations reveal no admixtures of T2 or higher triplet levels. The pathway of the nonradiative electronic relaxation in 2‐aminopyridine is found to be independent of the initially prepared vibrational states up to 1000 cm−1 of vibrational energy. We find no evidence o...

High resolution photoemission and Auger parameter studies of electronic structure of TiN oxides

Abstract: The electronic structure of metallic Sn, polycrystalline Sn oxides, and (110) natural single crystal SnO2 (cassiterite) was studied by high resolution measurements of core and valence photoemission, photoinduced Auger spectra, and energy loss spectra. The advantage of in vacuo scraping in obtaining SnO surfaces is shown in comparison with surface cleaning by ion sputtering. Valence band x‐ray photoemission spectroscopy spectra are interpreted by a cluster‐type discrete variational Xα molecular orbital model, revealing the change in the electronic structure which leads to an increased conductivity. The use of the Auger parameter approach in gaining information on the initial and final state effects, ligand polarizability, and final state hole–hole repulsion energies are demonstrated for the case of tin oxides.

Dipole‐bound excited states of the I−⋅CH3CN and I−⋅(CH3CN)2 ion–molecule complexes: Evidence for asymmetric solvation

Abstract: Dipole‐bound excited states are reported for the I−⋅CH3CN and I−⋅(CH3CN)2 cluster ions, located just below their vertical electron detachment energies (determined using negative ion photoelectron spectroscopy). The absorption cross sections for excitation to these states are observed to increase with increasing dipole moments of the solvent molecules in the I−⋅M series (M=methyl iodide, acetone, acetonitrile). Photoexcitation at the peak of the transition to the dipole‐bound state results exclusively in the dipole‐bound fragment ion, M−. The photoelectron spectrum of the CH3CN− fragment was also recorded by sequential two‐photon absorption in the I−⋅CH3CN parent, indicating that the excess electron is indeed weakly bound (≤10 meV) with very little intramolecular distortion evident upon electron detachment. The I−⋅(CH3CN)2 cluster displays two absorption bands, one below each of the two features in the photoelectron spectrum. The most intense band correlates with the weaker, lower binding energy photoelectron band. Excitation of this strong absorption band results in production of both CH3CN− and (CH3CN)−2 fragments, while excitation of the weaker absorption band only results in electron detachment. We interpret these results in the context of two structural isomers for I−⋅(CH3CN)2: one with the solvent molecules surrounding the I− and another with both solvent molecules on the same side, asymmetrically solvating I− in a configuration with a large electric dipole moment in the neutral cluster over this anionic geometry.

Characterization of reconstructed SiC(100) surfaces using soft-x-ray photoemission spectroscopy

Abstract: The surface quality of βSiC films grown on Si(100) by chemical vapor deposition has been assessed through synchrotron photoemission measurements of the valence band and of the linewidths and surface‐induced structure in Si 2p core‐level spectra. For these n‐type samples, band bending is small on the c(2×2) and (3×2) surfaces but larger on the (2×1), which also exhibits an increased Si 2p linewidth and evidence of elemental Si patches. All three reconstructions show emission from gap states extending from the valence band maximum to the Fermi level.

Electronic structures of transition-metal mono-oxides in the self-interaction-corrected local-spin-density approximation.

Abstract: The electronic structures of transition-metal mono-oxides are calculated using the self-interaction-corrected (SIC) local-spin-density approximation (LSDA). The LSDA-SIC has many solutions and various possible solutions are examined. It is shown that the total energies of these solutions are strongly affected by the choice of exchange-correlation energy functionals. Alternatively, if the solutions are chosen so that all orbitals are localized as Wannier functions, the energy gaps are overestimated by 1.5--3 eV. However, in these solutions, the relative positions of occupied transition-metal d bands and oxygen p bands are consistent with the analysis of photoemission spectroscopy by the cluster configuration interaction theory.

Atomic-Layer Chemical-Vapor-Deposition of SiO$_{\bf 2}$ by Cyclic Exposures of CH$_{\bf 3}$OSi(NCO)$_{\bf 3}$ and H$_{\bf 2}$O$_{\bf 2}$

Abstract: Atomic-layer chemical-vapor-deposition (AL-CVD) of SiO2 has been achieved by cyclic exposures of CH3OSi(NCO)3 and H2O2 at room temperature. The deposition rate was saturated at about 2.0 A/cycle i.e., equal to the ideal quasi-monolayer/cycle. The surface roughness after 100 deposition cycles was found to be less than ±10 A by atomic force microscopy (AFM). Film properties were also evaluated by auger electron spectroscopy (AES), X-ray photoemission spectroscopy (XPS), and Fourier transform IR (FT-IR) spectroscopy.

Two‐Photon Photoemission Spectroscopy of Electronic States at Metal Surfaces

Abstract: In two-photon photoemission a photon from a pulsed laser excites an electron from a state below the Fermi level to an unoccupied intermediate state below the vacuum level. A second photon of the same pulse ionizes the intermediate state. The energy distribution of the photoelectrons yields information on the position and the lifetime of both the initial and, in particular, the intermediate state. Most of the work in two-photon photoemission spectroscopy has been devoted to the investigation of image potential states (image states). They are bound states of electrons in front of metal surfaces in the potential well built by the image potential and the surface. The situation is analogous to the hydrogen atom, with a binding energy of the lowest state of 850 meV in first approximation. Up to four members of the Rydberg series are detected. Experimental results on the binding encrgy of image states in (111) and (100) surfaces of Ag, Au, Cu, and Pd agree with calculations by a one-dimensional scattering model. The results for Co, Ni, and Fe are not yet understood. The effective mass is close to unity except for Ag(111). Lifetimes between 5 and 170 fs arc measured. They can be explained by decay via electron-hole pair creation in the bulk. Image states on metal overlayers yield information on film growth and surface morphology. Dielectric overlayers are been investigated. Adsorbate states of atoms and molecules adsorbed on metal substrates can also be studied by two-photon photoemission spectroscopy. Results for Na, O, CO, and NO on Cu(111) are reported.

Threshold Photoelectron Spectroscopy of Cl2 and Br2 up to 35 eV

Abstract: Department of Physics and Department of Chemistry, State University of New York at Albany, Albany, New York 12222, EPSRC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, U.K., Institute for Molecular Science, Myodaiji, Okazaki 444, Japan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K., Department of Physics, Schuster Laboratory, Manchester University, Manchester MI3 9PL, U. K., and Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavik, Iceland

Formation of epitaxial Fe3−xSi1+x (0≤x≤1) silicides on Si(111)

Abstract: Epitaxial Fe3−xSi1+x films have been grown on Si(111) by codeposition at room temperature. Their structural and electronic properties have been investigated by means of low‐energy electron diffraction (LEED), x‐ray photoelectron diffraction (XPD), and x‐ray photoemission spectroscopy (XPS). These films, with compositions ranging from Fe3Si to FeSi, exhibit a (1×1) LEED pattern. Both XPD and core level binding energy measurements indicate that single Fe3−xSi1+x phases (with 0

An experimental study of the valence shell photoelectron spectrum of hydrogen sulphide

Abstract: The complete valence shell photoelectron spectrum of hydrogen sulphide has been studied in the 10 to 25 eV binding energy region using He I, He II and synchroton radiation. Using monochromated synchroton radiation, photoelectron angular distributions and branching ratios have been determined in the 14 to 120 eV photon energy range. The results suggest that most of the prominent inner valence features should be associated with ionisation from the 4a 1 molecular orbital. High resolution spectra have been obtained over a similar binding energy range using He I and He II excitation. Several narrow photoelectron bands have been observed in the region between 22.5 and 24.5 eV, and are associated with vibrational progressions belonging to two different electronic states. The He I excited spectrum has allowed a detailed analysis to be made of the vibrational and rotational fine structure exhibited in the photoelectron bands associated with ionisation from the 2b 1 and the 5a 1 orbitals. For the A 2 A 1 state, comparisons are made with the rovibronic structure predicted by Duxbury et al.

Long-range structural relaxation in the Staebler-Wronski effect

Abstract: X-ray photoemission spectroscopy (XPS) has been used to study the structural changes of device-quality a-Si:H between the annealed state (A) and the light-soaked state (B). The XPS spectra show a reversible shift of about 0.1 eV of the Si 2p peak to lower binding energy without a corresponding shift of the Si 2s peak in going from state A to state B. Density functional calculations on a Si 6 H 14 prototype molecule are also presented which suggest that the Si core energy levels may indeed be shifted by comparable amounts when small structural rearrangements are forced upon the structure. The change in the Si 2p peak is too large to be caused by a rearrangement of the amorphous lattice restricted to the immediate surroundings of the defects responsible for the Staebler-Wronski effect (defect density: 17 cm −3 ). It is proposed instead that the formation of dangling bonds under exposure to light is also accompanied by long-range structural rearrangements of the amorphous network. Results obtained by other groups are discussed which, in our opinion, also indicate the presence of long-range structural rearrangement during the Staebler-Wronski effect.

Atomic diffusion‐induced deep levels near ZnSe/GaAs(100) interfaces

Abstract: Luminescence spectroscopy measurements of ZnSe/GaAs(100) heterojunctions grown by molecular beam epitaxy reveal the formation of deep levels near ‘‘buried’’ interfaces upon thermal annealing. A pronounced emission at 1.9–2.0 eV appears at temperatures in the 300–400 °C range depending on the ZnSe growth conditions with a constant activation energy of 1.10 eV. X‐ray photoemission spectroscopy indicates a correlation between this deep level and atomic diffusion of Ga and Zn across the heterointerface. Zn‐rich ZnSe growth conditions dramatically reduce this emission, highlighting the importance of local interface composition on thermal stability.

Structural characterization of oxidized titanium surfaces

Abstract: Oxidized titanium surfaces resulting from various processes have been structurally characterized by means of scanning force microscopy, x‐ray photoemission spectroscopy (XPS), x‐ray diffraction, and electron energy‐loss spectroscopy (EELS) with losses in the 0–100 eV range. It has been found that the surface morphology has a granular structure for electropolished titanium and for titanium evaporated on mica at low substrate temperature (570 K), but changes to flat terraces for the films evaporated at higher temperature (770 K). Angular‐dependent XPS has revealed the presence of a Ti2O3 suboxide at the Ti/TiO2 interface for electropolished titanium. Dry oxidation has been performed at 770 and 970 K on both weakly and highly crystallized evaporated titanium films oriented along (0001). In the case of underlying crystallized metallic titanium, the resulting TiO2 films are crystallized with the anatase (004) orientation for oxidation at 770 K and with rutile (200) orientation for oxidation at 970 K. EELS spectra interpreted in terms of the molecular orbitals of a (TiO6)8− cluster show that the local octahedral environment of titanium atoms is preserved on native oxides, even if these oxides are not crystallized.

X-ray photoemission spectroscopy study of band bending at the interface of metal with poly(p-phenylene vinylene)

Abstract: We report on our recent x-ray photoemission spectroscopy investigations of the interface formation of metals with poly(p-phenylene vinylene) (PPV) prepared under various conditions. We have found that during deposition the metal reacts with residual hydroxyl groups in the polymer. In addition, we have found that Schottky barrier formation and the associated band bending depend strongly on surface preparation. In the case of Al deposition, samples converted in situ, containing 5% surface oxygen, show band bending that depends on the thickness of the metal overlayer, with effects arising after as little as 1 A of metal. On the other hand, a sample converted ex situ, with 10% surface oxygen, is insensitive to aluminium deposition. We feel that surface impurities and adsorbed species may delay Schottky barrier formation by acting as a butter layer that prevents the PPV substrate from interacting with the growing layer of Al. By constrast, the Ca/PPV surface surface exhibits delayed band bending, with strong interactions between surface oxygen and Ca. Our results indicate that band bending at the metal/PPV interface is governed by the metallicity of the metal overlaryer, which itself is influenced by the interface reaction of the deposited metal with the PPV substrate or the surface residual impurities. Finally, the degree of band bending observed did not correlate directly with the differences in work functions between the metals and PVV

Adsorption of Na on Si(100)2×1 at room temperature studied with photoelectron spectroscopy

Abstract: Room-temperature adsorption of Na on the Si(100)2\ifmmode\times\else\texttimes\fi{}1 surface was studied with high-resolution core-level spectroscopy and angle-resolved photoemission. Photoelectron spectra from the Si 2p and Na 2p core levels and from the Si valence band have been analyzed for increasing Na coverage. An abrupt decrease of the band bending by \ensuremath{\sim}0.3 eV was observed for the initial adsorption due to the onset of occupation of a surface band above the surface band gap. Further Na deposition onto the surface results in a smeared-out appearance of the Si 2p spectra, indicating multiple surface shifts corresponding to an inhomogeneous surface. Evidence for two adsorption sites was observed in the Na 2p spectra for higher Na exposures. At coverages around of the saturation coverage a good 4\ifmmode\times\else\texttimes\fi{}1 periodicity was observed by low-energy electron diffraction (LEED). The Si 2p spectrum of the Na-saturated Si(100)2\ifmmode\times\else\texttimes\fi{}1 surface shows a strong, well-resolved, Na-induced component with a surface core-level shift of \ensuremath{\sim}0.33 eV toward lower binding energies. Based on its intensity and the 2\ifmmode\times\else\texttimes\fi{}1 LEED pattern observed for the saturated surface this component is assigned to Si atoms forming symmetric dimers on the surface.

Effects of H+, Cl− and CH3COOH on the photocatalytic conversion of PtCl62− in aqueous TiO2 dispersion

Abstract: In this work, the effects of H+, Cl− and CH3COOH on the photocatalytic conversion of PtCl62− in an aqueous TiO2 dispersion were studied by means of X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), UV-visible diffuse reflectance spectroscopy and UV-visible absorption spectroscopy. The results obtained show that PtCl62−, in the form of its hydrolysis product, is photocatalytically converted into a solid deposit on the TiO2 surface; the quantity and composition of the solid deposit vary with the degree of PtCl62− hydrolysis. When both H+ and Cl− concentrations are sufficiently high to depress the hydrolysis of PtCl62−, no deposit is observed on the TiO2 surface. The effect of CH3COOH on the composition of the photocatalytic product of PtCl62− on the TiO2 surface is closely related to the degree of ionization of CH3COOH.