Showing papers on "Photoemission spectroscopy published in 1994"

Negative-electron-affinity effects on the diamond (100) surface

Abstract: A negative electron affinity (NEA) was found both experimentally by photoemission spectroscopy and theoretically by ab initio calculations for the 2\ifmmode\times\else\texttimes\fi{}1 reconstructed diamond (100) surface. This surface is the dominant growth surface for thin diamond films and can be obtained by chemical-vapor deposition. Various surface-preparation methods which result in a NEA are described. Theoretical results indicate that the observed NEA is associated with a monohydride-terminated surface, while the hydrogen-free surface exhibits a positive electron affinity.

Valence-band discontinuity between GaN and AlN measured by x-ray photoemission spectroscopy

Abstract: The valence‐band discontinuity at a wurtzite GaN/AlN(0001) heterojunction is measured by x‐ray photoemission spectroscopy. The method first measures the core level binding energies with respect to the valence‐band maximum in both GaN and AlN bulk films. The precise location of the valence‐band maximum is determined by aligning prominent features in the valence‐band spectrum with calculated densities of states. Tables of core level binding energies relative to the valence‐band maximum are reported for both GaN and AlN. Subsequent measurements of separations between Ga and Al core levels for thin overlayers of GaN film grown on AlN and vice versa yield a valence‐band discontinuity of ΔEV=0.8±0.3 eV in the standard type I heterojunction alignment.

Charge redistribution and electronic behavior in a series of Au-Cu alloys.

Abstract: A series of Au-Cu alloys of various stoichiometries and order have been studied using x-ray photoemission spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) Significant electronic changes are associated with alloying and with changes in the local environment The Au 5d electron charge depletion has been determined independently from XPS core-level and M\"ossbauer isomer shifts and from x-ray absorption near-edge structure (XANES) measurements Strong correlations were found using both methods in the parameters investigated here, especially in the elucidation of the charge-transfer mechanism It is found that at the Au site there is a loss of d charge, upon alloying, which increases as Au becomes more dilute in Cu This along with XANES evidence of d charge gain at the Cu site, conduction (primarily 6s) charge gain at the Au site, and overall charge gain at the Au site verifies the charge compensation model in which Au loses d charge but is overcompensated by a gain of conduction charge This observation is in line with electronegativity (Au is more electronegative than Cu) and electroneutrality (the overall gain of charge is small) arguments It is also found that there is a strong linear correlation between the 5d hole count at the Au site and (1) the M\"ossbauer isomer shift, (2) alloy d-band width, (3) ``Au'' apparent spin-orbit splitting in the alloy d band By comparing the ordered and disordered species (which have different local environments) it is found that the charge transfer depends primarily on the local environment (coordination number of like and unlike atoms) and not on atomic separation The charge transfer (d-band depletion) was larger in the ordered species and can be explained by the number of Cu and Au nearest neighbors as compared to the disordered phases

α-Al2O3 (0001) Surfaces: Atomic and Electronic Structure

Abstract: New results on the R± 9° reconstructed α-Al2O3 (0001) surface, which can be obtained after heating at high temperature (1400°C) under vacuum, are presented. The atomic structure has been studied by combining low-energy electron diffractometry and grazing incidence X-ray scattering. The surface structure is found to be perfectly commensurable with the underlying bulk lattice. The surface consists of hexagonal zones of two, nearly perfect, close-packed Al (111) planes separated by a defect of hexagonal periodicity with a parameter of 26.44 A. This model is consistent with previous surface studies of this reconstruction. The electronic structure has been investigated using valence band photoemission spectroscopy, X-ray absorption spectroscopy at the O K edge, electron energy loss spectroscopy, and X-ray-induced Auger electron spectroscopy. Interpretation of these experimental data in the frame of a self-consistent, tight-binding calculation leads to the conclusion that the R± 9° reconstructed surface is more covalent than the (1 × 1) surface. Significant changes in the; Al-O hybridizations are observed; these are likely due to a difference in the interatomic distances along the [0001] axis (relaxations). The increase of covalent character is mainly due to a strong decrease of the Madelung field on the reconstructed surface.

Metallic alloys : experimental and theoretical perspectives

Abstract: Preface. 1. Photoemission Spectroscopy of Alloys. 2. Experimental Studies of Alloys. 3. Theory of Alloys and Phase Transitions. 4. Theory of Surfaces and Interfaces. 5. Theory of Magnetism in Alloys and Layers. 6. Theory of Electronic States.

Collisional and electric field effects in the delayed pulsed field ionization zero‐kinetic‐energy photoelectron spectrum of argon

Abstract: The pulsed field ionization (PFI) zero‐kinetic‐energy (ZEKE) photoelectron spectrum of argon has been recorded in the region of the transition from the ground neutral state (1S0) to the first two ionization limits corresponding to the two spin–orbit levels (2P3/2 and 2P1/2) of the ground state of the ion. The high‐n Rydberg states (85

A comparison of photoelectron spectroscopy and two-photon ionization spectroscopy : excited states of Au2, Au3, and Au4

Abstract: Photoelectron spectra of Au−n with n=2–4 are reported. Due to the relatively high photon energy used in our experiment (hν=6.424 eV) and the energy resolution of about 50 meV, various transitions into excited states of the neutral clusters are resolved. It is demonstrated that photoelectron spectra can serve as a map of the electronic states of a cluster, while the high resolution of the resonant two‐photon ionization (R2PI) method gains information about the symmetry of the states. The comparison with similar data of Ag−n clusters indicates the influence of relativistic effects and the large spin–orbit splitting for Au.

Photoemission studies of the ZnO/CdS interface

Abstract: The highest conversion efficiencies of solar cells based on Cu(In,Ga)Se2 have been achieved using multilayer CdS/ZnO front contacts. The formation of the heterojunction interface between polycrystalline ZnO and CdS has been studied with x‐ray and ultraviolet photoemission spectroscopy. The valence band offset between ZnO and CdS has been determined to be 1.2 eV. No chemical reactions at the interface between ZnO and CdS have been detected up to 200 °C. In order to obtain a standard reference for the band discontinuities the valence band offsets of ZnO and CdS relative to Ge have been measured.

A new ion detection scheme for ion spectroscopy equivalent to zero kinetic energy photoelectron spectroscopy

Abstract: A new scheme for the detection of ions produced by delayed pulsed field ionization of long‐lived high‐n Rydberg states (ZEKE Rydberg states) is presented. The equivalence of this new ion detection scheme with high resolution zero kinetic energy photoelectron (ZEKE) spectroscopy is proven by the simultaneous measurement of electrons and ions originating from the same ZEKE Rydberg states, taking into account the effects of pulsed and static low electric fields on the ZEKE Rydberg states. Using this ion detection, a rotationally resolved mass analyzed threshold ionization (MATI) spectrum of benzene with the same resolution (the rotational structure of the benzene cation) as the simultaneously recorded ZEKE photoelectron spectrum has been obtained.

Synthesis and Characterization of Amorphous Si2N2O

Abstract: Amorphous silicon oxynitride powder was synthesized by nitridation of high-purity silica in ammonia at 1120°C. The resulting material was X-ray amorphous, and its chemical characteristics were determined by X-ray photoelectron spectroscopy (XPS) and 29Si nuclear magnetic resonance (NMR). The XPS analysis showed a shift to lower binding energies for the Si2p peak with increasing nitrogen content. Upon initial nitridation, the full width at half maximum (FWHM) of the Si2p peak increased, but decreased again at higher nitrogen contents, thus showing the formation of a silicon oxynitride phase with a single or small range of composition. The 29Si NMR analysis showed the formation of (amorphous) Si3N4 (Si–N4) and possibly two oxynitride phases (Si–N3O, Si–N2O2). It is concluded that while XPS, FT-IR, and nitrogen analysis may show the formation of an homogeneous, amorphous silicon oxynitride (Si2N2O) phase, the formation of phase–pure, amorphous Si2N2O is extremely difficult via this route.

Core‐level photoemission and the structure of the Si/SiO2 interface: A reappraisal

Abstract: Photoemission spectroscopy of spherosiloxane cluster derived Si/SiOx interfaces has allowed the direct assignment of observed spectral features to specific chemical moieties. The implications of these assignments for structural models of the Si/SiO2 interface are explored. Models specifically constructed to be consistent with photoemission data are shown to be incorrect after reanalysis of core‐level shifts based on the recently synthesized model systems. A new model for the Si/SiO2 interface is proposed which is consistent with the current understanding of photoemission for Si/SiOx interfaces as well as with results from numerous other experiments.

Low-temperature structure of lithium nesosilicate, Li4SiO4, and its Li1s and O1s X-ray photoelectron spectrum

Abstract: Li 4 siO 4 , M r =119.84, λ(Mo Kα)=0.71073 A 150 K, monoclinic, P2 1 /m, a=11.532 (1), b=6.075 (1), c=16.678 (1) A, β=99.04 (4) o , V=1153.9 (2) A 3 , μ(Mo Kα)=5.3 cm -1 , Z=14, F(000)=812, D x =2.414 Mg m -3 , R=0.028, wR=0.025 for 2255 observed reflections with I>2.5σ(I). The crystal structure of lithium nesosilicate has been reinvestigated at 150 K and its room-temperature Li 1s and O 1s XPS spectrum has been measured

Rotationally resolved threshold photoelectron spectroscopy of O2 using coherent XUV: formation of vibrationally excited ions in the Franck–Condon gap

Abstract: The rotationally resolved photoelectron spectrum of high vibrational levels of O2+ in the Franck–Condon gap was investigated using pulsed field ionization, zero kinetic energy photoelectron spectroscopy. By using a coherent extreme ultraviolet light source for single-photon excitation, the ν+ = 6–24 vibrational levels of O2+X2Πg were studied. This is the first time levels higher than ν+ = 14 have been observed with rotational resolution. The highest level studied in the present work had a vibrational energy corresponding to 70% of the well depth. Along with the novelty of the spectroscopic technique, the present results reveal interesting and new ionization dynamics. All levels observed are Franck–Condon forbidden, and are not observed in a conventional photoelectron spectrum. There was no direct relation between the vibrational bands and the autoionizing states observed in the photoionization efficiency spectrum in the same energy region, and the rotational line intensities observed in the various bands ...

A study of fec2 and fec2h by anion photoelectron spectroscopy

Abstract: delivered by two pulsed molecular beam valves (R. M. Jordan Co.) and is synchronized with the vaporization laser pulse. The plasma reactions of CHq and Fe produce a variety of cluster species which, together with the He carrier gas, undergo a supersonic expansion and are skimmed twice to form a collimated beam. FeC2- and FeC2H- are the smallest Fecontaining anions, and the FeC2- signal is quite weak. FeC2H2is also present, but its abundance is rather weak and unstable. About 70 cm downstream from the cluster nozzle, the negative ion species are extracted by a 1 kV high-voltage pulse into a 130 cm long flight tube for mass analyses. The TOF mass spectrometer has a large extraction volume and a modified Wiley-McLaren extraction stack with an added free-flight region between the two acceleration stages? Theoretically, a very high mass resolution can be achieved in this setup with the large extraction volume. We have obtained a mass resolution (MI AM) of more than 300, sufficient to resolve all the isotopic species pertinent to the present study. A three-grid mass gate is used to select only the desired cluster, 56FeC2- and 56FeC2H-, to enter the PES interaction zone. The 56FeC2H- ions can be mass-gated cleanly while the 56FeC2signal may be contaminated by a negligible amount of %FeC2H2-. The mass-selected cluster packet is decelerated by a new momentum deceleration procedure7 down to a very low velocity before photodetachment. This minimizes the Doppler broadening on the photoelectron kinetic energy distribution, a crucial step in achieving high-energy resolution with the magnetic bottle-type analyzer. The third harmonic of a second Qswitched Nd:YAG laser is used in the current study for the photodetachment. There is a cutoff for the transmission of lowenergy electrons at about 0.4 eV. The spectrometer is calibrated with the known spectrum of Cu-. The energy resolution (AE) in a TOF spectrometer is not constant with respect to the electron kinetic energy (Ek). AE is proportional to Ek3/2. An energy resolution of 30 meV at 1 eV electron kinetic energy is achieved in the current spectrometer with the new deceleration procedure, as measured from the photoelectron spectrum of Cu- at 3.49 eV photon energy. This resolution is no longer limited by the Doppler broadening. When the ion signal is weak, full deceleration cannot be achieved to reach this resolution.

Schottky effect at a metal‐polymer interface

Abstract: We report the observation of the Schottky effect at the interface between a metal and a semiconducting polymer by means of internal photoemission spectroscopy. The bias dependence of the barrier provides information on the electrical properties of the polymer.

Photon counting with a hybrid photomultiplier tube (HPMT)

Abstract: In comparison with a photomultiplier tube, the resolving power for photoelectron peaks is largely improved by a silicon diode bombarded with electrons from a light-detecting photocathode. With photoelectrons accelerated 10 13 keV, the measured variance of the electronic noise is 1.29 keV and the total variance of the photoelectron peaks is 1.81 keV. The continuum part of the photoelectron spectrum is explained by a backscattering process on the diode surface. The contrast function shows that up to 14 photoelectron peaks can be resolved.

Alkali-Metal-Induced Highest Fermi-Level Pinning Position above Semiconductor Conduction Band Minimum

Abstract: The room temperature deposition of small amounts of Cs on the InAs(110) surface induces the highest Fermi-level pinning position (~ 0.6 eV) above the conduction band minimum ever met for any semiconductor. The Fermi-level movement is monitored by core level photoemission spectroscopy using synchrotron radiation. This striking behaviour is explained in terms of donor-type surface states induced by few Cs atoms present on InAs(110) and suggests the existence of a two-dimensional electron gas at the surface.

Gap anisotropy in bi2sr2cacu2o8+delta by ultrahigh-resolution angle-resolved photoemission

Abstract: Ultrahigh-resolution (DELTAE up to 10 meV) angle-resolved photoemission spectroscopy on single crystals of Bi2Sr2CaCu2O8+delta (Bi-2212) show a highly anisotropic superconducting energy gap: the gap is minimized, with a value close to zero, along the GAMM

Surface precipitation process of epitaxially grown graphite (0001) layers on carbon‐doped nickel(111) surface

Abstract: The surface precipitation process of epitaxially grown graphite (0001) layers on the C‐doped Ni(111) surface has been investigated by in situ x‐ray photoemission spectroscopy measurements. Peak intensity measurements at various temperatures clarified the change of the carbon chemical state from the segregated carbon through the monolayer graphite to the multilayer graphite. The simple thickness estimation in the transition region from the monolayer graphite to the multilayer graphite indicates that once the estimated thickness is calibrated, controlling the thickness of the graphite thin film in the monolayer range by a precise adjustment of the temperature may be possible. Changes of the chemical states of carbon on the Ni(111) surface with temperature were clarified by high‐resolution measurements with a monochromated Al Kα source. From the observed chemical shift of the C 1s peaks, a negative charge transfer from the monolayer graphite to the substrate can be postulated. This C–Ni interaction is consid...

Effects of surface oxygen vacancies on electronic states of TiO2(110), TiO2(001) and SrTiO3(001) surfaces

Abstract: We have studied effects of surface oxygen vacancies on electronic states of TiO 2 (110), TiO 2 (001) and SrTiO 3 (001) surfaces by means of angle-resolved photoemission spectroscopy. The surface oxygen vacancy induces a state at ∼1 eV below the Fermi level in the bulk band gap. In the angle-resolved photoemission spectra, this state has slight energy dispersion and doesn't cross the Fermi level when the emission angle is changed. These results indicate that the state induced by surface oxygen vacancies is not simply interpreted by delocalized model, and that the localized behavior also plays an important role for the metal-insulator transition of these materials.

Hole Generation without Annealing in High Dose Boron Implanted Silicon: Heavy Doping by B12 Icosahedron as a Double Acceptor

Abstract: A high hole concentration region of about 1×1021 cm-3 was generated without any post-annealing by the implantation of high doses of boron into silicon substrates. X-ray photoelectron spectroscopy (XPS) measurement and Fourier transform IR spectroscopy (FTIR) absorption spectra revealed that B12 icosahedra were created in as-implanted samples. A new model of the generation of holes is proposed in which B12 icosahedron acts as a double acceptor.

An x‐ray photoemission spectroscopy study of the role of sample preparation on band bending at the interface of Al with poly(p‐phenylene vinylene)

Abstract: We report on our recent x‐ray photoemission spectroscopy investigations of the interface formation of Al with poly(p‐phenylene vinylene) (PPV) prepared under various conditions We have found that during deposition Al 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 Samples converted in situ, containing 5% surface oxygen, show band bending that depends on the thickness of the Al overlayer, with effects arising after as little as 1 A Al By contrast, a sample converted ex situ, with 10% surface oxygen, is insensitive to aluminum deposition In view of the results obtained, we feel that surface impurities and adsorbed species may delay Schottky barrier formation by acting as a buffer layer which prevents the PPV substrate from interacting with the growing layer of Al In the in situ samples where band bending occurs, we find that it takes place after the formation of metallic Al