Energy dependence of electron energy loss processes in Ge 2s photoemission

Deep core Ge 2s photoelectron spectra from polycrystalline Ge films induced by monochromatic synchrotron radiation, of 4, 6 and 8 keV were measured and analysed using two different methods, the partial intensity analysis and the extended Hüfner method to determine the spectral contributions from different electron energy loss processes due to bulk extrinsic, intrinsic and surface excitations. The obtained photon energy dependence of the ratio of these contributions was compared as a function of the photoelectron kinetic energy. It was found that the relative contribution of intrinsic excitations increase with the photon energy.     

Photoemission studies of the energy dependence of the bulk plasmon loss intensity in Si and Al

Photoemission, using synchrotron radiation in the photon energy range 100 to 600 eV, has been utilized to study the energy dependence of the bulk plasmon loss intensity in Si and Al. The results are compared to the predictions of an extrinsic model for plasmon creation.     

Plasmon excitation in core-level photoemission

The surface and bulk plasmon satellites in photoemission from a core level are calculated, treating the photoelectron quantum mechanically and including plasmon dispersion. The long wavelength plasmon excitations are suppressed by interference between intrinsic and extrinsic processes, giving featureless satellites at low electron kinetic energy. Structure develops as the electron kinetic energy increases.     

X-ray photoemission studies on InSb clusters

The changes in core-level electronic structure of In_1−xSb_x nano-clusters have been studied using X-ray photoelectron spectroscopy. Though, clusters with mean composition InSb show shallow and deep core-level binding energies similar to those of bulk InSb, the Sb and In rich versions show a negative BE shift for the excess element and a positive shift for the minority element. The observed BE shifts have been explained considering a core and surface shell model for the structure of the clusters and possible surface atom core-level shifts.     

The observation of surface sensitive M2,3 absorption edge shifts by Reflected Electron Energy Loss Spectroscopy (REELS)

We have observed that an electron beam of kinetic energy 100–200 eV is scattered from some transition metal surfaces like Ti, Fe and Ni with a strong resonant energy loss corresponding to the M_2,3 core-level excitation. Because of the low kinetic energy, the scattering is principally from the surface atomic layer, and core-level spectroscopy of just the surface atoms is therefore possible. Chemisorption of oxygen on these transition metals caused substantial shifts in the M_2,3 threshold energies which appear to be related to charge transfer bonding with the oxygen.     

Photoelectron Energy Loss in Al(002) Revisited: Retrieval of the Single Plasmon Loss Energy Distribution by a Fourier Transform Method

A Fourier transform (FT) algorithm is proposed to retrieve the energy loss function (ELF) of solid surfaces from experimental X-ray photoelectron spectra. The intensity measured over a broad energy range towards lower kinetic energies results from convolution of four spectral distributions: photoemission line shape, multiple plasmon loss probability, X-ray source line structure and Gaussian broadening of the photoelectron analyzer. The FT of the measured XPS spectrum, including the zero-loss peak and all inelastic scattering mechanisms, being a mathematical function of the respective FT of X-ray source, photoemission line shape, multiple plasmon loss function, and Gaussian broadening of the photoelectron analyzer, the proposed algorithm gives straightforward access to the bulk ELF and effective dielectric function of the solid, assuming identical ELF for intrinsic and extrinsic plasmon excitations. This method is applied to aluminum single crystal Al(002) where the photoemission line shape has been computed accurately beyond the Doniach–Sunjic approximation using the Mahan–Wertheim–Citrin approach which takes into account the density of states near the Fermi level; the only adjustable parameters are the singularity index and the broadening energy Г (inverse hole lifetime). After correction for surface plasmon excitations, the q-averaged bulk loss function, <Im[??1 / ε(E, q)]> _q , of Al(002) differs from the optical value Im[??1 / ε(E, q?=?0)] and is well described by the Lindhard–Mermin dispersion relation. A quality criterion of the inversion algorithm is given by the capability of observing weak interband transitions close to the zero-loss peak, namely at 0.65 and 1.65 eV in ε(E, q) as found in optical spectra and ab initio calculations of aluminum.     

Characteristic energy loss spectra of copper crystals with surfaces described by LEED

Energy distributions of electrons back-scattered from copper (100) and (110) surfaces have been obtained for incident electron energies in the range 30 to 350 eV. The relations between optical measurements and the characteristic energy losses, as well as the effect of interband transitions on the bulk and surface plasmon frequencies in metals which do not have ideally free electron plasmas are discussed. By chemisorbing increasing amounts of oxygen on the clean surface, the surface plasmon loss peak was identified in the copper energy loss spectrum from its intensity dependence on the dielectric constant at the surface. This peak has been identified by previous authors as the bulk plasmon loss of a single s-electron plasma oscillation. Our identification of the surface plasmon loss peak implies that the d-electrons in copper do participate in the plasma oscillation and that the bulk plasmon frequency is shifted from its free electron value because of interband transitions.     

Reflection electron energy loss spectroscopy of aluminum

Reflection electron energy loss spectra (REELS) of Al(111) single crystal and of the aluminum polycrystalline (poly Al) film were measured at 200 eV and 1000 eV electron energies for a variety of experimental geometries and were mutually compared. No anisotropy was found for the poly Al, as expected. Polar intensity plots evaluated from the elastic (no loss) and inelastic first surface plasmon- and first bulk plasmon-loss intensities of the Al(111) surface show clearly discernable peaks for both considered electron energies. Their positions on the angular axis are the same for the elastic as well as for the inelastic, surface and bulk plasmon-loss peaks. The polar plots of intensities of the elastically and inelastically reflected electrons were compared to calculated intensities of photoelectrons emitted from the Al 2s core level to the same kinetic energy. Peak positions in the theoretically determined polar plots of electron intensities agree with those obtained experimentally in REELS.     

Photon and electron stimulated desorption of adsorbates from W {100}

We have performed photon and electron stimulated desorption (PSD and ESD) studies of F⁺, Cl⁺ and O⁺ ions from a W {100} surface and have measured the kinetic energies for a small range of emission angles of these ions as well as the effect of different electron and photon excitation energies. We find that the PSD and ESD processes show essentially the same threshold energies and produce the same ion energy and angle distribution, and so evidently involve the same mechanism. For the case of O⁺ we also find clear evidence that substrate core-level excitation initiates the desorption process as proposed by Knotek and Feibelman for adsorbates bonded to surface atoms with a maximal valency configuration. The substrate levels are not seen to play a role in the F⁺ and Cl⁺ desorption, indicating a very different type of bonding for these species on the W surface.     

Photoelectron diffraction at low energies: the applicability of single scattering calculations

The energy dependence of the 4f core level photoemission intensity from Ta(100) has been modeled at low electron kinetic energies using the single scattering cluster (SSC) approximation. The results show a strikingly similar shape to that of a full multiple scattering calculation, particular at electron kinetic energies less than 75 eV. A comparison with the experimentally measured surface/bulk core level intensity oscillations indicates that the first interlayer spacing of the Ta(100) surface is contracted by 11 ± 3%, consistent with previous determinations. These results suggest that SSC calculations can model low energy photoelectron diffraction data from surface shifted core levels with sufficient accuracy to enable a quantitative structural determination of clean, unreconstructed surfaces.     

Electron energy loss and secondary emission mechanisms in BaO

Electron Loss Spectroscopy (ELS), X-ray Photoemission (XPS), Secondary Emission Energy Distribution, and Secondary Electron yield data have been obtained on both evaporated films and sprayed-on coatings of BaO. Using the ELS correlated with the XPS data, bulk and surface plasmon losses as well as excitonic and interband transition electron loss mechanisms have been identified. It was found that at low primary beam energies (<100 eV), structure in the secondary emission energy distribution could be correlated with a conduction band energy structure. This structure was consistent with the model used to explain the loss transitions. The structure in the energy distribution curves shows little, if any, correlation with plasmon decay mechanisms and other two-step electron emission processes. On the contrary, for the case of BaO (at least at low primary energies), the energy distribution data and structure in the secondary yield vs. primary beam energy data indicate that most secondaries are produced by direct excitation of secondaries by the primary electrons.     

Variation of relative intensities between surface and bulk plasmon losses due to crystal orientations for aluminium in low energy electron reflection loss spectroscopy

The contrast change of secondary electron images due to the crystal orientations is observed by the ultra high vacuum scanning electron microscope (UHV-SEM) for crystal grains of clean surface of polycrystalline Al in the primary energy E_p of 200 eV to 5 keV. The low energy electron loss spectra are measured by the cylindrical mirror analyzer. The relative intensity ratio between surface and bulk plasmon loss spectra was dependent on the crystal orientations. The SEM images taken by the surface and bulk plasmon signals at E_p = 230 eV show the inverse contrast depending on the grains. The inversion of the relative intensities between the surface and bulk plasmon losses is explained qualitatively by taking into account of variation of the penetration depth of the incident beam caused by the electron channeling.     

Effective attenuation length dependence on photoelectron kinetic energy for Au from 1 keV to 15 keV

Hard X-ray PhotoElectron Spectroscopy (HAXPES) is an extremely powerful tool for the electronic, compositional, and chemical characterization of bulk materials and buried interfaces. Its success is based on the dramatic increase of the electron effective attenuation length (EAL) with increasing photoelectron kinetic energy. EALs are well established for electrons with kinetic energies up to several keV (below 3 keV). However, few data are available for kinetic energies up to 15 keV. In the present study we have determined the EAL dependency on kinetic energy for gold from 1 keV up to 15 keV. Two different approaches have been used. The first approach consists of following the signal rate from a core level for a fixed kinetic energy as a function of overlayer thickness (overlayer method). The second approach consists of following the signal rate from a core level as a function of the incident photon energy, i.e., electron kinetic energy, for a fixed overlayer thickness (depth profile method). An EAL dependency of EAL (nm) = 0.022 × E_kin (eV)^0.627 has been obtained from both methods. Hence, the EAL, for gold, is 4.7, 7.3 and 9.4 nm for 5 keV, 10 keV and 15 keV electron kinetic energies, respectively. A comparison between the experimental data and the EALs predicted by practical expressions available in the literature is also performed.     

Surface electron accumulation in indium nitride layers grown by high pressure chemical vapor deposition

Surface termination and electronic properties of InN layers grown by high pressure chemical vapor deposition have been studied by high resolution electron energy loss spectroscopy (HREELS). HREEL spectra from InN after atomic hydrogen cleaning show N-H termination with no indium overlayer or droplets and indicate that the layer is N-polar. Broad conduction band plasmon excitations are observed centered at 3400 cm⁻¹ in HREEL spectra with 7 eV incident electron energy which shift to 3100 cm⁻¹ when the incident electron energies are 25 eV or greater. The shift of the plasmon excitations to lower energy when electrons with larger penetration depths are used is due to a higher charge density on the surface compared with the bulk, that is, a surface electron accumulation. These results indicate that surface electron accumulation on InN does not require excess indium or In-In bonds.     

Investigation of intrinsic plasmon energy losses associated with the Fe 1s core level in metallic iron

A recently developed Cu Kα₁ (hν = 8047.8 eV) X-ray source/ESCA300 electron spectrometer combination has been used to investigate the intrinsic plasmon energy losses associated with the Fe 1s core level (binding energy = 7111 eV) in metallic iron. The surface and bulk intrinsic plasmon energy losses were separated and it was found that using the theoretically calculated extrinsic energy loss cross-section to represent the bulk intrinsic energy loss cross-section gave an overall intrinsic loss probability which is approximately the same as if a Lorentzian type cross-section is used. However, this approach does not separate the surface and bulk intrinsic losses properly and is not a good approximation for peak shape analysis in the near peak region. A more realistic approximation is provided by using a Lorentzian type energy loss cross-section to represent the bulk intrinsic energy losses. It has also been shown that for the Fe 1s core level of metallic iron the probability that a photoelectron will suffer an intrinsic energy loss is higher at the surface than in the bulk. Also for this core level the excitation probability for the intrinsic plasmons is greater than that of the extrinsic plasmons. Hence ignoring the intrinsic plasmons would cause considerable error in peak shape analysis in the near peak region.     

Growth and electronic structure of Mn on Al(1 1 1)

Growth and electronic structure of a 3d transition metal Mn on a free-electron-like metal Al(1 1 1) have been studied by photoelectron spectroscopy and low energy electron diffraction (LEED). A LEED is observed at 6 ML Mn coverage, that is related to the α phase of bulk Mn. From the intensity of the adlayer and substrate core-level peaks and angle dependent studies, evidence of primarily layer by layer growth is observed. The Mn 2p core-level for 0.1 ML Mn coverage appears at 0.3 eV lower binding energy with respect to bulk Mn. With increasing Mn coverage, the Mn 2p binding energy increases. An extra component is observed at the lower binding energy side of the Al 2p spectra, that is related to interface alloying. The asymmetric line-shape of bulk-like Mn becomes symmetric for lower Mn coverages.     

Application of high-energy synchrotron-radiation XPS to determine the thickness of SiO2 thin films on Si(100)

The escape depth of photoelectrons depends on their kinetic energy. We apply this relationship to measure film thicknesses from X-ray photoelectron spectroscopy (XPS) measurements with tunable-energy synchrotron radiation (SR). For this purpose, a “high-energy SR-XPS” instrument has been constructed and used to characterize thermally oxidized thin films on Si(100) single crystals. In order to observe photoelectrons emitted from deeper regions than with conventional XPS, Si 1s photoelectrons with an energy up to 4000 eV were measured with X-ray energies up to 5800 eV. The oxide thickness was estimated from measurements of the relative Si intensities from the oxide and the substrate at various photon energies. Our results suggest that the SR-XPS system is useful for measuring the thickness of thin films.     

CuPd interface charge and energy quantum entrapment: A tight-binding and XPS investigation

Materials at heterojunction interfaces demonstrate many physical and chemical properties that are indeed fascinating with mechanisms that need yet to be explored. We show herewith that the “interface charge and energy quantum entrapment due to bond order distortion and bond nature alteration” perturbs essentially the Hamiltonian and hence the binding energy of the CuPd alloy interface. Analyzing the X-ray photoelectron emission of the thermally induced evolution of the Cu 2p and Pd 3d core-level energies at the Cu-Pd interface before and after thermally alloying revealed that the Pd 3d and Cu 2p interfacial potential traps are 0.36 and 0.95 times deeper than the potential wells of the corresponding bulk constituents standing alone.