Low Kinetic Energy Side Research Articles

Photoemission Characterization of Interface Dipoles and Electronic Defect States for Gate Dielectrics

We have demonstrated how inner potential changes due to electrical dipole moments at interfaces in dielectric stacks are directly evaluated by measuring the cut-off energy of secondary photoelectrons in the lower kinetic energy side, in which near-surface band bending is taken into account from an energy calibration of core-level photoelectrons. We have also showed how useful photoelectron electron yield spectroscopy (PYS) is to quantify the energy distribution of filled gap states and defect states even in widegap materials and their heterostructures such as a SiO2/GaN stack.

Ultrafast Charge Transfer Processes Accompanying KLL Auger Decay in Aqueous KCl Solution.

X-ray photoelectron and KLL Auger spectra were measured for the K^{+} and Cl^{-} ions in aqueous KCl solution. While the XPS spectra of these ions have similar structures, both exhibiting only weak satellites near the main line, the Auger spectra differ dramatically. Contrary to the chloride case, a very strong extra peak was found in the Auger spectrum of K^{+} at the low kinetic energy side of the ^{1}D state. Using the equivalent core model and abinitio calculations this spectral feature was assigned to electron transfer processes from solvent water molecules to the solvated cation. The observed charge transfer processes are suggested to play an important role in charge redistribution following single and multiple core-hole creation in atoms and molecules placed into environment.

Chemical State Analysis of Si-Doped CNT on SiC by Hard X-Ray Photoelectron Spectroscopy

The carbon nanotubes (CNTs) on 6H-SiC and Si-doped CNTs on 6H-SiC were analyzed by hard x-ray photoelectron spectroscopy to identify the chemical bonding character of the doped Si in the CNT layer. We performed the depth profiling of the sample by changing photoelectron's take-off-angle (TOA). The spectral component associated with the doped Si is clearly seen in the Si 1s photoelectron spectra of Si-doped CNTs on 6H-SiC. The Si 1s peak shifts toward lower kinetic energy side from TOA = 80° (bulk-sensitive) to 8° (surface-sensitive), which implies the formation of the sp2-like structure in Si-doped CNTs. [DOI: 10.1380/ejssnt.2011.54]

Non-Destructive Depth Profiling by XPS Peak Shape Analysis

It is well known that the usual procedure for quantification by electron spectroscopy that is based on measured peak intensities is highly unreliable. An improved method is to take into account that the peak shape in a wide energy range, on the low kinetic energy side of the peak varies considerably with the surface morphology on the nano-meter depth scale. This observation has in recent years been applied in the formulation of a by now well known method for quantification that is based on quantitative analysis of measured peak shapes. The technique is sensitive on the 1 - 10 nm depth scale and it is non-destructive. The method suggested for the extraction of quantitative information from the large variation of the inelastic background with atom depth distribution will be reviewed briefly here. An example of practical application of the technique is also shown.

Many-body effect in the coincidence L 3 and M 3 photoelectron spectroscopy spectra of Cu metal

The coincidence L 3 and M 3 photoelectron spectroscopy (PES) main lines of Cu metal are calculated by a many-body theory. There is no peak-energy shift between the singles PES main line and the coincidence one. The asymmetric narrowing of the coincidence PES main line on the low kinetic energy (KE) side is very small. This is in accord with recent experimental findings. In Cu metal, the shakeup satellite intensity is small and the main-line satellite separation energy is much larger than the core–hole lifetime width. The interference via the final-state interaction is negligible. In the PES main line, the imaginary part of the self-energy by shakeup excitations, which is very small compared to the core–hole lifetime width, decreases very slowly in linear with photoelectron KE. The branching ratio of Auger decay of a single hole state then increases very slowly in linear with photoelectron KE so that the deviation of the coincidence PES main line from the singles one is very small. The 939 eV structure seen only in the coincidence L 3 PES spectrum of Cu metal is attributed to the enhancement of the inelastic peak of a smaller energy loss due to electrons of a smaller average emission depth measured in coincidence with the elastic Auger peak. The structure will not be enhanced in the singles PES spectrum. The background subtraction in the coincidence spectrum cannot be the same as that in the singles one. Such consideration is necessary before we can conclude about the asymmetric narrowing on the low KE side. A unique capability of APECS by which one can determine the photoelectron KE dependent part of the imaginary part of the self-energy is pointed out.

The shift of L 2 level photoelectron line of Cu metal measured in coincidence with the L 2–L 3V–VVV Auger-electron line

The L 2 photoelectron line of Cu metal measured in coincidence with the L 2–L 3V–VVV Auger line, moves to lower kinetic energies when the Auger analyzer moves to lower kinetic energy tail of the L 3–VV ( 1G) Auger line. Creagh et al. [Surf. Sci. 432 (1999) 297] proposed the possibility that the shift is due to the post-collision interaction (PCI). However, the present many-body theory shows that even without the presence of the PCI, the L 2 coincidence-photoelectron line shifts from the XPS spectral peak position towards the lower kinetic energy side, irrespective of the Auger-electron analyzer’s positions. The energy shift depends on the Auger-electron analyzer’s position. The coincidence-photoelectron line is asymmetrical. The present theory predicts the shift which agrees with the observed energy shift. The shift is due to the change in the partial L 2–L 3V CK transition rate which depends critically on the CK-electron energy. We comment on the PCI effect in the light of the recent experimental study of the L 2–L 3V CK transition rate near the L 2 ionization threshold.

Dynamical Auger Emission Induced by Multistate Vibronic Coupling in the Core-Excited States of theBCl3Molecule

The Auger emission following the resonant excitation of the BCl3 molecule has been investigated using monochromatized synchrotron radiation. The B 1s excitation to the unoccupied Anti-bonding 4e(') orbital enhances the shoulder structure in the low kinetic energy side of the photoemission from the 2e(') valence bonding orbital. Based on a series of quantum mechanical calculations, this shoulder structure is interpreted as the dynamical Auger emission which reflects the B-Cl stretching nuclear motion and appears as a result of the purely multistate vibronic coupling effect among the Jahn-Teller split B 1s(-1)4e(') E' states and the closely lying B 1s(-1)3a(')(1) A'1 state.

Quantitative XPS: non-destructive analysis of surface nano-structures

The usual procedure for quantification by electron spectroscopy that is based on measured peak intensities is shown to be highly unreliable. It is pointed out that the peak shape in a wide energy range, on the low kinetic energy side of the peak, varies considerably with the surface morphology on the nano-meter depth scale. This observation has in recent years been applied in the formulation of a new method for quantification that is based on quantitative analysis of measured peak shapes. The technique is sensitive on the ∼1–10 nm depth scale and it is non-destructive. Some aspects of the physical basis of this technique are discussed and examples are given where the method has been applied in practice.

Photoemission study of Na and Cs adsorption on MgO(100)1 × 1

Adsorption of Na and Cs on vacuum-cleaved MgO(100)1 × 1 surfaces has been studied with core and valence band photoemission spectroscopy. We conclude that both Na and Cs interact weakly with the surface except for small quantities of the initially adsorbed metals. Only trace amounts (≤ 0.04 monolayers (ML)) of Cs could be found on the surface after doses that exposed the surface to many monolayers of Cs. Although larger amounts of Na (≤ 1 ML) could be condensed on the surface, most of the Na is weakly physisorbed and easily desorbs at room temperature. However, a small fraction (0.02−0.045 ML) of both Na and Cs dosed onto MgO(100) appears to chemisorb on defect sites as indicated by an inability to desorb this fraction. The photoemission spectra for MgO(100) exposed to Na and Cs are unaffected by their presence except at high Na doses, where small changes are observed on the low kinetic energy side of the valence band and the high kinetic energy side of the Mg2p feature.

Applicability of the Tougaard ratio D in the analysis of nanometric TiO 2 overlayers

Recent investigations have shown that X-ray photoelectron spectroscopy (XPS) spectra of metallic specimens may contain information about the elemental depth distributions in the surface region. The increase in the spectral background on the low kinetic energy side of a photoelectron peak is due to inelastically collided photoelectrons, and is thus strongly dependent on the depth of origin of the photoelectrons. The aim of this investigation was to evaluate the applicability of the simple Tougaard peak-to-background ratio D in the analysis of nanometric TiO 2 and carbon overlayers. Prior to the overlayer evaluations, the ratio D 0 of the Ti 2p emission in a homogeneous TiO 2 matrix was determined from an in situ scraped single crystal TiO 2 specimen. The experimental result obtained for D 0 in the TiO 2 matrix was 13 ± 0.5 eV, i.e. lower than reported the “universal ratio” D 0 of 23 eV. For the overlayer thickness evaluation, the mean path lengths R of the substrate photoelectrons were calculated from the peak-to-background ratios of the substrate photoelectron peaks for test samples with nanometric TiO 2 and carbon overlayers on molybdenum substrates. The results were compared with results from conventional XPS layer calculations, XPS sputter profiling and X-ray microanalysis. The results from the different methods were found to agree within 25%.

Chemical effects in the X-ray emission (F K ? ) and X-ray photoelectron spectra of alkali-metal and alkaline-earth-metal fluorides

X-Ray emission (XE) and both core and valence-band X-ray photoelectron (XP) spectra have been measured for LiF, NaF, KF, BeF2, MgF2, CaF2, SrF2 and BaF2. Both the F 1s ionisation energies and the F Kα X-ray energies decrease as the cation atomic number increases within a group of the Periodic Table. Knowledge of the F 1s binding energy enabled the valence-band XP spectra to be aligned on a common energy scale for direct comparison with the F Kα peak profile. Both the XE and XP spectra showed broader bands for the lighter cations, which for Li, Be, Mg and Ca could be resolved into two distinct components. Symmetric broadening of the valence-band peaks was rationalized as due to anion–anion interaction whilst additional structure on the low (kinetic) energy side and low (X-ray) energy side of the main XP and XE peaks was related to covalent character in the cation–fluoride bond. Differences in the relative intensities of these components for XE (reflecting F 2p character only) and the XP spectra were ascribed to the presence of F 2s and possibly cation s character in the more tightly bound orbitals of the valence band. Measurement of the BeKα and Mg Kβ XE spectra gave a direct indication of the covalent nature of the bonding in BeF2 and MgF2.

Electronic excitations and their relaxations in clean and oxidized zirconium surfaces measured in eels

Electron energy loss Spectroscopy on clean and oxidized Zr surfaces shows features related to collective and single electron excitations. The 4 p–4 d transition resembles that encountered in isolated atoms. On oxidized surfaces, the interatomic transitions dominate. At the low kinetic energy side, spectra contain contributions due to the decay of plasmons, inter- and intra-atomic Auger processes, and autoionization emission.

A study of theXVV auger line shape of Fe−Ni alloys

A comparison is performed between theoretical and experimental shapes of theXVV (X=M2,3 andL3) Auger spectra of ferromagnetic Fex−Ni1−x alloys forx=0.48 and 0.27 in order to check if recent theories, which take into account the Coulomb correlation between the two final Auger holes, would be able to reproduce shape and position of the experimental peaks in an accurate way. For the case under investigation, it seems that, at least for the low-energy (M2,3VV) spectra and notwithstanding the usual neglecting of matrix element effects, the theory gives results in close agreement with the experimental data. The comparison with the high-energyL3VV spectra is somewhat hampered by an asymmetric broadening of the experimental data on the low kinetic-energy side due to satellite structures, but still gives good agreement on the high kinetic-energy side.

Core-level photoemission spectra from Cs monolayers on Si(1 1 1)2 × 1

Band shape in the Cs 4 d core level of cesium atom monolayers adsorbed on Si(1 1 1)2 × 1 has been studied by photoemission spectroscopy using synchrotron radiation. Asymmetry appeared on the low kinetic energy side at high coverages of cesium atoms. This is caused by the energy loss due to the overlayer plasmon in the cesium atom monolayer of photoemitted Cs 4 d electrons.

Radiation Effect in Hexatriacontane Thin Film

The secondary electron emission spectra of hexatriacontane thin film were measured as a function of irradiation time with an electron beam of about 19 eV. The spectral features which originate in the conduction band features in the crystalline state shifted to the low kinetic energy side linearly with the logarithm of the irradiation time. The shift is explained as an increase in the electron affinity due to the decomposition of hexatriacontane molecules.

Practical peak area measurements in X‐ray photoelectron spectroscopy

AbstractAn assessment of the reliability of peak area measurements using the simple linear background subtraction approach has been made by comparing the results obtained by a number of workers from a set of simulated spectra mixing the Fe 2p peaks from Fe metal and Fe2O3. The simulated spectra are generated from Gaussian peaks with a step background similar to that of Shirley, but modified to reproduce the sloping background on the low kinetic energy side of the peak. The results show that systematic errors in the total area of the Fe 2p peak may be as great as 40% using the workers best estimate of the background line. An alternative approach involving the areas of both 2p3/2 and 2p1/2 peaks reduces this error to 17%.

Some aspects of structure and bonding in the perchlorodiphenylaminyl radical revealed by ESCA

The ESCA core level spectra of the perchlorodiphenylaminyl radical have been measured and are interpretated in terms of a significant unpaired spin density on nitrogen. Comparison is drawn with conclusions based on the corresponding ESR data. Satellites are observed to the low kinetic energy side of the direct photoionization peaks and these are interpretated in terms of π → π * shake up transitions accompany core ionizations.

Studies on the shake-up satellites of auger spectra for potassium and calcium compounds

Abstract Strong satellite peaks were observed in the KL 23 L 23 Auger electron spectra for potassium and calcium in their compounds. Satellite peaks for the potassium KL 23 L 23 Auger electron spectra were located at the lower kinetic energy side of the main peak and their intensities were functions of the nature of the bonding ions. Satellites due to the electrons with larger kinetic energies than the Ca KL 23 L 23 main Auger line were observed for calcium oxide, calcium sulfate and calcium carbonate. All of the satellites of KL 23 L 23 Auger electrons for both elements were interpreted on the basis of the charge-transfer shake-up process in terms of the Mulliken's concept of charge-transfer interaction. The equivalent core approximation was applied for the description of a doubly-ionized core state.

Observation of time dependent relaxation effects in the photoelectron spectra of some metal carbonyls

Photoelectron measurements on transition metal carbonyls in the gas phase were made at various photon energies. Satellite peaks on the low kinetic energy side of the valence orbitals are observed and are attributed to shake up of the CO derived levels. The variation of the intensity of the shake up, relative to the main peak with the kinetic energy at which the shake up peaks are observed, provides evidence for time dependent relaxation of the final state. This data indicates that the shake up intensity is constant for kinetic energies greater than ≈20 eV.