Auger Electron Appearance Research Articles

Auger electron appearance potential spectroscopy study of CrNx films

AbstractThin films of chromium nitride deposited with varying concentrations of nitrogen on stainless steel substrates have been studied using Auger electron appearance potential spectroscopy (AEAPS). This technique provides information on the density of states in the conduction band. The chromium L2,3 levels have been investigated. The normalized AEAPS chromium spectra in the films have been compared with the elemental chromium spectrum. The signal strength at the Fermi level (EF) is observed to be larger in the films. This represents an increase in the density of states at EF for the films as compared with that of elemental chromium. However, in the films, the density of states is found to decrease with increasing nitrogen concentration. The binding energy of the chromium L3 level for the films exhibits a positive chemical shift with respect to elemental chromium. These spectral data indicate a direction of charge transfer from chromium to nitrogen. In the films, the charge transfer is observed to decrease with increasing nitrogen content. We also present a scheme to estimate the Coulomb correlation energy using the chromium L3 core‐level binding energy data obtained from AEAPS and x‐ray photoelectron spectroscopy (XPS). A decrease in the correlation energy with increasing nitrogen content is observed for the films. We also have estimated the total density of unoccupied states above EF from the experimental data. This total density is observed to be more in the films than in elemental chromium. In the films, the total density of states decreases with nitrogen concentration. The AEAPS spectral results have been correlated with our earlier XPS results on these films. Copyright © 2001 John Wiley & Sons, Ltd.

Elastic contribution to the high-energy Auger electron appearance potential spectrum

Abstract The elastic contribution of Auger electrons to the appearance potential spectroscopy (APS) signal has been studied both experimentally and theoretically. The analytical approach is based on a solution of a kinetic equation satisfying the corresponding boundary conditions. An analytical expression for the total contribution of Auger electrons, which are not scattered inelastically, has been derived. The Auger electron yield has been shown to be a universal function of the ratio of the inelastic to the transport mean free paths of signal electrons and the inelastic mean free path of primaries. The overlayer technique has been used to investigate the mean escape depth of Auger electrons escaping from a target without losing a considerable amount of their kinetic energy. For this purpose a high-energy APS signal has been recorded as a function of the cut-off energy and the overlayer thickness. Ti and Ni were chosen as the overlayer and the substrate materials respectively and the Ti and Ni L-subshells were excited. The experimental data compare well with the corresponding analytical results. In particular, it was found that due to the attenuation of the primary beam the mean escape depth of the elastic APS signal electrons turned out to be smaller, by a factor of two, than that of the same Auger electrons in the usual Auger electron spectroscopy (AES). As a result the surface sensitivity of the high-energy APS may be higher by a factor of about two compared to AES. © 1997 Elsevier Science B.V.

New aspects of the deexcitation mechanism in Auger electron appearance potential spectroscopy

Abstract Contributions of electrons with different energies to Auger electron appearance potential spectroscopy (AEAPS) have been investigated. Appearance potential spectra amplitude dependences have been measured for Ti, Cr, Mn and Ni by means of a previously exploited method using a spherical three-grid retarding field system. The effective dimensionless coefficient σ eff , representing the number of slow electrons emitted at the disappearance of one elastically reflected electron at a threshold potential, ranges from 2.7 to 3.5 for L 2,3 subshells and from 9 to 11 for L 1 subshells. These results have been accounted for in terms of fast, highly probable Coster–Kronig transitions which turned out to be the only possibility for relaxation of the L 1 subshells of the materials under investigation. This explanation has been supported by a direct comparison with both former and recent experimental results. © 1997 Elsevier Science B.V.

On the information depth of auger electron appearance potential spectroscopy

The information depth of AEAPS in comparison with AES was estimated using the overlayer technique. The strengths of the AES and AEAPS L 3 and L 1 signals of Cr and Ti were measured as a function of the Cr overlayer thickness on a Ti substrate. No significant differences between the information depths of both methods have been found. It follows that the information depth of AEAPS is basically limited by the mean free path of the primary electrons. Slight disagreement between the results obtained for the L 3 and L 1 subshells of Ti and Cr may indicate that the information depth of AEAPS could be modified by the kind of relaxation mechanism and the subsequent transport process.

Study of the Ti/Si interface using x-ray photoelectron and Auger electron appearance potential spectroscopies

Thin films of titanium silicide have been grown by evaporative deposition of high-purity Ti on a Si 〈111〉 substrate under high-vacuum conditions and annealing at ∼300 °C for 15 min. The growth of TiSi2 on the top 1000 Å of Si was studied for its stoichiometry as a function of thickness by x-ray photoelectron spectroscopy (XPS) and Auger electron appearance potential spectroscopy (AEAPS). A sputter profiling technique using Ar ions was employed to determine the composition of the overlayer as a function of depth. Results of the study indicate the presence of both TiO2 and SiO2 on the top layers in addition to Ti and Si (which may be bonded to each other). The concentrations of TiO2 and SiO2 are found to decrease with depth, the former decreasing much more rapidly than the latter. The atomic percentage composition measurements indicate the formation of TiSi2 in the Ti overlayer. However, no appreciable shift is observed in the core binding energies of the Ti 2p3/2 and Si 2p peaks. AEAPS, which probes the local density of conduction band states, should be more sensitive to the chemical shift. The AEAPS measurements indeed show a positive shift of approximately 3 eV for the Ti 2p3/2 peak in Ti/Si. Moreover, this peak exhibits a broadening as compared to that of elemental Ti. These results are indicative of the bonding between Ti and Si in the TiSi2. The formation of TiSi2 at 300 °C is corroborated by x-ray diffraction measurements.

A study of LaH 3 by auger electron appearance potential spectroscopy

The Auger electron appearance potential spectra (AEAPS) of M 4,5 levels of lanthanum in the elemental state and in LaH 3 are presented. AEAPS measures the total differential secondary electron yield from the surface constituents as a function of incident electron energy. In the presence of a 3 d hole, the 4 f level is pulled down below the Fermi energy, E F. In the metallic La spectrum this is seen as a shoulder on the low energy side of the main peak, but in the LaH 3 spectrum this shoulder disappears and is replaced by two peaks, again on the low energy side. The high binding energy satellite reported in the M 5 level XPS spectra in La insulating compounds has been attributed to the lowering of the 4 f level above the ligand band. Since no high binding energy satellite is observed in the M 4,5 level AEAPS of LaH 3, the lowest energy peak is assigned as the transition of the 3 d electron to the 4 f level pulled down below the hydrogen induced band. The other peak is related to the partially filled hydrogen induced band below E F. Tentative energy level diagrams for these transitions are proposed.

Surface atom core level energy shifts for Ti, Cr, Fe, and Co polycrystalline surfaces

The 2p core level Auger electron appearance potential spectra (AEAPS) for Ti, Cr, Fe, and Co metal surfaces have been investigated. Each of the 2 p 3 2 and 2 p 1 2 - level spectra exhibits surface and bulk-like state peaks in the unoccupied band of these metals. The surface states appear as a shoulder on the leading edge of 2p-level AEAPS spectra and are localized near the surface. The surface state peaks are shifted to lower energies by 0.7–1.4 eV relative to the bulk peaks.

Surface atom core level energy shifts for Ti, Cr, Fe, and Co polycrystalline surfaces

The 2p core level Auger electron appearance potential spectra (AEAPS) for Ti, Cr, Fe, and Co metal surfaces have been investigated. Each of the 2 p 3 2 and 2 p 1 2 - level spectra exhibits surface and bulk-like state peaks in the unoccupied band of these metals. The surface states appear as a shoulder on the leading edge of 2p-level AEAPS spectra and are localized near the surface. The surface state peaks are shifted to lower energies by 0.7–1.4 eV relative to the bulk peaks.

Study of the 4f-levels in rare earths by appearance potential spectroscopy

Abstract The yttrium M 4.5 -level Auger electron appearance potential spectrum (AEAPS) in the 150–190 eV energy range is presented. AEAPS is a surface sensitive technique that measures the differential secondary electron yield as a function of incident electron energy. The spectral features of AEAPS are determined by the width of the empty conduction band and the degree of localization of the conduction band wave functions. Yttrium (Y) is considered to be one of the rare earth metals with an absence of a 4 f state. In lanthanum (La), which has an identical valence electron configuration-to Y, the 4 f levels are localized. Comparison of the 3 d -level spectrum of Y with that of La shows that though the main peaks remain the same, additional fine structures appear in the spectrum of La. The existence of these structures is correlated with the presence of empty localized 4 f levels in La.

Empty electronic states in solid and liquid nickel

The density of empty electronic states in solid and liquid nickel has been determined employing Auger electron appearance potential spectroscopy. The observed increase of the density-of-states at the Fermi level by a factor of 1.4 in the liquid is in good agreement with available resistivity data. An increase of d-holes from 0.6 to 1.0 upon fusion is also found and interpreted in terms of a reduced d- d-interaction in the liquid state.

On the deexcitation mechanism in Auger electron appearance potential spectroscopy

Abstract Contributions of electrons with different energies to the Auger Electron Appearance Potential Spectroscopy signal were measured for Ti, Cr, Mn and Fe by means of a method using a 3-grid retarding field system. This system permits to register a signal formed by electrons of energies either from zero to a given energy E G or from that energy to the primary energy E p . The importance of slow electrons was found in all cases. Numbers of slow electrons emitted at the disappearance of one elastically reflected electron at threshold potentials were calculated. They extend from 2.7 to about 3.7 for L 2,3 shells and are higher, namely 7–11, for L 1 shells. This results, as well as the fact that the Disappearance Potential Spectroscopy signal overcompensates the Auger Electron Appearance Potential Spectroscopy signal usually at several tens of eV for L 2,3 levels while at some hundreds of eV for L 1 levels, are discussed. Conclusions concerning the information depths of the methods are drawn.

On the deexcitation mechanism in Auger electron appearance potential spectroscopy

Contributions of electrons with different energies to the Auger Electron Appearance Potential Spectroscopy signal were measured for Ti, Cr, Mn and Fe by means of a method using a 3-grid retarding field system. This system permits to register a signal formed by electrons of energies either from zero to a given energy E G or from that energy to the primary energy E p. The importance of slow electrons was found in all cases. Numbers of slow electrons emitted at the disappearance of one elastically reflected electron at threshold potentials were calculated. They extend from 2.7 to about 3.7 for L 2,3 shells and are higher, namely 7–11, for L 1 shells. This results, as well as the fact that the Disappearance Potential Spectroscopy signal overcompensates the Auger Electron Appearance Potential Spectroscopy signal usually at several tens of eV for L 2,3 levels while at some hundreds of eV for L 1 levels, are discussed. Conclusions concerning the information depths of the methods are drawn.

Quasi-Instantaneous Measurements of Titanium Compounds Using Auger Electron and Auger Electron Appearance Potential Spectroscopies

The Auger electron and Auger electron appearance potential spectra of Ti, TiCx, TiNx and TiO2 were observed quasi-in-stantaneously, and the energy shift values for the L3M2,3M4,5 Auger peak were found to be 1.5 eV, 4.6 eV and 5.3 eV for TiCx, TiNx and TiO2, respectively. These values correspond to the energy separation between band (2eg→2p3/2) and band (2t2g→2p3/2) in XES (X-ray emission spectra) for TiN or the absorption edge in SXAS (soft X-ray absorption spectra) for TiC and TiO2. The threshold values of the L3 AEAP spectra in titanium compounds show good agreement with the energy position of the L3-absorption edge in SXAS. The energy separation between the main peak and the shoulder peak at high energy for TiNx and TiO2 in AEAP spectra corresponds to that of empty states in SXAS.

Changes in Electrical Resistance and Auger Electron Appearance Potential Spectra of Titanium Thin Films by Oxygen Diffusion

The electrical resistances of titanium evaporated films with a thickness of about 100 Å were measured after exposure to oxygen. Increases in the resistance up to an oxygen exposure of 10L follow a nearly straight line, but the increases become smaller as the Tempax glass (borosilicate glass) substrate temperature during titanium evaporation rises. This is thought to be caused by the difference in grain size of the films. A diffusion model of the phenomena is given. Auger electron appearance potential spectra, which give information about chemical bondings, show no evidence of the existence of any stable compounds in the films.

Auger Electron Appearance Potential Spectrum of Ni

Electrons are accelerated onto a solid polycrystalline Ni surface. These primary electrons interact with the surface to produce one of two phenomena: x-ray flourescence, or Auger electron emission. Auger Electron Appearance Potential Spectroscopy (AEAPS) is a process by which the Auger component of the secondary electron flux is analysed to extract qualitative information about the electronic structure of the empty conduction band states. In AEAPS, the threshold behavior of the Auger transitions is examined by taking the first derivative of the secondary electron current with respect to the incident electron energy using the potential modulation technique. Width of the empty band, degree of localization of the conduction band wave functions, overlapping of the 3d band with the 4s, and satellite phenomena are among the important parameters ascertainable by AEAPS. The present AEAPS measurements of L3,2 levels of Ni are compared with the reported data from Soft X-ray Appear nce Potential Spectroscopy (SXAPS) of Ni. SXAPS is a complimentary process of AEAPS in that it examines the secondary x-ray fluorescence from the surface in much the same way as AEAPS examines the total electron yield. AEAPS, used alone, or in conjunction with SXAPS, is considered to be a powerful technique for the systematic investgation of series of metals, such as the transition, or lanthanide series.

Surface states and oxygen chemisorption on Ti(0001)

The system ${\mathrm{O}}_{2}$ on Ti(0001) has been investigated using appearance potential spectroscopy (APS), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and work-function measurements obtained by the field-emission retarding potential technique. A strong band of states just above the Fermi level was found to be localized near the surface. This band of surface states broadened with increasing temperature and was suppressed by exposure to oxygen. Although weaker, a similar band of surface states was previously observed on polycrystalline titanium samples and more recently predicted for the (0001) surface to occur at the Fermi level and to extend a few tenths of an eV on either side of it [Peter J. Feibelman, J. A. Appelbaum, and D. R. Hamann, Phys. Rev. B 20, 1433 (1979)]. The sensitivity of the surface-state signal to sampling depth was demonstrated by continuously varying the sampling depth between the 5-10 \AA{} inelastic mean free path of the incident electrons in Auger electron appearance potential spectroscopy and approximately half that value for disappearance potential spectroscopy. The work function remained approximately constant for very low oxygen coverage and then steadily increased with additional exposure from a clean-surface value of 4.58\ifmmode\pm\else\textpm\fi{}0.05 eV, leveling off at approximately 5.3 eV as the surface became saturated with oxygen. The initially diffuse low-coverage $p(2\ifmmode\times\else\texttimes\fi{}2)$ LEED superstructure formed upon exposure at room temperature was significantly improved upon flashing the sample to 250\ifmmode^\circ\else\textdegree\fi{}C. This was accompanied by an abrupt decrease of 0.15 eV in the work function, although AES revealed no decrease in the amount of oxygen on the surface. A similar decrease in the work function upon a 250\ifmmode^\circ\else\textdegree\fi{}C flash anneal was observed for other exposures as well, suggesting some reordering of the surface adatoms. These results are interpreted in terms of two chemisorption states for the oxygen: a tightly bound $\ensuremath{\alpha}$ state at low coverages characterized by a well-ordered $p(2\ifmmode\times\else\texttimes\fi{}2)$ LEED pattern and a work function below that of the clean surface, and a $\ensuremath{\beta}$ state characterized by a disordered structure and a work function higher than the clean surface value. Heating converts the $\ensuremath{\beta}$ state to the $\ensuremath{\alpha}$ state.

The inversion of autoconvolution integrals

In this paper, we describe a procedure for inversion of autoconvolution integrals arising in several surface scientific methods. The solution is obtained by rigorous application of cubical spline functions. The method offers excellent numerical stability. Error propagation is discussed, and deconvolution examples are given for a characteristic isochromat, a soft X-ray appearance potential and an Auger electron appearance potential measurement of the iron L III level.

A modified Auger system for Auger electron appearance potential spectroscopy

We present an apparatus for Auger electron appearance potential spectroscopy measurements derived from a conventional Auger electron spectrometer. This non-dispersive apparatus is shown to be very sensitive with a high spatial resolution. The utility of the technique has been tested by measuring the 2P levels for some 3d transition metals.

ＣＯガス吸着によるＦｅ‐ＡＥＡＰＳスペクトル形状の変化

It was shown that AEAPS (Auger Electron Appearance Potential Spectroscopy) is useful for the observation of the change of unfilled density of states on solid surfaces. The changes of Fe-L3 spectra after CO exposure and heat treatments were observed. The Fe (L3 dip/L3 peak) is a good indicator of the change. The changes correspond to the splitting of 3d and 4s band densities. It is considered that the oxide layer on the iron surfaces may be formed at more than 100L CO exposure. The unfilled density of states calculated from clean Fe spectrum shows good agreement with the results obtained by X-ray isochromat spectroscopy.

Solid Surface Analysis using the Secondary Electron Yield Spectrum.

Auger electron appearance potential spectroscopy (AEAPS) apparatus for the study of solid state surfaces was constructed and the working conditions were extensively examined for surfaces of the 3d transition metals (Ti∼Ni). For the clean surface of polycrystalline Ti, the spectra of the first and second derivative signals were in good agreement with theoretical results obtained from the unfilled 3d binding state at Fermi energy EF. The spectra of Ti, Fe and Ni were obtained on both clean and oxidized surfaces at room temperature. The AEAPS and soft Xray appearance potential spectroscopy (SXAPS) spectra were compared on the same Mn and Co clean surfaces. Relative L3 AEAPS signal strengths were measured for clean 3d transition metal (Ti∼Ni) surfaces.