A digital scanning Auger electron microscope

A new instrument, a scanning Auger microscope with digital scanning and a concentric hemispherical analyser, is described together with some preliminary demonstration results. The sample, a titanium ribbon, was hand polished and resistively heated. After a short heating to remove some of the overlay carbon and oxygen, the sample showed some irregular areas of very low SEM contrast which were attributed to calcium from the L_{2, 3}VV Auger process at 288 eV. Using this calcium peak a scanning Auger map of the areas showed high contrast and the corresponding titanium Auger maps demonstrate that this indeed was an overlayer of calcium. The probe resolution was 50 nm and a single picture was produced in 3 min.     

Obtaining density of states information from self-deconvolution of Auger band-type spectra

A systematic revision of the techniques to isolate Auger peaks in experimental spectra obtained with LEED/AES analyzers has been carried out: Background subtraction, deconvolution with the elastic peak and inversion of self-convolution. New techniques have been devised and applied to the Auger spectra of graphite, aluminum, magnesium, magnesium oxide and silicon. Self-deconvolution of these spectra leads to a transition density function that has been compared with the density of states of the valence band near the surface, as given by other competitive techniques (XPS, UPS, XES) and by theory. This comparison shows that in those meterials, Auger electron spectroscopy (AES) is a sensitive probe of the electron environment of surface atoms.     

Interatomic Auger processes and the density of states

The correlation between the line shape of Auger peaks and the density of states near the surface has been the subject of recent controversy. In certain cases, it has been possible to obtain the density of states by numerical deconvolution of a KVV peak (Amelio, 1970) or directly using a KLV peak (Cardona et al., 1973). However, the extension of this technique to transition metals (Cu, Zn) has encountered serious difficulties, related to the perturbation created by the presence of localized charges either in the initial or in the final state, although it is not yet clear why this perturbation is strong only in certain cases. The purpose of the present communication is to show a series of results that can throw some light on the abovementioned problem. The main point is that Auger processes of interatomic type, as those occurring in the INS technique of Hagstrum, are free of these perturbations. Recently, the authors have studied the line shape of the Auger peaks of O, C, N and S adsorbed on Cu, Ni and Fe. These results show that only that part of the Auger structure originated by interatomic transitions between substrate and adsorbate atoms can be related to the local density of states (LDOS). The rest of the structure, due to normal intraatomic processes, is dominated by the spectral terms in the final configuration of the ion. This new interpretation allows a separation of perturbation effects and clarifies the contribution of the LDOS to the peak line shape. In this communication, we present the line shape analysis of the L_2,3 VV and KVV Auger peaks of Mg and O in MgO. Due to the strong ionic character of this compound, the L_2,3 VV peak of Mg⁺⁺ is mainly due to interatomic processes between Mg⁺⁺ and O⁼ ions, whereas the KVV peak of O is mainly due to interatomic processes. This analysis shows that good agreement exists between the L_2,3VV Mg⁺⁺ Auger peak and the self-convolution of MgO density of states, whereas the KVV Auger peak of O⁼ is dominated by the spectral terms of the final configuration. Only a small peak in the high energy side of the latter peak can be related to the density of states and could be interpreted as an interatomic transition between two neighboring oxygen ions, in agreement with the interpretation given by others.     

Quantum coherence in the time-resolved Auger measurement

We present a quantum mechanical model of the attosecond-XUV (extreme ultraviolet) pump and laser probe measurement of an Auger decay [Drescher et al., Nature (London) 419, 803 (2002)]] and investigate effects of quantum coherence. The time-dependent Schr?dinger equation is solved by numerical integration and in analytic form. We explain the transition from a quasiclassical energy shift of the spectrum to the formation of sidebands and the enhancement of high- and low-energy tails of the Auger spectrum due to quantum coherence between photoionization and Auger decay.     

Effect of surface plasmon oscillations on Auger electron emission

The integrated areas of the Al L₂₃VV and O KL₂₃L₂₃ Auger peaks and the Al surface plasmon energy ?ω_S are reported for the Al(001) surface as a function of exposure to O in the exposure range 0–114 L(1 L=1langmuir=10^?6Torr sec). It is shown that for exposures below a critical value of 15 L, ?ω_S is constant within experimental error while the O Auger peak area increases linearly. For exposures above 15 L, ?ω_S decreases linearly from 10.5 eV to 8.5 eV and the O Auger peak area undergoes relatively slow linear increases correspondingly. The Al Auger peak area decreases by 30% per 1 eV decrease of ?ω_S. The results are discussed with reference to theory relating Auger transition intensities to the spectral density function.     

PCA-Based Analysis of X-Ray-Excited Auger Spectra from Non-ordered Ag(110)

Principal-components analysis (PCA) followed by factor analysis enables one to decompose the structure of Auger lines originating from large effects such as energy shifts induced by chemical effects. The aim of the present contribution is to show that PCA can also be effectively used for detection of composed structure in a set of Auger spectra even if the observed changes in line shape are very subtle. The analysed set of X-ray-excited MNN Auger spectra from Ag(110) shows a clear correlation between peak shifts and peak widths. This correlation can be explained as a result of the composed structure of the recorded Auger lines. It is suggested that the resultant Auger lines may consist of a number of constituents, each referred to Ag atoms differing in the value of their co-ordination number.     

High resolution Auger electron spectroscopy of metallic copper

Part of the LMM Auger spectrum from metallic copper has been studied in a high resolution X-ray photoelectron spectrometer. Fine structure not earlier reported has been observed. The main L₃M_4,5M_4,5 peak is very narrow, 1.0 eV, although the valence band is involved in the transition. The agreement between experimental and calculated Auger electron energies is very good. Since fine structure is found to be an intrinsic property in Auger spectra the interpretation of “satellite” peaks as due to electron—plasmon interactions should be used with care. The L₃M_4,5M_4,5 peak is very sensitive to the copper surface conditions. Surface oxygen affects the peak in a characteristic way.     

On the ‘plasmon energy gain’ in the Be Auger spectrum

The Auger spectrum and the high energy satellite structure from evaporated Be films were investigated. There was no evidence of a plasmon gain peak 18 eV above the main Be KVV Auger peak, in disagreement with the observation of Jenkins and Zehner.¹ The high energy satellite peaks associated with the Auger peaks must be interpreted on the basis of double ionization of the initial state.     

Hydrogen adsorption on silicon (1 1 1) surfaces studied by Auger electron spectroscopy

The adsorption of atomic hydrogen on the (1 1 1) planes of silicon single crystals is studied by Auger Electron Analysis, using the decrease of the silicon Auger peak heigh as a measure of the hydrogen coverage. The zero coverage sticking coefficient is found to be 3 × 10^?4.     

Binary collision model for neon Auger spectra from neon ion bombardment of the aluminum surface

A model is developed to account for the angle-resolved Auger spectra from neon ion bombardment of the aluminum surface recently obtained by Pepper and Aron. The neon is assumed to be excited in a single asymmetric neon-aluminum collision and scattered back into the vacuum where it emits an Auger electron. The velocity of the Auger electron acquires a Doppler shift by virtue of the emission from a moving source. The dependence of the Auger peak shape and energy on the incident ion energy, angle of incidence and on the angle of Auger electron emission with respect to the surface is presented. Satisfactory agreement with the angle resolved experimental observations is obtained. The dependence of the angle-integrated Auger yield on the incident ion energy and angle of incidence is also obtained and shown to be in satisfactory agreement with available experimental evidence.     

Auger electron peak shape and structure of adsorbate layers

A detailed study is made of the evolution of the N_6.7O_4.5O_4.5 Au Auger electron peak with Au coverage on a Mo(110) surface up to saturation for adsorption at 300 and 900 K. A number of features may be attributed to inelastic scattering of Auger electrons but the more prominent details are related to the various structures observed in this adsorption system. As a consequence of the structure sensitivity of the Auger peak shape the use of the derivative signal may give misleading information regarding the film growth.     

Study of a Nonequilibrium Auger-Transition Using Emission Auger Spectroscopy

Journal of Applied Spectroscopy - A nonequilibrium Auger transition consisting of a vacancy in the valence layer being filled within ~10–14 s by a probe electron that receives the energy...     

Auger electron microscopy: An overview

The aim of Auger electron imaging is to obtain quantitative surface elemental distribution maps at high spatial resolution. The realization of this goal is complicated by many instrumental effects and by spurious data processing contributions giving rise to an image contrast unrelated to the specimen surface composition. The critical properties of scanning Auger microscopy that may cause such a false information or imaging artefacts are reviewed. Instrumental or data processing related effects appear in the case of the beam current variation, of the background slope effect, and of the use of a combined peak to background ratio. The second set of artefacts are mainly due to the significant differences between the penetration depth of the exciting primary electrons and the escape depth of the Auger electron signal. In this case the net effect is a surface elemental contrast which is dominated by the substrate or by the overlayer rather than by the surface under investigation. In addition, there are also topographical effects of the specimen under test which normally affect the Auger yield and hence the contrast in the image. Methods for the successful suppression of some of these artefacts are outlined. They are based on the creation of reference images from complementary signals acquired by additional detection channels in parallel with the Auger signal of interest.Invited lecture presented at the Seminar on Secondary Electrons in Electron Spectroscopy, Microscopy, and Microanalysis, Chlum (The Czech Republic), 21–24 September 1993.     

Observation of a new atomic-like peak in the ion-induced Auger spectrum of Si

We have observed Si L-shell Auger electrons from a silicon surface bombarded with 20 keV Ar⁺ ions at different angles of incidence. The measurements of angle-resolved electron energy distributions allowed us to separate the contributions to the Auger electron spectra coming from de-excitations inside and outside the solid. At grazing angles of incidence and observation, the Auger electron distributions resemble those from gas-phase atomic collisions. In these conditions we observed an atomic-like Auger peak with an energy close to that of the high energy edge of the Si L_2,3VV transition.     

Disorder broadening of alloy Auger spectra

Auger spectroscopy promises the means to separate initial and final state contributions to the disorder broadening of core XPS spectra in disordered alloys. Auger disorder broadening, deduced from recent ab initio results, is predicted to be greater than XPS disorder broadening for Cu₅₀Pd₅₀ and Ag₅₀Pd₅₀ alloys. Simulations are used to assess whether this effect is observable experimentally despite the greater lifetime broadening of Auger spectra. A number of cases where narrow core–core–core Auger transitions should allow clear experimental identification of this effect are identified. The prospects for determining environment-resolved Auger spectra using APECS have been investigated.     

Quantitative Auger electron spectroscopy; A comparison of techniques for adsorbed tin on iron

Three independent approaches are discussed for the quantitative calibration of tin adsorbed on the surface of iron monitored by Auger electron spectroscopy. Two of the approaches involved measurement of the substrate and condensate Auger electron intensities at stages during the condensation of tin or stannic oxide on pure iron. The condensate quantity was directly monitored by a quartz crystal oscillator and the growth form by measurements of the elastic scattering. Allowances were made for changes in the tin Auger electron peak structure due to its combination with the oxygen in the oxide condensate. These two calibrations compare very closely with a calibration obtained from the cleavage face of a single crystal of α-iron containing 7.9 wt % of tin in solid solution. A calibration in terms of relative peak intensities is obtained for tin on iron and it is concluded that the quantifica- tion procedures are applicable, even for strongly bonded compounds.     

Auger emission in degenerate GaSb crystals

The dependence of the peak energy of the Auger band on the doping in n-type and p-type degenerate GaSb crystals is investigated, in good agreement with experiments.It is suggested that the continuum for n-type GaSb is only by a factor of 20 stronger than for p-type material, and the maximum value of the stimulated Auger emission in the n-type or p-type GaSb photoluminescence is 4 times weaker than the corresponding maximum value of the near-band-edge emission.     

Progress in instrumentation,data reduction,and depth profiles in Auger electron spectroscopy

Progress in the use of Auger electron spectroscopy is discussed. Specifically limits to spatial resolution in the scanning Auger microprobe resulting from backscattered electrons is illustrated. Determination of chemical state by peak shapes and energies are discussed along with the use of correction factors in quantitating Auger electron data. Finally, the use of inverse Laplace transforms of angle-resolved electron spectroscopy data to generate composition depth profiles of sputtered GaAs is illustrated. It is concluded that Gibbsian surface segregation during sputtering caused depletion of As near the surface of GaAs.     

Difficulties in the detection of surface impurities on platinum using Auger electron spectroscopy

The difficulties in the detection of surface impurities on platinum by Auger electron spectroscopy are demonstrated by the analysis of samples which showed varying degrees of contamination in field emission microscope investigations. Spectra indicate that S contamination can easily be overlooked because of an overlap of the main S transition at 152 eV with the Pt transitions at 150 and 158 eV. A similar example involving the overlap of the 92 eV Si peak with the 93 eV Pt peak is investigated. The presence of P is found to be easily obscured by electron beam induced effects. Several oxygen-P cross transition signals can arise, each of relatively low intensity. Together, these signals give the appearance of background noise. The results of these analyses are applied to investigations of the so called “clean” reconstruction of Pt {100}. In reviewing over seventy papers on {100} reconstruction, including those of Ir and Au, not one Auger spectrum could be found providing unequivocal evidence of surface cleanliness.