A fast and simple method for background removal in Auger electron spectroscopy

A purely formal method for background removal in electron beam induced Auger electron spectroscopy is presented. The method has been developed for practical purposes. It is typically used to remove the background of a complete recorded spectrum, no fit is necessary to remove the backscattered electrons background. An overcompensation of the background, resulting in negative values of the background removed spectrum is not possible, all values of the background removed spectrum are positive or zero. Since the Auger peaks are separated by zeros after background removal, the method is well suited for peak finding and identification.Dedicated to Professor Dr. H. Seiler on the occasion of his 65th birthday     

Data Preprocessing in Peak Shape Analysis of Auger Electron Spectra

 Factor analysis is an established method of peak shape analysis in Auger electron spectrometry. The influence of different commonly used data preprocessing tools onto the results of factor analysis is demonstrated on AES depth profiles of multilayers and implantation profiles. For the analysis of Auger electron spectra it has been traditional to differentiate spectra by Savitzky and Golay’s method to remove background and to elucidate changes in peak shape. For phosphorus implanted in titanium it is shown that background removal works not ideal so that inelastic losses of the Ti(LMM) Auger peak can affect the result of factor analysis for the P(LVV) peak located at ca. 250 eV lower in kinetic energy. The contribution of such losses to the background can be corrected by shifting the spectra so that the high energy side above the peak equals zero. Numerical differentiation can introduce correlated error into the data set. To diminish edge effects the reduction of filter width at the edges and cutting off the outermost data points is recommended. The precision of spectrum reproduction is considered as a crucial test for the number of principal components. The reliability factor is investigated as a measure for the goodness of spectrum reproduction.     

Intensity calibration for monochromated Al Kα XPS instruments using polyethylene

We describe a method for the intensity calibration of XPS instruments using polyethylene as the reference material. Previous methods have employed noble metals, such as gold, silver, and copper. Polyethylene has a number of advantages over these. It has far fewer photoelectron and Auger electron peaks than such metals, ie, the spectrum largely comprises inelastic background over a wide and continuous range of kinetic energies. The XPS spectrum can be described by a mathematical function enabling simple and noise-free implementation of the reference spectrum. Polyethylene can be cleaned ex situ using a sharp knife or razor blade to remove trace oxygen and, due to its chemical composition, should not be affected by adventitious carbon contamination. Thus, an ion source for sputter cleaning is not required, although an electron flood source for charge compensation is required. The drawback to using polyethylene is that the photoelectron yield is far lower than gold or silver, and this necessitates longer acquisition times and removal of dark noise. Longer acquisition times carry the risk of damaging the polyethylene surface, and we show that, even if damage does occur, it has a negligible effect on the XPS background intensity. The reference spectrum is valid for monochromated Al Kα XPS instruments with a monochromator-sample-analyser angle close to 60°.     

Monte Carlo simulation of full energy spectrum of electrons emitted from silicon in Auger electron spectroscopy

A Monte Carlo simulation including surface excitation, Auger electron‐ and secondary electron production has been performed to calculate the energy spectrum of electrons emitted from silicon in Auger electron spectroscopy (AES), covering the full energy range from the elastic peak down to the true‐secondary‐electron peak. The work aims to provide a more comprehensive understanding of the experimental AES spectrum by integrating the up‐to‐date knowledge of electron scattering and electronic excitation near the solid surface region. The Monte Carlo simulation model of beam–sample interaction includes the atomic ionization and relaxation for Auger electron production with Casnati's ionization cross section, surface plasmon excitation and bulk plasmon excitation as well as other bulk electronic excitation for inelastic scattering of electrons (including primary electrons, Auger electrons and secondary electrons) through a dielectric functional approach, cascade secondary electron production in electron inelastic scattering events, and electron elastic scattering with use of Mott's cross section. The simulated energy spectrum for Si sample describes very well the experimental AES EN(E) spectrum measured with a cylindrical mirror analyzer for primary energies ranging from 500 eV to 3000 eV. Surface excitation is found to affect strongly the loss peak shape and the intensities of the elastic peak and Auger peak, and weakly the low energy backscattering background, but it has less effect to high energy backscattering background and the Auger electron peak shape. Copyright © 2014 John Wiley & Sons, Ltd.     

Neue Methode zur quantitativen Analyse von AES-Spektren

Summary The quantitative analysis of Auger electron spectra may lead to problems using Auger peak-to-peak heights (APPH), especially in connection with chemical peak deformation and peak overlap. To eliminate these problems a method has been developed and was applied to metalnonmetal compounds. An integral spectrum is fitted with reference spectra and correction spectra, background differences are compensated. To deal with chemical effects a digital filter process is used. In order to test this method a copper-palladium alloy series has been measured and evaluated according to this method. The results show that a more accurate quantification could be obtained than by using APPHs and sensitivity factors. As a further advantage, relative sensitivity factors are no longer necessary due to peak/background standardization.     

Physical foundation of quantitative Auger analysis

The possibility of using an Auger peak height in the dN (E) /dE spectrum and an integrated N (E) spectrum as a measure of the Auger current is discussed and necessary relations are presented. The methods of the background determination are reviewed and discussed.

The relation between the Auger current and the atomic cancentration of a corresponding sample component is derived and the state of art in the field of theoretical and experimental determination of factors appearing in this relation (ionization cross-section, Auger transition probability. backscattering factor, and inelastic mean free path of Auger electrons) is presented.

Approaches to the quantitative Auger analysis (QAA) of homogeneous, isotropic samples, including corrections for matrix factors, are presented and discussed. Problems arising when heterogeneous samples are analyzed are discussed and practical approaches to such an analysis are presented.

The role of crystalline effects (the dependence of the Auger signal from crystalline samples on the direction of the primary electron beam and angular distribution of Auger electron emission from such samples) in QAA is discussed and examples of such crystalline effects are presented together with their physical foundation.

Some rules are suggested allowing the quantitative Auger analysis to be performed with the smallest possible error.     

Quantitative Auger electron spectroscopic analysis of carbides,nitrides and carbonitrides

A novel method for the quantitative evaluation of Auger electron spectra based on peak areas is presented. Sample and reference spectra in integral mode are filtered by an area conserving digital filter. This transforms the peak shapes influenced by chemical effects into standard peak shapes. After filtering a linear combination of reference spectra, differentiated spectra accounting for peak shifts and some low order polynomials to account for variations in the background is fitted to the sample spectrum by a least squares method. The need to approximate the spectrum of the secondary electron background explicitly for direct calculation of peak areas is thus eliminated. Filters of different widths are applied to reduce errors by chemical effects. The composition of the sample is computed from the composition of the reference samples and the coefficients obtained from the fit.To demonstrate the validity of this technique it has been applied to both, Gaussian model peaks and spectra of titanium carbonitrides. A further test on an alloy series is under investigation. The results show that the method works as predicted and gives accurate quantification.     

The Auger electron spectrum of water vapour

The Auger electron spectrum of water vapour has been recorded and analyzed. For the analysis, an approximate formula for calculating the intensities of the Auger electron lines is derived. It is shown, that the calculated intensities along with theoretical energies of the Auger transitions account well for the observed spectrum. In particular, new assignments in terms of transitions to triplet final states are suggested.     

负载零价铁膨胀石墨的合成及对水中NO3-去除的效果与机制

以网络状孔型结构发达的膨胀石墨（EG）为载体,采用化学沉积法制备了负载零价铁（ZVI）的膨胀石墨（EG-ZVI）.利用扫描电子显微镜（SEM）、X射线衍射（XRD）仪及X射线光电子能谱仪（XPS）等对负载及反应前后的EG-ZVI进行表征,探究了EG-ZVI对硝酸根（NO₃^-）的去除效果并对其反应产物及机理进行了分析.结果表明,亚微米级零价铁已负载到EG石墨表面,且分布均匀;与EG相比,EG-ZVI对NO₃^-的去除能力显著提升,其去除率是EG的2.3倍.得益于铁碳原电池效应,EG-ZVI对pH依赖性比零价铁低,即使在pH=9的条件下,NO₃^-去除率依然能达到65%以上,是单独用零价铁处理时的1.83倍;EG-ZVI去除NO₃^-是吸附和还原过程共同作用的结果,符合三级动力学模型,其还原过程由负载在EG表面的零价铁发生腐蚀提供电子,从而还原NO₃^-产生以NH₄⁺-N为主的含氮化合物;EG-ZVI对NO₃^-具有较强的还原吸附作用,并能解决零价铁在反应过程中生成惰性层或金属氢氧化物导致去除效率低的缺陷,使其在含NO₃^-废水的处理中具有较高的应用潜力.     

Radiative auger effect and extended X-ray emission fine structure (EXEFS).

Radiative Auger spectra are weak X-ray emission spectra near the characteristic X-ray lines. Radiative Auger process is an intrinsic energy-loss process in an atom when a characteristic X-ray photon is emitted, due to an atomic many-body effect. The energy loss spectra correspond to the unoccupied conduction band structure of materials. Therefore the radiative Auger effect is an alternative tool to the X-ray absorption spectroscopy such as EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near Edge Structure), and thus it is named EXEFS (Extended X-ray Emission Fine Structure). By the use of a commercially available X-ray fluorescence spectrometer or an electron probe microanalyzer (EPMA), which are frequently used in materials industries, we can obtain an EXEFS spectrum within 20 min. The radiative Auger effect, as an example, demonstrates that the study on atomic many-body effects has become a powerful tool for crystal and electronic structure characterizations. The EXEFS method has already been used in many industries in Japan. Reviews about the applications and basic study results on the radiative Auger effect are reported in this paper.     

Auger decay of molecular double core-hole state

We report on theoretical Auger electron kinetic energy distribution originated from sequential two-step Auger decays of molecular double core-hole (DCH) state, using CH(4), NH(3), and H(2)CO molecules as representative examples. For CH(4) and NH(3) molecules, the DCH state has an empty 1s inner-shell orbital and its Auger spectrum has two well-separated components. One is originated from the 1st Auger transition from the DCH state to the triply ionized states with one core hole and two valence holes (CVV states) and the other is originated from the 2nd Auger transition from the CVV states to quadruply valence ionized (VVVV) states. Our result on the NH(3) Auger spectrum is consistent with the experimental spectrum of the DCH Auger decay observed recently [J. H. D. Eland, M. Tashiro, P. Linusson, M. Ehara, K. Ueda, and R. Feifel, Phys. Rev. Lett. 105, 213005 (2010)]. In contrast to CH(4) and NH(3) molecules, H(2)CO has four different DCH states with C1s(-2), O1s(-2), and C1s(-1)O1s(-1) (singlet and triplet) configurations, and its Auger spectrum has more complicated structure compared to the Auger spectra of CH(4) and NH(3) molecules. In the H(2)CO Auger spectra, the C1s(-1)O1s(-1) DCH → CVV Auger spectrum and the CVV → VVVV Auger spectrum overlap each other, which suggests that isolation of these Auger components may be difficult in experiment. The C1s(-2) and O1s(-2) DCH → CVV Auger components are separated from the other components in the H(2)CO Auger spectra and can be observed in experiment. Two-dimensional Auger spectrum, representing a probability of finding two Auger electrons at specific pair of energies, may be obtained by four-electron coincidence detection technique in experiment. Our calculation shows that this two-dimensional spectrum is useful in understanding contributions of CVV and VVVV states to the Auger decay of molecular DCH states.     

Reminiscences with Martin P. Seah

Starting in the mid-1960s, the detection and display of peaks in Auger electron spectroscopy (AES) were improved by using modulation of the electron energy analyzer coupled with electron detection using a lock-in amplifier. This allowed a derivative of the electron energy distribution, N(E), to be obtained directly at the output of the lock-in amplifier thereby removing most of the effect from the relatively large, slowly varying, electron background signal due to secondary and backscattered electrons. For relatively low modulation amplitudes, the peak-to-peak intensity of the Auger features increased linearly with modulation amplitude (for a deflection-type analyzer), improving the signal-to-noise ratio. However, with relatively large modulations, the Auger peak shapes distorted, and the peak-to-peak heights eventually decreased in size, and this nonlinearity would cause problems in quantitative analysis. A universal curve was developed for singlet Auger peaks to approximate corrections due to this peak distortion, but an approach to exactly correct for such distortions was largely ignored by the AES community. This approach was called Dynamic Background Subtraction and is even relevant today as some Auger instruments using modulation and lock-in amplifiers are still being manufactured. This review paper describes approximate and exact corrections for modulation effects in AES data.     

X-ray and electron induced Auger processes for I2 adsorption on uranium

The accurate determination of the kinetic energy of X-ray induced Auger electrons, which is necessary in XPS experiments, e.g. for calculating the Auger parameter, is sometimes hampered by peak interferences or by the high secondary electron background. The latter is of special importance for low kinetic energy electrons like e.g. the U(OPV) and U(OVV) Auger electrons. These problems can be circumvented by using electron induced Auger transitions (AES). However, since XPS and AES use different reference points for the energy scales, both scales have to be matched. This can be done by measuring the kinetic energy of an appropriate Auger transition in XPS and relating this value to the maximum of the second derivative of the same peak in AES.     

Projection of Si 1s photoexcited orbitals into resonant Auger electron spectra in KLL decays of Si(CH3)4 and SiF4

Spectator resonant KL(23)L(23) Auger electron spectra have been measured in the Si 1s photoexcitation region of Si(CH(3))(4) using monochromatized undulator radiation combined with a hemispherical electron spectrometer. The broad peak with high intensity in a total ion yield spectrum, coming mainly from excitation of a 1s electron into the 6t(2) vacant orbital, induces a spectator Auger decay in which the excited electron remains in its excited orbital. The component on the higher energy side of this peak through 1s excitation into a Rydberg orbital produces resonant Auger decays in which the excited Rydberg electron moves into a slightly higher Rydberg orbital, or is partly shaken up to a significantly higher Rydberg orbital. These findings of Si(CH(3))(4) indicate a clear contrast to those for SiF(4), in which the 1s excitation into a Rydberg orbital induces a shake-down phenomenon as well as a shake-up one. The results of these molecules exhibit a clear splitting effect among excited orbitals which are smeared out by overlapping due to lifetime widths and due to densely populated levels in the 1s electron excitation spectrum. This is consistent with the calculation on photoexcitation within the framework of density functional theory.     

Application of positron annihilation induced Auger Electron Spectroscopy to the study of surface chemistry

Positron annihilation induced Auger Electron Spectroscopy (PAES), makes use a beam of low energy positrons to excite Auger transitions by annihilating core electrons. This novel mechanism provides PAES with a number of unique features which distinguishes it from other methods of surface analysis. In PAES the very large collisionally induced secondary electron background which is present under the low energy Auger peaks using conventional tecniques can be eliminated by using a positron beam whose energy is below the range of Auger electron energies. In addition, PAES is more surface selective than conventional Auger Spectroscopy because the PAES signal originates almost exclusively from the topmost atomic layer due to the fact that the positrons annihilating with the core electrons are trapped in an image correlation well just outside the surface. In this paper, recent applications of Positron Annihilation Induced Auger Electron Spectroscopy (PAES) to the study of surface structure and surface chemistry will be discussed including studies of the growth, alloying and inter-diffusion of ultrathin layers of metals, metals on semiconductors, and semiconductors on semiconductors. In addition, the possibilities for future application of PAES to the study of catalysis and surface chemistry will be outlined.     

Experimental auger electron spectrum of ammonia

The first electron excited Auger electron spectrum for gas-phase ammonia has been obtained and is compared with an existing theoretical prediction. Decomposition of the spectrum using a gaussian approximation reveals close agreement in both intensity and energy for the higher energy components provided the theoretical data is shifted toward lower energies by ≈ 2.7 eV. As the transitions involve progressively deeper final state levels there is a general trend of increasing widths and increasing discrepancy towards lower energy with respect to their predicted positions. Such behavior has been seen in other experimental—theoretical comparisons of gas-phase Auger results and while the variation in widths is interpreted in terms of transitions to a manifold of final state vibrational levels, the nature of the differential shift in the lower energy components is unclear.     

SCF discrete-variational Xα calculations of the NH4Cl Auger spectrum

Formalism for calculations of the singlet and triplet Auger energies within the Xα model is presented. Using this method the KVV Auger spectrum of NH₄Cl is calculated and the theoretical intensities are determined within a one-center approximation. The calculated spectrum reproduces quite well all the main features observed for solid NH₄Cl. A new interpretation of the spectrum is suggested.     

X-ray absorption and resonant Auger spectroscopy of O2 in the vicinity of the O 1s-->sigma* resonance: experiment and theory

We report on an experimental and theoretical investigation of x-ray absorption and resonant Auger electron spectra of gas phase O(2) recorded in the vicinity of the O 1s-->sigma(*) excitation region. Our investigation shows that core excitation takes place in a region with multiple crossings of potential energy curves of the excited states. We find a complete breakdown of the diabatic picture for this part of the x-ray absorption spectrum, which allows us to assign an hitherto unexplained fine structure in this spectral region. The experimental Auger data reveal an extended vibrational progression, for the outermost singly ionized X (2)Pi(g) final state, which exhibits strong changes in spectral shape within a short range of photon energy detuning (0 eV>Omega>-0.7 eV). To explain the experimental resonant Auger electron spectra, we use a mixed adiabatic/diabatic picture selecting crossing points according to the strength of the electronic coupling. Reasonable agreement is found between experiment and theory even though the nonadiabatic couplings are neglected. The resonant Auger electron scattering, which is essentially due to decay from dissociative core-excited states, is accompanied by strong lifetime-vibrational and intermediate electronic state interferences as well as an interference with the direct photoionization channel. The overall agreement between the experimental Auger spectra and the calculated spectra supports the mixed diabatic/adiabatic picture.     

Summary of ISO/TC201 standard: ISO/TR 18394:2006—surface chemical analysis—Auger electron spectroscopy—derivation of chemical information

ISO/TR 18394 provides guidance for the identification of chemical effects on x‐ray or electron‐excited Auger electron spectra as well as for applications of these effects in chemical characterization of surface/interface layers of solids. In addition to elemental composition, information can be obtained on the chemical state and the surrounding local electronic structure of the atom with the initial core hole from the changes of Auger electron spectra upon the alteration of its local environment. The methods of identification and use of chemical effects on Auger electron spectra, as described in this Technical Report, are very important for accurate quantitative applications of Auger electron spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.     

Auger photoelectron coincidence spectroscopy using synchrontron radiation

The technique of Auger-photoelectron coincidence spectroscopy (APECS) is described and illustrated with a case study of the Cu(100) 3p and M₂₃VV spectra. APECS offers many advantages over the conventional singles spectroscopy such as isolating overlapping spectral features, reducing secondary electron background, and revealing new decay modes. In the coincidence Cu Auger spectra discussed here, the multiplet structure of the quasi-atomic 3d⁸ Auger final state is clearly observed, as well as different intensities for the multiplet components for the p_1/2 and p_3/2 transitions. Furthermore, the spectra reveal evidence for a Coster-Kronig decay channel for 3p_1/2 core holes, and illustrate that the sum of the Auger electron and photoelectron kinetic energies is conserved. Possible technical improvements that can increase the counting efficiency are also discussed.