Quantitative AES Research Articles

Quantitative AES. V. Practical analysis of intensities with detailed examples of metals and their oxides

A study is made of the metal and oxygen Auger electron intensities of a range of metals and their stoichiometric oxides from the high-resolution digital Auger electron database. The changes in peak shape for the oxygen Auger electrons is analysed and it is shown that, for a constant oxygen peak area, the direct spectrum peak heights and the differential peak-to-peak intensities each scatter over factors of 3 and 5, respectively. However, by broadening the spectra with a Gaussian function of up to 20 eV width, the scatters in the differential peak-to-peak heights can be reduced to a relative standard deviation of ∽5%. The broadening and differential method is not as accurate as a full analysis of peak area with all the necessary corrections, but it is simple and rapid and is about five times more accurate than existing simple differential procedures. The use of the broadened differential peak-to-peak intensities will need new sensitivity factors for quantification, which may be derived from traditional values and the data presented here. The relative intensities of the L2,3M2,3M2,3, L2,3M2,3M4,5, L2,3M4,5M4,5 metal peaks for Ti to Zn in the metallic and oxide states are also analysed and are shown to correlate well with theoretical predictions. However, small differences for the areas of these peaks between the metallic and oxide states are seen, leading to a relative standard deviation of 2.9% in the intensity ratios. These differences correlate with a simple model for the effect of oxygen on the effective number of valence electrons available in the metal M4,5 level contributing to the relevant Auger electron transitions. This results in a small change in the relative intensities of the metal peaks on oxidation. Measurements for the oxygen Auger electrons show a complementary effect. These results show that the relative area of a single intense peak in the multiplet structures for the metals and for oxygen varies by typically 2.9% and 6.7%, respectively, when normalized against the respective total peak areas. These uncertainties add to the overall uncertainties in quantifying data in AES analysis where a single peak in the group is used, e.g. in analysis using traditional differential spectra. © 1998 John Wiley & Sons, Ltd.

Quantitative AES. VI. Backscattering and backgrounds—an analysis of elemental systematics and corrections of absolute intensity

An analysis is made of the backscattered background in true spectra from an AES digital Auger database in order to understand the systematics with atomic number. This is important for modelling details of the spectra and for determining the peak areas for Auger electron intensities. It is shown that the shape of the background in the emission energy range near 2500 eV, for spectra using electron beams of 5 or 10 keV energy, are accurately described by an exponential. This exponential has a characteristic energy of 1988 eV for 3⩽Z⩽83, for a 5 keV beam, but is a more complex function of Z, the atomic number, for a 10 keV beam. The absolute intensities at 2000 eV emission energy for the 5 keV beam and 2500 eV emission energy for the 10 keV beam are described by analytical functions, the deviations from which are 5% and 6%, respectively, attributed mainly to the effects of sample roughness. These deviations may be used to correct the measured databank Auger electron intensities generated using 5 keV and 10 keV electron beams. The ratio of these corrected intensities agrees with theory to 0.99±0.05. Without the roughness correction derived from this study of backgrounds, this ratio deteriorates to 1.02±0.07. © 1998 John Wiley & Sons, Ltd.

Determination of the inelastic mean free path of electrons in GaAs and InP after surface cleaning by ion bombardment using elastic peak electron spectroscopy

Abstract The inelastic mean free path (IMFP) of electrons is an important material parameter needed for quantitative AES, EELS and non-destructive depth profiling. The distinction between the terms for IMFP and the attenuation length (AL) has been established by ASTM standards. A practical experimental method for determining values of the IMFP is elastic peak electron spectroscopy (EPES). In this method, experimentally determined ratios of elastically backscattered electrons from test surfaces and from a Ni reference standard are compared with the values evaluated theoretically. The present paper reports systematic measurements of the IMFP by EPES for GaAs and InP. They are carried out in two laboratories using two different electron spectrometers: a CMA in Budapest and DCMA in Warsaw. Prior to measurements, the samples were amorphized by high-energy Ar + ions (100–400 keV), and the surface composition was determined by quantitative XPS. Argon cleaning produces enrichment of samples in the surface layer in Ga (80%) and In (70%), respectively. The experiments refer to a such modified sample surface that was considered in Monte Carlo calculations. The experimental data were analyzed using calibration curves from Monte Carlo calculations which account for multiple elastic scattering events. This approach has been used previously for elemental solids and is now extended to amorphized binary compounds. The experimental values of IMFP obtained in both laboratories exhibited a reasonable agreement with the available literature data in the 0.1–3.0 keV energy range. With respect to the information depth of EPES, the experimental results refer to the bulk composition within a reasonable extent.

Chemical ordering and reconstruction of Pt 25Co 75(100): an LEED/STM study

The surface of a disordered Pt 25Co 75(100) alloy has been investigated using quantitative LEED, AES and UHV-STM at room temperature. Atomic-resolution images reveal that it reconstructs with close-packed rows shifted by half the interatomic distance, from hollow to bridge sites. The density of shifted rows increases with the surface Pt concentration, leading to (1 × 5), (1 × 6) and (1 × 7) patterns. Segregation and chemical ordering lead to the formation of c(2 × 2) domains between the shifted rows. Chemical resolution was achieved with STM: the apparent height of the Pt atoms in the STM topographs is about 0.1–0.4 Å above that of Co, whereas LEED shows that Pt atoms are geometrically ∼0.04 A ̊ higher. The composition was determined down to the fourth layer. An oscillatory segregation profile is observed, with Pt-rich layers (〈 C 1〉 = 62.6% Pt, 〈 C 3〉 = 53.5%) and Pt-depleted layers (〈 C 2〉 = 6.9%, 〈 C 4〉 = 2.7%). Chemical ordering is present in the third layer and the four-layer surface slab stabilises with a structure and a composition quite similar to that of the L1 2 PtCo 3 phase. As regards the composition and ordering of the top layer, there is a remarkable agreement between chemically resolved STM analysis and LEED analysis.

Calculated Elemental Sputter Yield Correction Factor in Quantitative Auger Electron Spectroscopy—A Poor Approximation

Ion bombardment usually leads to a change in the equilibrium sputtered surface composition of compound semiconductors. This change is matrix dependent. To compensate for this sputter-induced effect in quantitative AES, a sputter yield correction factor is usually employed. This factor is based on the relative component sputter yields of the atomic species in the compound. In an approximation of this factor, experimental elemental sputter yields have been commonly used. However, these experimental values do not always exist for all the atomic species. Therefore, this study used the Sigmund sputter theory to calculate the elemental sputter yields. Different nuclear stopping powers and α values were employed in the calculations. The AES-determined Ar+-sputtered surface compositions of several binary semiconductor compounds were obtained from a compilation of published data: for each compound the mean value was taken. A comparison between the experimental and predicted (using the sputter yield correction factor) surface compositions was not good, indicating that this approximation to the sputter yield correction factor in quantitative AES is not the correct one. © 1997 John Wiley & Sons, Ltd.

Surface Composition of the CoPd Alloys Studied by Electron Spectroscopies

Palladium-based alloys and bimetallic systems are of importance in heterogeneous catalysis. Recently, attention was devoted to studies of the catalytic properties of the Co-Pd binary system. An important issue to consider is the surface composition of this alloy. The aim of the present work was to determine the surface composition of the polycrystalline CoPd alloys in a wide range of constituent concentrations (Co30Pd70 at.%, Co50Pd50 at.% and Co70Pd30 at.%) at temperatures up to 900°C. Quantitative AES and XPS analyses were used for this purpose. To determine the surface composition, the values of the inelastic mean free path (IMFP) for the studied alloys are necessary. These were derived from the elastic peak intensity. The reported values of the IMFP for Pd are in agreement with the data available in the literature. Reasonable consistency of the IMFP values for CoPd alloys and Co was also observed. The surface compositions of the CoPd alloys at room temperature determined by AES and XPS are in good agreement. It has been found that Pd segregates to surfaces of the Co30Pd70 and Co50Pd50 alloys at temperatures above 300 °C.

Applications of quantitative AE methods: Dynamic fracture, materials and transducer characterization : 37483 Sachse, W. Solid Mechanics Research for Quantitative Nondestructive Evaluation, Proceedings of the ONR Symposium on Solid Mechanics Research for QNDE, Northwestern University, Evanston, Illinois (United States), 18–20 Sep. 1985 pp. 41–66. Martinus Nijhoff Publishers (1987)

Applications of quantitative AE methods: Dynamic fracture, materials and transducer characterization : 37483 Sachse, W. Solid Mechanics Research for Quantitative Nondestructive Evaluation, Proceedings of the ONR Symposium on Solid Mechanics Research for QNDE, Northwestern University, Evanston, Illinois (United States), 18–20 Sep. 1985 pp. 41–66. Martinus Nijhoff Publishers (1987)

Universality Classes of Inelastic Electron Scattering Cross-sections

Quantitative XPS and AES analysis of surfaces requires models to do fast calculations of the energy loss of electrons at any energy as they move in solids of general and non-uniform composition. This is possible with the Universal cross-section that was introduced a decade ago. A critical review is given of the Universal cross-section in the light of the results of research carried out since then. In particular its validity is compared to that of experimental cross-sections determined by analysis of reflected electron energy-loss spectroscopy (REELS) spectra. It is shown that for applications in quantitative surface analysis by XPS and AES, the solids can be divided into classes according to the full width at half-maximum of the dominating shape of the inelastic scattering cross-section. The Universal cross-section is quite accurate for solids with a cross-section width 20 eV. For solids with a cross-section width of 10–15 eV, the Universal cross-section is still fairly good for the description of the far-peak region (30 eV from the peak energy) but it is less accurate to account for the near-peak region (10 eV from the peak energy). For solids with a cross-section width 5 eV, the REELS cross-section is always more accurate than the Universal cross-section. A Three-parameter Universal cross-section was defined, which gives a good fit to narrow experimental cross-sections. Parameters for different solids were determied and it was shown that the polymers form a separate universality class. © 1997 by John Wiley & Sons, Ltd.

Determination of Al xGa 1− xAs Auger sensitivity factors

Al x Ga 1− x As Auger sensitivity factors have been determined by using PHI610 scanning Auger microprobe with pure elemental standards Al, Ag and matrix GaAs. The quantitative results of Al x Ga 1− x As measured by the present method are in very good agreement with X-ray double crystal measurements. It is shown that by using sensitivity factors obtained from the self-instrument, the accuracy of the quantitative AES analysis can be considerably improved compared with that using elemental relative sensitivity factors given by the PHI handbook or internal standard method.

Monte Carlo model of electron elastic scattering in solids

An analog Monte Carlo model is presented simulating electron elastic scattering in solids and capable to treat problems of quantitative AES and XPS. Its analytical and numerical verification is described. The model is used to investigate the dependence of the elastic reflection coefficient on the inelastic mfp. It was found that the slope decreases with increasing inelastic mfp. The single elastic scattering coefficient is shown to be proportional to the total, not to the inelastic, mean free path. An analytical expression of the single elastic scattering coefficient is derived and supported by Monte Carlo calculations.

Quantitative AES Analysis of Fracture Surfaces in the Presence of O- and Ccontaining Residual Gas

Quantitative AES Analysis of Fracture Surfaces in the Presence of O- and Ccontaining Residual Gas

Quantitative AES Analysis of Fracture Surfaces in the Presence of O- and C-containing Residual Gas

The chemical composition of thin segregation layers is calculated using elemental Auger spectra and considering the adsorption of residual gas components during analysis. We take into account that the coverage with oxygen is different for the different elements in the thin layer and that the shape of the Auger peaks is changed by the coverage. Application of the method is demonstrated for quantitative analysis of in situ fractured grain boundaries of B-doped Ni 3 Al bicrystals.

Use of the Peak-to-background Ratio for Quantitative Auger Analysis of Semi-insulating Polycrystalline Silicon Layers

The work presents the results of quantitative evaluation of Auger electron spectra utilizing the heights of Auger peaks normalized with respect to the background in N(E) vs. E distribution curves. The quantification method used was verified on a series of semi-insulating polycrystalline silicon samples with a variable content of oxygen. The results obtained were compared with those yielded by other procedures of quantitative AES, and by secondary neutral mass spectrometry.

Recent developments in quantitative AE : Sachse, W. Proceedings of the IUTAM Symposium on Elastic Waves and Ultrasonic Nondestructive Evaluation, University of Colorado, Boulder, Colorado (United States), 30 Jul. – 3 Aug. 1989. pp. 285–294 Edited by S.K. Datta, J. D. Achenbak and Y.S. Rajapakse. North - Holland, (1990). ISBN 0 444 874 852

Recent developments in quantitative AE : Sachse, W. Proceedings of the IUTAM Symposium on Elastic Waves and Ultrasonic Nondestructive Evaluation, University of Colorado, Boulder, Colorado (United States), 30 Jul. – 3 Aug. 1989. pp. 285–294 Edited by S.K. Datta, J. D. Achenbak and Y.S. Rajapakse. North - Holland, (1990). ISBN 0 444 874 852

Surface reactivity of the borided Pd(111) with respect to hydrogen, ethyne and ethene

In the present AES-LEED-TDS study, the surface reactivity of the borided Pd(111) sample exposed to H 2, C 2H 2 or C 2H 4 at 300 K was investigated. Quantitative AES surface analysis allowed determination of the concentration of boron. Interaction of hydrogen and the hydrocarbons with clean and borided Pd(111) surfaces has been compared. The results were interpreted in terms of the catalytic role of surface boron on palladium.

Monte Carlo simulation of background in AES: a comparison with experiment

Monte Carlo simulations of the energy distribution of backscattered electrons including cascade secondary electrons have been carried out. This calculation yields the background observed in direct mode by AES. We found that, based on a dielectric model of electron inelastic scattering and secondary excitation, the simulation describes very well the background of the experimental EN( E) spectra for Si, Cu, Ag and Au measured with a novel cylindrical mirror analyzer which has been developed by Goto for providing standard AES spectra. The results reveal that the Monte Carlo simulation has potentiality for providing an intimate insight into more accurate theoretical background for the quantitative AES study and for establishing the data base of standard spectra.

Recent progress in quantitative and high spatial resolution AES

Progress in high resolution Auger electron spectroscopy (HR-AES or scanning Auger microscopy, SAM) during the past few years is characterized by the use of efficient field emission electron sources, parallel detection capabilities and improved data acquisition, storage and processing, thus enhancing spatial resolution (to about 10 nm), signal to noise figure and quantification of elements in different chemical bonding states, e.g. by routinely using factor analysis. Optimized ion sputtering facilities, particularly sample rotation, enable depth profiling with high, depth independent resolution. The basic features of SAM are discussed with respect to EPMA (electron probe micro-analysis), emphasizing fundamental limitations and future developments.

Matrix factors affecting quantitative analysis of AES for binary alloys

To improve the accuracy of quantitative AES analysis, matrix effects disturbing the linearity between the intensity of an Auger signal and the atomic concentrations have to be considered. Matrix factors accounting for these effects refer to (1) electron back-scattering, (2) attenuation length, (3) Auger line shape, (4) initial surface roughness, (5) sputter-induced surface roughness during depth profiling, (6) preferential sputtering, (7) elemental sensitivity and (8) atomic density. Measurements are presented on three systems of binary alloys (Ag/Cu, Ni/Cr and Pb/Sb), whose elemental concentrations range from 0 to 100%. They indicate that the relation between the measured and the true concentration can be expressed by a single formula with two parameters describing summarily the various matrix effects mentioned above. We derive the separation of the contributing factors (1) to (8) from the equation of the Auger electron intensities for a binary system, and their inclusion into the two-parameter approach mentioned. Values for these parameters are empirically estimated for the systems under investigation. The significance of individual factors (1) to (8) in these systems is discussed. The matrix factors mentioned do not fully describe all experimental data obtained from these alloys.

Practical interest of multicomponent standards for quantitative AES

AbstractIn this paper, it is first emphasized that the parameters of the classic expression of the Auger signal can be grouped into three terms that are independent of each other: the first relative to recording conditions, the second to an i‐element Auger transition and the last to sample chemistry. Consequently, it is shown that, using a relevant multicomponent standard set, it is possible to free quantitative AES of alloys from experimental conditions. The set of standards has to contain all the elements of the sample; each standard has to contain at least one element of the sample and one element common with another standard. By using the described method, a library of mulitcomponent standard spectra can be created, which can easily be used by other laboratories.

DETERMINATION OF AlxGa1-xAs AUGER SENSITIVITY FACTORS

AlxGa1-xAs Auger sensitivity factors have been determined by using PHI 610 scanning Auger microprobe with pure elemental standards Al, Ag and matrix GaAs. The quantitative results of AlxGa1-xAs measured by the present method are in very good agreement with that by X-ray double crystal measurement. It is shown that by using sensitivity factors obtained from the self-instrument, the accuracy of the quantitative AES analysis can be considerably improved compared with that using elemental relative sensitivity factors given by the PHI handbook or internal standard method.