Results of a joint auger/ESCA round robin sponsored by astm commitree E-42 on surface analysis: Part I. Esca results

We report results of a round robin involving binding-energy (BE) and relative-intensity measurements on high-purity samples of gold and copper by X-rayWe have conducted an extensive round robin involving ESCA or XPS measurements on high-purity samples of Ni, Au, and Cu. The Ni sample was used to estabThis round robin had three principal objectives. First, it was intended to assess the overall accuracy of BE and intensity measurements in XPS measuremSecond, it was intended to establish an empirical intensity calibration of one type of XPS instrument versus another from the reported intensity ratiosThird, it was desired to determine the peak position and peak shape for the carbon 1s photoelectron line originating for “adventitious” carbonaWe have made an extensive analysis of trends in the BE and intensity data. We have computed deviations of individual BE measurements from the median vaThe two types of plots just described based on BE and intensity data can be regarded as “response functions” that describe the performance of indivWe have invoked Occam's Razor to distinguish instruments with response functions which vary monotonically with electron energy from those instruments wThe reported spreads in BE and relative-intensity data reported here indicate that improved calibration and operating procedures are required for XPS m     

Quantitative Auger electron spectroscopy analysis with co-evaporated AuCu films

A method for the quantitative Auger electron spectroscopy (AES) analysis by using a co-evaporation technique is extended to the AuCu system following the previous work. The calibration curves for lower Auger energy have peaks at 60 eV for Cu and at 69 eV for Au, and for higher Auger energy peaks at 239 eV for Au and at 920 eV for Cu. It is found that a simple linear relation does not exist in the results for AES measurements and the bulk analysis by atomic absorption spectroscopy (AAS) because of the back-scattering effect and the overlap of the spectra at lower energies in the Au-Cu system. It is also found that the adsorption of oxygen caused by electron beam bombardment has a significant influence on the AES results. The calibration curves obtained after a correction for oxygen adsorption are successfully applied to the determination of the composition at the surface of a sputtered AuCu alloy.     

The surface composition of the nickel-copper alloy system as determined by Auger electron spectroscopy

The Ni-Cu alloys were prepared by evaporation of the specpure metals in UHV onto a quartz substrate. Spectra were obtained from clean as well as from gas covered surfaces. The Auger signal intensity of a monolayer of both metals was determined for the low energy electrons (102–105 eV) and for the high energy electrons (716–920 eV). The overlapping peaks of Cu and Ni in the low energy region (102–105 eV) were evaluated by comparing them with computer simulated alloy spectra. The results of the sintered alloys are interpreted by means of a model by Gallon and Jackson, using the experimentally determined signal intensity of a monolayer. Several surface enrichment data were used to predict the experimentally observed Auger signal intensities. A clear indication of surface enrichment of Cu was obtained; this is in good agreement with previous conclusions based upon hydrogen adsorption and work function measurements. An explanation is suggested why previous work with AES and CO chemisorption did not reveal any surface enrichment.     

The surface composition of the gold-palladium binary alloy system

The surface composition of the AuPd alloy system has been determined by Auger electron spectroscopy. Measurements were performed on polished polycrystalline alloy foils. After cleaning, the intensities of the 71 eV and 2024 eV gold Auger transitions, and the intensity of the 330 eV palladium transition were measured, and then converted to atom concentrations in the surface layer. The surfaces of the annealed samples were found to be significantly enriched in gold with respect to the bulk. This result disagrees with the regular solution theory prediction. After extensive sputtering of the AuPd alloys with 1.5 keV Ar⁺ ions, a slight surface enrichment with palladium was found, as predicted by the simple theoretical model for sputtering.     

Inelastic effects and structure in the auger electron emission spectra of V2O5(010) and V(100) surfaces: Study of chemical shifts

Characteristic ionization losses and plasma losses occurring in the AES spectra of V₂O₅ (010) and V(100) surfaces are discussed. The former are used to evaluate chemical shifts of the VL₂ and VL₃ levels in V₂O₅. Values of approximateley 2.5 eV are found. These are smaller than the values obtained with ESCA (± 5 eV). It is described how the electron beam used in AES is thought to be responsible for this effect. ESCA data from partly reduced V₂O₅ samples tend to confirm the proposed model, based on oxygen loss and decomposition of V₂O₅ single crystals under the influence of the electron beam.The plasma losses of the larger Auger peaks are discussed. It is shown how some fine structure in the spectra can be partly explained by their presence. The plasma losses were simulated with numerical techniques based on the use of a signal averager.Signal averaging, curve fitting and related numerical techniques improve the resolution of AES spectra. Spectra of V₂O₅(010) and V(100) obtained in this manner are discussed. With respect to the transitions involving the valence band it is shown that the complex valence band structure is one of the causes of the observed discrepancies between theoretical and experimental AES data. Furthermore there is an uncertainty concerning the way in which ionization correction should be applied in this case. This correction is thought to increase with the degree of localization of the valence electrons.Auger peak intensities in function of the primary energy were found to show a maximum at about 3.5 times the critical potential, as was expected from theory if a moderate amount of backscattering is taken into account. Finally the intensity variations of the Auger peaks under continuous electron bombardment show the rate of oxygen loss at a V₂O₅ surface due to the primary beam.     

Electron-induced KLL Auger electron spectroscopy of Fe,Cu and Ge

KLL Auger spectra excited by electrons with energies in the 30–35 keV range of Fe, Cu and Ge films were measured, using thin free-standing films. It was possible to obtain spectra with an energy resolution of about 1 eV. The observed spectra can not be described satisfactorily by just the multiplet splitting of the final state as calculated for an isolated atom. Additional features, due in part to intrinsic (shake satellites) and in part to extrinsic (energy loss of the escaping electron) processes formed a large fraction on the observed intensities. In particular a number of distinct satellite structures that are not predicted by the atomic Auger process are observed. For Fe and Cu the satellite peaks can be explained in terms of shake-up processes from the 3d_5/2–4d_5/2 states. Similar satellite structures observed in Ge are partly attributed to plasmon creation and partly to shake-up processes. It is demonstrated that both the thickness dependence of the observed intensity distributions and transmission electron energy loss measurements contain invaluable information for the interpretation of these spectra.     

Stability of ZnO{0 0 0 1} against low energy ion bombardment

The steady-state surface compositions of the polar (O and Zn terminated) faces of ZnO{0 0 0 1} produced by low energy (0.3-2 keV) Ar⁺ ion bombardment were studied by Auger electron spectroscopy and electron energy loss spectroscopy. The alterations produced by the ion bombardment using different ion energies were monitored by calculating the intensity ratios of the low and high energy Zn Auger peaks (59 eV and 994 eV, respectively); Zn and O Auger peaks (59 eV and 510 eV, respectively). Based on the dependence of these ratios on the ion energy and termination of the surface, we could conclude that the stability of the Zn face is higher against the low energy argon ion bombardment-induced compositional changes than that of the O face.     

Intensity and energy calibration in AES: The effect of analyser modulation

To a large extent, Auger electron spectra (AES) are recorded and portrayed in the differential mode using the potential-modulation method with a phase-sensitive detector. The precise intensities of the peaks and their energy positions depend on the modulation amplitude in a nonlinear manner. This dependence, in turn, is also a function of the resolution and detailed construction of the analyser. Thus, accurate quantification and accurate assignments of peak energies depend on a detailed knowledge of the effect of analyser modulation in AES. The theoretical response of spectrometers to a Gaussian line is here analysed and compared with measurements for “singlet” peaks using several different commercial spectrometers. This shows that, in certain circumstances, the traditional methods of intensity and energy measurement can be transferred with accuracies as high as 3% and 0.05 eV, respectively. A method for calibrating the effective modulation amplitude, vital for accurate quantification, is presented in the following paper.     

An AES study of surface segregation of AgAu alloys with ion bombardment and annealing

Equilibrium segregation and selective sputtering in the surface of AgAu alloys have been investigated systematically with argon ion bombardment and with annealing by means of AES measurements. Slight enrichment of Ag was observed on the alloy surfaces after the annealing of the alloys at 550°C, while relatively large enrichment of Au was observed on the ion-bombarded surfaces with the use of Au (240 eV) and Ag (300 eV) Auger electrons. With the aid of other Auger electrons with different escape lengths, it was found that the concentration varies with distance from the surface within the sampling depth of the Auger electrons. On the basis of the above facts, the depth profiles were proposed for the annealed and the ion-bombarded surfaces. The uppermost surface layer is enriched more with Ag than the apparent AES observations on both the ion-bombarded and the annealed surfaces. The proposed depth profiles on both the surface layers were compared with previous results by different authors.     

Quantitative AES and XPS: convergence between theory and experimental databases

Experimental spectral databases have been recorded for AES and XPS using fully calibrated instruments. These instruments have been calibrated so that the spectra have the true shape and peak area intensities may be integrated to give absolute yields for AES and relative yields for XPS. Removal of all the backgrounds requires care but may be completed by using information from both databases. The resultant yields may be compared with theory. The correlations for AES are the more complex and involve the total intensities for all transitions originating in each shell. The correlations are excellent using significant changes to the traditional approach. These involve the use of the Casnati et al. ionisation cross section and the restriction of the number of electrons for use in the inelastic mean free path calculations to electrons of 14 eV or less binding energy in the s, p or d sub-shells. The average ratio of experiment to theory is 1.04 with a standard uncertainty of the mean of 4%. Results for XPS are excellent using Scofield’s ionisation cross section together with the above rules for the inelastic mean free path calculations. Improvements for certain elements are still needed for removing the inelastically scattered Auger and photoelectrons in both databases. To assist analysts in using such databases a simpler measure of Auger electron intensity is developed involving differential spectra broadened with a Gaussian function of 15–20 eV width. The peak-to-peak intensities from these broadened spectra are reasonably closely related to the peak area of the direct spectra except in a few exceptional cases. The unbroadened differential spectra show strong contributions from the spectrometer resolution and changes in the chemical state which are avoided by the spectral broadening. To simplify calculations for the analyst when studying homogeneous materials by AES and XPS, the relative sensitivity factors are re-defined to be for an average matrix instead of the pure element. This leads to a matrix-less equation for calculating compositions from the spectra.     

Quantitative aes: the problems of the energy dependent phase shift and modulation amplitude and of the non-ideal behaviour of the channel electron multiplier

In pursuit of the objective to obtain identical Auger electron spectra from different types of instruments, the Varian 10 keV CMA system has been studied. On one particular unit it is shown that the high frequency of 17 kHz, chosen for modulating the spectrometer to produce differential spectra, causes two major errors, each of which affects the relative intensities of peaks in the spectrum. Firstly, it is shown that the phase of the AC signal changes by 56° through the spectrum to 2000 eV, and secondly the amplitude of the AC modulation varies by 100% over the same range. The latter causes low energy peaks to be reduced to half of their true intensity, but the former can cause intensities at either high or low energy to be reduced even more strongly. It is recommended that instruments should be tested and operated, if necessary, at a lower frequency such as 1.7 kHz at which the problems should disappear. A further factor affecting intensities in both the direct and differentiated spectra is the operation of the bias control for the cone of the channel electron multiplier. Field penetration of the cone causes an unwanted behaviour. It is recommended that the cone be fitted with a transparent wire mesh to remove this field penetration and then the bias may be operated at the required potential of at least 200V.     

Energieverlustspektren der Alkalihalogenide und der Metalle Cu,Ag und Au und Vergleich mit optischen Messungen

Energy loss spectra of 50 keV electrons have been measured in transmission of the alkali halides and of Cu, Ag, and Au at zero scattering angle. These spectra are compared with the quantity ?Im(1/ε) determined from optical measurements. The general agreement is good, especially in the low energy loss region. In the case that the foil thicknesses could be measured, the absolute intensity of ?Im(1/ε) is found to be equal or up to three times larger than from optical data. In addition to the sharp low energy peaks (excitons), characteristic sharp peaks are found at higher energies: 60 eV for the Li-halides, 33 eV for the Na-halides, and 20 eV, 17 eV, and 13 eV for the K-, Rb-, and Cs-halides respectively. These peaks are very little shifted by the different anions in the crystal lattice and may be due to excitons caused by alkali band transitions. We have further examined several peaks in the loss spectra of LiF, KCl, KBr and the Na-halides: 1. The angular dependence of the loss intensity is in agreement with theory. 2. The dispersion of the peaks is much smaller than for plasma losses. 3. The temperature dependence of the peak energy has been found quite different for different peaks.     

The backscattering factor for systems with a non-uniform surface region: Definition and calculations

It has been recently shown that the backscattering factor (BF) in Auger electron spectroscopy (AES) noticeably depends on the in-depth structure of the surface region. This is a particularly important problem in sputter depth profiling monitored by AES since the signal intensity cannot be described with a single BF value. The BF depends on the removed amount of material and thus varies with sputtering time.In the present work, the definition of the BF is generalized to extend its applicability to systems with an in-depth composition profile. The generalized definition of the BF was applied to the special case of a depth profile, i.e. a buried thin layer. It has been shown that the BF for this case is expressed by the excitation depth distribution function (EXDDF) which is equivalent to the “Phi-Rho-Z” function used in electron probe microanalysis (EPMA). Different algorithms for calculating the BF are discussed. Calculations of the BF for buried layer were performed for the Ag M₄N₄₅N₄₅ Auger transition in a thin layer of silver located at different depths in three matrix materials: Si, Cu, and Au. It was found that, indeed, the BF noticeably varies with the depth of the layer in the analyzed volume, although the extent of this variation depends on the matrix material. For Si, the variation is observed for the lowest primary beam energies considered, i.e., 1 and 2 keV. For Cu, a distinct depth dependence of the BF is visible at 10 keV and lower energies, while for Au, the BF varies with depth even at the highest considered energy, i.e. 30 keV.     

Segregation at CuAu alloy surfaces

Experimental results for Auger measurements on clean evaporated CuAu alloy films having (111)-orientation are presented. Signals from Auger transitions at 72 eV, 239 eV, and 2024 eV from Au in the alloys were normalized to signals from pure Au references. The experimental data were converted to atomic layer compositions using a model which allowed the first two atomic layers to differ in composition from the bulk and using estimates of the energy dependence of the electron attenuation length derived from published results. Significant enrichment of the first layer with Au was found over the entire range of composition studied.     

Comparison of escape depths between Auger electron and disappearance potential spectroscopy electron

The escape depths of the characteristic electrons of the Auger electron and the quasielastically reflected electron were determined by Auger electron spectroscopy (AES) and disappearance potential spectroscopy (DAPS), respectively, for a Cr overlayer onto Ti and Fe substrates. For the case of Cr on Fe, in-situ measurements of AES and DAPS were carried out. From the results, the mean free paths of 455, 575 and 710 eV electrons through Cr were obtained as 9.6, 13 and 15 Å, respectively. The attenuation length of a 2.5 keV primary electron of AES through Cr was also obtained and the value was 62 Å. In addition, the mean free paths of electrons with the same energy depend on the scattering materials of Cr, Mo and W (material dependence). The phenomena are useful for a quantitative electron spectroscopy of surfaces.     

Electron escape depth in silicon

The ESCA electron escape depth in silicon is determined from the peak areas in the electron spectra from evaporated thin films. For electron energies in the region 320 eV to 3.6 keV values from 13 to 83 Å are obtained. The escape depth in silicon dioxide is determined for the energies 1.6 and 3.6 keV. Binding energies and Auger energies are determined in silicon and silicon oxides.     

Secondary ion and Auger electron emissions from Ar+-ion-sputtered FeAl alloys

Some FeAl alloys, and pure Al and Fe samples, are sputtered in ultrahigh vacuum with Ar⁺ ions between 4 and 15 keV. As previously observed with CuAl alloys, the intensity of the principal Al Auger peak at 63.5 eV is a parabolic function of the Al concentration. Symmetric collisions Al → Al are thus much more efficient for Auger emission from pure aluminium than asymmetric collisions Ar → Al, the proportion of which among effective collisions hardly reaches 20% at 15 keV. Whatever the initial ion energy, the intensities of singly charged atomic secondary ions Al⁺ and Fe⁺, normalized to pure metals, are equal to the atomic concentration of the corresponding elements, whereas the normalized intensities of the multiply charged ions Al²⁺ and Al³⁺ are roughly equal to the square of Al concentration. These results agree with the occurence of two mechanisms in intrinsic ion emission: an electronic excitation process during the separation of the outgoing particle from the target (singly charged ions) and a collisional process from the symmetric collisions Al → Al only, with multiple Auger de-excitation outside the target (multicharged ions of light elements).     

Scaling properties of azimuthal anisotropy in Au+Au and Cu+Cu Collisions at sqrt[s NN]=200 GeV

Differential measurements of elliptic flow (v2) for Au+Au and Cu+Cu collisions at sqrt[sNN]=200 GeV are used to test and validate predictions from perfect fluid hydrodynamics for scaling of v2 with eccentricity, system size, and transverse kinetic energy (KE T). For KE T identical with mT-m up to approximately 1 GeV the scaling is compatible with hydrodynamic expansion of a thermalized fluid. For large values of KE T mesons and baryons scale separately. Quark number scaling reveals a universal scaling of v2 for both mesons and baryons over the full KE T range for Au+Au. For Au+Au and Cu+Cu the scaling is more pronounced in terms of KE T, rather than transverse momentum.     

The effect of the earth's magnetic field on auger analysis

Semi-quantitative analyses of thin films or surfaces are commonly obtained from the peak intensities in the differentiated Auger spectrum. To reduce effects of surface roughness, beam focus and electron current, ratios of peak heights are used rather than absolute values. In performing analysis of CdSe single crystal and thin film samples in a commercial Auger analyzer fitted with a cylindrical mirror analyzer (CMA), the ratio of the Cd(376 eV) to Se(1315 eV) peaks was found to vary by as much as 15% when the diameter of the incident electron beam was increased from 5 to 60 μm. The effect was found to be due to an energy-dependent shift of the electron beam caused by the earth's magnetic field. The electron transmission of the CMA was measured as a function of the primary electron beam spot position on the sample. The transmission decreases rapidly once the spot falls outside an area with a radius ～25 μm. Due to this response, the relative shift in position caused by the magnetic field produces variations in peak ratios when the spot size is changed. This effect will produce inaccurate analysis if the Auger peaks differ significantly in energy and the primary electron beam spot size is large, and accounts for the observed 15% variation in $\text{Cd}\text{Se}$ ratio.     

An Auger and X-ray photoelectron spectroscopic study of sodium metal and sodium oxide

Photoelectron and Auger electron measurements have been made on polycrystalline films of sodium metal evaporated in ultra high vacuum, and on Na₂O produced by in-situ oxidation by dry oxygen. Most of the spectra were recorded using Mg Kα (1254 eV) radiation but excitation by 5 keV electrons or monochromatized Al Kα (1487 eV) X-rays was used for specific purposes. Core and valence electron binding energies, photoionization cross-sections relative to Na 1s, KLL and KLV Auger energies and transition probabilities are reported. Energy losses in the metal and oxide are discussed and the relative intensities of surface and bulk plasmon losses have been used to calculate mean electron escape depths in the metal. When corrections were made for experimental geometry, escape depths of 10 Å at 180 eV and 31 Å at 1200 eV were obtained. An escape depth of 23 Å at 980 eV was obtained by Na 1s-Na K-Auger intensity correlation and this is consistent with the plasmon data. Data on Auger satellite lines are presented and, in particular, evidence has been obtained which indicates that a high energy satellite should not be attributed to a plasmon gain mechanism. Valence band influences on the KLV Auger spectra are discussed with reference to the XPS spectrum and other sources of valence band information. Unexpected structure was found in the KLV spectra of the metal which, pending thorough interpretation, offsets the sensitivity and resolution advantages which these spectra otherwise offer for valence band studies.