Auger Peak Intensities Research Articles

Method for correcting peak overlaps in quantitative Auger electron spectroscopy of Cr-containing oxides

This paper provides a new method for improving the quantitative accuracy of Auger electron spectroscopy (AES) when measuring the surface composition of mixed metal corrosion oxides that contain chromium (Cr). Without this correction, the Cr Auger peak intensity (in the conventional first derivative display mode) is attenuated by its overlap with the oxygen (O) Auger line, leading to significant relative errors (∼30%–70% too low) in the Cr content. A peak fitting method was used to derive basis spectra in the direct mode display (before differentiation) and simulate a wide range of O to Cr ratios. A mathematical function was derived to define a Cr intensity correction factor, as a function of the O/Cr ratio. This method can be conveniently applied to data sets containing many spectra, such as depth profiles. Once corrected, the composition depth profiles obtained by AES are consistent with x-ray photoelectron spectroscopy profiles obtained from similar areas. The approach presented can also be used to generate correction factors for overlapping peaks in the Auger spectra of other material systems.

An almost knowledge-free approach to XPS intensity evaluation where use of atomic photoemission cross sections suffices for yielding material-specific inelastic background

A general procedure for peak intensity evaluation on the inelastic background of X-ray photoelectron spectra (XPS) has been developed, which is made up of tasks in three nested levels, level I, II and III. The task in level I is a simple background optimization for the case when the inelastic mean free path (IMFP, denoted λ(E)) and other constraints are given in advance [M. Jo, Surf. Interface Anal., 2003, 35, 729–737]. In order to find appropriate constraints, a batch job, which is a series of single jobs with systematically varied constraints, is constructed and iteratively solved in level II by improving the constraints from the viewpoint of how successfully the background is subtracted. It is found that, by the task in level II, for polycrystal Cu metal, the spectrum after inelastic background subtraction still contains a baseline component proportional to E−k, where E (>50eV) is the electron kinetic energy and the exponent k around 2.4. This baseline is attributed to the high-energy tail of primary excitation spectrum of true secondary electron emission. The relative core intensities thus determined are in good agreement with the Scofield’s photoexcitation cross sections. The relation of core and Auger peak intensities is consistent as demonstrated by the fact that the sum of Cu 2p and 2s equals that of all Cu LMM peaks. The level III calculation is designed for the general case where λ is not available before the analysis. The relative energy dependence of λ except for the absolute value can be estimated by iterating the level II tasks with updated λ(E), based on the assumption that the peak intensities should be proportional to Scofield’s values. The relative deviation of thus determined λ(E) in level III from those by the well-known calculation (Tanuma, Powell, and Penn, TPP) is less than 1.7% between E=600 and 2000eV if their values at 800eV are assumed to coincide. The normalized peak shapes and relative intensities after background subtraction in this level are also very similar to those obtained in level II starting with λ(E) by TPP that is currently known to be reliable. These findings indicate that an almost knowledge-free analysis, which is free from the detail of the solid’s properties such as chemical composition and nevertheless retains the material-dependent results, became possible for the first time, as long as one keeps away from the discussion of absolute intensity.

Influence of Subsurface Oxygen in the Catalytic CO Oxidation on Pd(111)

The formation of palladium subsurface oxygen, i.e., Pd(111) saturated with oxygen below the surface, has been demonstrated. This compound is much more stable than palladium surface oxide or bulk palladium oxide, which easily reacts with CO at elevated temperatures. The catalytic activity of an atomically clean Pd(111) surface and of Pd(111) affected by subsurface oxygen (coverage Θ = 0.00–0.46 ML) was studied during the CO oxidation reaction. The measurements were performed for a temperature range of 353–523 K, partial pressures of reactants in the range of 10–7–10–5 mbar, and different subsurface oxygen content. A distinct, previously not observed feature in the dependency of the reaction rate vs temperature was found. Furthermore, it was found that subsurface oxygen reduces the catalytic activity of the Pd(111) surface significantly, which is attributed to reduction of the sticking coefficient of CO and oxygen. In the temperature range applied for investigation of catalytic CO oxidation, the rate of the formation of subsurface oxygen is negligible. However, the formation of this phase starts to be pronounced above ∼600 K. The presence of subsurface oxygen was confirmed by the KLL Auger transition excited by monochromatic X-ray radiation, not by electron beam. It was observed that the classical Auger measurement with an electron beam destroys the subsurface oxygen compound. The combination of the CO titration with Auger peak intensity measurements allows the quantitative analysis of the subsurface oxygen. The problem of subsurface oxygen detection and its impact on the adsorption of CO and oxygen on Pd(111) are discussed. It was suggested that subsurface oxygen and surface oxide can be manufactured simultaneously. Then the obtained activity in the CO oxidation reaction will be a result of two competitive effects, i.e., promoting from surface oxide and inhibiting from subsurface oxide. This might be a source of the discrepancy in results on the influence of surface oxide on the CO oxidation reaction measured by different researchers.

Accurate quantification of Cu(In,Ga)Se2 films by AES depth profiling analysis

Quantitative analysis of Cu(In,Ga)Se2 (CIGS) films with non-uniform depth distributions was investigated by Auger electron spectroscopy (AES) depth profiling. The atomic fractions of the CIGS films were measured by relative sensitivity factors determined by a total number counting method from a CIGS reference film certified by isotope dilution–inductively coupled plasma/mass spectrometry. In the AES depth profiling analysis of the CIGS films, the intensities of Auger electron peaks in Cu, In, Ga and Se were determined by integrating the individual Auger peak intensities in the whole depth range of the AES depth profiles. The atomic fractions measured by AES analysis were linearly proportional to the certified values. The uncertainty in the AES depth profiling analysis of CIGS films was much smaller than that in the secondary ion mass spectrometry depth profiling analysis and that in the international comparison of national metrology institutes for the quantification of Fe–Ni alloy films.

Surface state of Y3(Al,Ga)5O12:Tb phosphor under electron beam bombardment

The surface state of Y3(Al,Ga)5O12:Tb phosphor was determined before and after 27h of prolonged electron beam exposure. The electron irradiation was carried out at a base pressure of 2.3×10−8Torr and an oxygen pressure of 1.0×10−6Torr. New surface layers were formed after the chemisorbed species were removed as a result of electron stimulated surface chemical reactions. The rate of the removal of the chemisorbed species from the phosphor's surface during prolonged electron irradiation was affected by the background working atmosphere as measured with Auger electron spectroscopy combined with cathodoluminescence (CL) spectroscopy. The CL intensity of the Y3(Al,Ga)5O12:Tb stabilized after removal of the chemisorbed species and stayed constant during further electron irradiation, indicating that this phosphor is good for the technology used in field emission displays. There was an increase in the Al, Y, O and Tb Auger peak intensities pointing to the formation of a complicated surface structure that was probably a combination of more than one chemical compound. X-ray photoelectron spectroscopy results suggested electron-beam induced formation of new interleave oxide layers, such as AlOx, YOx and Y(Al,Ga)O3 on the surface. These oxide layers acted as a protective layer inhibiting further CL intensity degradation during prolonged electron irradiation.

Approach to Quantitative Evaluation of Electron-Induced Degradation of SiO2 Film Surface with Different Amounts of Carbon Contaminations

Effects of carbon contaminations existing on the SiO2 film surface on the electron-induced damage of SiO2 were investigated. Carbon contaminations, the amount of which is only ∼0.05 nm thickness, are found to act as a protective layer for the degradation. The changes in the elemental and oxide Si-LVV Auger peak intensities due to the electron-induced damage are confirmed to be reasonably described by two- and one-step decomposition models, respectively. The critical dose, at which a certain amount of the damage occurs, is larger for the higher beam current density of primary electrons and the larger initial amount of carbon contaminations. The present results confirmed that the dependences of the degradation on the amount of carbon contaminations and the beam current density are attributed to the diffusion of oxygen and recombination of broken bonds between Si and oxygen, which are enhanced by the increase in the local temperature due to the electron irradiation. Consequently, the small amount of carbon contaminations was found to strongly affect results of the quantitative Auger electron spectroscopy analysis of metal oxides when the electron-induced damage of samples occurs. The present analytical approach, where the critical doses are measured for the SiO2 surfaces with different amounts of carbon contaminations, is effective to evaluating and estimating the electron-induced damage. [DOI: 10.1380/ejssnt.2011.277]

Theoretical aspects of studies of oxide and semiconductor surfaces using low energy positrons

This paper presents the results of a theoretical study of positron surface and bulk states and annihilation characteristics of surface trapped positrons at the oxidized Cu(100) single crystal and at both As- and Ga-rich reconstructed GaAs(100) surfaces. The variations in atomic structure and chemical composition of the topmost layers of the surfaces associated with oxidation and reconstructions and the charge redistribution at the surfaces are found to affect localization and spatial extent of the positron surface-state wave functions. The computed positron binding energy, work function, and annihilation characteristics reveal their sensitivity to charge transfer effects, atomic structure and chemical composition of the topmost layers of the surfaces. Theoretical positron annihilation probabilities with relevant core electrons computed for the oxidized Cu(100) surface and the As- and Ga-rich reconstructed GaAs(100) surfaces are compared with experimental ones estimated from the positron annihilation induced Auger peak intensities measured from these surfaces.

Metal thin-film nanophases and their interface with silicon

Composition, electron density and morphology of metal thin-film nanophases were studied by AES, EELS, AFM and conductivity measurements during MBE deposition of Fe and Co on Si(100) and (111), respectively. AES data demonstrated a layer by layer growth of Fe with a segregation of submonolayer coverage of Si at T = 20 °C and, after annealing at T = 250 °C, a fixed and increased value of the Si-to-Fe Auger-peak ratio in the ranges of d = 0.03−0.12 or 1.2 nm and 0.3−0.6 nm, respectively. EELS spectra indicated a redistribution of valence electrons at the Fe/ Si(100) interface at d = 0.03−0.12 nm. With further increase of Fe thickness, EELS spectra showed transitions to Fe nanophases with lowered concentration of valence electrons near d = 0.12−0.3 nm and d = 0.6−1.2 nm. After annealing AFM images showed the relief stability of stepped Si(100) surface in the range of d = 0.03−0.3 nm, the disappearance of the stepped relief type after 0.3 nm and formation ridge-like islands in Fe film at d = 1.2 nm. Auger peak intensity and conductivity versus the thickness for nanophases of Co on Si(111) showed variations of growth mechanism in accordance with variations of interface layer state.

AES depth profiling with a tilted sample in front of a cylindrical mirror analyzer: quantification and profile shape changes according to an extension of the conventional MRI model

AbstractThe influence of the tilt angle of a sample in front of a cylindrical mirror analyzer (CMA) on AES depth profiles is calculated with the conventional mixing‐roughness‐information (MRI) depth model and an extended MRI model. While the conventional model works with an average electron escape depth value, the extended model takes into account the intensity from different segments along the azimuth angle corresponding to different escape depth values before summing up for the total, measured intensity. The deviation between both approaches is generally less than 4%, even for the worst case at 47.7° tilt angle. The shape of the profile is slightly different for both approaches. Because, for a CMA with coaxial gun, the sample tilt angle varies as the electron beam incidence angle, the influence of the latter has to be additionally taken into account for quantification of AES. In reasonable agreement with experimental results it is shown that above 45° the Auger peak intensity of Cu (914 eV) increases up to about a factor of two for an incidence angle of 85°. Copyright © 2006 John Wiley & Sons, Ltd.

Many-electron effect in the Si K-LL resonant Auger-electron spectroscopy spectra of the Si delta layer in GaAs

The Si K-LL resonant Auger-electron spectroscopy (RAES) spectra of silicon delta dopped layers in GaAs with very thin capping layers show both normal Auger decay and resonant Auger decay, when the core-level electron is excited to the conduction band. The resonant Auger peak kinetic energy (KE) shows no dispersion with photon energy, except when excited by the highest energy photons [M.D. Jackson, J.M.C. Thornton, D. Lewis, A. Robinson, M. Fahy, A. Aviary, P. Weightman, Phys. Rev. B71 (2005) 075313]. The RAES spectra are analyzed using a many-body theory. The presence of resonant Auger decay and no dispersion of resonant Auger peak KE with photon energy is explained in terms of the relaxation of a metastable excited core-hole state to a stable one on the time scale of core-hole decay. The excited electron in the conduction band either delocalizes rapidly leaving the ionized Si to decay by a normal Auger decay or drops to a state localized in the Si delta layer before the core-hole decays so that the RAES spectrum has both normal Auger decay and resonant Auger decay. As a result of the relaxation, the resonant Auger peak KE does not show any dispersion with photon energy. The variations with photon energy of the normal or resonant Auger peak intensity, KE, and width are explained in a consistent manner by a many-body theory.

VALENCE ELECTRONIC STRUCTURE OF OXYGEN-MODIFIED α-Mo2C(0001) SURFACE: ANGLE-RESOLVED PHOTOEMISSION STUDY

Adsorption of oxygen on α- Mo 2 C (0001) is investigated with Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and angle-resolved photoemission spectroscopy (ARPES) utilizing synchrotron radiation. It is found that C KLL Auger peak intensity does not change during O 2 exposure, indicating that the depletion of C atoms does not proceed. It is deduced from ARPES and LEED results that adsorbed oxygen atoms from a well-ordered (1 × 1) lattice on the α- Mo 2 C (0001) surface. The ARPES study shows that oxygen adsorption induces a peculiar state around Fermi level (E F ). Off-normal-emission measurements prove that the state is a half-filled metallic state.

Positron trapping at quantum-dot-like particles on metal surfaces

Measurements of the positron annihilation-induced Auger electron (PAES) spectra from the Fe–Cu alloy surfaces with quantum-dot-like Cu nanoparticles embedded in Fe reveal a decrease of the Fe M 2,3VV positron annihilation-induced Auger signal intensity and an enhancement of the Cu one for surfaces created by enriching the Cu content of the Fe–Cu alloy. These experimental results are analyzed by performing calculations of positron surface states and annihilation characteristics at the Fe(1 0 0) surface with quantum-dot-like Cu nanoparticles embedded in the top atomic layers of the host substrate. Estimates of the positron binding energy and annihilation characteristics reveal their strong sensitivity to the nanoparticle coverage. Theoretical core annihilation probabilities are compared with experimental ones estimated from the measured Auger peak intensities. The observed behavior of the Fe and Cu PAES signal intensities is explained by theoretical calculations as being due to trapping of positrons in the regions of Cu nanoparticles embedded in the top atomic layers of Fe.

Formation of Sb submonolayer phases on high index Si(5 5 12) surface

This work is the first report on the submonolayer adsorption of antimony (Sb) on the high index Si(5 5 12) surface, studied in UHV by in situ surface sensitive probes such as Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and low energy electron diffraction (LEED). The Auger growth curve which is a plot of the Sb(MNN)/Si(LVV) Auger peak intensity ratio with deposition time, provides a calibration of a deposition rate of 0.06 ML/min. A coverage of 0.2 ML is obtained by four different routes (a) adsorption at room temperature (RT), (b) adsorption at high substrate temperature (HT) of 680 °C, (c) annealing the RT adsorbed surface to 800 °C and (d) annealing the high substrate temperature (680 °C) adsorbed surface to 800 °C. The (2 2 5) facets are observed for the RT adsorbed system at coverage of 0.2 ML, while the pathway adopted to attain the 0.2 ML coverage by adsorption at elevated temperature shows two different atomic arrangements in the formation of the (3 3 7) facets. The HT adsorbed surface suggests the formation of nano-wire-like features while the 0.2 ML coverage obtained by annealing the HT as well as RT adsorbed surface suggests an anisotropic Sb atomic arrangement.

A comparison between Auger and photoelectron emission detection in an X-ray standing-wave analysis of adsorbates

We studied the contribution of secondary electrons emission to the Auger intensity when the adsorption edge is low compared to the photon energ y. We performed a normal incidence X-ray standing-wave (XSW) experiment comparing photoemission K 2s and Auger K-LVV yields of Ni(1 1 1)- p(2×2)K system. The analysis of both profiles permits us to estimate that 5% of the total secondary electrons contributes to the K-L 23VV excitation strongly modifying the line-shape of the potassium Auger profile. In this case, the use of the Auger peak intensity profile as a monitor during an XSW experiment for surface structural studies gives completely unreliable results.

Growth types and surface topography of Co, Cu and Co/Cu multilayers studied by AES and STM/SFM

A thin film of cobalt, copper and Co/Cu multilayers deposited on Si(1 0 0) has been studied by an in situ combination of Auger electron spectroscopy and scanning tunnelling/scanning force microscopy. We show that thickness-dependent Auger peak intensity measurements, taken in situ during deposition of constituents of Co/Cu multilayers, combined with microscopy can bring valuable information about growth type of the system components.

Studies of positron states and annihilation characteristics at the reconstructed (1 0 0) surface of GaAs

The high-resolution positron-annihilation-induced Auger spectrum from GaAs(1 0 0) displays six As and three Ga Auger peaks below 110 eV corresponding to M 4,5VV, M 2M 4V, M 2,3M 45M 45 Auger transitions for As and M 2,3M 4,5M 4,5 Auger transition for Ga. PAES data is analyzed by performing quantum mechanical calculations of positron surface states and annihilation characteristics for As- and Ga-rich reconstructed (1 0 0) surfaces of GaAs. It is shown that by comparing theoretical positron annihilation probabilities for relevant core electrons with experimental ones estimated from the measured Auger peak intensities, it is possible to identify the chemical composition of the topmost layers of the GaAs(1 0 0) surface.

Growth and dissolution kinetics of Au/Pb(1 1 1): an AES-LEED study

The growth of Au on a Pb(111) surface is studied by AES-LEED at room temperature (RT). After deposition of 1/3 Au monolayer (ML), LEED observations reveal a p(√3×√3)R30° superstructure. Beyond this coverage, no superstructure is observed. From Au and Pb Auger peak intensities, we deduce that the growth corresponds to the formation of an inter-metallic compound AuxPby continuously growing under a floating Pb ML. The dissolution kinetics of one Au ML recorded at various temperatures systematically show a plateau (a slowing down) at a surface concentration corresponding to 1/3 ML. From a quantitative evaluation of the AES data we propose that the p(√3×√3)R30° superstructure corresponds to a surface alloy with composition AuPb2 buried under 1 Pb ML. Such a surface alloy has been previously measured after annealing of a Au(Pb) 0.45at.% solid solution [Surf. Rev. Lett. 4 (1997) 1139], we propose that the slowing down observed during the dissolution kinetics is also the signature of this unexpected segregation behaviour of Au. It could be related to the fact that the surface energy of this inter-metallic compound (AuPb2) is lower than the surface energy of both constitutive elements (Au and Pb).

Quantitative AES depth profiling of a Ge/Si multilayer structure

AbstractAuger electron spectroscopy (AES) depth profiling of a Ge/Si multilayer structure with Ge layers of 2.2, 4.3 and 2.2 nm thickness in Si was performed with Ar+ ions of 1.5 and 3 keV energy and at 74° incidence angle to the normal to the surface. The plot of the Ge LMM Auger peak intensity as a function of the sputtering time was analysed quantitatively using the MRI (mixing–roughness–information depth) model. As a result, the optimum MRI parameters w (mixing length), σ (roughness) and λ (here, used as information depth) were found to be 1.7, 0.8 and 1.6 nm, respectively, for 1.5 keV Ar+ bombardment. For an appropriate evaluation of the 3 keV Ar+ data, w had to be increased to 3.0 nm and σ to 1.4 nm, respectively, while λ was kept constant. In the first run, the MRI calculation results could not be fitted optimally to the profiling results, and therefore the assumed in‐depth distribution of Ge had to be changed accordingly. After a few modifications of the latter and of the roughness parameter, an optimized fit was obtained by changing either the thickness of the layers or the average Ge concentration within the layers, or, most favourably, by a combination of both approaches. Comparison with densitometer measurements on transmission electron microscopy images confirm the thicker layer structure. Copyright © 2002 John Wiley & Sons, Ltd.

Electron stimulated carbon adsorption in ultrahigh vacuum monitored by Auger electron spectroscopy

Electron stimulated carbon adsorption at room temperature has been studied in the context of radiation induced surface modifications in the vacuum system of particle accelerators. The stimulated carbon adsorption was monitored by Auger electron spectroscopy (AES) during continuous irradiation by 2.5 keV electrons and simultaneous exposure of the sample surface to CO, CO2, or CH4. The amount of adsorbed carbon was estimated by measuring the carbon Auger peak intensity as a function of the electron irradiation time. Investigated substrate materials are technical oxygen-free electrolytic copper and TiZrV nonevaporable getter thin film coatings, which are saturated either in air or by CO exposure inside the Auger electron spectrometer. On the copper substrate electron induced carbon adsorption from gas phase CO and CO2 is below the detection limit of AES. During electron irradiation of the nonactivated TiZrV getter thin films, electron stimulated carbon adsorption from gas phase molecules is detected when either CO or CO2 is injected, whereas the CH4 partial pressure has no influence on the carbon Auger peak intensity evolution.

Studies of the Auger spectrum from the (100) surface of GaAs using positron annihilation induced Auger electron spectroscopy

Measurements of the first high-resolution positron annihilation induced Auger spectrum from GaAs(100) are presented. The spectrum displays six As and three Ga Auger peaks below 110 eV, including a strong As M 4,5VV peak at 28 eV and a less intense Ga M 2,3M 4,5M 4,5 peak at 53 eV. The Auger peak intensities are used to obtain experimental annihilation probabilities for relevant core-level electrons. Experimental results are compared with first-principles calculations of positron surface states and annihilation characteristics of surface trapped positrons.