Escape Depth Of Auger Electrons Research Articles

Surface dynamics of graphite probed by soft x-ray linear dichroism in Auger-emission yield

Element-specific investigation of the dynamics of surface atoms is not a trivial task. Here, the authors show that time-resolved near-edge x-ray absorption for strongly anisotropic excitation resonances -- combined with Auger-electron detection -- yields such information, whereby the small Auger-electron escape depth provides surface sensitivity. The method is applied to highly oriented graphite in a pump-probe scheme, based on femtosecond laser excitation and x-ray probe pulses with variable polarization direction and delay time, both of which influence transient changes in the $K\phantom{\rule{0}{0ex}}V\phantom{\rule{0}{0ex}}V$ Auger yield.

Modification of the surface and bulk electronic properties of ytterbium nanofilms under the action of adsorbed CO molecules

The modification of the surface and bulk electronic properties of ytterbium nanofilms under the action of adsorbed CO molecules has been investigated. It has been shown that adsorption of these molecules is accompanied by a fundamental rearrangement of the electronic structure of the films. This rearrangement is initiated by the formation of a donor-acceptor adsorption bond by a lone electron pair localized at the carbon atom of the CO molecule. The formation of this bond is accompanied by the lowering of the Yb 5d level below the Fermi level. After this lowering reaches a considerable value, the 4f electrons start to transfer to the 5d level; i.e., the 4f14 → 4f13 transition occurs. The transition opens a way to the N45N67N67 Koster-Kronig supertransition in the Auger spectra. As a result of these processes, the peaks observed in the spectra before adsorption of the CO molecules disappear and new peaks appear. Therefore, the adsorption of the CO molecules brings about qualitative changes in the Auger spectra of ytterbium. It has been found that the rearrangement of the ytterbium electronic structure due to the action of CO molecules involves both the surface of the ytterbium films and their near-surface layers. The thickness of these layers exceeds the Auger electron escape depth and can reach more than 11 monolayers.

Auger investigations of thin SiC films

Problems of the evaluation of Auger depth profiles of thin SiC layers are caused by a number of effects. These are coverage with an adsorbate layer, preferential sputtering, the change of the peak shapes by chemical bonding states and the broadening of the interfaces by atomic mixing, Auger electron escape depth and original surface, interface and sputtering induced roughness. These effects are investigated and their contribution to the degradation of the depth profile is considered. Atomic mixing simulations including electron escape depth correction are able to reproduce the Auger depth profiles. In special cases the simulation must be convoluted with a resolution function caused by roughness. Thus quantitative conclusions about the layer structure, film composition, impurity distribution etc. are possible.

Elastic contribution to the high-energy Auger electron appearance potential spectrum

Abstract The elastic contribution of Auger electrons to the appearance potential spectroscopy (APS) signal has been studied both experimentally and theoretically. The analytical approach is based on a solution of a kinetic equation satisfying the corresponding boundary conditions. An analytical expression for the total contribution of Auger electrons, which are not scattered inelastically, has been derived. The Auger electron yield has been shown to be a universal function of the ratio of the inelastic to the transport mean free paths of signal electrons and the inelastic mean free path of primaries. The overlayer technique has been used to investigate the mean escape depth of Auger electrons escaping from a target without losing a considerable amount of their kinetic energy. For this purpose a high-energy APS signal has been recorded as a function of the cut-off energy and the overlayer thickness. Ti and Ni were chosen as the overlayer and the substrate materials respectively and the Ti and Ni L-subshells were excited. The experimental data compare well with the corresponding analytical results. In particular, it was found that due to the attenuation of the primary beam the mean escape depth of the elastic APS signal electrons turned out to be smaller, by a factor of two, than that of the same Auger electrons in the usual Auger electron spectroscopy (AES). As a result the surface sensitivity of the high-energy APS may be higher by a factor of about two compared to AES. © 1997 Elsevier Science B.V.

AES depth profiles of thin SiC-layers - simulation of ion beam induced mixing

Thin carbonized SiC layers were investigated by Auger depth profiling. If the layers are smooth, ion induced mixing and Auger electron escape depth are the dominant factors that degrade the depth profiles. With the help of the Monte Carlo simulation code T-DYN by Biersack and an escape depth correction program, simulations can describe the measured Auger depth profiles. For quantitative comparison of simulations and measurements the Auger data must be quantified. The sputter time scale must be converted into depth using a known or best fitting sputter rate. The Auger peak-to-peak height must be converted into concentration using sensivity factors which do not include the sputter correction for selective sputtering. Since these sensitivity factors are normaly estimated at a sputtered single crystal, selective sputter correction must be excluded later. A good quantitative agreement was found for a simulated adsorbate/SiC/Si layer system with a measured Auger depth profile of a carbonized SiC layer on Si. This allows a description of the layer structure and of the composition of the layers.

Interpretation of Auger depth profiles of thin SiC layers on Si

Silicon carbide thin films, prepared by carbonization of Si-wafers are analysed by Auger depth profiling. The influence of atomic mixing is simulated with a Monte Carlo model. By using mixing simulations the dependence of the two mixing parameters (width of the mixing zone and recoil depth) on ion beam energy, incidence angle and ion mass can be calculated. For comparison of the simulated data with Auger measurements an Auger electron escape depth correction is necessary. The simulated and λ-corrected data of several layer structures show good qualitative agreement with Auger depth profiles of thin carbonized SiC-layers.

The interface between sputter-deposited gold thin films and ion-bombarded sapphire substrates

Pull-tests show that the adhesion strength of gold film sputter-deposited onto annealed and pre-sputtered sapphire is over 15 times higher than that of the film deposited onto as-received sapphire. The effects of ion beam bombardment on the sapphire substrate were investigated in situ with AES. Metallic aluminum was detected on the surface of sapphire substrates after irradiating for 3 min with 7 keV Ar +-ions. These results agree with TRIM calculations that yield preferential ion-beam etching. No metallic aluminum was formed at the surface when the ion bombardment was done at 3 keV for 3 min. The Au Al 2O 3 interface was investigated using a new technique that takes advantage of the variable film thickness in the vicinity of a sputter-etched crater. This method not only allows for a non-destructive search of the interface but it also avoids a substantial ion damage to this region. Higher energy Auger peaks, which have higher Auger electron escape depth, revealed that a AuAlO compound formed at the interface. Formation of this compound, which is responsible for the strong metal-ceramic bonding, is due to ion-induced cleaning and reduction of the sapphire surface prior to gold film deposition.

Effect of elastic scattering and cylindrical mirror analyser geometry on the evaluation of escape depths of auger electrons

AbstractWe have studied the effect of elastic scattering and analyser geometry on calculations of electron escape depths. A theoretical analysis of the electron escape depth has been presented that shows how it is related to the solid angle for detection and the geometrical configuration of a cylindrical mirror analyser. To estimate the extent to which elastic scattering can modify the values of the escape depth, Monte Carlo simulations of the emission of Auger electrons were carried out for Cu MVV (59 eV), Cu LMM (916 eV), Au NVV (238 eV) and Au MNN (2025 eV) signals. From the calculated depth distributions of signal electrons, we have derived a correction factor for the electron escape depth in terms of the corresponding inelastic mean free path which includes both the effects of elastic scattering and analyser geometry. We have found that elastic scattering reduces the value of the escape depth by several tens of per cent and, depending on the target, signal electron energy and analyser geometry, the ration between the escape depth and the inelastic mean free path varies from 0.4 to 0.6.

Atomic mixing, surface roughness and information depth in high‐resolution AES depth profiling of a GaAs/AlAs superlattice structure

AbstractHigh‐resolution AES depth profiling of a GaAs/AlAs superlattice structure with 8.8/9.9 nm single‐layer thickness by sputtering with 1.0 keV and 0.6 keV Ar+ ions at an incidence angle of 80° resulted in optimum depth resolution values for 0.6 keV Ar+ ions of Δz = 2.0 nm for the low‐energy Al (68 eV) intensity profile and of Δz = 2.0 nm for the high‐energy Al (1396 eV) intensity profile. A simple model description of the influence of atomic mixing (w), surface roughness (σ) and Auger electron escape depth (λ) was applied to calculate the measured profiles. An excellent fit is obtained by a reasonable set of these parameters: for 0.6 keV Ar+ ion sputtering, w = 1.0 nm, σ = 0.6 nm, λ (Al 68 eV) = 0.4 nm and λ (Al 1396 eV) = 1.7 nm.

Quantitative analysis of radiation-induced grain-boundary segregation measurements

Radiation-induced and precipitation-induced grain-boundary segregation profiles are routinely measured by scanning-transmission electron microscopy using energy-dispersive X-ray spectroscopy (STEM-EDS). However, radiation-induced grain-boundary segregation (RIS) profiles achieved at low and moderate temperatures are exceedingly narrow, typically less than 10 nm full width at half maximum. Since the instrumental spatial resolution can be a significant fraction of this value, the determination of grain boundary compositions poses a formidable challenge. STEM-EDS and Auger electron spectroscopy (AES) measurements are reported, performed on controlled-purity alloys of type 304L stainless steel irradiated with 3.4 MeV protons to 1 displacement per atom at 400°C. Because of statistical noise and the practical lower limit on the step size in STEM, deconvolution of the measured data does not yield physical results. An alternative analysis of STEM data is presented. Numerical calculations of RIS profiles are convoluted with the instrumental broadening function and modified iteratively to fit the data, yielding a “best estimate” profile. This “best estimate” is convoluted with the Auger intensity profile to yield a simulated AES measurement, which is compared with the actual AES measurement to provide an independent test of the validity of the “best estimate”. For impurities with a narrow segregation profile and an Auger electron escape depth of one monolayer, a combination of STEM and AES data allows a determination of the width of the segregated layer. It is found that, in an ultrahigh-purity alloy doped with P, the latter is essentially contained in a single monolayer.

Chemical composition by Auger sputter-depth profiles of GaAs oxide layers formed after chemical or Ar + ion etchings

The oxide formation on the GaAs(100) surface after different etching procedures has been studied using Auger sputter-depth profiles. The etching procedures were either chemical etching (H 3PO 4/H 2O 2/H 2O or NH 3/H 2O 2/H 2O) or low-energy (100 eV) Ar + ion beam etching. An attempt to a quantitative determination of the concentration profiles has been made using the sequential layer sputtering (SLS) model, taking into account the statistical nature of the ion sputtering and the Auger electron escape depth. The chemical composition and the oxide coverage on the GaAs surface have been deduced from the concentration profiles of the elements Ga, As and O. The oxide composition has been also studied after a thermal annealing and compared to an atmospheric native oxide.

Use of AES to determine sputter‐induced concentration profiles in Cu–Ni alloys

AbstractThe difference in escape depth of Auger electrons of different energies was used to determine sputter‐induced concentration profiles in Cu‐Ni alloys. Alloys with Cu/Ni concentrations of 70/30, 50/50 and 30/70 at.% were bombarded with 2 keV Ar+ ions. After equilibrium has been reached, an AES spectrum for the alloy was recorded using a 12‐bit analogue‐to‐digital converter interfaced with a computer. Spectra for the pure elements, Cu and Ni, were recorded under identical conditions and were used to generate Auger spectra for the alloys that could be compared with the experimental data. The layer concentration values for which the best fit was obtained were used to construct a concentration profile. The results indicate a Cu‐enriched first layer followed by a depleted zone. It is concluded that the bombardment of Cu‐Ni alloys by 2 keV Ar+ ions leads to the segregation of copper and that AES can be used to determine sputter‐induced profiles if the necessary experimental precautions are taken.

Comparison of AES analyses of transition‐metal nitride coatings with carrier‐gas heat extraction analyses

AbstractIn spite of some work on the quantification of Auger spectra of transition‐metal nitride thin films (hard coatings), a correlation with the true nitrogen content is rather difficult. In order to get a measure of the validity of the generally applied calibration of AES results by means of Handbook relative sensitivity factors, TiNx, ZrNx, NbNx and MoNx films of different nitrogen content were deposited by reactive magnetron sputtering on thin high‐purity platinum foils and were analysed subsequently for their nitrogen content by carrier‐gas heat extraction analysis (GA). The GA results were compared with the above‐mentioned AES evaluation. Whereas the agreement between GA and AES is fairly good for TiNx (based on a special AES evaluation procedure), it depends significantly on the use of either high‐ or low‐energy Auger peaks for Zr, Nb and Mo. Corrections of AES elemental sensitivity factors for changes in density, Auger electron escape depth and primary electron back‐scattering only show a minor influence in the direction of higher nitrogen contents. Comparison of GA and AES results yields sensitivity correction factors for the used metal peak energies in the systems studied.

Matrix effect in quantitative Auger analysis of binary alloys: Comparison between the measured and the calculated values

Matrix factors in quantitative AES analyses of binary alloys of AlNi, AuCu, AuNi, CuNi, CuPt and NiPt were studied. Using the in-situ scraping method, matrix factors were experimentally determined with an accuracy better than 5% or 10%. For comparison, matrix factors were calculated using the methods of: Reuter and Seah and Dench (R-SD); Reuter and Tokutaka, Nishimori and Hayashi (R-TNH); Reuter and Tanuma, Powell and Penn (R-TPP); Shimizu and Ichimura and Seah and Dench (SISD); SITNH and SITPP. By comparing the calculated values with the experimental results, we conclude that, the choice of the backscattering factor data, either after R or after SI, only marginally affects the resulting matrix effect despite the fact that their data differ considerably from each other. The most important effect stems from the choice of escape depth. When SD's data are adopted, the calculated values differ significantly from the experimental results except in the case of CuNi. By comparison, when TNH's data (in all of the studied cases) or TPP's data (except in the case of Al-Ni) are adopted, the calculated values are in good agreement with the observed values. This result also implies that the escape depth of Auger electrons depends on the materials in the manner suggested by TNH and TPP rather than the one suggested by SD.

An Auger investigation of the effects of rapid thermal annealing of GaAs on Si with or without intermediate Ge layers

Scanning Auger analyses have been performed on angle lapped GaAs/Ge, Ge/Si, GaAs/Si, and GaAs/Ge/Si structures grown by vacuum evaporation and metalorganic chemical vapor deposition techniques. Elemental Auger line scans are shown to have an inherent spatial resolution of 60 Å as a result of the angle of lapping, electron beam diameter, and Auger electron escape depth. To minimize the effects of artifacts such as residual lapping and Ar+ sputtering damage and electron backscattering, the interfacial width Wi for each species i, is defined by the difference (in Å) between the point where the Auger intensity is 75% of the full intensity in that layer (Ii=0.75Ii0) and the point where the Auger intensity is 25% of the full intensity in that layer (Ii=0.25Ii0). These structures have interfaces that are 60–150 Å wide. The GaAs/Si and GaAs/Ge interfaces are broadened due to As depletion near the interface, resulting in a total interface width larger than the interface width of any individual species. Rapid thermal annealing at 900 °C for 5 s, resulted in narrower interfaces and specifically reduced the As depletion.

Effects of Auger electron elastic scattering in quantitative AES

The Monte Carlo algorithm was developed for simulating the trajectories of electrons elastically scattered in the solid. The distribution of scattering angles was determined using the partial wave expansion method. This algorithm was used to establish the influence of Auger electron elastic collisions on the results of quantitative AES analysis. The calculations were performed for the most pronounced KLL, L 3 MM and M 5 NN Auger transitions. It turned out that due to the elastic collisions the Auger electron signal is decreased by up to 10%. The corresponding decreased of the escape depth of Auger electrons reaches 30% as compared with the value derived from the inelastic mean free path. The values of the inelastic mean free path resulting from the overalyer method may be strongly affected by elastic scattering of Auger electrons.

Segregation of Mg to the (0001) Surface of Single‐Crystal Alumina: Quantification of AES Results

The actual atomic concentration of Mg segregated to the basal plane of MgO‐doped single‐crystal alumina surfaces is calculated using an Auger electron spectroscopy quantification method which takes the backscattering factors and Auger electron escape depth into account. The results suggest that Mg segregates as effectively as Ca to AI2O3 interfaces. Plausible reasons why Mg has not been detected previously in the grain boundaries of MgO‐doped poly crystalline Al2O3, while Ca has always been found instead, are discussed.

Surface concentration changes in Cu-Zn alloys under ion bombardment

We have measured the depth profile of a Cu-48.2at%Zn alloy by means of Auger Electron Spectroscopy combined with Ar + ion bombardment. We have also measured the steady state surface concentration for different ion energies, and the transient variation of surface concentration when ion energies were changed from 0.5 to 5 keV and vice versa. We found that the effect of altered layer thickness and the Auger electron escape depth can produce an incorrect measurement of the preferential sputtering energy dependence. We also found that the presence of a strong preferential sputtering can shadow AES measurement of concentration variations along a depth profile.

Escape depth of auger electrons; the elastic scattering effect

We have performed Monte Carlo calculations of the contribution to KLL (1396 eV) and LVV (68 eV) aluminum Auger electrons peaks from the surface atomic layers. These allowed us to calculate the escape depth of Auger electrons considering the effects of elastic scattering among electrons and ion cores. We found some disagreement with the calculation based on an exponential attenuation law.

Escape depth of auger electrons; the elastic scattering effect

Escape depth of auger electrons; the elastic scattering effect