Elastic Peak Intensity Research Articles

Intercomparison of methods for separation of REELS elastic peak intensities for determination of IMFP

AbstractThe consistency and accuracy of inelastic electron mean free paths (IMFPs) determined from comparison of the intensity of elastically reflected electrons with theoretical calculations were studied. The variation with experimental geometry, spectrometer energy resolution and the procedure for background subtraction was studied. Four different types of spectrometers— double pass cylindrical mirror analyser (CMA) pre‐retarded CMA, hemispherical analyser (HSA) and retarding field analyser (RFA)— with widely different geometries and energy resolutions, placed in four different laboratories in three countries, were used. Four background subtraction methods (Shirley, linear, Tougaard and ELPSEP) were applied to isolate the elastic peak intensity from the reflected electron spectra. It was found that the IMFPs determined with data from the double pass CMA deviated considerably more (by a factor of 2) from the average values as well as from theoretical IMFP values than IMFPs determined with data from the other spectrometers. The data from the double pass CMA were therefore excluded from the statistical analysis. The root mean square (RMS) deviation of the IMFP from a function fitted to the data was 1.6–2.8 Å, depending on the background subtraction method, and it is smallest for the Tougaard method. The RMS deviation from IMFP values calculated by Tanuma et al. is 2.1–3.2 Å, again with the smallest value for the Tougaard method. The percentage deviation of the results using a Tougaard background from Tanuma et al. values is 12.6%. The results point to the conclusion that the major contribution to the inaccuracies in IMFPs determined with this method is not the background subtraction procedure but rather the lack of accuracy of the presently available models for elastic electron scattering, i.e. atomic elastic scattering cross‐sections and effects of crystallinity that are not included in the presently applied models. Copyright © 2001 John Wiley & Sons, Ltd.

Background subtraction III: The application of REELS data to background removal in AES and XPS

An analysis is provided of the use of reflected electron energy loss spectroscopy (REELS) data for the removal, by deconvolution, of the inelastically scattered electron background in Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) measurements in order to deduce the peak area intensities. Traditionally, the REELS data have either been used directly or the background intensity in the REELS data have been scaled by dividing by a factor A with respect to the elastic peak intensities, prior to the deconvolution. The scaling leads to an increase in the measured peak area approximately proportional to A. Measurements of the angular variation of the REELS data for scattering angles θ in the range 133°⩽ θ⩽153° show for the first time that the inelastically scattered intensity can vary by a factor of 2.5, for an angular change of 16°, with respect to the elastic peak background intensity. This angular variation has structure which relates directly to that of the differential elastic scattering cross-section. A simple relation for A, for losses greater than 80 eV, is thus found involving the differential elastic scattering cross-section and the transport cross-section. This leads to a result for A, for our instrument, which has a minimum of 0.9 at Rb (37) and a maximum of 1.6 at Ta (73), consistent with experimental data. For losses less than 80 eV, the scaling changes exponentially with energy from 1/ A at high values of energy loss to 1 at zero energy loss with a characteristic decay energy of around 26 eV. Using these results, an angle-averaged set of REELS data may be generated for spectral deconvolution in AES and XPS.

Experimental determination of the inelastic mean free path of electrons in GaP and InAs

The inelastic mean free path (IMFP) of electrons is a fundamental material parameter needed for surface analysis. The IMFP data of semiconducting III–V compounds have been calculated by Tanuma et al., Kwei et al. and Gries. The present paper completes previous research and reports experimental studies on the IMFPs of GaP and InAs. Elastic peak electron spectroscopy (EPES) was used with Ni, Au and Ag reference samples. Both InAs(100) and GaP(100) have been studied in three laboratories, using different types of electron spectrometer analysers and energy ranges: HSA (0.2–5 keV), DCMA (0.2–2 keV), DESA 100 (0.5–2 keV). The surface of the samples was cleaned and amorphized by Ar+ ion bombardment with controlled parameters (energy, dose, etc.) to eliminate crystallinity effects. The surface composition of the ion-bombarded samples was determined in situ by XPS and Auger electron spectroscopy. Their crystallinity and incoherent elastic scattering were tested with Renninger plots of the elastic peak intensity, rotating the sample around its axis perpendicular to the surface. The IMFPs were calculated by a Monte Carlo algorithm using the NIST database of elastic scattering cross-sections. A reasonable agreement was found with theoretical IMFP values calculated by other authors. Copyright © 2000 John Wiley & Sons, Ltd.

Neutron scattering study of boson peak in glassy 3-methylpentane

The neutron scattering experiments of glassy 3-methylpentane were performed with two high resolution spectrometers LAM-40 at KEK and HER at ISSP. A clear boson peak was found around 2.5 meV in the energy spectrum by LAM-40. The scattering-vector dependence of the boson peak intensity measured by HER indicated that the boson peak of 3-methylpentane can be treated as a single harmonic oscillator. The mean square atomic displacement for the oscillator was proportional to the temperature and much larger than that obtained from the scattering-vector dependence of the elastic peak intensity.

Neutron-scattering study of spin-density wave order in the superconducting state of excess-oxygen-dopedLa2CuO4+y

We report neutron scattering measurements of spin density wave order within the superconducting state of a single crystal of predominately stage-4 La2CuO4+y with a Tc(onset) of 42 K. The low temperature elastic magnetic scattering is incommensurate with the lattice and is characterized by long-range order in the copper-oxide plane with the spin direction identical to that in the insulator. Between neighboring planes, the spins exhibit short-range correlations with a stacking arrangement reminiscent of that in the undoped antiferromagnetic insulator. The elastic magnetic peak intensity appears at the same temperature within the errors as the superconductivity, suggesting that the two phenomena are strongly correlated. These observations directly reveal the persistent influence of the antiferromagnetic order as the doping level increases from the insulator to the superconductor. In addition, our results confirm that spin density wave order for incommensurabilities near 1/8 is a robust feature of the La2CuO4-based superconductors.

Crystalline effects in elastic peak electron spectroscopy

Elastic peak electron spectroscopy (EPES) is confined to incoherent elastic scattering, appearing in the elastic peak intensity Ie. Ion bombardment cleaning disorganizes the surface layer and might produce a component enrichment. EPES was used for the experimental determination of the inelastic mean free path of electrons on ion bombarded Si, GaAs, InP, InSb and GaSb crystals. Imperfect amorphization can produce diffraction or directional elastic peak electron spectroscopy (DEPES) effects influencing the elastic peak intensity. They appear in the Renninger plots of Ie rotating the sample around its azimuthal axis. The problem was met with the inelastic mean free path studies on InSb and GaSb crystalline samples working with a hemispherical analyser. The surface composition after Ar bombardment was checked by in situ X-ray photoelectron spectroscopy and electron energy loss spectroscopy. Using 2keV Ar+ sputtering, the enrichment of In or Ga was avoided, whereas In metallic phase was formed by 10keV ions. The energy values of diffraction on Si(111), and III–V (100) crystals was calculated for a cylindrical mirror analyser and for a hemispherical analyser. Incoherent elastic scattering occurs in the minima of Renninger plots. The goal is to avoid diffraction and DEPES effects. They are eliminated by Ar+ ion bombardment. The thickness of the amorphized layer was controlled by the Ar+ ion energy and dose.

Determination of the transmission and correction of electron spectrometers, based on backscattering and elastic reflection of electrons

Quantitative surface, interface and thin film analysis is based in practice on reference standard samples. The elastic reflection coefficient r e of a surface is a material parameter that can be determined from the elastic peak intensity. Absolute values of r e have been published by several authors, mainly working with a retarding field analyser. Koch published the angular distribution r e( Θ) %/sr for a number of elements covering the E=400–2400 eV energy range. Goto developed a cylindrical mirror analyser for elastic current measurements and published results on graphite, Ni, Ag, Cu, Au and Si. The transmission of the ESA 31 (ATOMKI) and DESA 100 electron spectrometer of Staib were determined from the backscattering spectra of standard samples. Comparison of experimental r e( E, Z, 138°) data of Koch with those of Goto exhibited nearly constant ratio close to 5 that slowly decreased with E. The transparency of Goto's CMA was found nearly 20%, near to his estimated value. The elastic current data can be affected by the spectrometer energy resolution integrating the loss spectrum adjacent to the elastic peak. Spectrometer correction for that is needed. The transmission (response) of the ESA 31 and DESA 100 was determined. The elastic peak can be used as internal reference standard for quantitative AES and electron energy loss spectroscopy.

Dependence of experimentally determined inelastic mean free paths of electrons on the measurement geometry

The inelastic mean free path of electrons (IMFP) can be derived from the ratio of elastic peak intensities measured for the studied sample and a reference material. This method is frequently described with the acronym EPES (elastic peak electron spectroscopy). The corresponding calculations should be based on a realistic theoretical model of electron transport in solids. If such a model is available and if the elastically backscattered intensities are measured correctly, the resulting IMFP values should be independent of the experimental configuration used to measure the intensity ratios. In this paper, the role of experimental geometry is systematically checked for elements with a wide range of atomic numbers: Si, Cu, Ag, and Au. A pronounced dependence of the derived IMFPs on emission angle, α, has been observed in the range 5°≤ α≤20°. This dependence becomes noticeably weaker for larger emission angles, 25°≤ α≤45°. The scatter of the IMFPs corresponding to the range 25°≤ α≤45° seems to decrease with increasing energy, especially above 200 eV. The observed effects are discussed in terms of the validity of the theoretical model and possible instrumental problems. An effective Monte Carlo strategy has been proposed for the needed EPES calculations which made the present computations manageable since calculations of over 600 calibration curves were necessary for processing the experimental data.

The inelastic mean free path and the inelastic scattering cross-section of electrons in GaAs determined from highly resolved electron energy spectra

GaAs samples have been studied with a hemispherical analyser of high resolution (type ESA 31). The analyser covers the energy range 10–5000 eV with controlled energy resolution. Prior to measurements, sample surfaces have been exposed to Ar + ions in order to amorphise the surface layer. This procedure resulted in Ga enrichment (70–85 at.% Ga as determined by XPS). The elastic peak and EELS spectra were measured in the loss range E− E l of 50 eV. The elastic peak intensity ratios of GaAs sample and the Ni reference were used to determine the IMFP in GaAs. The relations between these ratios and the IMFP have been determined from Monte Carlo simulation of the elastic backscattering effect. The values of the IMFP resulting from this procedure are in reasonable agreement with the literature data. The inelastic scattering cross-sections have been determined using the Tougaard procedure. The energy loss distributions λ i K are presented in the 0.2–5.0 keV range.

Determination of the inelastic mean free path (IMFP) of electrons in germanium and silicon by elastic peak electron spectroscopy (EPES) using an analyser of high resolution

The IMFP of electrons is a fundamental material parameter of surface analysis by AES, XPS, EPES and EELS. In surface analysis calculated IMFP values are used. Their experimental determination is rather difficult. The IMFP of amorphous Ge and polycrystalline Si was determined by comparing the elastic peak intensity ratios with electrolytic Ni reference sample of 1 nm surface roughness, achieved by dedicated Ar + ion bombardment cleaning and examined by STM. Experimental results obtained with a hemispherical analyser type ESA 31 developed by ATOMKI Debrecen have been evaluated by Monte Carlo analysis, based on Jablonski's differential elastic scattering cross sections and elaborated for the HSA analyser angular window. Due to the 5 × 10 −5 energy resolution of the ESA 31 no spectrometer correction was needed. The ratio of the background to the elastic peak was < 1% at 4 keV. The energy range was extended to 5keV using Ashley's IMFP data. Reasonable agreement was found with our previous CMA experimental results in the overlapping energy range and with Ashley. They exhibit systematically higher values by 15–20%. EPES proved to be an efficient tool for IMFP measurements.

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.

Inelastic-neutron-scattering studies on glassy and liquidCa0.4K0.6(NO3)1.4

Triple-axis inelastic-neutron-scattering measurements have been carried out on the ionic glass material Ca0.4 K0.6(NO3)1.4. Our measurements of the temperature dependence of the position of the main elastic diffraction peak in the liquid give a linear coefficient of thermal expansion of (2.3±0.2)× 10−4, which is markedly greater than the value of 1.2× 10−4 from density measurements. This shows that the expansion is accompanied by structural rearrangements. The Debye-Waller factor, as determined from the temperature dependence of the intensity of the main elastic diffraction peak, shows an anomaly at the calorimetric glass transition temperature, Tg=335 K, but the anomaly at the critical temperature Tc predicted by mode-coupling theory is not seen. In contrast, the change in the effective root-mean-square displacement, ⟨r2⟩effT−⟨r2⟩eff300 deduced from the Q dependence of the temperature variation of the quasielastic scattering over a wide range of wave vector Q from 2 to 4 A−1, shows a break of slope at a temperature that can be identified as Tc, but there is no anomaly at Tg. Our results give Tc=368±5 K. The inelastic scattering for constant Q scans at frquencies ν=ω2π<~0.5 THz deviates from harmonic-phonon-like behavior at temperatures above Tg. This can be identified as a fast relaxation or β process, as predicted by mode-coupling theory. Such a deviation is not observed at higher frequencies. In the present work, we establish the Q and ν dependences of the β process.

Extended fine structure in elastically scattered electron spectra: application to a study of Bi on an a-Si surface

A new surface structure sensitive technique was used to study bismuth adsorption on an a-Si surface. The technique is based on the extended (oscillating) fine structure in the energy dependence of the elastic peak intensity. It was found that the mean interatomic distance at theBi/Si interface increased with Bi coverage. It is noteworthy that, in the submonolayer region, the mean interatomic distance increases linearly with Bi coverage. We thus propose to treat the Bi/Si interface as a “quasi-2D-solid solution”.

Experimental determination of the inelastic mean free path (IMFP) of electrons in Cr, Mo, Ge and Si based on the elastic peak intensity ratio with a Ni reference sample

The inelastic mean free path (IMFP) of electrons has been determined for selected elemental solids using elastic peak electron spectroscopy (EPES) and a Ni standard. The IMFP was evaluated for the range of 500–3000 eV on Cr, Mo, Ge and Si materials. The Ni standard surface has been prepared by electrolysis and HV vapour deposition. Its quality was verified by AES and STM. The theoretical model relating the elastic peak intensity to the value of the IMFP was based on relativistic scattering cross sections and the multiple elastic scattering events were simulated by a Monte Carlo procedure. Reasonable agreement of the obtained IMFP values and their dependence on the energy with the data by Tanuma et al. and by Ashley et al. was found.

Photoyield measurements of CVD diamond

Abstract We investigated the electron affinity of the (100)-oriented single-crystal chemical vapor deposited (CVD) diamond surfaces. The photoyield spectra of (100) surfaces exposed to hydrogen plasma start from 5.4 eV incident photon energy, which is lower than the band gap of diamond (5.5 eV), using UV synchrotron radiation. This shows that the vacuum level at the (100) surface exists below the conduction band of the bulk. Also, the electron affinity of the (100) surface is found to be negative from changes in elastic and secondary electron emission peak intensities.

Quantification of the electron beam damage of thin films

Owing to diverse possible mechanisms of the e-beam damage, some changes either in the crystalline structure, chemical composition or film thickness occur as the proper “damage” of the virgin state. Changes in the chemical bonds are often connected with some change in thickness or structure so that the other two are the most important items to be detected, preferably in real time of the electron-microscopical or electron-spectroscopical examination. We propose to measure the film thickness on the basis of the most probable energy loss due to the electron pass through the film and back, i.e. on the basis of the position of the broad background maximum in the electron spectra of stratified specimens. A change in crystallinity can be sensed through the elastic peak intensity. Placing a measurement window between these features in the energy scale and measuring the energy filtered background during the irradiation, we get a pronounced dependence of the signal on the dose which unambiguously reveals the damage limit in the form of a curve knee. In some cases, two stages of damage are detectable.

Development in Auger depth profiling technique

The usage of Auger depth profiling technique in its conventional form was limited considerably by surface roughening accompanying the ion sputtering. The depth resolution thus decreased with the thickness of the removed layer. Introducing rotation of the specimen and using glancing-incidence-angle ions the surface roughening was greatly reduced. Applying these conditions, one can study the ion mixing effect, which is another limiting factor for the depth resolution. A Mo 3.6 - Si 3.3 nm multilayer system deposited on a Si substrate has been studied by AES depth profiling with specimen rotation and glancing-incidence-angle ions (87° to the surface normal) in the 1–2 keV electron and 1–5 keV ion energy ranges, using a special ion gun. Depth profiling was followed by plasmon loss and elastic peak electron spectroscopy (EPES). EPES enables variation of the sampling depth, while the AES sampling depth was ≈0.4 nm. Plasmon loss spectra have been recorded on the Mo and Si layers and are considerably different from those obtained on pure Mo and Si, respectively. Simulation of the elastic peak intensity versus profiling depth resulted in good agreement with experimental data. The calculation was based on a single elastic scattering approach and with Liau's ion mixing model.

Modified procedure of quantitative analysis by AES

AbstractMuch controversy in the literature is associated with the definitons of terms describing electron transport in solids, i.e. the inelastic mean free path, the attenuation length, the escape depth, etc. This may cause some confusion in determining the corresponding correcting factors for calculations of quantitative AES. There are indications that the proper parameter for the correcting procedure is the inelastic mean free path (IMFP). None the less, difficulties may be encountered in estimating the values of the IMFP for studied samples, particularly for binary or multicomponent systems.A novel procedure of quantitative analysis is proposed in which knowledge of the IMFP is no longer necessary. Actually, the correction procedure normally used requires the ratio of the IMFP for the sample and the standard. One can take advantage of the fact that the IMFP is related to the absolute elastic peak intensity. In that case, determination of the ratio of elastic peak intensities for the sample and the standard makes possible calculation of the ratio of the IMFP. Inasmuch as the measurements of the absolute elastic peak intensities offer considerable experimental difficulties, the determination of the ratio of intensities is relatively easy to perform. Thus, recording the ratio of elastic peak intensities, in addition to the ratio of Auger electron intensities, provides complete information for the correction procedure of quantitative analysis. Such a procedure has two important advantages: the ratio of the IMFP is determined for the studied samples instead of taking these values from the literature, and the actual concentration dependence of the IMFP is accounted for. The calibration plots facilitate recalculation of the ratio of elastic peak intensities to the ratio of the IMFP. Somewhat lengthy calculations are necessary to determine such a plot; however, they are made only once for a given alloy. Application of the proposed method of quantitative analysis is illustrated by using the AuPd alloys.

Elastic electron backscattering from surfaces with overlayers.

The most pronounced effects of elastic scattering in solids are observed in electron backscattering experiments, which, as a result, offer the possibility to test the completeness of the theoretical models of electron transport. For this purpose, elastic backscattering from the Au/Ni(111) system was studied in the present work. Since the total-elastic-scattering cross section for Au is considerably larger than that for Ni, one would expect from simple arguments that the observed elastic peak intensities increase as the Au overlayer thickness is increased. However, a considerable intensity decrease (by a factor of up to 5) with increasing overlayer thickness is observed at energies up to at least 500 eV. This unexpected behavior can be explained within the proposed theory as due to a particular arrangement of differential-scattering cross sections within the experimental geometry used. At higher energies the backscattered intensities increase, in agreement with expectations. In all cases, the predicted intensities compare very well with experimental observations. The low-energy-electron-diffraction pattern for polycrystalline gold shows a characteristic dark ring whose origin can be ascribed to the complex structure of the differential-elastic-scattering cross section of gold. The decreased elastic-backscattering probability within the dark ring is responsible for the unusual behavior of intensities observedmore » in the overlayer experiments. The position of the dark ring is well explained by the proposed Monte Carlo algorithm.« less

Vibrational and collective excitations in the Bi/GaAs(110) system

We present a high resolution electron energy loss spectroscopy investigation of the Bi/[n-type GaAs(110)] system. Analysis of the elastic peak intensity, of the substrate's Fuchs-Kliewer phonon and dopant-induced free carrier plasmon, shows the influence exerted on GaAs by the first semiconducting Bi monolayer and by the subsequent semimetallic clusters at higher coverage.