Attenuation Length Of Photoelectrons Research Articles

Determination of photoelectron attenuation lengths in thin oxide films on iron surfaces using quantitative XPS and elastic recoil detection

Thin oxide films play an important role in the corrosion of metals. Using XPS it is possible in principle to obtain information on the chemical state of near-surface atoms, the stoichiometry of the surface layer and its thickness. A problem is the quantification of XPS spectra, due to the large uncertainty in the value of both electron attenuation lengths as well as sensitivity factors. By applying the Tougaard background subtraction method and comparing only peak intensities of the Fe 2p and O 1s peaks with those of Fe, FeO and Fe2O3 reference samples, we determined the stoichiometry of the oxide layer grown on Fe(100) and Fe(110) at 200°C and room temperature to be Fe0.90±0.05O. We combined XPS with the high-energy ion-beam technique of elastic recoil detection (ERD). By comparing the absolute coverages of oxygen determined by ERD with the intensities of different XPS peaks, we were able to determine the attenuation lengths for kinetic energies of 776 eV (Fe 2p) and 957 eV (O 1s) to be 9.3×1015 and 9.9×1015 Fe0.9O molecules cm−2, respectively. © 1998 John Wiley & Sons, Ltd.

Model for Analysis of XPS Electron Take-off Angle Experiments in Layer-structured Samples: Determination of Attenuation Lengths in a Well-characterized Langmuir-Blodgett Film

A series of equations have been proposed to calculate take-off angle dependency of the elemental composition of layer-structured models from the experimentally obtained peak intensities of XPS. We have measured the take-off angle dependency of XPS peaks of O 1s, C 1s, Si 2p and Cd 3d from a well-characterized one-layer Langmuir–Blodgett (LB) film of polymerized cadmium 10,12-pentacosadiynoate on a silicon wafer substrate and have applied the newly proposed equations to deduce attenuation lengths of photoelectrons for every element in the layer-structured organic materials. From the best fitting between the theoretically calculated and the experimentally obtained data by the least-squares method, attenuation lengths for a monolayer film of polymerized cadmium 10,12-pentacosadiynoate have been evaluated to be 3.4, 3.7, 4.0 and 4.5 nm for the respective photoelectrons of O 1s (955 eV), Cd 3d5/2 (1082 eV), C 1s (1202 eV) and Si 2p (1388 eV) with the kinetic energy dependency ofE0.75. It is essential to use polymerized LB films and to confirm the surface structure of the film directly by suitable techniques such as atomic force microscopy, scanning electron microscopy or transmission electron microscopy.© 1997 by John Wiley & Sons, Ltd.

Model for Analysis of XPS Electron Takeoff Angle Experiments in Layerstructured Samples: Determination of Attenuation Lengths in a Wellcharacterized LangmuirBlodgett Film

A series of equations have been proposed to calculate take-off angle dependency of the elemental composition of layer-structured models from the experimentally obtained peak intensities of XPS. We have measured the take-off angle dependency of XPS peaks of O 1s, C 1s, Si 2p and Cd 3d from a well-characterized one-layer Langmuir–Blodgett (LB) film of polymerized cadmium 10,12-pentacosadiynoate on a silicon wafer substrate and have applied the newly proposed equations to deduce attenuation lengths of photoelectrons for every element in the layer-structured organic materials. From the best fitting between the theoretically calculated and the experimentally obtained data by the least-squares method, attenuation lengths for a monolayer film of polymerized cadmium 10,12-pentacosadiynoate have been evaluated to be 3.4, 3.7, 4.0 and 4.5 nm for the respective photoelectrons of O 1s (955 eV), Cd 3d5/2 (1082 eV), C 1s (1202 eV) and Si 2p (1388 eV) with the kinetic energy dependency of E0.75. It is essential to use polymerized LB films and to confirm the surface structure of the film directly by suitable techniques such as atomic force microscopy, scanning electron microscopy or transmission electron microscopy.© 1997 by John Wiley & Sons, Ltd.

XPS depth profiling by changing incident X-ray energy

Excitation energy variable XPS measurements were performed using high-flux soft X-rays from synchrotron radiation. This method enables non-destructive XPS depth profiling by changing the photoelectron attenuation length. In the analysis of a HF cleaned silicon wafer, the present method revealed the presence of a very thin (∼ 1A˚) surface layer of partially oxidized silicon (Si δ+) which would not be detected by conventional Al or Mg Kα X-ray sources.

Spectroscopic Ellipsometry Investigation of Amorphous Silicon Nitride Thin Films

It has been reported earlier that the attenuation lengths of photoelectrons from very thin amorphous silicon nitride films (3 to 11 nm) increase with film thickness. These changes were attributed to systematic variations in the film density. Spectroscopic ellipsometry has been used here as an independent technique to evaluate the refractive indexes of several of the same films. The ellipsometric results are consistent with the earlier finding that the relative densities of these films vary with thickness. The density variations may be attributed to either structural changes in the films or differences in the oxygen content of the films with film thickness.

Experimental influences on the results of concentration depth profiling by ARXPS

AbstractAngular resolved XPS (ARXPS) makes it possible to determine film thicknesses, attenuation lengths of photoelectrons and depth profiles of compositions. The usual algorithms for evaluating thin film results (rectangular depth profile of composition) do not account for influences of count‐rate statistics, finite solid angle of electron acceptance, elastic scattering of electrons and surface roughness on the results. Computer simulations of ARXPS performed on specimens with rectangular depth profiles provide a better understanding of deviations from expected depth responses of composition due to these influences. The dominant influence of surface roughness on the results is demonstrated by a comparison of ARXPS performed on SiO2‐films on monocrystalline and on polycrystalline Si‐wafers.

Film thickness measurements of SiO2 by XPS

AbstractThe preferred XPS methodology for measurement of SiO2 film thickness on polished silicon surfaces is discussed. A precise measurement of the photoelectron attenuation length was made using nuclear reaction analysis (NRA) to calibrate the film thicknesses. Anodic oxide films on Si, which are very reproducible in thickness, are used as test samples. Appropriate corrections for the problem of adventitious carbon are shown. Under some conditions, the phenomenon of photoelectron diffraction has a significant effect on the measurements, and improvements to the precision by rotating the sample about the normal during data acquisition are demonstrated.

Determination of attenuation lengths of photoelectrons in aluminium and aluminium oxide by angle‐dependent x‐ray photoelectron spectroscopy

AbstractAngle‐dependent x‐ray photoelectron spectroscopy (AD‐XPS) measurements have been carried out on thin aluminium oxide layers formed on pure aluminium by oxidation in oxygen at 25°C and at 250°C. The amounts of oxygen of the oxide overlayers were measured by nuclear reaction analysis (NRA). The equipment thicknesses of the oxide layers were 14 Å and 21.5 Å for the oxides formed at 25°C and at 250°C, respectively. The AD‐XPS measurements were carried out in the range of take‐off angles 12–122°.The signals emitted by Al3+ in the oxide and by Alm in the metal versus take‐off angle can be, within experimental error, fittex by an exponential function in the range of take‐off angles ∼30–120°. This confirms the applicability of an exponential law for the attenuation of photoelectrons for the studied system in the range of take‐off angles ∼30–120°. The AD‐XPS measurements were used to determine the attenuation lengths of the electrons emitted by aluminium (kinetic energy ∼1180 eV) in the oxide (λox) and in the metal (λm). The attenuation lengths derived from the experiments are: λox = 20 Å and λm = 18 Å. These values are lower than the theoretical inelastic mean free paths owing to the contribution of elastic scattering. For the lowest investigated take‐off angles (<30°) the measurements deviate from the exponential law. The deviation is attributed to the effects of elastic scattering and of the angle of acceptance of the photoelectrons.As a complement to this work, an intercomparison of AD‐XPS measurements was performed in four laboratories on the two reference samples. In the range of take‐off angles 40–120°, the results can be fitted by the exponential law for the attenuation of the photoelectrons. However, differences of 10–20% were observed in the apparent overlayer thickness. Careful analysis of the data revealed that this was primarily caused by a systematic error introduced in the peak fitting when a symmetrical peak is used, instead of an asymmetrical peak, for the Alm signal.

Quantitative XPS analysis of SiNx(:H) films using simple methods without sputtering

AbstractThe use of Ar ion sputtering to remove the native surface oxide on SiNx(:H) is known to cause underestimation of its N/Si ration in XPS or AES analysis owing to preferential sputtering. We have carried out an XPS analysis of the N/Si ratio of plasma‐enhanced chemical vapour deposition SiNx:H (x <1.5) films using two simple methods without sputtering. In the first method samples were analysed with no surface treatment. In the second method the surface oxide was removed by chemical etching with HF solution. Curve fitting of Si 2p spectra was carried out to remove the influence of the surface oxide in both methods. There were good linearities between the N/Si ratios obtained by these methods and those estimated by RBS and absorption edge measurement. In particular, after correction for an error caused by differences in the attenuation length of photoelectrons, both methods proved to give results of sufficiently high precision for practical analysis.

Use of X-ray photoelectron spectroscopy in the measurement of protein orientation on surfaces

The use of X-ray photoelectron spectroscopy (XPS) for determination of the orientation of proteins on surfaces is demonstrated. A myoglobin derivative in which a pentaamineruthenium(III) group is attached to a histidine residue is used for this purpose, with the ruthenium and the heme iron atoms acting as a double marker. The relative intensities of the X-ray photoelectron spectra for these two markers are shown to be a sensitive measure of orientation in the protein film, since the RuFe distance is comparable to the photoelectron attenuation length. On aluminum, indium-tin oxide and oriented graphite surfaces, XPS indicates that the RuFe axis of the myoglobin derivative is preferentially oriented with the ruthenium furthest from the substrate.

Particle size information from dispersed phase photoemission intensity ratios

A simple method for obtaining particle size information from photoemission results is formulated by considering the intensity ratio for two dispersed phase core levels with different kinetic energy. This analysis predicts a high sensitivity for detecting particle size differences for sizes in the range of about 0.4 λ ≤ d ≤ 2 λ, where λ is the photoelectron attenuation length, i.e., particle sizes in the range of about 10–50 Å. A major advantage of this approach over earlier methods involving the dispersed phase to support phase intensity ratio is a reduced dependence on surface roughness and physical properties of the catalyst. Example applications suggest that many opportunities exist for applying this analysis in studies of multicomponent practical catalysts.

Attenuation lengths of photoelectrons in hydrocarbon films

Abstract : Quantitative analysis of data obtained by X-ray photoelectron spectroscopy (XPS) requires a knowledge of the escape depths of electrons from the surface of a sample. In order to derive the composition of a homogeneous material from the intensities of the photoelectrons originating from different elements, one needs to know not only the relative atomic cross-sections, but also the variation of the attenuation length, gamma, with the energy of the photoelectrons. The ability to derive an elemental depth profile of a layered material from the variation in the photoelectron intensity with the angle of emission requires a knowledge of the absolute value of gamma. The recent growth of interest in thin organic films has generated an immediate need for accurate, reliable values of gamma in organic materials in general, and in thin, densely packed hydrocarbon films in particular. In this paper we have determined the attenuation length of electrons with energies in the range 940-1400 eV in self- assembled monolayers of n-alkanethiols adsorbed on gold. (JS)

ESCA measurement of surface atom oxidation states on chemically modified Mo(100) surfaces

Quantitative surface atom oxidation states are measured in a conventional ESCA spectrometer. Spectra that solely reflect the chemical nature of the surface atoms are generated from a linear combination of spectra measured at two take-off angles. Proper selection of the multiplicative constants results in removal of bulk atom effects from the measured spectra. A simple method which is based on photoelectron attenuation lengths is used to calculate these coefficients. The surface Mo atom oxidation numbers for a series of chemically modified Mo(100) surfaces have been obtained by this procedure. Charge transfer effects resulting from the adsorption of electronegative surface modifiers (B, C, O and CO) are observed. At constant adatom coverage, the Mo oxidation number is linearly dependent on the adatom electronegativity. For a series of partially oxidized Mo(100) surfaces the extent of charge transfer to the metal atoms is not a simple function of the surface oxygen coverage. A model for the initial oxidation of Mo(100) is presented which is in agreement with previous structural and electronic state studies. At oxygen coverages below one monolayer, a chemisorbed oxygen phase exists which produces a constant Mo oxidation number of 1.3. At higher coverages, a rapid increase in the Mo surface atom charge indicates the formation of a surface oxide layer. The electron deficient Mo surface atoms are acidic sites for adsorption of gas phase Lewis bases. The relative strengths of the acidic sites correlate with the metal atom oxidation numbers. This result suggests that the observed adsorption selectivity and reactivity of these surfaces can be discussed in a traditional acid-base context.

The energy dependence of the photoelectron attenuation length via the oxidation of silicon

The energy dependence of electron attenuation length in the range 20–60 eV has been determined through synchrotron radiation photoemission spectroscopy of the 2 p core states in oxidised silicon (111) surfaces. The attenuation length is found to be approximately constant in this energy

Attenuation length of photoelectrons in thin films of SiO 2 grown on Si

Attenuation length of photoelectrons in thin films of SiO 2 grown on Si

Thickness dependence of the quantum and attenuation length of photoelectrons in thin indium films: J Peisner et al, Phys Stat Sol (a), 4 (3), 1971, K187–K191

Thickness dependence of the quantum and attenuation length of photoelectrons in thin indium films: J Peisner et al, Phys Stat Sol (a), 4 (3), 1971, K187–K191

Thickness dependence of the quantum yield and attenuation length of photoelectrons in thin indium films

Thickness dependence of the quantum yield and attenuation length of photoelectrons in thin indium films

The attenuation length of photoelectrons in thin films of uranium

AbstractFilms of uranium up to about 1000 Å thick were evaporated on to quartz substrates at pressures less than 5 × 10−10 Torr. The attenuation length L of photoelectrons in the films was estimated by comparing the photoelectric emission current when the film was illuminated from the vacuum side (i1) to that when the film was illuminated from the substrate side (i2). Theory indicates a linear relationship between log (i1/i2) and thickness, and this was confirmed by experiment for illumination with light of photon energies from 3.5 to 5.0 eV. The constants of the relationship yield values for L between 60 and 120 Å, the attenuation length decreasing with increasing photon energy. Measurements of the electrical conductivity of the films as a function of thickness yielded a value for the mean free path of the conduction electrons of about 290 Å. The results suggest that the principal energy‐loss mechanism for the excited electrons is one of electron‐electron scattering.