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X-Ray Photoelectron Spectroscopy in Analysis of Surfaces


  1. Steffen Oswald

Published Online: 15 MAR 2013

DOI: 10.1002/9780470027318.a2517.pub2

Encyclopedia of Analytical Chemistry

Encyclopedia of Analytical Chemistry

How to Cite

Oswald, S. 2013. X-Ray Photoelectron Spectroscopy in Analysis of Surfaces . Encyclopedia of Analytical Chemistry. .

Author Information

  1. Leibniz-Institut für Festkörper- und Werkstofforschung Dresden, Dresden, Germany

Publication History

  1. Published Online: 15 MAR 2013

1 Introduction

  1. Top of page
  2. Introduction
  3. Sample Requirements and Sample Preparation
  4. Measuring Strategy
  5. Applications for Typical Material Classes
  6. Special Methodologies
  7. Comparison with Other Techniques
  8. Outlook
  9. Related Articles
  10. References

XPS is an analytical technique that uses photoelectrons excited by X-ray radiation and released from the material into vacuum for the characterization of surfaces. The electrons in the (solid) sample are characterized by their binding energies (BE), which depend on the element of its origin. Using only the elastically scattered electrons (electrons without energy losses) emitted from the sample, information about the composition of the sample can be derived from their energy spectrum. The energy spectrum is measured in an electron spectrometer and the measured kinetic energy (KE) of the electrons are determined by their BEs in the investigated material and the energy of the X-ray photons used for excitation.

The surface sensitivity of the technique is determined by the relatively low escape depth (0.5–2 nm) of the only elastically scattered electrons. All elements of the periodic table can be detected by different characteristic BE peak positions (with the restriction that the light elements H and He may only be found with very low intensity in the valence band). The detection limits depend on the element and are typically in the range 0.1–0.5 at%. As the excitation does not use electrically charged particles, the method is dedicated to the measurement of electrically nonconducting materials, too. Radiation damage of the surface is small in comparison with electron beam excitation. In addition, eventual sample contamination is lower than by techniques implying excitation with external electrons (cf. Ref. 1). Chemical information may be obtained by analyzing typical BE shifts of the photoelectron peaks, which depend on the chemical bonding of the elements under investigation.

However, because of the high surface sensitivity and to avoid energy losses of electrons in the analyzer region, the measurements require UHV conditions typically in the range 10−9-10−8 Pa. Sample information is often disturbed by surface contamination (mostly hydrocarbons or (hydr)oxides), which requires special sample preparation or preconditioning. In multicomponent samples, spectral overlap from several elements may occur, further complicated by X-ray excited Auger electron (XAES) lines.

The origin of the technique is connected with the discovery of the photoelectric effect by Hertz in 1887. In the early years of the twentieth century, additional theoretical and experimental work was done. In 1914, Rutherford determined the following equation:

  • equation image(1)

which describes the measured electron energy Ek as a difference of X-ray energy hν and the electron BE Eb. The instrumentation was improved gradually by various research groups; however, the modern XPS era is closely connected with the work of Siegbahn et al., dating from about 1954. They also introduced the acronym electron spectroscopy for chemical analysis (ESCA), which is often used.2 Industrial equipment became available in the 1970s and today XPS machines are among the most used surface analytical tools.

Valence band spectroscopy, which is often concerned only in context with ultraviolet (UV) excitation, should be regarded as an integral part of the XPS method. Although the interpretation of valence band spectra needs a lot of expertise, this technique is also briefly discussed here.

The XPS method and its applications are described in considerable detail in monographs and review articles that summarize both the earlier2-8 and the more recent9-15 years of XPS development. Additionally, the article by Turner (X-Ray Photoelectron and Auger Electron Spectroscopy) covers the whole field of XPS: physical principles, instrumentation, and quantification. Consequently, this article considers the physics and theory only to the extent necessary to acknowledge the general application strategy and to interpret the examples discussed.

2 Sample Requirements and Sample Preparation

  1. Top of page
  2. Introduction
  3. Sample Requirements and Sample Preparation
  4. Measuring Strategy
  5. Applications for Typical Material Classes
  6. Special Methodologies
  7. Comparison with Other Techniques
  8. Outlook
  9. Related Articles
  10. References

The sample requirements are determined from the design of the particular electron spectrometer used and the physical principles of the technique itself. The minimum sample size is determined from the acceptance area of the analyzer or the size of the exciting X-ray beam, respectively, and the maximum sample size by the construction of the entry lock and the sample manipulating system (today up to 300 mm). Most modern spectrometers have variable acceptance areas between about 50 µm and 1 mm; therefore, the typical sample size (also for handling purposes) is in the range of 5 mm × 5 mm. The samples should be as smooth as possible in order to have a defined surface region for electron emission.

Usually, the XPS measurements have to be done under UHV conditions. Note that for a gas molecule at room temperature and a pressure of 10−4 Pa, and assuming a sticking probability of 1, the time for monolayer adsorption is only in the order of some seconds. Thus, for minimum electron collision with the gas molecules, to avoid energy losses, and for minimizing the recontamination of the prepared surfaces, pressures in the range 10−8 Pa should be reached at least. Therefore, samples should generally have a low vapor pressure. Sample preparation with organic glue or epoxy resin should be avoided. Dedicated adhesive materials are available and can be used in minimum quantities for special preparation purposes. Powders may be impressed into ductile metal foils (In or Au). An alternative is the use of special UHV-compatible double-sided adhesive tape for sample fixing. Generally, the samples should be outgassed in the fast entry chamber of the spectrometer for a suffcient time, especially in the case of porous materials. Biological samples may be freeze-dried or, if sensitive to dehydration, the samples may be cooled in situ (by liquid nitrogen) during analysis.

Surface contamination is one of the main problems of sample preparation for surface analysis, unless the topmost natural surface layer itself is the topic of investigation (e.g. adhesion, catalysis, and corrosion). In particular, the so-called adventitious carbon contamination from hydrocarbons plays an important role. Therefore, clean conditions (gloves, tweezers, exsiccators, glove boxes, laminar boxes, etc.) and special cleaning procedures (rinsing in solvents, ultrasonic cleaning) should be routinely used. Despite such sample precleaning, contamination can never be completely avoided. Because even parts of monolayers of adsorbed species can negatively influence the surface-specific electron spectroscopic measurements, in situ cleaning is necessary in many cases. Medium-energy (1–5 keV) noble gas (Ar, Kr, and Xe) ion sputtering is generally used; however, the ion bombardment induces morphological, structural, and chemical changes in the surface16. An alternative can be the use of special transfer chambers for the transport of sensitive samples from glove boxes under noble gas atmosphere. For the preparation of clean single-crystal surfaces, sputtering–heating cycles are often applied. As an alternative, mechanical methods can be used to prepare fresh surfaces under UHV conditions if the sample properties and geometry are suitable. In principle, cleaving (of single crystals), scraping (with diamond-coated tools), or fracturing (special sample geometry, cooling with liquid nitrogen) may be used. Guidelines for specimen handling and preparation may be found in some reviews.17, 18 In recent years, more and more special equipment is applied to overcome the so-called pressure gap in chemistry and life sciences. These methods are also indicated as “near-ambient pressure XPS” (NAP-XPS), which is mostly applied not only with synchrotron radiation19, 20 but also with laboratory sources21 (see also discussion in Section 5.6).

Many XPS facilities are connected to equipment for in situ sample preparation, such as molecular beam epitaxy (MBE), pulsed laser deposition (PLD), chemical vapor deposition (CVD), and magnetron sputtering. The advantage of such combinations is the possibility to investigate the films, multilayers, heterostructures, or materials prepared without taking them under atmospheric pressure, which may induce sample contamination or other surface modifications.

3 Measuring Strategy

  1. Top of page
  2. Introduction
  3. Sample Requirements and Sample Preparation
  4. Measuring Strategy
  5. Applications for Typical Material Classes
  6. Special Methodologies
  7. Comparison with Other Techniques
  8. Outlook
  9. Related Articles
  10. References

3.1 Elemental Information

Information about the elements incorporated in the surface region is obtained from the measured BE values of the separate lines in the electron spectrum. Complete series of spectra are given in several publications (a standard reference book is that by Moulder et al.22). Table 1 summarizes the BE values of the two most intense lines for each element. Note that the XPS nomenclature uses the quantum numbers of the electronic core levels. In addition, note that the p, d, and f levels are splitted by spin–orbit coupling into doublets with characteristic energy differences (Table 1 also lists energy splitting) and intensity ratios (1:2, 2:3, 3:4). However, for quantum-mechanical reasons these ratios may vary from the exact theoretical ones slightly, for example, for the 3 d transition metals.23, 24 Another origin for variation of branching ratios stems in photoelectron diffraction (PED) effects. Electrons emitted from inside the sample are elastically scattered by the upper atomic layers in their way out to the vacuum. This scattering amplitude depends on the KE. Therefore, the components of a spin–orbit split doublet, which have different KE, may have different intensity ratio than that expected from atomic physics. This phenomenon is manifesting especially in the casr of single-crystal samples. Commonly Mg Kα (1253.6 eV) or Al Kα (1486.6 eV) are used for excitation because their natural line width is relatively small (0.7 eV for Al Kα; 0.56 eV for Mg Kα), photoelectron lines of all elements may be measured, and the relatively low KE of the measured photoelectrons allows high-energy resolution and good surface sensitivity. For special reasons, especially to obtain more bulk-specific information or to generate effectively higher energy Auger lines, X-ray sources with higher energy (e.g. Ag Lα, Si Kα, Ti Kα, synchrotron radiation) have been used. The progress in this field is due to the increasing number of available beamlines around the world (Section 5.5). Nevertheless, the energy range given in Table 1 is restricted to the energy region up to 1400 eV available with the laboratory sources and also covering the whole range of elements in the periodic system.

Table 1. BE Values (in Electron Volts, Values Rounded, for Al Kα X-Rays) for the Two Most Intense XPS Lines in the Order of the Elements of the Periodic System (Values are Taken from Moulder et al.22)
Atomic numberElementLineBESplittingLineBESplitting
8O1s531 2s23 
9F1s685 2s30 
10Ne1s863 2s41 
11Na1s1 072 2s64 
12Mg1s1 303 2s89 
13Al2p73 2s118 
30Zn2p3/21 022233p3/2892
31Ga2p3/21 117273 d19 
32Ge2p3/21 217313 d5/2291
33As2p3/21 324353 d5/2421
34Se3 d5/25613p3/21636
35Br3 d5/26913p3/21827
36Kr3 d5/28713p3/22088
37Rb3 d5/211123p3/22409
38Sr3 d5/213423p3/227011
39Y3 d5/215623p3/229912
40Zr3 d5/217923p3/233013
41Nb3 d5/220233p3/236115
42Mo3 d5/222833p3/239418
43Tc3 d5/225343p3/342520
44Ru3 d5/228043p3/246222
45Rh3 d5/230753p3/249724
46Pd3 d5/233553p3/253327
47Ag3 d5/236863p3/257331
48Cd3 d5/240573p3/261834
49In3 d5/244484 d17 
50Sn3 d5/248584 d25 
51Sb3 d5/252894 d33 
52Te3 d5/2573104 d5/2411
53I3 d5/2619114 d5/2492
54Xe3 d5/2670134 d5/2612
55Cs3 d5/2726144 d5/2773
56Ba3 d5/2781154 d5/2903
57La3 d5/2836174 d5/21033
58Ce3 d5/2884184 d5/21093
59Pr3 d5/2932204 d115 
60Nd3 d5/2981224 d121 
61Pm3 d5/21 034264 d129 
62Sm3 d5/21 081274 d129 
63Eu3 d5/21 126294 d128 
64Gd3 d5/21 186324 d140 
65Tb3 d5/21 241354 d146 
66Dy3 d5/21 296374 d152 
67Ho4 d160 4p3/230944
68Er4 d167 4p3/232147
69Tm4 d175 4p3/233351
70Yb4 d182 4p3/234148
71Lu4f7/2724 d5/219610
72Hf4f7/21424 d5/221111
73Ta4f7/22224 d5/222612
74W4f7/23124 d5/224313
75Re4f7/24024 d5/226014
76Os4f7/25134 d5/227914
77Ir4f7/26134 d5/229715
78Pt4f7/27134 d5/231517
79Au4f7/28444 d5/233518
80Hg4f7/210144 d5/236120
81Tl4f7/211844 d5/238521
82Pb4f7/213754 d5/241222
83Bi4f7/215754 d5/244024
90Th4f7/233394 d5/267637
92U4f7/2377114 d5/273643

As can be seen from this table, there is a wide spread of energies, which may be used for elemental identification. However, there are spectral overlaps for specific combinations of elements, especially because Auger electron lines (not included in Table 1) are also excited by X-rays. The effect of overlapping Auger lines may be mostly avoided by an energy shift when changing the X-ray source. However, these XAES lines are often useful for both elemental identification and as a source of chemical information25 (Section 3.2).

As an example, Figure 1 shows an Al Kα excited spectrum of an Sb2O4 powder coated with a thin Au film for energy calibration. Besides a complete overlap of O1s and Sb2p3/2, one can see the Auger lines, C contamination, Au lines, and satellite lines (from satellite energies of nonmonochromatized Al Kα radiation). Despite the dependence on the element (Equation 3), the measured intensities vary systematically depending on the transmission function of the electron analyzer, here with 1/Ek for the hemispheric analyzer which was used. An increasing background to lower KE (higher BE) comes from both inelasticity scattered electrons and the electrons excited by bremsstrahlung radiation from the X-ray source. Using a monochromatized source instead, the bremsstrahlung radiation and satellite peaks can be avoided, and additionally sharper peaks are obtained. Monochromatization is mostly done using Bragg reflection at single crystalline optical elements, additionally sophisticated systems can be used for creating a point source for small spot X-ray excitation.

thumbnail image

Figure 1. Spectrum of an Sb2O4 sample. The sample was coated with a thin Au film for energy calibration.

For exact peak position analysis, the BE scale has to be calibrated.4, 6, 9 This is done experimentally using the Fermi energy level of the spectrometer EF = 0 as a reference and by measuring (i) the well-defined peak positions of core levels of clean metals (e.g. Au4f = 84.0 eV, Cu2p3/2 = 932.67 eV) or (ii) the valence band cut-off of metals with a high density of states near EF = 0 (e.g. Ni). Thus, the basic Equation 1 is modified to the following equation:

  • equation image(2)

with ΦS as the work function of the spectrometer.

Concentration quantification is done using the intensity of a suitable (sufficient intensity, no spectral overlap with other elements) characteristic line as a measure for the number of atoms of a certain element in the analysis region. The basic relationship to calculate the total photoelectron intensity Iix of the core level peak i of element x for a homogeneous sample is given by the following equation10:

  • equation image(3)

where B is the instrumental factor (X-ray flux, angular acceptance, and total transmission of the spectrometer), σ is the photoabsorption cross-section for the level including asymmetry parameter, λ is the total electron escape depth for the core-level energy and sample material, T is the transmission coefficient of the electrons through the surface, and n is the atomic density of element x.

For exact calculations, all these terms have to be assumed to be dependent on space coordinates and the appropriate integrals have to be solved.9, 11

For routine concentration calculations, usually some assumptions are made to end in a simply applicable algorithm:

  • the analysis region is assumed to be a homogeneous mixture of the elements;

  • the excitation probability of the core level used does not depend on the environment of the atom (low matrix effect); and

  • the electron background near the selected peak has to be removed before the intensity calculation is carried out.

The background subtraction can be done in several ways, from simple linear dependence to physically determined functions.26 Modern software reduction packages (e.g. Refs 27-29) have implemented a number of different background shapes; however, in routine analysis, the method after Shirley30 is still mostly used, which calculates an integrated background under the peaks. An example is given in Figure 2 for the complex Mo3d peak structure at a Mo oxide sample.

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Figure 2. Demonstration of a Shirley background subtraction for an MoO2 sample which was measured by XPS without further in situ sample treatment.

Even if both the peak height and the peak area can be used as intensity values for calculations,22 only the use of peak area can be seriously recommended (see also X-Ray Photoelectron and Auger Electron Spectroscopy). Peak area calculations lead to more realistic values, because the peak shape is changed by chemical effects (see Figure 2 and as discussed below) and will produce completely wrong results for components with different bonding states for the peak height method. The peak area may be estimated by simple peak integration considering the background curve or using more sophisticated peak fitting procedures (see Section 3.2 and X-Ray Photoelectron and Auger Electron Spectroscopy). Using these assumptions, the simplest calculation for the concentration c is given by the following equation:

  • equation image(4)

where Ix is the intensity (peak area or peak height) and Sx is a relative sensitivity factor of the element x.

This simple algorithm is also used in the mentioned commonly used data reduction packages.

In a first approximation, the sensitivity factors can be taken from an element or a standard sample measured under equivalent conditions; alternatively, values from reference measurements for the appropriate analyzer type (using the same transmission function)22 or from theoretical calculations31 are routinely used. Analytical experience derived from working with a wide range of materials shows that, based on this simple strategy, a precision of about 10 % can be reached; however, significant errors may occur if the sample is inhomogeneous within the range of electron escape depth.

Further improvements of the results may be obtained by applying the following steps9:

  1. measuring element standards under exactly the same measuring conditions as for the unknown sample;

  2. measuring similar well-defined compound standards;

  3. performing escape depth correction and correction of changed atomic density (matrix correction);

  4. simulation of concentration gradients if they are well known (overlayer correction); and

  5. performing a multicomponent fitting where each individual line has its own associated inelastic background 32, 33. If the associated background of a given component is very weak, this is a clear sign of the “surface” nature of the component in question: electrons producing this component originate from the outermost layer of the material and therefore present no inelastic losses.

A critical review of the limits of quantitative analysis is given by Powell and Seah,34 which covers all the experimental, physical, and mathematical aspects.

3.2 Chemical Information

One of the main advantages of XPS is the possibility of obtaining chemical information relatively easily by analyzing changes of the BE of the photoelectrons. This is possible because only one electron level is involved in the emission process, in comparison to AES where three energy levels have to be considered. The general shape of the photoelectron peaks, as shown in Figure 2 for a multicomponent peak, is determined by experimental and physical effects. First, the observed BE values are determined by the final ionized state of the emitting atom. Experimental broadening occurs typically as the result of the limited energy resolution of the spectrometer and the energetic width of the X-ray source, and may be described by a Voigt function, which is a convolution between Lorentz and Gauss lineshapes 35. The Lorentz lineshape simulates the atomic emission of X-rays, whereas the Gauss lineshape simulates the transmision function of the electron energy analyzer and thermal broadenings. Broadening due to the lifetime of the resulting hole is given by a Lorentzian function, and has to be further convoluted with the experimental broadening. Especially for metals, there occurs an additional asymmetric broadening to higher BE, generated by an interaction with unfilled electron levels of the valence band above the Fermi level (discussed in Section 5.2). The resulting width is usually defined as full width at half maximum (FWHM) and, for optimum measuring conditions and sharp metallic lines (e.g. Ag 3 d5/2), is about 0.5 eV.

The exact energetic peak position of an emitted core-level electron on the BE scale now depends on the environment of the emitting atom. In single-element (e.g. single-crystalline or polycrystalline) materials used for standardization, this environment is defined by the same element with its characteristic bonding state and near-neighbor distances. In multielement materials (alloys, oxides, carbides, etc.), this environment is altered and so is the electronic structure of the emitting atom. Several more or less empirical models were proposed during the early stages of XPS development, such as the “charge potential method,”2, 36 the “valence potential method,”37 and the “equivalent cores approach.”38, 39 These models, summarized by Brundle and Baker4 and by Briggs,6 consider the chemical characteristics of the bonding partners as electronegativity, charge transfer, or reaction energies. These original approaches have been further developed and extended, also using quantum-mechanical methods.10, 11 The principles of chemical shift estimation are discussed here for the charge potential method, which describes the change ΔE in the BE of a core level by the changes in the electron density and the resulting potential changes:

  • equation image(5)

In Equation 5, Δq is the difference in valence charge of the atom (assumed to be located on a hollow sphere) multiplied by a factor k, and ΔV is the change in the effective potential assuming the surrounding atoms to be point charges. It is clear from the first term that the decrease in valence charge (e.g. oxidation) leads to higher BE values and the increase (e.g. reduction) to lower BE values. However, the second term has also to be considered; it has an opposite sign. This charge transfer situation has to be modeled using complex calculations; fast estimations are often possible using simple considerations of electronegativity differences,36, 40 as shown in Figure 3 for some boron compounds.41 Similar conclusions are also possible in the case of oxides.42

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Figure 3. Dependence of B1s BE on electronegativity difference of the binding partners in several compounds. (Reproduced from Ref. 41. Copyright 1999, John Wiley & Sons, Ltd.)

From this simple picture, some general rules can be derived:

  1. Most core levels of metals are shifted to higher BE in oxides (or fluorides, chlorides, etc.) because of the high electron affinity of O (F, Cl, etc.). These shifts will usually increase with increasing formal oxidation numbers.

  2. The carbon line is shifted to higher BE in oxygen-containing compounds and to lower BE in carbides.

  3. The core-level shifts in similar compounds will decrease with higher atomic radius, that is, higher Z in the same group of the periodic table.

However, for real-world samples, the specific electronic and crystallographic structure of the compound must always be considered. Thus, in practice, it is not generally possible (although it is proposed from time to time) to apply BE reference energy values from a standard sample with well-defined valence states to an unknown compound for quantitative chemical state characterization.

Another approximation for the characterization of chemical effects is the so-called (modified) Auger parameter method. By measuring an additional XAES level (the three-electron energy levels participating in the Auger process should be indexed lmn) of the same element and doing some approximations,43, 44 the difference of this KE Ek(lmn) and the measured core-level BE Eb(i) may be assigned to the parameter α (Equation 6):

  • equation image(6)

and the change in this parameter may be determined by the following equation:

  • equation image(7)

where ΔRea is the so-called extra atomic relaxation energy, where static and dynamic relaxation have to be considered.10 This relaxation is associated with the polarization of a dielectric medium (here by the core-hole states), and thus the XPS experiment is closely connected with the chemical surrounding of the emitting atoms. However, the parameter Δα is not influenced by surface charging (Section 4.3) or other factors affecting BE reference levels. This can be a useful method for fingerprinting sample states. In the so-called Auger parameter plot (Figure 4)22, bonding types are characterized by clusters and may differ in both Auger parameter values and BE values.

thumbnail image

Figure 4. An Auger parameter plot for various copper compounds. Typical Cu valence states are shown. (Reproduced from Ref. 22. Copyright 1995, Perkin Elmer.)

For quantification of the described chemical shifts in sample series, mathematical methods may be used. The quantities of overlapping parts of core-level peaks may be estimated by peak fitting procedures,45 which are mostly based on nonlinear least-squares (NLLS) algorithms, as summarized by Gans.46 The starting parameters (the number of peaks, energy separation, peak shape model, etc.) influence the results and require operator knowledge. Procedures are implemented in the manufacturers' measurement software and in standard graphic software, which is critically reviewed by Siegbahn (see X-Ray Photoelectron Spectroscopy and Auger Electron Spectroscopy: Introduction). Figure 5 demonstrates such a fitting for the MoO2 sample given in Figure 2. The peak area ratio (Mo3d3/2 : Mo3d5/2 = 1:1.5) and the peak separation (here 3.1 eV) were fixed for the calculation. Three 3 d doublets, which arise from different oxidation states, are necessary for good reproduction of the spectrum.

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Figure 5. Demonstration of a multicomponent fit for the MoO2 measurement given in Figure 2. Three 3 d doublet spectra are necessary for a good reproduction of the measured spectrum.

Changes occurring in a series of measurements may be typically classified by mathematical methods as linear least-squares (LLS) fitting, principal component analysis (PCA), or factor analysis (FA).45-49 The mathematical principles are summarized elsewhere.50, 51 The LLS algorithms use standard measurements or typical sample states included in the measured data set as target spectra, whereas FA (commonly based on PCA algorithms) can detect chemical changes without (or with little) a priori information about the sample. Figure 6 demonstrates this for a sputter depth profile of an Re[BOND]Si[BOND]Re layer system.52 Interface phases connected with silicide formation, mainly induced by the ion sputtering effects, can be derived by such a mathematical procedure. The calculated spectra are characterized by both changes in intensity ratios and small energy shifts.

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Figure 6. Depth profile of an Re[BOND]Si[BOND]Re[BOND]Si-substrate layer system (a) decomposed by FA into several phases (i.e. principal components). (b) The portions (loadings) of these several spectral components show strong changes in the interface regions. (c) The spectral shape of the components (scores) varies in peak energy/shape and peak area ratio and can be assigned to several silicide phases, in addition to Re and Si.

3.3 Local Information

Local information from a surface analytical technique such as XPS can be both depth information (because of the small information depth) and lateral information (using spectrometers with appropriate lateral resolution). The depth-profiling methods can be classified into destructive (using depth erosion methods and lateral resolving spectroscopy) and nondestructive methods (only in the near-surface region).

For destructive depth-profiling, medium-energy (1–5 keV) noble gas (Ar, Kr, and Xe) ion sputtering is mostly used. Depending on the ion source and the necessary scan size (typically 2 mm × 2 mm, determined by the lateral resolution of the spectrometer), the erosion rates are in the range 1–10 nm min−1. Thus, elemental depth profiles can be determined by sequential measurements of elemental intensities after several sputtering depth intervals. The depth resolution is determined by the escape depth of the electrons and sputtering effects, and lies in the range of 3–5 nm. In practice, the measuring depth is limited to approximately 1 µm, because the total measuring time is not only determined by the time of the sputtering cycles but also mainly by the time for sequential detection of the electron spectra. Therefore, it effectively depends on the sputter-cycles-per-depth density, the number of measured elements, the desired spectral resolution, and the detection limit. As an example, Figure 7 shows a depth profile of a nonconducting Ba[BOND]Sr[BOND]titanate superlattice.53

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Figure 7. Kr+ sputtered (1.5 keV ion energy) depth profile of a Ba[BOND]titanate[BOND]Sr[BOND]titanate superlattice. The double layer thickness of the nonconducting structure is only 21 nm.

Depth calibration of the intensity versus sputtering time measurements is difficult because the sputtering rates depend on both material (composition, microstructure) and the sputtering conditions (ions, angles, and current density). The conversion of the sputtering time-scale into a depth-scale, if necessary, has to be done using depth (material)-dependent sputter rates for the particular experimental conditions.54 Single-layer sputtering rates are usually determined for layers of known thickness or from crater depth measurements by surface profilometry.55, 56 Alternative methodologies have also been discussed.57, 58

One of the main problems of depth profiling is the occurring of sputtering artifacts, which include preferential sputtering, atomic mixing, recoil implantation, bond-breaking, amorphization, and crystallization.59-62

Preferential sputtering arises from the different sputtering yields of the individual elements in multicomponent samples. As a result, the surface composition (which is measured by XPS) is altered and does not reflect the bulk (or layer) composition. Atomic mixing and recoil implantation additionally change the concentration gradients in the region influenced by ion implantation (up to 10 nm depth depending on ion sputtering conditions). These three effects can be modeled with reasonable results by Monte–Carlo computer simulations for a wide range of materials.63 The possible bond-breaking and structural changes limit the chemical information obtainable from depth profiling; however, in many cases, ion depth profiling is the only method to obtain such information from thin layers or buried interfaces.

In summary, with ion sputtering, the virgin (bulk) sample state is not necessarily being analyzed, and the nature and extent of damage strongly depends on the sample. Therefore, the influence of the sputtering effects has to be established for every new kind of material.

Nondestructive depth-profiling techniques make use of the limited escape depth of the electrons (summarized in detail by Seah and Dench64) and are therefore limited to the first few nanometers of the surface region. However, this region is often of particular interest in surface science (contamination, segregation, passivation, adsorption, etc.).

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) is the most used technique.65-67 By sampling a series of measurements for different emission angles, the effective information depth d (for 95 % of signal intensity) varies with the take-off angle β of the electron analyzer to the surface normal (Equation 8):

  • equation image(8)

The relative changes of peak area and peak shape give qualitative information on changes in both elemental composition and chemical bonding. Using appropriate surface layer models and mathematical algorithms, one can estimate the surface layer structure.68-70 Because of the increasing importance of surface and interface characterization, ARXPS is back in research focus and so it is discussed separately in Section 5.4.

A second nondestructive method uses changes in the electron background coming from the nonelastically scattered electrons.71 The low-KE tail of a specific characteristic core level of one selected element is measured over a range of tens of electron volts, because these losses are characteristic for the depth distribution of this element in the surface region. Approximating the shape of this energy region with a computer program, by assuming a characteristic depth dependence (e.g. QUASES72), allows the estimation of some typical parameters (thickness, concentration, and nature of surface layer growth) of the surface structures.

Lateral information is rarely used in XPS because of the relatively poor lateral resolution (50–3 µm, depending on the instrumentation) at relatively long measuring times. Several experimental principles have to be considered. Traditionally, for large-area X-ray radiation, either a small-area lens (with deflection unit) in front of the electron analyzer or an imaging analyzer with channel plates is used; currently, focused X-ray sources with beam scanning or mechanical movement of the sample are also applied. Thus, line scan measurements or mapping of surface areas can be obtained. Although most of the interesting phenomena regarding modern nanostructured materials are associated with nanometer-scale dimensions, some applications arise from the possibility of chemical imaging, which is the detection of complete spectra at each measuring point and spectral interpretation with respect to bonding changes. This is demonstrated in Figure 8 with a map of laser-treated Si3N4 ceramics.73 Spectral analysis by FA (Section 3.2) shows that in addition to Si[BOND]N/Si[BOND]O bonding (not distinguished at the energy resolution used, which was optimized with respect to reasonable measuring time for the small spot measurements) in the center of the laser spots Si[BOND]Si bondings resulting from molten and decomposed Si3N4 were found. Recent progress in small spot XPS powered by the use of synchrotron radiation is discussed in Section 5.5.

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Figure 8. FA results from chemical mapping of Si2p spectra. The secondary electron (SE) micrograph (d) shows the investigated area of an Si3N4 sample with separated laser spots. The spectral components PC1 (oxidized Si, silicon-nitride) and PC2 (elemental Si) are characterized by FA (b). As shown in the lateral distributions of the two components (a and c), calculated from a 16 × 16 point XPS surface mapping, nonbonded elemental (Si[BOND]Si) silicon is found in the centers of the spots.

3.4 Valence Band Information

Although early XPS2, 12 work mainly dealt with the elemental and chemical information from core-level photoelectron lines, valence band photoemission studies soon became increasingly important.4, 6, 74 This went hand in hand with the application of UV radiation as an excitation source, establishing the method of ultraviolet photoelectron spectroscopy (UPS). The valence band region (generally in the BE range from 0 to about 20 eV) covers the information about the outermost electron shell(s). The spectra in this region are usually rather complex and therefore are less easy to interpret than the core-level peaks. In molecules, the spectra reflect changes in the chemical bonding between the different atoms, which is of particular interest for organic materials.75 In the solid state, the valence region has a band structure and the spectra characterize the density of the occupied electronic state (DOS), of considerable importance in the investigation of electronic transport properties in materials. Such band structure investigations may be complemented by the method of inverse photoemission spectroscopy (IPES) that measures UV light emitted from electrons excited to the unoccupied density of states near the Fermi level.25 Valence band measurements with UV radiation are characterized by high excitation cross-sections, good energy resolution, and in plane k (i.e. momentum) conservation (angular measurements at single crystals), by very high surface sensitivity (leading to problems with contamination), and by dependence on the unoccupied density of states at the final-state energy.6 For excitations with conventional high-energy X-ray sources, the intensities and energy resolution of the valence band measurements are weak; however, a direct measurement of the DOS is possible that does not depend on final-state effects.10 Because of the complexity of this topic, only a small number of application examples are discussed separately in Section 5.1.

4 Applications for Typical Material Classes

  1. Top of page
  2. Introduction
  3. Sample Requirements and Sample Preparation
  4. Measuring Strategy
  5. Applications for Typical Material Classes
  6. Special Methodologies
  7. Comparison with Other Techniques
  8. Outlook
  9. Related Articles
  10. References

In the many years of XPS use, this technique found a broad spectrum of applications in nearly all scientific fields. The aim of this section is to give an idea of typical applications in several fields of natural and material science, rather than a comprehensive account, together with some complementary techniques. Because there is partly some overlap between the used classifications, the outline of this section is not always consistent. Most examples are taken from the period 1980–2000, supplemented by some more recent work in the “2013-revision” of this article. Much of the pioneering work, taken from worldwide sources, has been well reviewed elsewhere.4-6, 9, 25

4.1 Metals and Alloys

Measuring the elemental composition of metal surfaces (layers or bulk material) is not a typical XPS task because elemental information for electrically conducting material can be obtained more quickly by other methods, such as AES. However, if chemical changes are of interest, XPS is of advantage, because the peak shifts are often much easier to interpret in terms of chemical bonding. Metals have been studied for a long time with respect of the chemical potential during alloying,76-78 work function measurements,6, 79 and segregation phenomena,80-82 chemisorption studies,83, 84 electrochemistry and corrosion science,85-87 or fundamental studies in catalysis.88, 89

4.1.1 Segregation

Segregation at surfaces or interfaces is a phenomenon that strongly influences a material's properties, such as brittleness or reactivity. Some XPS studies provide information about the segregant elements as well as information about their bonding. Zhang and Macdonald90 have shown for a W[BOND]Ni system how a combination of ARXPS and depth profiling can help to understand the chemical nature of the segregated surfaces. Danoix-Souchet and D'Huysser91 have observed Cu[BOND]Ni elemental and chemical changes during annealing — in addition to enrichment of the minor elements, a phase transition of the residual surface oxides from hydroxide to oxide could be found. Different MoSi2 composite materials were studied by Yi et al.92 by in situ fracturing. As shown in Figure 9, at the fractured surfaces an increasing amount of oxygen and Si[BOND]O bonding was observed. With information derived from scanning electron microscopy (SEM) of the fractured surfaces, it was concluded that the grain boundaries were covered with SiO2. Boron segregation, forming borides, was also found.

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Figure 9. Spectra recorded at in situ fractured surfaces of several MoSi2 composite materials: (a) pure MoSi2, (b) and (c) with Zr oxide. The grain boundaries at (b) and (c) are covered with an SiO2 film. (Reproduced from Ref. 92. Copyright 1997, John Wiley & Sons, Ltd.)

4.1.2 Interfaces

Thin interlayer regions or interfaces determine the properties of most layer systems and can be studied by depth-profiling analysis. Metal–metal interfaces are often used in electronic contacts. Marinova et al.93 showed an example of an Al[BOND]Ni[BOND]Al on SiC contact system where both the elemental changes and peak shape changes connected with phase formation were explained on the basis of depth-profiling measurements. Working with Pd[BOND]Co superlattices, Lesiak et al.94 determined the interface phases using a pattern recognition method.

For an Re[BOND]Si[BOND]Re[BOND]Si-substrate system, interface silicide phases were found and quantitatively extracted with help of FA,52 as shown in Figure 6. These phases were produced mainly by the ion sputtering process because they were not found in transmission electron microscopy (TEM) cross-sections of the virgin sample.

Interface formation is also a preferred field for application of ARXPS. Here, a part of a series of systematic studies of the growth of Ta-,95, 96 W-97 and Ti-98 based diffusion barrier layers on Si and SiO2 substrates is discussed. Identification of interface chemical phases such as silicides and/or oxides was possible from peak shifts. ARXPS was helpful for studying the thickness and sequence of the interface layers that develop during film growth at deposition series. Figure 10(a)–(d) shows for two examples, Ta and Ti on SiO2, spectra recorded at such deposition series98. Oxide and silicide formation is followed for both systems from the observed peak shape changes. Figure 11(a)–(d) shows angle-resolved measurements (points) and the results of approximation calculations (lines) with the given layer models for beginning of Ta and Ti layer growth. In both cases, silicide and oxide interlayers are identified, however, with different layer sequence.

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Figure 10. Spectra recorded at different deposition times for Ti (a,b) and Ta (c,d) metal on SiO2 substrate. Peak shape changes point to oxide and silicide formation at the interfaces. (Reproduced with permission from Ref. 98. Copyright 2010, Springer-Verlag.)

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Figure 11. Angle-resolved XPS measurements (points) at the beginning of Ti and Ta deposition could be approximated by model calculations (lines) (a,b) using the model (c,d). Silicide and oxide is formed in both cases, but with different layer sequence. (Reproduced with permission from Ref. 98. Copyright 2010, Springer-Verlag.)

4.1.3 Corrosion

The investigation of corrosion is a typical XPS application because chemical information plays an important role. Research activity in this field ranges from basic research on well-defined surfaces to practical applications relating to construction materials. A review of the application of surface analytical techniques (including XPS) for corrosion investigation is given by Quaddakkers and Viefhaus.99 The XPS technique is characterized as a well-established method for the detection of corrosion products. Also useful is AES if applied with the high-energy resolution and with the higher lateral resolution for local analysis. The application of XPS in corrosion science covering many metals has also been reviewed by McIntyre et al.100

An example of a systematic study of the formation of passivation layers is given by Maurice et al.101 for the Fe[BOND]18Cr[BOND]13Ni system. Starting from the analysis of the natural oxide layer, the passivation layer formed in an electrochemical cell, for various exposure times, was studied ex situ in the XPS apparatus. Both depth dependence of the layer by ARXPS and chemical states (with respect to oxide and hydroxide state) were estimated by curve fitting (Figure 12). The investigations were accompanied by AFM (atomic force microscopy) measurements of the surface structure.

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Figure 12. Spectra after 2 h of electrochemical passivation of an Fe[BOND]18Cr[BOND]13Ni sample. The ARXPS measurements, supplemented by the peak fit, show differences in the oxide/hydroxide ratio in the uppermost passive layer. (Reproduced with permission from Ref. 101. Copyright 1998, The Electrochemical Society.)

4.1.4 Catalytic Reactions

Catalysis can be studied with XPS in UHV only remotely. Gas adsorption investigations are of principal interest in this context. The motivation behind the use of surface analytical techniques is that in heterogeneous catalysis the mechanisms of interest all are surface reactions. Most investigations in this field are dedicated to the study of the basic reaction mechanisms of catalytic processes. However, there is a strong interest in getting closer to more realistic conditions, which is discussed in Section 5.6.

Adsorption studies are usually performed under well-defined conditions using single-crystal surfaces.102-104 A typical example is given by Sandell et al.102 with a study of adsorption and reaction mechanisms of several gases (CO, NO, O2, and CO2) on an ordered Pd[BOND]Mn single-crystal surface prepared by Mn deposition on a Pd(100) single crystal and subsequent heating. Figure 13 shows changes in the Pd3d5/2 spectra during adsorption. The multiple components (see the curves at 20 L CO) were separated by peak fitting and assigned to several surface and bulk contributions.

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Figure 13. CO adsorption on Pd. The adsorption started from a clean Pd(100)[BOND]Mn[BOND]c(2 × 2) surface. This ordered surface structure is equivalent to the (100) surface of an ordered Pd3Mn bulk alloy. Coverage using 1 and 20 L (1L = 1 Langmuir, a dose of 10–6 torr s) of CO completely changes the spectra. By heating, the CO coverage can be removed. (Reproduced from Ref. 102. Copyright 1999, Elsevier.)

Besides such adsorption studies, investigations using catalysts with more technological backgrounds were also done on precious metals (e.g. Pd105, 106). From the technological point of view, oxide (supported) catalysts are of more interest and are discussed in Section 4.3.

4.2 Semiconductors

Because of the very small lateral structures of semiconductor devices, XPS is not a favored technique for failure analysis or process control. Thus, AES, as a surface analytical method with high lateral resolution, was used early in this field.107 XPS with its poorer lateral resolution is, nevertheless, used for investigations of new materials and their electronic properties, and for the development of new thin-film deposition techniques. Some early examples are XPS studies of surface treatment108-110 and for new device technologies.111-113

An important problem with semiconductors is the BE calibration. On the one hand, semiconductors have sufficient electrical conductivity for stable measurement using charged-particle methods and on the other hand, the position of the Fermi energy EF, which is the energy reference in XPS (Section 3.1), is not always well defined. In the undoped semiconductor, EF lies in the middle of the band gap, whereas when doped EF is shifted to the acceptor or donor states at the edge of the valence band or conducting band, respectively.114, 115 The degree of shift depends on doping concentration and temperature. As discussed in detail by Egelhoff,116 when in contact with the metallic part of the spectrometer, a (more or less well defined) Schottky contact may be formed; however, at the vacuum side of the sample (which is under investigation), band bending may occur, which can considerably influence the measured BE. This problem has been discussed for B- and Al-doped SiC single crystals.41 UHV-fractured and -sputtered surfaces were compared. Energy shifts that could be assigned to the influence of surface states occurring at the amorphized sputtered surfaces were corrected. Thus, realistic BE values for Al and B could be obtained with respect to their chemical bonding in the SiC environment. It was shown that the Ar2p energy reference, from the noble gas atoms implanted by ion sputtering, was misleading in this case.

4.2.1 New Electronic Materials

The material SiC (discussed above) has been assessed as a candidate for high-temperature applications.117, 118 Also of interest are the potential and problems for studying its electronic properties by XPS. Other materials, such as InP, AlGaAs, or GeSi, are studied typically with respect to their surface and interface reactions.119-124 In InP, for example, the surface Fermi level is separated at cleaved surfaces for p- and n-type doping due to the Fermi level pinning.119 Figure 14 shows Fermi level changes as a result of exposure to several gases and subsequent heating.

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Figure 14. Changes in surface Fermi level for p- and n-doped InP. Differences for (a) hydrogen polysulfide/hydrogen sulfide mixture and (b) ozone exposure are shown. (Reproduced from Ref. 119. Copyright 1992, Elsevier.)

Chemical core-level shifts after treatment with S2Cl2 were examined by Peisert et al.,120 including the use of the Auger parameter method. Figure 15 shows peak shape changes during heating that have been estimated by peak fitting.

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Figure 15. Results of surface treatment of InP(001) with S2Cl2 at room temperature (RT) and with subsequent heating. The BE scale is referenced to the valence band maximum (VBM) and not, as usually, to EF to minimize the influence of bandgap changes on the results. (Reproduced from Ref. 120. Copyright 1996, John Wiley & Sons, Ltd.)

Investigations of new deposition methods125, 126 allow the study of film formation or new interface properties.

4.2.2 Contact Structures

These play an important role in determining electrical properties and microelectronic technology. Rinta-Möykky et al.127 showed the selective oxidation of Te at the interface for a PdTe alloy contact on ZnSe during interface formation, that is, during its preparation. A Pd/SiC Schottky structure was measured with depth profiling, including peak shape analysis.128 Silicide formation could be observed during heating.

4.2.3 Surface Contamination

The characterization and minimization of surface contamination is also an application of XPS. Moon et al.129 studied a cleaning procedure for organic contamination. The effectiveness of the cleaning process, especially when using UV radiation, can be observed through analysis of the C1s peak (Figure 16).

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Figure 16. Efficiency of ozone surface cleaning for hydrocarbon contamination of Si wafers: (a) spectra of residual C1s contamination and (b) C1s intensities for several exposure conditions. (• = taken from experiment (a), ▴ = experiment after a period of one year with another wafer). The positive effect of UV radiation is clearly shown. (Reprinted with permission from Ref. 129, Copyright 1999, American Institute of Physics.)

The XPS technique has been used to optimize etching processes to obtain low defect densities130 and for studying Fermi level pinning for doped Si wafers.131

Quantitative analysis of such thin overlayers and of the material underneath is assisted by mathematical methods. These have been further developed for ARXPS (Section 5.4), including the effects arising from surface topography.132-135

4.2.4 Surfactant-Assisted Growth

As XPS is a highly sensitive technique, the identification of surfactants is straightforward. An example is given in Figure 17. Half a monolayer of antimony is deposited on GaAs(110) before the growth of Co layers. As the Co thickness is increased, the substrate core levels (Ga 3 d, As 3 d) decrease in intensity, whereas the Sb 4 d level remains approximately constant, indicating clearly that antimony is always located at the sample surface, irrespective to the thickness of the Co layer.

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Figure 17. Core level electron distribution curves for surfactant-assisted growth of Co on GaAs. (Reproduced with permission from Ref. 136. Copyright 2005, American Physical Society.)

4.3 Insulators

Surface analytical techniques that use charged particles, such as AES or SIMS (secondary ion mass spectrometry), generally do not work for insulators because their electrical nonconductivity results in surface charging. However, XPS uses electromagnetic radiation for excitation and is therefore favored for the investigation of surface properties of insulators. Insulator material of interest include inorganic material such as various oxides,137, 138 oxide catalysts,88 glasses, and ceramics.139

Nevertheless, the emitted electrons lead to remarkable surface charging of the sample, a feature that has to be considered in any investigation. In most cases, however, the use of nonmonochromatized X-ray excitation leads to relatively small and stable energy shifts at the BE scale. This is because secondary electrons emitted from the X-ray tube effectively stabilize the surface charging situation. External flood guns, which produce low-energy electrons, can be used for additional charge stabilization. This is necessary especially when using monochromatized X-ray radiation.140 This approach also minimizes the shift observed when using nonmonochromatic radiation. Another method is to use a metallic grid (e.g. from TEM) or metallic aperture on top of the insulating sample for charge exchange. The problems with surface charging are complex and depend on sample geometry, differential charging, and peak shape changes.141-143

For quantification of chemical information, the residual shift of the BE scale has to be corrected with respect to a reference BE. There are several traditional methods9: (i) adventitious carbon contamination, C1s at 285 ± 0.4 eV, although there can be relatively broad peaks from various C[BOND]X species144; (ii) internal reference lines of well-defined energy; (iii) implanted noble gas, mostly Ar, although errors may arise from electronic interaction with the surrounding chemical environment; and (iv) artificial mixtures/overlayers, where the problems are homogeneity, contamination, and particle size. A promising method is the deposition of Au nanoparticles from a suspension onto the surface (Au 4f7/2 = 84.0 eV).145 Such referencing techniques have also been successfully used for automatic shift correction of measurement series146 (such as depth profiles with varying surface conductivity) before further data analysis by FA or LLS.

Insulator analysis is a wide-ranging application area, because of the many material classes involved. It includes materials with covalent bonds (e.g. oxides, nitrides, and carbides) and with ionic bonding (e.g. chlorides and fluorides).

4.3.1 Catalysts

Catalysts, discussed in Sections 4.1.4, 4.5.3 and 4.5.4, are often applied in oxide state or supported on oxide material. Mathematical methods (curve fitting, FA, deconvolution) are used for quantification of small peak shifts or peak shape changes in the XPS spectra.147-150

In many investigations, standard spectra are compared with spectra of chemical states formed during treatment of the catalytic surfaces. Davidson et al.147 investigated changes of Ni oxide species on Ce-based material in comparison with NiO, Ni2O3, and Ni(OH)2 standard materials. A transformation from Ni(OH)2 to NiO during heating in air was observed. The chosen BE reference was C1s at 285 eV.

4.3.2 Ceramic Materials

These are investigated to identify changes in their surface properties. Figure 18 shows results for a laser-ablated surface of an Si3N4 ceramic sample. In this example, large-area Nd : YAG radiation in a halogenic atmosphere was being tested for future micromachining applications. A sputter depth profile (Ar+ ions) of the changed surface region of about 2 µm depth was measured. Because of changing conductivity, different peak shifts were observed during depth profiling. An automatic correction with respect to Ar2p from sputtering was done, using the method proposed by Oswald and Baunack73, 146, which proved occurring peak shifts of up to 6 eV. The result, which was derived by FA, shows three main (principal) components with different depth profiles: PC1 is surface oxifluoride, PC2 is an Si-rich interlayer formed from molten and decomposed silicon nitride, and PC3 is bulk silicon nitride.73 However, the resulting spectra (scores) do not have ideal peak shapes because of the relatively high charging shifts, in which there is a possibility of differential charging. The decomposition of silicon nitride as observed in PC2 was also found during local laser radiation in the center of the laser spots. A chemical map of a sample area about 2 mm × 2 mm measured with 100 µm lateral resolution is shown above in Figure 18 and discussed in Section 3.3.

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Figure 18. Results of FA at depth profiling of a laser-treated Si3N4 ceramic sample. (a) The shift necessary for charge correction in connection with the chemical depth profile of the three identified compounds (PC1–3). (b) The spectral shapes of these components point to the following identification: PC1, surface oxifluoride; PC2, intermediate Si[BOND]Si phase; and PC3, bulk Si nitride.

Ion-implanted silicide ceramic has been investigated to detect modification of its tribological properties.151 Line scan and depth-profile measurements established that the wear reduction mechanism is not solid lubrication, as previously assumed, but comes from structural changes in the surface region.

4.3.3 Oxide Materials

Mostly oxides are insulating materials. However, the electrical properties of oxides vary, depending on structure, from wide-gap insulating to semiconducting and to superconducting properties at low temperatures.

As an important dielectric material, silicon oxide has been studied with respect to its surface stoichiometry.152 Several methods (peak area calculation, lineshape fitting, and Auger parameter) were compared, showing the Auger parameter method as being easy, fast, and very reliable. This fact has been confirmed for many oxide systems.153, 154 Cluster formation and oxidation of Ge, implanted in silicon dioxide films after thermal treatment and the redistribution processes, were studied by XPS following the characteristic peak shape changes that occurred.155 These changes were probed further by FA.156

Stoichiometric SiO2 is a very stable oxide and is used as a passivation layer. It is also very stable during ion sputtering procedures, whereas other oxides show marked changes after ion sputtering. Okude et al.157 demonstrated this for tin oxide thin films.

As a semiconducting oxide doped with other components, SnO2 is used for electrical applications such as gas sensors and indium–tin oxide (ITO) transparent contacts. Examples of gas sensors are given by various authors.158-160 Figure 19 shows XPS results159 for a Pt-doped material. Peak fitting procedures show that several compounds are formed at the surface, changing the electrical properties.

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Figure 19. Spectra of a Pt/SnOx film prepared by dc magnetron sputtering. Synthetic peak fit spectra are given in comparison with the measured spectra. At the Sn3d5/2 peak (a) suboxide formation is visible. The peak fit results at O1s peak (b) and the Pt4f peak (c, peak doublet 3,4) show that hydroxide formation takes place at Pt. At the O1s peak, some adsorbed water and an O2-component attributed to SnO2 were also found. (Reprinted with permission from Ref. 159, Copyright 1994, American Institute of Physics.)

It is necessary to know the formal valence states of oxides in order to study their properties and structure. To derive this information from XPS measurements, many problems have to be considered. Although BE reference values from external standards161, 162 can be used, there may be variations arising from the BE not being in correlation with a formal chemical valence.137 Another problem is that the valence states at the sample surface may differ from bulk, as a result of contamination or preferential oxidation.163, 164 By way of illustration, Figure 20 shows spectra for Mo compounds prepared under different conditions. The species Mo6+ and Mo4+ in the MoO3 and MoO2 (scraped) samples are well defined. In the Mn2Mo3O8 (scraped) sample Mo should also be Mo4+(2MnO[BOND]3MoO2), but the peak is significantly separated from MoO2 due to the MnO environment. It may be that this is an intermediate oxide state that also occurs at the MoO2 surface (shoulders to high-BE side). Rapid oxidation to Mo6+ is found after short exposure to air of the Mo4+-containing samples.

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Figure 20. Spectra of different Mo-containing oxide single crystals (a) scraped under UHV conditions and (b) after short treatment in air. Relatively well-defined valence states (the sharp peaks) are found on the scraped samples. Oxidation to Mo6+ occurs immediately in air.

Barr11 has shown that analysis of the oxygen O1s peak shift can give good information about the metal–oxygen bonding. This is also true for O2s information at TiAlO films.154

Complex oxides are of interest for new electronic applications and as oxide superconductors. The depth profile of the Sr[BOND]Ba titanate superlattice shown in Figure 7 can only be measured while using a charge compensation (here a low-energy electron flood gun). Despite the sputtering effects, relatively sharp depth-profile tails point to sharp interfaces. Calculations53 showed that the interface roughness was about 2 nm. High Tc superconductors (especially with a critical temperature, Tc, higher than 77 K, the boiling point of liquid nitrogen) are materials attracting much basic research and are of technological significance. Padalia and Mehta165 have reviewed XPS studies of YBCO oxide superconductors. The main topic of these investigations is the detailed study of the Cu2p spectra including the satellite structures typical of the active Cu[BOND]O superconductor layers in comparison to Cu oxide standard samples.

In this context, it should be mentioned that radiation damage may also occur during XPS measurement of particular samples (Section 4.4). This may arise from both secondary electrons from the X-ray window (nonmonochromatized source) and from photoelectrons and Auger electrons emitted from the sample (also with monochromator). Iijima et al.166 discussed this for the partial reduction of CuO surfaces in different experimental conditions.

4.4 Polymers and Organic Materials

The materials discussed here can be insulating, semiconducting, or electrically conducting and are based on the main element carbon and its compounds. This covers a wide range of materials, including new polymers widely used as construction and functional materials, organic materials including problems of biocompatibility, and new inorganic carbon-based materials such as diamond or a : C[BOND]H (hydrogenated amorphous carbon) layers, carbon nitride, or the conjugated carbon systems (fullerenes, nanotubes, etc.). Because of the relatively low damage by X-rays to polymers or organic materials, XPS was applied to this field early in its development. Clark et al. did a lot of experimental and theoretical work in the 1970s (e.g. on core levels167), which covered a wide range of materials (summarized by Clark168 and by Dilks169). The benefit of valence band information to distinguish different functional groups was also recognized early on.170-174 In biology and medicine XPS also found application.175-177

Investigations of changes in the C1s peak are the main interest. During the early stages of XPS development36, a wide spread of BE was reported,22 from 281 (carbide) to 293 eV (fluorine). However, these material systems consist mainly of the light elements (C, H, N, O, etc.) and thus are sensitive to radiation damage.75 Toth et al.178 discussed this for polyvinyltrimethylsilane with respect to surface ion cleaning and electron flooding for charge compensation. The example in Figure 21 shows results from CN layers demonstrating that only a very small ion dose (Ar+, 3.5 keV, 2 min, 0.3 µA, 2 mm × 2 mm) leads to drastic peak changes and a strong degradation of nitrogen. A promising approach especially for the carbon-based materials is the use of cluster ion sources based on the use C60 fullerenes179 or large Ar (with more than 1000 atoms) clusters.180, 181 In this way, very low energy per incidence particle and very low damage even in depth profile investigations can be reached.

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Figure 21. Spectra of sputter-deposited CN layers (surface composition C : N ≈ 1 : 1) before and after low-dose Ar+ ion sputtering for surface cleaning. The N1s spectra (a) and C1s spectra (b) are normalized and thus do not represent the true intensity relations. The sputtering leads to a strong decrease of N (to 20 at%), but the N1s peak shape is not influenced. Strong peak shape changes are found for C1s. While in the as-deposited state, a mixture of C[BOND]C, sp2, sp3 and C[BOND]O (from surface contamination) bonds with a preference for sp2 was found, drastic changes occur after sputtering and the surface is dominated by the C[BOND]C bonds.

If handled carefully, XPS and UPS provide one of the most powerful methods for the analysis of such sensitive materials, in contrast to electron beam techniques such as AES and EPMA (electron probe microanalysis). Because of its surface sensitivity, XPS is mostly used for investigation of surface treatments or interface properties in compound materials. An overview of XPS applications to polymers up to the late 1980s was given by Briggs.75

4.4.1 Polymers

Polymers and their surface treatments is one main area of XPS application. The cleaning of polyester products is discussed by Briggs et al.182 Figure 22 demonstrates how the complex C1s spectrum of a commercial product can be fitted to the base material spectrum of the polyester and to the spectrum of residuals of a Permalose polymer. This was also confirmed by time of flight/secondary ion mass spectrometry (TOF/SIMS) measurements. The thickness of the Permalose contamination is estimated to 2.4 nm from attenuation length considerations.

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Figure 22. (a) Fingerprint spectra of a polyester-based fabric. The peak overlay procedure shows it contains both (b) the PET base spectrum and (c and d) Permalose polymer spectral components. (Reproduced from Ref. 182. Copyright 1996, John Wiley & Sons, Ltd.)

Similar surface modification experiments have been published with respect to plasma polymerization,183 surface segregation,184 and laser treatment185 for different polymers. Mähl et al.186 have dealt with the fitting of multicomponent C1s peak structures to polymers. Polymer–metal interfaces are of growing interest. During evaporation of aluminum on polyaniline films, Lim et al.187 studied the reaction mechanism by evaluating peak shape changes of the N1s peak. Similar work has been done for the metals Cu, Ti,188 Cu, Fe, Al,189 Al,190 and Cr.191 Riga et al.192 used valence band spectroscopy to identify the specific structure of well-oriented polyethylene mats after adsorption on Au. Structural information can also be derived for Langmuir–Blodgett films.193 Adhesion phenomena are closely connected with the interaction of organic polymers and metal oxides.194

Conducting and doped polymers have been studied by valence band XPS or UPS to establish changes of electronic structure and thus conducting or doping mechanisms.195-197 Salanek198 has summarized investigations on conjugated polymers. As an example, the UPS valence band spectra (Section 5.1) of Figure 23 show how the electronic properties of a polymer may be controlled by sodium doping. The charge carrier density near the Fermi energy increases with dopant concentration, leading to higher conductivity.

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Figure 23. Valence band spectra of an Na-doped polymer. Increasing doping leads to a higher DOS at the Fermi edge and a higher conductivity. The energy scale is calibrated with respect to the vacuum level by comparison with spectra of the free polymer molecules considering the typical spectral features (A, B, C). Therefore, changes in EF can be observed. (Reproduced from Ref. 198. Copyright 1997, John Wiley & Sons, Ltd.)

The possibilities of chemical imaging for polymers were demonstrated by Forsyth and Coxon.199 Using the different C1s energies for C[BOND]C, C[BOND]O, and C[BOND]F as the source for mapping, a surface contamination of polytetrafluoroethene could be detected with a lateral resolution below 5 µm.

4.4.2 Biomaterials

Investigations in this area mostly include the problem of dehydration in a vacuum. Little reported work deals with the measurement of hydrated samples at low temperatures.75, 200 More successful are investigations of naturally dehydrated materials, such as wool,201 bone, teeth, or cellulose-containing materials from plants including wood.202 In this work, the combination of XPS and TOF/SIMS was also demonstrated, which is often used for polymer analysis. Merrett et al.203 reviewed the application of surface analysis for biomaterials bringing XPS in context with many other complementary techniques.

The chemical changes of flax fibers after NaOH treatment have been studied.204 The flax fibers were fixed on the sample holder for analysis with UHV-compatible adhesive tape; however, a longer outgassing period was necessary. Strong oxidation with a peak shift to C[BOND]O was found. Other examples in this field deal with proteins on surfaces205, 206 and aerosol particles.207

Many applications concern biocompatible surfaces208, 209 because of their increasing importance for modern medical implants. The sol–gel technique seems to be promising for oxide interlayer deposition on Ti. Cirilli et al.210 have compared sol–gel-prepared and conventional plasma-sprayed Ti oxide layers. By studying the O1s peak shape, it was found that in the sol–gel-prepared oxide layer fewer OH and H2O components are incorporated, which indicates that these films consist of better defined stoichiometric oxides.

The application of equipment for near-ambient pressure investigation may also be useful in this field.211

4.4.3 Inorganic Carbon-Based Materials

The most studied feature here is the ratio of sp3 to sp2 hybridization, because this controls the material structure. As demonstrated by Meral et al.,212 for diamond-like carbon films using XPS with high-energy resolution, the sp2 (284.4 eV, graphite) and sp3 (285.2 eV, diamond) spectra may be separated directly and quantified by curve fitting. Figure 24 shows that the proportions of these two components depend on the power density used for the laser deposition technique. This separation was used to detect the sp3 content, which had to be maximized for optimum layer properties. For the carbon nitride system with respect to the theoretically predicted C3N4 phase, the sp2 to sp3 ratio of the C[BOND]N bondings plays an important role (sp3-like carbon bonds are expected in C3N4). As concluded by Boyd et al.213 for the C[BOND]N system, the sp2 (C1s = 285.9 eV, N1s = 400.6 eV) and sp3 (C1s = 287.0 eV, N1s = 398.8 eV) bondings are also well separated. From Figure 21, it can be seen that this reactively sputtered layer consists mainly of sp2 carbon–nitrogen bonds and that the degradation due to sputtering leads to a substantial increase of C[BOND]C bonding by preferential sputtering of N.

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Figure 24. C1s spectra of laser-deposited diamond-like carbon films compared with graphite and diamond. With increasing laser power density, a higher quantity of sp3 bonds is found in the films. (Reproduced from Ref. 212. Copyright 1998, Elsevier.)

One area of basic research is connected with conjugated carbon systems, namely the fullerenes, nanotubes, and onions (so-called because of their appearance as multiwalled carbon spheres). Photoelectron spectroscopy is used to study changes in the electronic structure during doping or phase formation. Using core-level measurements, Poirier et al.214 observed structural changes depending on dopant concentration. Figure 25 demonstrates this feature for Rb doping.

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Figure 25. Spectral changes of Rb3d-doped fullerenes depending on Rb concentration, as demonstrated by the coordination geometry: O, fcc octahedral sites; T1, tetrahedral sites; and T2, body-centered phase. (Reproduced with permission from Ref. 214, Copyright 1993, the American Physical Society.)

Valence band measurements are used to study the occupied states near the Fermi energy. For RbC60, changes were found in the lowest unoccupied molecular orbital (LUMO) states, depending on phase structure.215 Additional studies216-218 combining photoemission measurements with electron energy loss spectroscopy (EELS) provide new information about both occupied and unoccupied electronic states in this new class of materials.

4.4.4 Spectral Databases

Spectral databases should become useful especially for polymers and organic materials because of the great variability in their structure and chemical bonding (i.e. for fingerprint investigations). As already in place for static SIMS for several years,219 collections of XPS spectra are now being offered. However, the expectations should not be too high because the spectral shape, especially for complicated systems, depends on many experimental factors, such as X-ray source parameters, take-off angle, spectrometer type and pass energy, and BE reference/charge neutralization. The reliability of such reference spectra depends on the analytical problem and has to be assessed by the individual user.

For some time, XPS spectra, including sample and measuring parameters, have been published periodically in a particular journal.220 A database specializing in organic polymers has also been published.221 Two commercial databases in electronic form come from the National Institute of Standards and Technology (NIST), USA222 (with free access up to version 3.5) and from XPS International (founded in Japan, now moved to the USA; The last-mentioned company now also distribute the bulky spectra collection in the handbook series of B. V. Crist.223 Many of such activities are also offered on the Worldwide Web: (, (, (, and ( Naturally, such a list cannot be complete.

A free database from the Surface Analysis Society of Japan is available, including a data processing system from the Internet ( This database can be combined with own spectra. In this context, it should be also mentioned that there exists a project for implementing a standardized data format225, 226 for the transfer of spectral data between different machines in surface science.

The NIST also published some very useful databases227 with electron-solid interaction data: “NIST electron elastic scattering cross-section database” (SR 64), “NIST electron inelastic-mean-free-path database” (SR 71), and “NIST electron effective-attenuation-length database” (SR 82).

4.5 Functional Materials

In common sense, functional materials get their functionality from special interaction features with their surrounding based on their physical or chemical properties, special structures or sizes, and preparation or combination with each other.228 Thus, they have in general a large overlap with the materials discussed in the previous sections because they are of the same origin. Nevertheless, their specific properties also require dedicated analysis, which can partly also cover surface chemistry characterization with XPS. Most topics are also in context with the actual demand for sustainability in modern economy and thus of very high interest. However, in this section, only some of the basic ideas can be touched.

4.5.1 Energy Applications

This is a key topic for the next decades because of limited resources and climate changes. XPS relevant topics in this field are especially found in energy conversion in fuel cell, storage in batteries, or thin-film solar cells.

Corcoran et al.228 reviewed the use for XPS for fuel cells with particular focus on several metallic electrocatalysts, which play a key role in optimization for application. In addition, the surface properties of the mostly C-based membrane electrodes229 and its degradation230, 231 during use are of interest. Often special electrochemical cells are used to avoid influence from atmosphere.228, 232

The development of modern Li ion batteries is closely connected with the electrochemical reactions at both electrodes.233, 234 On the anode side, XPS investigations often point to the “solid electrolyte interphase” (SEI) and damage by Li plating;235, 236 on the cathode side, besides SEI studies, mostly the changes of the redox-partner of Li are investigated.237, 238 However, stability aspects of the electrolyte also play an important role.237, 239 Figure 26 demonstrates changes at an LixCrMnO4 anode material after 100 charging/discharging cycles to reduced (4.88 V) and full (5.2 V) cell voltage.240 Charging to full voltage (broken line) leads to high starting capacity, but worse reversibility (Figure 26a). Cells charged only to 4.88 V shows good reversibility, but at a lower capacity level. The bad reversibility for 5.2 V is connected with irreversible Cr6+ formation (Figure 26b), and a lithium-free defect layer (Figure 26d and e) is formed. Here, “quasi in situ” analysis with Ar-filled transfer chambers was applied. Basic investigations are often done with thin-film solid-state model cells,241-244 because of the better compatibility with the necessary UHV conditions during analysis. Such thin-film cells,245, 246 however, also have the potential to be used as miniaturized batteries “on a chip.”

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Figure 26. Behavior of the discharge capacity when cycling an LiCrMnO4-based battery cell 100 cycles (a) and XPS spectra (b–e) for the discharged states when charging the cell up to different end voltages of 4.88 (solid lines) and 5.2 V (broken lines). When charging to 5.2 V, the capacity loss is connected with a lithium free surface region dominated by Cr6+ and Mn3+. (Reprdocued from Ref. 240. Copyright 2010, John Wiley & Sons, Ltd.)

The field of solar cell technology is also widespread. Here, XPS can be used for process control,247 chemical analysis,248, 249 energy level alignment,250 or depth-profiling investigations.251

4.5.2 Nanoparticles

Nanostructured materials of different nature are more and more used to enhance the parameters of common materials or because they have itself special properties. A priori such materials have a large surface-to-volume ratio and thus their surface properties, interactions with the surrounding, and its characterization play an important role. An extensive review about a large variety of different nanomaterials was recently collected by Barron.252 Mostly large-area measurements at agglomerates of the particles are done. A large number of XPS investigations can be found for carbon nanostructures, such as carbon-nanotubes or nanofibers. Especially the study of the functionalization of the surfaces of carbon-nanotubes is a topic of particular interest, where XPS is extensively applied.253-256 Here, both the chemical elemental composition at the surfaces and changes in the chemical bonding by peak shape changes are studied. Figure 27 shows an example where single-wall C-nanotubes (SWNCT) were doped with nitrogen during preparation with CVD.255 Different nitrogen content was controlled by changing the atmosphere used during nanotube growth. Characteristic changes in both the spectral shapes (a–e) of the incorporated nitrogen and in the total N concentration and portions of different detected N species (f) were found.

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Figure 27. N1s XPS signals (a–e) of SWCNTs prepared with different acetonitrile/ethanol mixtures; (f) content of different nitrogen forms incorporated into SWCNTs as functions of the acetonitrile concentration in the acetonitrile/ethanol mixtures. (Repoduced from Ref. 255. Copyright 2010, Elsevier.)

Otherwise, there are a lot of XPS-related studies on metallic or oxide nanoparticles257-260 and nanowires261-265 of a wide spread of different elements and materials, often connected with sensor, catalytic, or medical application. Thus, also in the other mentioned topics of Section 4.5, nanomaterials play an important role. In addition, nowadays, localized XPS analysis is also done using synchrotron radiation, as summarized in Section 5.5.

4.5.3 Catalysis

Catalysis is already mentioned in several previous chapters and after a long period of XPS use, it is still in the focus of interest, because catalytic reactions are surface reactions. Many review work give overviews from different point of scientific interest.228, 266-268 To reach high catalytical activity, high surface fraction is necessary; thus, most catalysts are used in form of nanoparticles (Section 4.5.2). Up to now, most of the XPS studies are still on noble metal269, 270 and oxide271, 272 catalysts; however, also in catalysis, there is a trend to overcome the pressure gap273-276 by special equipment and using synchrotron radiation (Sections 5.5 and 5.6).

4.5.4 Photocatalysis

Photocatalysis is a recent topic; it started by the beginning of this century. The key material for photocatalysis is titanium dioxide, a complicated material (it crystallizes into several forms: anatase, rutile, brookite, and amorphous phases are also easily prepared). This is a semiconductor with a bandgap of 3.2 eV with the following properties: when activated with UV light (energy above the bandgap), the surface is activated and exhibits unusual catalytic properties, especially for degradation and mineralization of organic compounds. Therefore, such surfaces present “self-cleaning” properties. Titania is also used in water or air decontamination, reduction of detergents or pesticides, odor elimination, etc. A comprehensive review of photocatalysis can be found in 277. In addition, titania surface exhibits different wetting behavior, ranging from super-hydrophilicity to super-hydrophobicity. These wetting properties can also be triggered UV light. The current efforts in the photocatalysis area are dedicated to reduce the bandgap of the material by doping or other phase modifications (e.g. nanostructuring) in order to render it efficient for activation with visible light.

As photocatalysis is a surface phenomenon, it is often studied by XPS. Chemical states of doping atoms, such as metals (Fe and Eu) 278 or nitrogen 32, 33. The chemical states of doping atoms are derived, which is a useful parameter to be related with structural modifications, optical absorption, and photocatalytic efficiency.

4.5.5 Tribology

Tribology is not really a topic of “functional material” itself, but is mentioned here in this relationship because it is a kind of functionalization of surfaces and thus it is closely connected with surface chemistry. Optimized tribology layers are of very high importance for the lifetime of engines and other industrial products using moving parts. It was already accepted that surface characterization is an important part of tribology science.279, 280 The tribochemical reactions are very complex and thus in particular XPS is a favorable technique because its capability to study surface chemistry by analyzing peak shifts. From the very large widespread of applications, here only some actual examples can be mentioned: role of additives in oil281, 282 ore grease,283 ionic liquids,284 PTFE films,285 hard coatings,286, 287 chemistry,288, 289 and depth profiles290 of specific tribofilms.

5 Special Methodologies

  1. Top of page
  2. Introduction
  3. Sample Requirements and Sample Preparation
  4. Measuring Strategy
  5. Applications for Typical Material Classes
  6. Special Methodologies
  7. Comparison with Other Techniques
  8. Outlook
  9. Related Articles
  10. References

5.1 Valence Band Spectroscopy

The method of valence band spectroscopy (Section 3.4) is an integral part of the XPS method. However, it has a particular role in terms of chemical analysis, because chemical bonding is directly involved in the valence band structure and the valence spectra are quite complex. Therefore, no direct insight into the elemental composition of the investigated sample is possible. Consequently, some of its applications are discussed here in this separate section. Because of the higher excitation probability, UV radiation from discharge lamps (He I = 21.2 eV, He II = 40.8 eV) is often used. UPS has been used for the investigation of light elements, gases, small molecules or hydrides,291 oxidation/chemisorption processes,83, 84, 292 and conducting polymers,170, 293 often in conjunction with XPS studies. Valence band investigations (also partly done by X-ray excitation) are useful for the identification of different structures in polymers where the C1s core-level peak shows no change.173, 294 These UPS results have been compared with DOS calculations for simple metallic systems.295, 296 Using approximation calculations for the molecular electronic structure commonly based on multiple scattered wave methods (usually described as Xα calculations),297, 298 valence band spectra estimations are possible for larger molecules. The combination of such measurements and DOS calculations are performed routinely to confirm theoretical considerations.299, 300 Figures 28 gives an example for that showing calculated and experimental spectra of different dimers of doped fullerenes.

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Figure 28. Valence band spectra for (C59N)2 (full circles) and (Rb1C60)2 (open circles) in comparison with density-of-states (DOS) calculations (solid lines). The normalized curves of the calculated DOS and the measured photoelectron intensities can then be used for qualitative comparison of the doping mechanism. (Reproduced with permission from Ref. 300, Copyright 1997, the American Physical Society.)

A sophisticated version of valence band studies is the angle-resolved (ultraviolet) photoelectron spectroscopy (ARPES) where it is possible to measure the energy distributions of the emitted electrons in combination with its momentum. Thus, it is possible by polar and azimuth-angle-depending experiments to map the complete band structure and Fermi surface topology. This is especially of interest for basic studies of superconducting materials and is pushed by new developments in the field of detectors, analyzers, and high-brilliance light sources (synchrotron).301, 302 Figure 29 shows as an example of such a photoemission map at the Fermi level of an iron–arsenide superconductor.303, 304

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Figure 29. Distribution of the photoelectron intensity at the Fermi level at a Ba1 − xKxFe2As2 superconductor measured with ARPES. (Reproduced with permission from Ref. 304. Copyright 2009, American Physical Society.)

5.2 Loss Spectroscopy

Because the X-ray-induced photoelectrons or Auger electrons have to cross the surface region before they are emitted into the vacuum, inelastic scattering may influence their energy. If the structural, electronic, or chemical nature of the material causes specific energy (region) of these losses to dominate, additional peaks may be identified and used for analysis. Processes for such additional (secondary) structures are very complex and are described in more detail elsewhere.9, 10, 22 In this section, although not completely correct, all additional structures at the low-KE (high-BE) side of the photoelectron peak are briefly discussed as losses:

  • Plasmon losses: These collective interactions with the conduction band electrons mainly occure at metals, and are divided into bulk and surface plasmons. These losses may also be discussed in connection with conventional EELS investigations.

  • Shake-up satellite lines: These may be observed if a significant part of the emitting atoms is in the final state, not in the ionic ground state but in an excited state (valence electrons excited in higher unfilled levels).

  • Asymmetric core levels: These are found in metals due to the excitation of conducting band electrons into unfilled bands above the Fermi energy (similar to the shake-up process). In nonmetals, vibrational broadening may be observed.

  • Multiplet splitting: This may occur if the atom has unpaired electrons in the valence level. Because of the residual spin, the final ionic state may have a different energy and an additional splitting may be observed.

In connection with the main XPS features, an analysis of such structures can lead to additional information. Cremer et al.154 have discussed the possibilities of loss analysis for ceramic material. There is also a direct comparison of XPS and EELS loss analysis.

A classic example of loss spectroscopy is boron nitride,305 because its cubic and hexagonal modifications may not be distinguished by XPS core-level analysis, but these are clearly identified by changes of the π − π* shake-up satellite. By way of illustration, Figure 30 shows spectra of hexagonal BN and cubic BN material.

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Figure 30. Difference between hexagonal (h-) BN and cubic (c-) BN by investigation of the π–π* transition 9 eV from the B1s peak. (Reproduced with permission from Ref. 305, Copyright 1997, American Institute of Physics.)

Other typical examples where losses are important and have similar intensities (sometimes larger) than the parent lines are rare earth oxides, which are prototype catalysts and exhibit also photoluminescence. A typical example is CeO2, where the 3 d XPS spectrum is quite complicated and only recently it was simulated with a mixture of Ce3+ and Ce4+ doublets, each one splitted in (parent + shake-up + shake-down components), therefore with six doublets, or 12 lines.306

5.3 Photoelectron Diffraction

As the nonelastic processes discussed in Section 5.2, elastic scattering of the emitted electrons is a general process in the surface region. Because of the wave nature of the electrons in the quantum-mechanical description, diffraction and interference phenomena are to be expected. By measuring the peak intensity for the BE of a core-level photoelectron peak in the hemisphere above the sample, diffraction patterns can be observed if the surface region is well ordered (e.g. for single crystals or epitaxial layers). One of the first detailed studies was the work on copper by Fadley et al.66 In practice, the sample is usually rotated with respect to the analyzer (and the X-ray source) for such measurements.66, 307 PED is a complementary technique to extended X-ray absorption fine structure (EXAFS) investigations.10, 308

Band dispersion properties can be investigated using UV excitation and photoelectron energies near the Fermi energy with angular dependent measurements (Section 5.4). Using a small energy window, Fermi surface mapping can be done — as a spherical cut through the three-dimensional Fermi surface.309 This work demonstrates the powerful combination of such UV excited with hard X-ray techniques. As an example, Figure 31 shows diffraction patterns of Au to study the growth mode on Bi-2212 (Bi2Sr2CaCu2O8 + x) superconductors, where an 8.4-nm-thick Au film was deposited onto Bi-2212. The measured Au4f diffraction pattern (Figure 31a) can be explained by a structural model (Figure 31b) of two Au(111) domains rotated by 180 °. This was concluded from Figure 31(c), which shows a very similar hexagonal pattern to that in (a). This pattern was reconstructed from an overlay of two Au(111) diffraction patterns as measured on Cu(111) (Figure 31d), but rotated by 180 °.

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Figure 31. Examples for Au4f PED patterns in connection with the study of contact structures on Bi-2212 superconductors. (a) Measured Au4f diffraction pattern of a thin Au film on Bi-2212, (b) proposed structural model: 2 Au(111) domains rotated by 180 °, (c) pattern very similar to (a) reconstructed from two Au(111) diffraction patterns rotated by 180 ° as measured at 18 monolayers Au on Cu(111) shown in part (d). (Reproduced from Ref. 309. Copyright 1996, John Wiley & Sons, Ltd.)

Other work310, 311 deals with structural effects in GaAs-based epitaxy; already mentioned also in the section 4.2.4 with a XPD study of bcc-Co grown on GaAs 32. As shown theoretically by Nefedov and Fedorova,312 it should also be possible to learn about overlayer thicknesses by investigating the shape of the diffraction patterns.

5.4 Angle-Resolved X-Ray Photoelectron Spectroscopy

Despite the general lack of information content in ARXPS measurement as stated by Cumpson,313 its application is widely accepted. This trend is supported by the increasing demand for nondestructive surface studies and by a number of theoretical studies in this field since 1995. The main challenge in ARXPS is that the measured data need always model calculations for interpretation. Figure 32 shows a simple example of an ARXPS application at a native Al-oxide layer.314, 315 The angle dependent Al-data show typical variation with angle and can be separated in oxide and elemental component for improvement of data analysis. The quantification calculation based on a simple multilayer algorithm assuming exponential signal decay with depth70, 315 shows a good match between experiment (points) and approximation (lines). Resulting surface structure here was an Al-oxide layer of 2.7 nm thickness, covered by a carbon contamination of 1.1 nm (relative surface coverage of 0.6).316, 317

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Figure 32. Changes in the Al-oxide and Al-element peak areas for an angle-resolved XPS measurement at a native Al-oxide (a). Approximation of the measured ARXPS data (points) with model calculations (lines) without (b) and with splitting (c) of the Al oxide/element portion result in 2.7 nm Al-oxide and 1.1 nm C-contamination. (Reproduced from Ref. 314. Copyright 2005, Elsevier.)

Such approximations are possible in any case, but a lot of error sources have to be considered (measuring statistic, roughness, elastic scattering, and quality of material parameters) critically for the interpretation of the results. A review of the state of the art of application and limits of ARXPS were published recently.338 Remarkable progress in the quality of data interpretation has been reached by simulation of ARXPS data with Monte-Carlo methods.318-320 Here especially the influences of sample roughness321-323 and elastic scattering320, 324 could be clearly demonstrated.

From the experimental point of view, the application of ARXPS is supported by the development of energy analyzers with two-dimensional detectors and parallel measurement of the whole polar angle region and improved (e.g. maximum entropy) data evaluation software.

5.5 Synchrotron Application

The increasing number of synchrotron light sources around the world led to an enormous rise of its application also in the field of XPS, because with special monochromators intense monoenergetic X-rays over a wide energy range can be generated.325-329 One disadvantage is the necessary installation of the equipment at the beam lines, which can be overcome, however, by well-organized sharing of resources between the individual research groups. There are, in general, three main trends to observe, the use of (i) hard and tunable X-rays for increasing the information depth, (ii) focused X-rays for local chemical microanalysis, and (iii) the very high beam brilliance for special applications.

Hard X-rays allow extending the depth region for XPS investigations.330, 331 This method usually abbreviated as HAXPS is useful for studies of buried interfaces or thicker overlayers.332, 333 Combined with ARXPS, the range of nondestructive depth profiling can be extended.334, 335 Another possibility is the use of different X-ray energies from the tunable synchrotron source for in-depth studies with varying the information depth at fixed angle,336, 337 which is complementary to ARXPS.

A very impressive application of synchrotron-based XPS is chemical imaging,328, 338 whereas both principles X-ray beam focusing with scanning and high-resolution imaging microscopy are applied.338 Figure 33 shows an example338 where Ni-silicide of submicron dimension could be distinguished in both morphology and chemical phase.

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Figure 33. (a) About 20 × 20 and 5 × 8 µm2 Ni3p maps and intensity profiles of round (1) and elongated (2) islands detected at Si surfaces after annealing a 2 ML Ni film. (b) Analysis of local spectra points to NiSi in the round (top) and NiSi2 in the elongated (middle) islands. (Reproduced from Ref. 338. Copyright 2009, Elsevier.)

The advantage of the high brilliance of the X-ray beam is used for high-resolution application,339, 340 real-time measurements,102, 341 PED342, 343 (Section 5.3), resonant photoemission,344 spin-resolving345, 346 studies, or environmental XPS (Section 5.6).

5.6 In Situ Preparation, Environmental X-Ray Photoelectron Spectroscopy

Because of the problems that can arise concerning surface contamination, sample preparation is an important issue. Classical preparation techniques such as sequential sputtering and heating (for single-crystal preparation17), scraping, fracturing, or cleaving (for bulk or grain-boundary investigations) can often be used directly in the analysis chamber under UHV conditions.18 The trend toward more realistic experimental conditions for surface and interface modification has led to the use of special preparation chambers, to protect the analytical system from damage or contamination. Such chambers are separated from the analysis chamber by gate valves. This makes it possible to conduct procedures such as layer deposition with various deposition techniques, heating to higher temperatures, treatments with reactive gases, and even electrochemistry without affecting the XPS measuring system. For contamination-free transfer of the sample after treatment, the preparation chamber is evacuated to UHV.18

Electrochemical experiments and other solid–liquid interaction studies have been tried, using specialized equipment347-353 to deal with the non-UHV conditions. These so-called environmental studies, described also as NAP-XPS,19-21, 211 have been supported by the better availability of synchrotron-based X-rays in recent decades, too. The high-pressure to UHV gap is bridged either by differential pumping systems354, 355 or membranes.356 In addition, special in situ electrochemical cells357 can be used, where the working electrode can be directly transferred into the spectrometer. Another way to avoid contamination is the use of special transfer chambers.240, 358-360 Such equipment can be used for changing samples between different vacuum processes or to extract them from experiments under glove-box conditions.

6 Comparison with Other Techniques

  1. Top of page
  2. Introduction
  3. Sample Requirements and Sample Preparation
  4. Measuring Strategy
  5. Applications for Typical Material Classes
  6. Special Methodologies
  7. Comparison with Other Techniques
  8. Outlook
  9. Related Articles
  10. References

Comparison with other techniques is a difficult task, because of the wide range of methods and their specifications. For the confident solution of most problems, a skillful combination of several techniques is usually necessary.

As a basis for comparison with other analytical techniques, the main characteristics of standard XPS are the following:

  • Features: High surface sensitivity (2–5 nm), elemental information from core levels, detection limit about 0.1 at%, chemical information from peak shifts, sputter depth profiling, and direct measurement of band structure (valence band).

  • Advantages: High surface sensitivity, chemical information easily to understand, nonconducting material possible, and low sample damage.

  • Disadvantages: Low lateral resolution and thus low depth-profiling sputter rates, problems with surface contamination, difficult preparation of bulk states, and difficult BE scale calibration at insulators and semiconductors.

The techniques outlined below and other surface-related techniques are summarized regularly in several monographs.3, 12, 25, 361-363

6.1 Auger Electron Spectroscopy

AES is the most complementary technique to XPS because of the similarity of the physical process employed. The Auger electrons are usually excited by a focused electron beam (3–10 keV), but X-ray excitation is also possible. In principle, AES has the same surface sensitivity and detection limits, although it is used more routinely for surface elemental analysis. High lateral resolution (20 nm) and faster depth profiling is possible. Chemical shifts are also present here, but they are more complicated to interpret. The AES technique is not suitable for insulating samples. Often multitechnique equipment AES/XPS is available on one machine using the same electron analyzer. In addition, AES is more effective in promoting sample contamination even in UHV 1.

6.2 Secondary Ion Mass Spectrometry

This technique, which uses ion sputtering and detection of the emitted secondary ions from the surface, is a method for local trace analysis (ppb range). Its characteristics include lateral resolution down to 50 nm (with liquid metal ion sources), monolayer depth resolution (additionally broadened by mixing effects), and detection of all elements (including H), and isotope separation is possible. Quantification is difficult because of strong matrix effects. Typical application field is monitoring of dopant depth profiles in microelectronics. With additional postionization, using secondary neutral mass spectrometry (SNMS), better quantification may be obtained. In static mode (low ion dose) and with TOF (time of flight) mass spectrometry (offering high mass resolution and mass range), molecular information may also be obtained; static SIMS is often used in connection with XPS for polymer analysis.

6.3 Atomic Force Microscopy and Scanning Tunneling Microscopy

These are methods for studying the local surface structure. Atomically sharp tips mounted on a very thin cantilever are used close to (nanometer distance) or in slight contact to the investigated surfaces. Mechanical (i.e. atomic forces) or electrical (i.e. tunnel current) interaction is used as the detected signal. Fine positioning of the cantilever is done by actuators piezo ceramics and the resulting deflections are measured with laser optics. Structural information of the surfaces down to atomic resolution is routinely available. Using special measuring modes (modulation, special tips), additional local (mechanical, magnetic, and chemical) information may be obtained. These methods are often used in connection with XPS to derive additional molecular and structural information. Atomic force measurements are also possible in the atmosphere or even in liquids. The main problem of AFM and scanning tunneling microscopy (STM) is the lack of chemical information: with AFM, it seems unlikely to obtain such information, whereas STM evidences local variations of the electron density (with sub-Angstrom lateral resolution), but it is difficult to state on the exact nature of the corresponding atoms in the case of a multicomponent sample. Eventually, scanning tunneling spectroscopy (STS) may be performed, which may be regarded as a combined UPS-IPS performed locally. However, the interpretation of such STS V–U curves is sometimes difficult and requires UPS and IPS performed macroscopically for confirmation.

6.4 Low-Energy Electron Diffraction and Reflection High-Energy Electron Diffraction

These are both electron diffraction techniques that can be used for surface structure investigations during sample cleaning (single crystals), thin-film preparation (epitaxy), and adsorption studies. They mainly differ in the impact energy and in the impact angle of the electrons. Because of the very low penetration depth of one or two atomic layers, low-energy electron diffraction (LEED) applications mainly concern adsorption or single-crystal studies. In reflection high-energy electron diffraction (RHEED), high-energy electrons (typical 5–20 keV) are used for diffraction at crystalline material. Surface sensitivity is reached here when working at grazing incidence and may be controlled by variation of the reflection geometry. This method is also used in combination with glancing angle X-ray diffraction. Another useful application of RHEED is the true in situ follow-up of the layer-by-layer growth by using RHEED oscillations, since the RHEED pattern is of higher intensity when the electrons are diffracted by a continuous layer, then their intensity decreases when the next layer starts to grow, reaching again a maximum when the next layer becomes continuously. In the MBE community, it is commonly asserted that RHEED oscillations are the only way to exactly estimate the number of layers grown.

6.5 Ion Surface Scattering

This is a rarely used method with extreme surface sensitivity (topmost layer). The elastic scattering of light ions (typically He+ or Ne+ in the energy range from 500 eV to a few kiloelectron volts) depends on the masses of surface elements. An energy spectrum of the scattered primary ions, depending on the angular situation, is recorded. The surface damage is low, but not always negligible. The main applications are surface segregation and adsorption studies. It is usually combined with XPS because of the use of the same analyzer (with changed polarity). Ion surface scattering (ISS) is sometimes called low-energy ion scattering (LEIS).

6.6 Positron Annihilation-Induced Auger Electron Spectroscopy

This is another highly surface-sensitive method, its sensitivity being practically limited to the outermost single atom layer. The internal potential of any solid sample, which may be regarded as a potential well for electrons, is converted into a potential barrier for positrons. When low-energy positrons are directed toward a sample, their interaction with valence band electrons stabilizes them in bound states located near the surface, with a lifetime of a few picoseconds, before annihilation. Positron annihilation occurs therefore with electrons from the outermost atoms of the sample only. When such annihilation produces a core hole in an outermost atom, this core hole induces Auger electron emission [Ref. 364]. Therefore, these Auger electrons may be used to investigate the composition of the outermost single atom layer. The disadvantage of this method resides on the weak intensities of conventional positron sources, to the necessity of gamma radiation shielding of the setup and eventual to the necessity of using big facilities, such as nuclear reactors.

6.7 Infrared Spectroscopy

In surface analysis, infrared spectroscopy is useful for investigation of the bonding of molecules on the surface. Two methods should be mentioned here: Fourier-transform infrared spectroscopy (FTIR, based on the absorption of the radiation) and Raman spectroscopy (based on characteristic inelastic scattering of light). Lateral information may be obtained using focused laser radiation. Surface analysis is done preferably using reflection geometry. Vibrational infrared modes give information on the bonding types in UHV, under any gas pressure or in the liquid phase, which allows in situ experiments even in electrochemical cells.

6.8 Electron Energy Loss Spectroscopy

This method has been discussed earlier in this article and is not a typical surface analytical technique. However, it is often used in connection with valence band XPS and UPS for basic studies of electronic band structure. Because high-energy electrons are used, EELS is commonly done in transmission on thin samples (typically 100 nm) in both standard transmission electron microscopes (mainly for local element or chemical analysis) and special UHV equipment (basic band structure studies).

6.9 Rutherford Backscattering Spectroscopy

This is a widely used nondestructive depth-profiling method with very good quantification capabilities using the scattering of high-energy (as opposed to ISS), low-mass ions (typically He+ in the 1 MeV range). The depth information has to be recalculated from the broadening of the elemental scattering peaks on the KE scale. The method has no lateral resolution (typically in the millimeter range, or using special beam collimators down to some micrometers) and is not suitable for light elements with high-Z samples.

6.10 Glow Discharge Spectroscopy

Glow discharge optical emission spectroscopy (GDOES) and glow discharge mass spectrometry (GDMS) are developing from bulk-only to depth-profiling methods because of new source design and fast data collection. Relatively low ion energies from the glow discharge enable good depth resolution; however, the surface sensitivity is limited by high erosion rates and high sample temperatures. No lateral information is available (millimeter range).

7 Outlook

  1. Top of page
  2. Introduction
  3. Sample Requirements and Sample Preparation
  4. Measuring Strategy
  5. Applications for Typical Material Classes
  6. Special Methodologies
  7. Comparison with Other Techniques
  8. Outlook
  9. Related Articles
  10. References

Photoelectron spectroscopy is an analytical technique that, during its several decades of use, is capturing an increasingly important role among the surface-related techniques. Both experimental and theoretical aspects are now well understood. For routine analysis, further development in terms of quantification, promoted by the revolution in computer techniques, is envisaged — more sophisticated escape depth models, calculation of chemical shifts and valence band features, and use of large spectral data banks. Demands on future applications will be connected with new trends in biotechnology, environmental research, life sciences, biology, and nanotechnology. New instrumentation is likely to address needs for lower spatial resolution, parallel detection, higher brightness, good energy resolution, and variable X-ray energies, and will be closely connected with new generations of synchrotron sources and X-ray optics.

Abbreviations and Acronyms

Auger Electron Spectroscopy


Atomic Force Microscopy


Angle-resolved (Ultraviolet) Photoelectron Spectroscopy


Angle-resolved X-ray Photoelectron Spectroscopy


Binding Energy


Chemical Vapor Deposition


Density of the Occupied Electronic States


Electron Energy Loss Spectroscopy


Electron Probe Microanalysis


Electron Spectroscopy for Chemical Analysis


Extended X-ray Absorption Fine Structure


Factor Analysis


Fourier-transform Infrared Spectroscopy


Full Width at Half Maximum


Glow Discharge Mass Spectrometry


Glow Discharge Optical Emission Spectroscopy


Inverse Photoemission Spectroscopy


Ion Surface Scattering


Indium–Tin Oxide


Kinetic Energy


Low-energy Electron Diffraction


Low-energy Ion Scattering


Linear Least-squares


Lowest Unoccupied Molecular Orbital


Molecular Beam Epitaxy


Nonlinear Least-squares


Principal Component Analysis


Photoelectron Diffraction


Pulsed Laser Deposition


Reflection High-energy Electron Diffraction


Scanning Electron Microscopy


Secondary Ion Mass Spectrometry


Secondary Neutral Mass Spectrometry


Scanning Tunneling Microscopy


Scanning Tunneling Spectroscopy


Transmission Electron Microscopy


Time of Flight


Time of Flight/Secondary Ion Mass Spectrometry


Ultrahigh Vacuum


Ultraviolet Photoelectron Spectroscopy




X-ray Excited Auger Electron Spectroscopy


X-ray Photoelectron Spectroscopy


X-ray Photoelectron Diffraction


  1. Top of page
  2. Introduction
  3. Sample Requirements and Sample Preparation
  4. Measuring Strategy
  5. Applications for Typical Material Classes
  6. Special Methodologies
  7. Comparison with Other Techniques
  8. Outlook
  9. Related Articles
  10. References
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