Anal. Chem. 1998, 70, 229R-250R
Surface Analysis: X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Noel H. Turner*
Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5342 John A. Schreifels
Department of Chemistry, George Mason University, Fairfax, Virginia 22030 Review Contents X-ray Photoelectron Spectroscopy Instrument Calibration Data Handling and Line Shape Analysis Background Subtraction Binding Energies Valence Band Spectra Shake and Multiplet Splitting X-ray-Excited Auger Electrons and Auger Parameter Semiconductors Polymers Films Instrumentation Auger Electron Spectroscopy Line Shapes Beam Interactions Background Quantitative Analysis Depth Profiling Instrumentation and Data Handling Scanning Auger Microscopy Combined XPS-AES Topics Analysis Instrumentation Analysis Depth Depth Profiles Angular Diffraction Electron Spectroscopy Coincidence XPS and AES Literature Cited
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This fundamental review is on the subject of X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), and Auger electron spectroscopy (AES) and will cover publications abstracted in Chemical Abstracts between October 1995 and October 1997. The review is written in three separate parts for the convenience of the reader: section A, XPS; section B, AES; and section C, combined XPS-AES topics. However, for those who use only one of these techniques, there may be items of interest in the other sections. XPS and AES are used widely for the analysis of surfaces. From about 1970 to the present time, these techniques have grown in acceptance by the scientific community. Much of this activity has been documented in earlier Fundamental and Application Reviews in Analytical Chemistry (1-24). S0003-2700(98)00013-4 CCC: $15.00 Published on Web 05/01/1998
© 1998 American Chemical Society
While this review is lengthy, it is not an all-inclusive bibliography of XPS and AES during the review period. The articles have been selected with the focus on improvement in the “state of the art” of these techniques and new trends. The goal of this review is to help analysts solve the problems that are encountered in using XPS and AES in a regular laboratory with commercially available equipment. In addition, reviews on related topics, e.g., ion-induced Auger transitions, are cited. A section on inelastic mean free paths (IMFP) and related topics is included in the combined XPS-AES part of this review. Finally, the names of the authors of the papers cited were not included in the text due to space limitations. X-RAY PHOTOELECTRON SPECTROSCOPY X-ray photoelectron spectroscopy (XPS) is probably the most widely used of the surface analysis techniques due to its capability of determining the surface composition without (in many cases) altering the composition during analysis. Additionally, it can give insight into the chemical environment of the surface constituents. This is invaluable in many fields of science, such as (but not limited to) catalysis, corrosion, and semiconductors. A book was published discussing the principles and applications of the technique (A1). Critical reviews compared the popular XPS data banks (A2) and discussed the determination of binding energies from fluorinated carbon compounds by XPS (A3). A review of XPS and its application to surface science (A4) appeared. A critical review covered a discussion of X-ray natural widths, level widths, and Coster-Kronig transitions probabilities which were collected from XPS and absorption spectroscopies (A5). An overview of electronic structures of solids, basic features that can be observed in XPS, and the typical instrumentation used was published (A6). Reviews discussed the applications, samples, analysis time, limitations, instrumentation, and related techniques (A7) and the principles and applications of laterally resolved XPS with monochromatic X-rays (A8-A11). The penetration of deposited Ag and Cu overlayers through alkanethiol self-assembled monolayers on Au (A12), the use of electron and vibrational spectroscopies in biological sciences (A13), the application of XPS and other techniques to the study of tropospheric heterogeneous reactions (A14), the initial interaction of adsorbates with the substrate (A15), the identification of mixed oxidation states in catalysts (A16), the physical characterization of nanoparticle interfaces (A17), the characterization of paper (A18), and ceramic surface chemistry (A19) were reviewed. Analytical Chemistry, Vol. 70, No. 12, June 15, 1998 229R
Other articles appeared that discussed recent developments in the study of core-hole electronic structure using synchrotron radiation (A20, A21), the use of XPS with synchrotron radiation for the surface microanalysis in the life-sciences (A22), and typical methods of performing electron spectroscopic analysis with synchrotron radiation (A23). Methods of performing theoretical calculations of core-level spectra in solids having f and d electrons with Anderson impurity model (AIM) (A24, A25) and recent developments in the understanding of XP spectra of Ce and Yb using the Kondo approach with the impurity Anderson Hamiltonian (A26) were outlined. The application of valence band (VB) spectra for chemical environment determinations of amorphous real samples was summarized (A27). The role molecular orbital (MO) cluster calculations can play on the interpretation of spectral results also was discussed. Instrument Calibration. It has always been difficult to compare data acquired from different laboratories because of instrumental variations in both the binding energy (BE) and intensity scales. Efforts have continued to improve consistency in reported data. A review of recent advances, which have led to improvements in spectral intensity measurements so that an accuracy of (2% irrespective of analyzer employed is possible, was published (A28). A homemade Cu/Ag/Au alloy was a convenient and suitable reference material for the calibration of X-ray photoelectron (XP) spectrometers (A29, A30). BEs were within (0.1-0.2 eV of the values obtained for the pure metal foils. Because of variations in composition across the alloy surface, the material should not be used to calibrate the intensity scale of an instrument. BEs from aluminum compounds were referenced to the Au 4f7/2 peak from 20-nm gold particles and compared to those referenced to the C 1s from adsorbed carbon (A31). Differences of up to 1.1 eV were observed, leading to the conclusion that smalldiameter Au particles would make a better BE reference. BE shifts, which depended on the thickness of Au deposited, were observed on poly(ethylene), poly(vinyl fluoride), poly(tetrafluoroethylene) (PTFE), poly(n-dimethylpyrrole), SiO2, and SiN (A32). An optimum film thickness was observed for each surface that was correlated with the extraatomic relaxation energy. Charging effects on alumina and glass without charge compensation were studied (A33). The position of the O 1s peak varied for about 60 min before reaching a steady-state value; most of the charging was eliminated with a Au mask that was placed directly above the sample. Lateral differential charging in a bulk insulator was examined using imaging XPS (A34). Variations in the charging shift and peak shapes across the surface were correlated with variations in the X-ray flux and interpreted as due to lateral charging. Differential charging also was studied by observing variations in spectra from a sample of patterned indium tin oxide on glass (A35). Charging also was related to the local sample environment, sample mounting, and spatial variation of the X-ray flux. The position of peaks from insulated metals depended upon the X-ray power, and the vacuum level of the insulated metal was aligned to that of the spectrometer when the BE/power curve was extrapolated to zero power (A36). By comparison of spectra from insulated metals and metals in direct contact with the spectrometer, it was possible to determine the work function of the spectrometer. Systematic variations in the 230R
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C 1s line shape indicated that this peak was not suitable as a BE reference for alumina surfaces (A37). Differences in spectra for a calcined surface, as the calcination temperature was varied, correlated well with its activity for the dehydrogenation reaction. An alternative charge referencing scheme was proposed that involved the O 1s peaks of these mixed Al oxides (A38). The narrow line shape made referencing easier and allowed intersample comparisons of chemical states. Data Handling and Line Shape Analysis. Overlapping peaks can be difficult to distinguish due to similar electronic environments. Data processing principles for element and chemical state identification as well as quantitative analysis were discussed (A39). With factor analysis (FA), it was possible to determine the number, positions, and widths of spectral components of vanadium during reduction of V/Al2O3 catalysts (A40). The procedure involved applying principle component analysis (PCA) and iterative target transformation FA to determine the number and positions of the components and nonlinear least-squares curve-fitting to determine the distribution of components. Corrections to this article were made (A41). FA of Al KVV and O KLL spectra revealed the presence of Al° as well as two other forms of Al (A42). Detailed angle-resolved XPS (ARXPS) indicated a three-stage growth mechanism: chemisorption, oxide island formation, and slow growth of an oxide overlayer. Three components (Cu, Cu2O, and CuO) were detected in oxide films on copper substrates when XPS results combined with ion sputtering were analyzed by FA (A43). A depth profile of each component was constructed and compared with results from Rutherford backscattering (RBS) and glow discharge optical emission spectroscopy. The fuzzy kNN approach of pattern recognition was applied to Pd/Co mixtures and multilayers (A44). Using shapes of spectra from well-defined samples (pure metals and their alloys), the method verified that alloying occurred in a Pd/Co multilayer. Overlapping peaks were resolved using the Kalman filter after optimal alignment by an annealing procedure (A45). The annealing algorithm involved an iterative search for the position of component peaks and was more accurate than other procedures in determining the relative atomic amount of the components in the mixture. Carbon spectra of plasma-oxidized poly(ethylene) were deconvoluted using the maximum entropy method after background subtraction by the Tougaard approach (A46). Seven components were resolved in the C 1s spectrum that were attributed to functional groups ranging from ethers to carbonates. Data were smoothed using the filter f(a ,ω) ) exp(-a2ω2), where a is a positive parameter, in Fourier space (A47). Filtration of noisy O 1s data, followed by iterative fitting of spectra, gave three peaks at approximately the same binding energies as were obtained from spectra with little noise. The simplex algorithm was applied to the decomposition of the Si L23VV and Si L1L23V spectra into their component peaks (pp-, sp-, and ss-like) after background removal (A48). Changes in the spectra after this analysis were correlated with incorporation of hydrogen into hydrogenated amorphous silicon. Chromium atoms in Cr2O3 and CrF3 were assumed to have two final-state screening conditions with XPS data, and the 2p spectra were assumed to be the summation of peaks for each of these states (A49). Use of a single asymmetric peak for each of
these states resulted in satisfactory fits to experimental data. However, the fitting of metal 2p spectra with a single asymmetric peak was criticized (A50). Observed spectra were reproduced using a multiplet pattern which had been previously reported for the free Cr3+ ion. Line shape analysis of 2H-TaS2 and some of its transition metal intercalates revealed that charge transfer (CT) into the conduction band due to intercalation affected the corelevel line shape (A51). The rigid-band model was applicable to the line shapes of these two-dimensional materials near the peak but could not be used farther away from it. Line shapes of polytypes of TaS2 also showed distortions from the 1T layers into the trigonal layers in 4Hb-TaS2 (A52). This was related to the local density of states (DOS) at particular Ta sites and was attributed to CT between layers. Decreases in the full width at half-maximum (fwhm) as well as the asymmetries of peaks were observed in the Pb 4f and Ba 3d spectra of Ban+1PbnO3n+1 compounds (A53). These variations were related to changes at the Fermi level as a result of a composition-induced metal/ insulator transition. A double peak structure in the Ni 2p XP spectrum of NiO, attributed to nonlocal screening, was not observed in the spectrum of La2NiO4 (A54). However, nonlocal screening effects were not necessary to interpret the Ni 3s spectra, and the double peak structure was attributed to CT, near degeneracy, and exchange mechanisms. Changes in line shape of the Ni 2p spectrum were observed when the coverage of epitaxially grown NiO varied from 1 to 20 monolayers (A55). These changes, which have conventionally been attributed to Ni3+, were instead shown to be sensitive to the surface structure and thus related to the nearest as well as next-nearest neighbor environment (nonlocal screening). A two-step protocol of curvefitting was outlined for the analysis of aging in plasma-deposited polymer films (A56). By long-term monitoring of spectra, it was possible to define better peak positions and to extract more information from observed spectra. Background Subtraction. Methods of removing the contribution of a background, which increases on the high BE side of a main peak, have continued. Simulations involving Monte Carlo techniques and a Poisson distribution for photoelectrons starting from a depth that was greater than and less than one mean free path length, respectively, were performed to study the effect of surface losses on the Au 4f spectrum (A57). A better agreement between the computed and the experimental spectra was obtained when surface contributions were taken into account. After the differential inverse mean free path for bulk excitations and differential probability for surface excitations were calculated, a new convolution formula for the background was derived and applied to Au 4f spectra for three takeoff angles (A58). Good agreement with experimental data was obtained only when surface effects were included. Angular distributions of photoelectrons also were studied by Monte Carlo simulations (A59). Surface effects were included using parameters that were calculated from the extended Drude dielectric function. Energy loss functions for Si 2p spectra were derived from reflection electron-loss spectra and the Landau formalism that had been modified to account for elastic scattering (A60). More accurate background subtraction of Si 2p spectra was obtained from this procedure than was gained when the optical loss function was used.
Deconvolution of the N 1s spectrum for a series of transition metal complexes provided the response function to remove the background from the transition metal 2p spectra using the Fourier transform method (A61). It was necessary for the N 1s spectra to be narrow peaks comprised of only one crystallographic site, no multiplet splitting, and no satellite peaks due to CT transitions. This method produced a deconvoluted ratio of 2p3/2 to 2p1/2 peaks that was closer to the theoretical value than that obtained by other approaches. A pragmatic approach using modifications to the Shirley method for the removal of XP background was proposed for chromium (A62). The method required an individual background for each peak; when trying to include shake-up satellites over a large BE range, it was difficult to use the procedure due to the decreasing background. A model that studied the effect of arbitrary depth, exit angle, and electron energy on the inelastic scattering cross sections of electrons in XP spectra was proposed (A63). It was possible to determine the relative contribution of surface loss features. Good agreement with Al 2p spectra was obtained for an exit angle of 0°, although the model overestimated surface losses at 60°. The angular dependence of the inelastic loss or peak shape parameter was determined for graphite overlayers on silver (A64). The parameter varied with film thickness, but an unexpected variation with angle was explained in terms of surface excitations. Background intensities in the XP spectra of Au/Si and SiO2/Si wafers were reduced by a factor of 2 when the incident X-ray source was placed at a glancing angle (A65). Reductions were also observed in the X-ray-excited Auger spectra. Binding Energies. The main contributions to binding energies and their shifts were studied experimentally as well as theoretically. The application of BE shifts for the determination of surface acidity (Bro¨nsted and Lewis) (A66) and the concept of electronegativity and its effect on BEs were reviewed (A67). The nature of the core-level BE was discussed with emphasis on adsorbates, metal surfaces, and alloys (A68). The model, which accounts for the energy before and after ionization, was suggested to correctly describe chemical shifts. Following the previously established linear relationship between theoretically determined charge and experimentally determined BE shifts, consistent correlations were observed for organometallic compounds involving several transition metal ions with AM1 calculations (A69). Straight lines were observed in all cases, and a linear relation was observed between the slope of the charge-BE relation and Pauling electronegativity. The effect of hydrogen on BEs, which cannot be directly detected in XPS, was discussed for molecular hydrides (A70). BE shifts from the formation of hydrogen bonding, -N-H-N- in proton sponge organic systems and -O-H-O- bonds in the adsorption of peptides on oxidized metals, also were discussed. Previously reported gas-phase BEs of a series of sp3-, sp2-, and sp-hybridized compounds were correlated with mean dipole moments that were obtained from infrared measurements (A71). A linear variation in these was observed, and the slope was inversely proportional to the degree of hybridization. The contribution of hybridization to the chemical shift of a core-level BE was determined theoretically for pyridine and pyrrole (A72). Energy calculations were performed for N with orbital configuration constrained and compared with results where they were Analytical Chemistry, Vol. 70, No. 12, June 15, 1998
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not constrained. The difference between the two was taken as the energy of N(2s) hybridization. The need to account for possible hybridization when determining sources of chemical shifts in core-level spectra was stressed. A broadening of the C 1s peaks with ion-sputtered graphite and diamond was attributed to changes in hybridization (A73). This broadening on the high BE side suggested a transition from sp3 to sp2 hybridization; peakfitted spectra allowed the determination of the relative proportions. The relative amounts of Ca2+ ions in different positions of the crystal lattice of bismuth superconductors was determined from the relative areas of Ca 2p XPS spectra after peak-fitting unresolved spectra (A74). The ratio varied with the bulk composition and gave the distribution of Ca2+ nonequivalent sites. A decrease in the Pb 4f BE and an increase in the relative amount of a nonbridging oxygen peak was observed with increasing PbO concentration in lead silicate glasses (A75). This was taken as evidence of a smooth transition to covalent bonding and the formation of a PbO4 pyramid polymeric chain network. Systematic shifts to higher BE were observed in the Gd 3d, 4d, and 4f levels of GdX (X ) P, As, Sb, Bi) (A76). The shifts could be explained by assuming a constant energy separation from the center of gravity of the Gd 5d state and a reduced crystal field splitting of the Gd 5d state in the series. Increases in BEs, compared to those of the bulk metal, were observed for elements of bimetallic clusters that were deposited onto graphite (A77). Subtraction of observed BE shifts for large clusters from those of small clusters gave the shift due to variations in cluster size. Variations in the Au 4f7/2 position as a function of electron escape angle were observed for the cluster compound Au55(P(C6H4-OCH3)3)12Cl6 (A78). Spectra were modeled in terms of a group of 12 apical and 43 nonapical Au atoms having shifts of 0.60 and 0.33 eV, respectively (relative to metallic Au); in the interpreted model, the origin of the changes was a response to screening during hole creation in the photoemission process. A decrease in BE associated with a change from Cr6+ to Cr3+ was observed when in situ-cleaved single crystals of K2CrO4 and K2Cr2O7 underwent ion bombardment (A79). This change was correlated with a loss of K in each compound, but this preferential sputtering of potassium was twice as high in K2Cr2O7. XP spectra of in situ-cleaved crystals of KMnO4 and K2MnO4 were taken to obtain the intrinsic spectra of these compounds and to compare them with dirty samples and spectra obtained after prolonged X-ray exposure (A80). Satellite structure was observed in the O 1s spectrum of bulk oxygen from these compounds. BEs of Pd 3d peaks from an alloy consisting of Al60Pd25Mn15, a quasicrystalline material, were 2.2 eV higher than those in pure Pd; this shift was attributed to a combination of initial and final state effects (A81). A very sharp Mn 2p3/2 peak was observed in the quasicrystal, and this shape appeared to be very sensitive to structural and environmental changes in the alloy. Mn KR X-ray emission and Mn 2p XP spectra for a series of high oxidation number manganese compounds were measured and compared (A82). Compounds with no unpaired electrons gave symmetrical spectra, while those with unpaired electrons were asymmetrical. Fractured samples of HgBa2CuO4+d, a high-temperature superconductor, produced Hg 4f spectra that indicated the presence of Hg2+ only, whereas scraping produced spectra similar to those that previously had been reported and interpreted as due to the 232R
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presence of Hg3+ (A83). Satellite structure in the Cu 2p3/2 corelevel spectra also revealed that copper was in the +2 oxidation state. When Li+ were intercalated (from an appended electrochemical attachment) into nanoporous TiO2, a peak 2.1 eV lower in binding energy developed in the Ti 2p spectrum that was attributed to the formation of Ti3+ (A84). This state could be reversibly oxidized back to Ti4+. After correction for any surface charging, curve-fitted Al 2p and O 1s spectra varied together with changes in the treatment of oxide films on Al surfaces (A85). The range of Al 2p BEs agreed well with the range reported in a BE database and was attributed to differences in the Fermi levels for the various treated surfaces. It was suggested that these relative BE positions might be a better way to characterize these surfaces. After curvefitting Al 2p spectra, it was shown that the resulting peak areas were characteristic of the relative amounts of tetrahedral and octahedral Al in non-silicate aluminum oxides and aluminosilicates (A86). It was necessary to use a high-resolution XPS instrument and an electron flood gun. The relative amounts of tetrahedral and octahedral states were determined by the same method for a series of phyllosilicates using monochromatic X-rays (A87). Experimental results were supported by calculations using density functional theory. Combined XPS and low-energy electron diffraction studies of electrochemically ex situ-deposited pyrazine on Au(111) and Ag(111) revealed differences between adsorption isotherms on these surfaces (A88). A shift in the N 1s binding energy on Au(111) was correlated with electrochemical potentials and indicated that pyrazine had two orientations (vertical and flat), but the absence of a similar shift for adsorption of pyrazine on Ag(111) indicated that this molecule deposited in the flat orientation only. Larger BE shifts were observed in the Mn 3s spectrum than in the Mn 2p for a series of manganese complexes (A89). Large variations in the peak shapes also were observed and attributed to multiplet splitting. Systematic changes in the W 4f BEs were observed from electrochromic HxWO3 films as a function of injected charge (A90). Higher BEs and broader peaks were observed with greater injected charge; this led to systematic changes in the intensity of low BE peaks, the asymmetry factor of core lines, and the intrinsic line width. Changes in the signal intensity of Rh relative to that of Pd indicated the surface segregation of Pd in a Pd93Rh7 alloy when the sample was annealed at 750 °C (A91). Spectral changes at different takeoff angles gave the surface core-level shift for Pd that was in agreement with the Z + 1 approximation. An interpretation on CT was criticized since it did not account for previous interpretations on rehybridization (A92). Bonding of Pd, Ni, Cu, and Au to surfaces of transition and (s,p) metals leads to a redistribution of electron density that changes the amount of charge in the d orbitals and affects the metals’ ability to adsorb CO. A reply claimed there was now agreement on several issues (A93). Occupancy changes near the Fermi edge were suggested to be very important in determining the bonding of CO and the dissociation of molecular hydrogen. Large F 1s chemical shifts were observed during XPS analysis of phosphate glasses (A94). Initially fluorine formed Al-F bonds in aluminophosphate glasses, while in tin phosphate glasses P-O-P bonds were created first, followed by the formation of
Sn-F bonds. Curve fitting of O 1s spectra of xZnO‚(1 - x)P2O5 (0.50 e x e 0.67) glasses was accomplished using two peaks (A95). The ratio of these two peaks was taken as a quantitative measure of the bridging to nonbridging oxygen ratio; experimentally determined results agreed with those predicted by a simple, compositionally dependent, structural depolymerization model. Variations of 1.3 eV in the F 1s BE of strontium and calcium copper oxyfluorides were observed (A96). These shifts were attributed to variations in the local structure around the fluorine atoms. Electrical conductivity properties of thin films (∼10 µm thick) of pentaphenyl ether and pentaphenyltrimethyltrisiloxane oils (common diffusion pump oils) were studied by XPS (A97). The films behaved as electrical conductors and required no charge compensation as long as the film thickness was less than 10 µm. The BE for the first component in the C 1s spectra was very close to 285 eV. Broadening of C 1s spectra from phenylated diaminoalkanes was related to changes in the melting point of the compounds (A98). This was explained in terms of closer molecular packing, which produced increases in the melting point. Reduction of CeO2 during XPS analysis was affected by the X-ray flux and exposure time and the extent of surface charging (controlled by an electron flood gun) (A99). Reduction was attributed to photon-stimulated desorption of oxygen via an Auger interatomic process. Rapid changes in the Pd 3d spectrum were observed with PdCl2/poly(vinylpyrrolidone) samples and not with PdCl2 films on glass as a result of exposure to either non- or monochromatic X-rays (A100). It was shown that care must be exercised to ensure that BEs are not the result of alterations due to X-ray exposure during survey scans. A high binding energy N 1s peak found in calcined carbonaceous materials was studied using model compounds and theoretical models (A101). It was shown that a nonoxidized “graphitic” nitrogen could be the source of peaks in the 401403-eV range, and great care should be exercised when assigning a high BE N 1s peak to oxidized nitrogen. Calculations of amine BEs were performed using density function theory to examine the bonding of preelectrolyzed Ni surfaces by comparing BEs for a series of model compounds (A102). The results indicated that the NH4+ probably is not in direct contact with metallic sites but is, instead, bonded through hydroxylated sites. Extended Fenske-Hall LCAO-MO calculations on H-type pseudoatom clusters were able to explain experimentally determined core-level BE shifts for a series of solid phosphorus compounds in terms of initial and final state contributions (A103). Relaxation trends were related to the nature of the nearest neighbor. The core 3s spectra of transition metals on graphite were studied using the Anderson impurity model and taking into account exchange terms between core-hole spin and the transition metal magnetic moment (A104). The 3s and 4s spectra of other 3d and 4d transition metals can be calculated using this Hamiltonian by varying the number of d electrons and selecting differing values of the total d-spin. The 3s spectra of NiO, CoO, FeO, and MnO were interpreted using an impurity cluster configuration-interaction ab initio model in which exchange and intra-atomic M-shell charge redistribution energies were calculated (A105). Good agreement between theory and experiment clarified the interplay among CT, exchange, and near-degeneracy effects. An analysis of these spectra,
giving equal weighting to exchange and CT energies, also was made (A106). CT effects and exchange effects were dominant for NiO and MnO, respectively. The initial stages for the oxidation of PbS were modeled using theoretically calculated XP spectra (A107). Extra peaks in the S 2p spectra were predicted, even with only one oxidation state present, and indicated that great care must be exercised in the spectral interpretation. Valence Band Spectra. Structures in the VB spectra often provide information about the electronic environment around an atom, even when chemical shifts in the core-level spectra are small. The fine structure in XP spectra in the 15-50-eV range as a result of the formation of inner valence molecular orbitals was reviewed (A108). A review of evidence for and against the single-ion description of photemission data from metallic Ce compounds was published (A109). Differences in the VB spectra of SiO2 sol-gel thin films were diagnostic for the loss of Si-C bonds during annealing at various temperatures (A110). Variations in the Si Auger parameter implied losses of ethyl and methyl groups during annealing. The core-level spectra (Sn 3d) of mixed SnO and SnO2 films treated in several ways showed little if any differences (A111). However, the O 1s and VB regions had easily observed differences. The fine structure in the XP VB spectra of 3d elements was computed using the single-ion approach (A112). Spectra were treated as dn-1 and dnL multiplets representing the screened and unscreened portions, respectively, of the final state. The development of a steplike structure at the Fermi level indicated an insulator-to-metal transition in amorphous films of FexSi100-x when the iron content was between 10 and 20 atomic % (A113). A shift in the Fe 2p3/2 orbital BE with respect to the pure metal was proportional to the relative amounts of iron. The interaction of implanted carbon with Pt samples was studied by observing changes in the X-ray and UV excited valence band spectra (A114). A change of the Fermi edge showed the formation of isolated Pt after high levels of carbon had been implanted, while intensity near the Fermi level demonstrated a weak electronic interaction between Pt and C at lower carbon levels. XP, electron energy loss, and inverse photoemission spectra along with Al KR, He I, and He II excited VB spectra of Cr2O3 were reported to determine hybridization (A115). A 3d occupation of 4.2 was found from peak height ratios of Cr L3VV Auger structure and ratios of the XPS and VB peaks. Ab initio band structure calculations of the R and β forms of PbO agreed with the experimentally determined VB spectra (A116). The Pb 6s2 lone pair was strongly hybridized with oxygen atoms, making a highly asymmetric electron charge distribution around the Pb. Shake and Multiplet Splitting. Shake-up processes and multiplet splitting often are the source of extra peaks and peak broadening in XP core-level spectra. Satellite spectra of the O 1s level of cleaved SrTiO3, BaBiO3, and LiNbO3 were reported at a higher resolution than in previous studies (A117). Loss structures were assigned to distinct excitations and were related to the optical loss function. The complex satellite structure accompanying the core lines of transition metal oxides was measured by XPS and compared with those of other core lines (A118). Transitions associated with these shake-up peaks were identified and used to determine changes in covalency. Angle-resolved Ti 2p and O 1s spectra of clean and hydroxylated rutile TiO2 (110) revealed that Analytical Chemistry, Vol. 70, No. 12, June 15, 1998
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the position of shake-up satellite structure shifted to lower BE at grazing emission angle (A119). The position of this satellite was suggested to be the result of a combination of one peak associated with the surface and one from the bulk. A decrease in the C 1s shake-up satellite structure was observed in the spectrum of C60 films as a function of irradiation time with a 500-W mercury lamp (A120). By assuming that this decrease was the result of a reduction in the number of p electrons, the number of [2 + 2] cycloadditions occurring for each C60 species was estimated. Similar C 1s shake-up spectra from biphenyl and p-terphenyl were observed experimentally and calculated using semiempirical calculations (A121). Observed peaks were assigned to inter- or intraring CT, and a general nomenclature could be used to describe all spectra. C 1s shake-up satellites of C60 and model compounds were studied experimentally and theoretically (A122). The excitation energy of the lowest energy satellite increased with the system size and consisted of nonlocal CT; at higher energy, the overall behavior was the same for all of the compounds and consisted of local CT. The origin of the satellite structure in the V 3s peak was studied using a cluster model that considers intraatomic electron correlation and electron delocalization equally (A123). The results suggested that the satellite was due to corehole screening rather than the magnetic moment of the vanadium atom. Agreement between high-resolution and spin-resolved XP spectra and theory was obtained for the 3d, 4d, 4f, and 5p levels of ferromagnetic Gd (A124). The close agreement indicated the importance of extraatomic phenomena such as shake-up, final state CT, and photoelectron diffraction. Including the effects of CT to the conduction band in the single-impurity cluster model had a considerable influence on the 2p core-level line shape of Ni compounds (A125, A126). The asymmetric line shape of NiS was well produced; the model was less successful in explaining the effects for more ionic compounds. Local magnetic moments of Fe atoms in Fe-based alloys were obtained using the magnitude of multiplet splitting in Fe 3s spectra (A127). Agreement between experiment and theory was obtained. The extent that CT and unscreened configurations mix was examined for MnO and NiO using ab initio Hamiltonians in which the matrix elements were determined (A128). The weak CT effect for MnO (weak satellites) and the extremely large CT effect for NiO were due to incomplete shielding from d electrons and an increase in the effective nuclear charge on moving to the right in the periodic table. Core-level peak positions and intensities of satellites relative to the main peak were calculated for di- and trivalent 3d transition metal compounds using the cluster model (A129). CT, d-d Coulombic repulsion, and ligand p-metal d hybridization energies were varied; the results were plotted to provide a practical look-up table for the possible combination of parameters that would correspond to observed data. A review of spin-resolved photoemission spectroscopy was published (A130). Magnetic linear dichroism in angle-resolved photoemission distributions (MLDAD) from Fe(001) was measured using Mg KR radiation (A131). Small variations were found for the Fe 2p1/2 and Fe 2p3/2 peaks, and changes were not observable for the Fe 3p, VB, and Auger peaks originating from a 2p1/2 hole. Magnetic circular dichroism and spin-resolved XP spectra were calculated for the 2p level of ferromagnetic Ni using a small-cluster many-body model (A132). Agreement with ex234R
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perimental results led to a better understanding of the satellite structure. Since there is zero spin-orbit interaction in s-level initial states, it was possible to study magnetic dichroism from the photoemission final state of Fe 3s and 2s levels (A133). The Fe 3s level exhibited a small magnetic dichroism, while the effect was not noted in the Fe 2s level. X-ray-Excited Auger Electrons and Auger Parameter. High-resolution X-ray-excited KLL and KLM Auger spectra and Auger parameter values of V, Cr, Mn, and Fe were reported for the first time and compared with Auger spectra from radioactive samples (A134). Contributions from inelastically scattered electrons were minimized since only thin films of the metals were studied. Systematic variations in the modified Auger paramater as a function of TiO2 coverage on MgO and Ag were observed (A135). The overlayer-substrate interaction was simulated using MO calculations and explained in terms of changes in the O 2pTi 3d energy gap. In another study, systematic variations in the modified Auger parameter as a function of coverage of TiO2 on substrates consisting of SiO2, MgO, Ag, and SnO also were observed (A136). Chemical state plots of the modified Auger parameter showed a substrate effect. Analysis of the Auger and photoemission spectra from core levels using either the pointcharge or the electrostatic model provided a simple quantitative picture of the local electronic structure of an atom (A137). Extra atomic contributions were estimated and accounted for the local geometries, number of nearest neighbors, and ligand polarizability. Reasonable correlations were obtained between Auger parameter shifts for nontransition metal oxides but were less satisfactory with transition metal oxides (A138). Apparently, metal atom screening for transition metals is unusally efficient. The conclusion that similar Auger parameters for oxygen and silicon from a silica standard and SiO2 coatings indicated the absence of SiOH bonds was criticized (A139). Peak shifts were too small, and, even if they were large enough, the film could be too thin, resulting in only a minor contribution to the overall O 1s peak. The modified Auger parameter was able to distinguish between different layers in thin oxide/nitride/oxide films of Si (A140). Clear variations in the Auger parameter were observed during ion milling, allowing the determination of the stoichiometry of each layer. Chemical shifts in Ge core-levels induced by substoichiometric amounts of nitrogen in a-GeNx(0 e x e 0.3) alloys were observed (A141). Linear variations of the modified Auger parameter with changes in the calculated charge (Sanderson’s scale) likewise were found. Variations in the Auger parameter of P as a function of its average coordination number were observed for a series of binary nickel phosphides (A142). These were explained as the result of the larger polarizability of neighboring nickel atoms. Computed Auger kinetic energy shifts of Pd-based alloys using the complete screening model of a core hole were compared with experimentally determined shifts (A143). It was possible to determine the relative Fermi energy in these alloys from these shifts. A complicated S 2p spectrum was observed when InP(001) was exposed to an (NH4)2S solution or S2 molecules in a vacuum (A144). Chemical state plots using the Auger parameter made it possible to identify the chemical states. Variations in the line shape of the KVV Auger peak of carbonaceous materials provided a simple way to determine the sp2:sp3 ratio (A145). After peak-fitting of the spectrum, the
separation between two peaks in the largest envelope was correlated with the hybridization. After the contributions from various types of bonding were separated, X-ray-excited Si KLL spectra provided an estimate of the thickness of nitridation films on Si powder (A146). A ratio of peaks led to a linear rate law for all films formed at temperatures ranging from 1127 to 1190 °C. Semiconductors. As the dimensions of semiconductor devices decrease, the necessity of understanding interactions of semiconductors during fabrication becomes increasingly important to ensure reliability. The use of surface science to study semiconductor/electrolyte interfaces was reviewed (A147). Studies involving SiC and the development of metal-semiconductor contacts were outlined, and future directions in contact surface studies were discussed (A148). The principles behind surface charge spectroscopy, which is a modification of normal XPS, and how it is applied to measure the potential distributions in ultrathin insulator/semiconductor interfaces was outlined (A149). With thinner oxide films on semiconductors, it is important to understand the effect of surface potential on the electrical properties. It is possible also to obtain information about the depth distribution of the various surface charge states. The heterojunction band offset from AlN/GaN(0001) was determined from differences in peak positions relative to the onset of the VB spectra (A150). XPS results were significantly different from recent theoretical predictions. The locations and energies of interface states in the band gap of an oxide-coated GaAs wafer were determined by measuring the position of the As 3d peak after biasing (A151). The Fermi level was 0.85 eV above the VB maximum. BE shifts as a function of applied voltage between Pt and Si from a Pt/Si-oxide/n-Si(100) sample were observed for both the oxide and substrate (A152). The shifts were attributed to bias-induced changes in the charge of the interface state. Direct determination of the VB discontinuity at GaP/GaN heterointerfaces was made from the BE difference between P 2p and the top of the VB in GaP, the separation between the N 1s and the top of the VB in GaN, and the difference between N 1s and P 2p peak positions in the heterojunction (A153). The top of the VB was determined by fitting experimental spectra with a theoretically determined DOS, since the VB was too broad to accurately determine its position directly. Large changes in the F 1s and Ce 3d XP spectra from CeO2 grown films using a fluorine-containing precursor were observed with X-ray exposure time (A154). This made determination of the chemical state of these two species in the film before any X-rayinduced change difficult. VB and core-level spectra of ultrathin SiO2/Si interfaces were studied (A155). The Si 2p region was missing a peak associated with the presence of Si2+. Difference spectra of the films and bulk Si led to a VB top for the oxide of 4.36 eV, regardless of film thickness (1.8-3.7 nm). Theoretical studies of the Si 2p spectra from the Si(001)-SiO2 interface showed that the number of nearest-neighbor oxygen atoms is the major factor in determining the magnitude of core-level shifts (A156). It was not necessary to invoke differential charging as a cause of BE shifts with increasing oxide thickness. Ge 3d corelevels from substoichiometric amorphous germanium-nitrogen alloys were studied for a series of samples having varying nitrogen content (A157). BEs and the fwhm increased with nitrogen
relative amounts, and results were consistent with a random bonding model. The stoichiometry and chemical states of SiOx films were investigated by using sensitivity factors, peak areas, and a curve fit of the Si 2p spectra with five peaks (A158). The fraction of Si associated with the lowest BE peak was higher than predicted from a random bonding model. The composition of SiOx thin films was calculated from BE shifts using a random bonding model and Sanderson’s electronegativity method (A159). Satisfactory agreement between the value predicted from this method and x from intensity ratios was observed. Using Si 2p3/2 chemical shifts of standard oxides and oxynitrides, observed peaks from oxygenand/or nitrogen-implanted n-type silicon wafers were decomposed into individual components using peak-fitting techniques (A160). The results were consistent with a random bonding model. No change in the BE of the N 1s peak for a series of oxynitride glasses (nitrogen content varied from 3.7 to 10.0%) suggested that nitrogen tends to substitute in nonbridging oxygen sites at these compositions (A161). A broadening of this peak at higher nitrogen amounts suggested that additional sites are produced that are related to “non-cross-linked” nitrogen. Submonolayer thicknesses of SiO2 and SiOx were determined from curve-fit areas for Si4+ and Siy+, empirically determined relative sensitivity factors, and equations that related intensities to film thickness for each oxide (A162). Reference materials gave the exact positions and widths of the Si4+ and Si° states. The traditional interpretation of shifts in the Si 2p spectrum at the Si/Si oxide interface was reviewed and questioned (A163). Recently published data using spectra from structurally welldefined clusters were discussed, along with new cluster results. The data suggest that a new way of interpreting spectra must be devised and well-defined structures be used to create a reliable model. BEs from adsorbed silatrane (a spherocluster) were measured to study the shifts at the Si(100)-SiO2 interface (A164). Shifts in the Si 2p spectra were observed when silatrane was adsorbed onto Si(100)(2 × 1) and were compared with reported results using a similar cluster. A shift of 1 eV to higher BE after the formation of SiF on the surface of Si(100) from an exposure to XeF2 was observed (A165). This led to a proposal of an extended scheme (includes remotely induced shifts) that explains all known data of Si 2p shifts. The temperature dependence of the Si 2p peak widths was measured with monochromatic X-rays (A166). Increases in line width up to 470 K were consistent with phonon broadening, while above 470 K a decrease in line width was observed and related to enhanced core-hole screening. The interaction of organic adsorbates on Si(100)(2 × 1) was studied using XPS and quantum mechanical calculations (A167). Results from both approaches supported the model that a nonbridging adsorbate existed as a result of the formation of Si-Si dimers on the surface. XP spectra after sputtering a SiO2 passivation film on a layered structure of an InP substrate revealed some damage from the ion beam, but it was still possible to reduce the thickness of the SiO2 layer so that the interface could be studied (A168). Etching with an acid solution destroyed an As-rich layer under the SiO2 overlayer. Differential BE shifts from ultrathin Si oxides of an Si4+ state compared to Si°, Si2+, and Si3+ states as a function of film thickness were observed (A169). Apparently, the outer SiO2 layer was Analytical Chemistry, Vol. 70, No. 12, June 15, 1998
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charging while the suboxide layers were not, and this could be a calibration method in preference to the use of electron flood guns when determining the chemical states of silicon wafers. Theoretical studies of Si 2p and O 1s peaks from O/Si(111)(2 × 2) using the DV-XR9LCAO isolated slab band method agreed with semiempirical peak assignments for an intermediate number of bound oxygen atoms (A170). Disagreement with the traditional semiempirical peak assignments was observed for Si atoms with dangling bonds. The composition of 0.50-µm trenches from polysilicon gates (etched using a chlorine-based plasma) was determined by XPS using a 1-mm X-ray beam (A171). Signal intensities of Si, C, O, and Cl varied with data collection angle. High-density plasma etching of 0.25-µm trenches from polysilicon films on Si(100) substrates (patterned using a hard oxide mask) also was studied (A172). Thin oxide films were observed on the sidewalls, and about twice the amount of chlorine was found on the sidewalls as on the oxide mask or gate oxide bottoms. XPS analysis of sputteretched, oxide-masked surfaces of polycrystalline SiGe revealed features in the spectra that were attributed to the tops, sidewalls, and bottoms of the etched structures (A173). When the 0.5-µm structures were aligned parallel to the electron energy analyzer, bottom features were detected, and, with perpendicular alignment, the sidewall features were found. Compositional depth profiles obtained from XPS and conventional powder X-ray diffraction results were simultaneously analyzed in a Monte Carlo-based algorithm to determine the strain level of Si1-xGex layers on Si(100) (A174). Partially strained epitaxial layers were obtained with laser pulses. Depth profiles of C+-implanted SiC thin layers characterized the composition profiles as a function of implanted C content (A175). A doublepeak feature was observed in the Si 2p peak and was intrepreted as an in situ crystallization. By monitoring the BE of the C 1s spectrum, it was shown that a C film (grown by molecular beam epitaxy) dissolved into the Si(001) substrate upon heating (A176). Since the total signal did not change significantly, it was shown that the subsurface layer must be concentrated and very close to the surface. Passivation of chemically etched Hg1-xCdxTe by deposition of chromium or aluminum was studied by using corelevel intensities as a function of film thickness (A177). At high coverages, a persistent Te signal led to the conclusion that a metal telluride “floats” on the surface of the deposited metal. Polymers. A review of the application of XPS for the research, development, and routine analysis of cellulosic and other polymeric materials was published (A178). Other reviews covered the recent application of XPS and secondary ion mass spectrometry (SIMS) to the characterization of polymeric biomaterials (A179) and polymer surfaces (A180) and methods for characterization of filler surfaces and filler-matrix interfaces in polymer composites (A181). Determination of the Lewis acid-base properties of polymer surfaces was reviewed (A182, A183). Using previously established correlations of Cl 2p BEs with Drago enthalpies of interaction, it was possible to assess acid-base interaction strengths. It was suggested that submicrometer analysis of acid-base properties with modern instrumentation was possible. Variations in the spectra of thin films of a diandicyanato bisphenol A, a prepolymer, were observed at different takeoff 236R
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angles and film thicknesses (A184). An adsorption model was proposed in which the first layer interacts with the surface through the triazine rings. The effect of low surface free energy perfluoroctyl chains in polymer mixtures on surface composition and hydrophobicity was studied by XPS and dynamic contact angle (A185). High relative amounts of F were detected on the surface, and the contact angle appeared to be strongly affected by this enhancement. The disappearance of a F 1s peak, the appearance of an O 1s peak, and a large shift in the C 1s peak position were observed when PTFE was treated with a solution containing benzophenone and sodium hydride and with ultraviolet radiation (A186). The degree of surface unsaturation that resulted from this treatment was estimated by bromination of the modified surface and measurement of the Br/C atomic ratio using XPS. Time-dependent XPS studies of the derivatization of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) with trifluoroethanol revealed that stoichiometric esterifaction occurred after approximately 12 h for PAA but was only about 60% for PMAA (A187). It was concluded that incorporation curves must be constructed for materials of unknown chemistries and structures to more fully characterize the number of carboxylic acid sites. XPS analysis of a poly(vinylidene fluoride) film after its treatment with LiOH revealed oxygen-containing species (A188). When this surface was treated with NaBH4 in 2-propanol, the decomposed O 1s spectrum indicated a substantial relative increase of the alcohol functionality. The transformation of gauche poly(ethylene terephthalate) to the trans glycol conformation was monitored using XPS core-level and VB spectra (A189). Changes in the C 1s signal position associated with glycol and the VB were observed during the transformation from crystalline to amorphous states. This change also was monitored using both FT-IR and XPS (A190). Changes in the BE of the C 1s signal associated with the glycol occurred at a faster rate than changes in the FT-IR spectrum, showing that the surface reaction rate was greater than that in the bulk. The dependence of the small XPS shift on conformation also was studied by ab initio calculations (A191). Close agreement was obtained between the predicted glycol BE shift (0.11 eV) and the experimental data. Imaging XPS of patterned self-assembed monolayers containing perfluorinated alkyl chains, using a “stamping” technique, gave information with a spatial resolution of 30 µm (A192). Different chemical states for fluorine could be observed, suggesting that a small fraction of molecules existed in a different local environment. PTFE transfer films on a silicon wafer also were examined by normal and imaging XPS (A193). The azimuthal dependence of the XP spectrum at low takeoff angles indicated a ribbonlike structure with hydrocarbon contamination on silicon in the channels between the ribbons. The surface composition of a film from methyl methacrylate-poly(ethylene glycol) methacrylate copolymers was essentially the same as the bulk composition according to results obtained by XPS and static SIMS (A194). A series of ions containing the methyl-capped poly(ethylene glycol) chain ends was easily observed by SSIMS, but not by XPS, demonstrating the complementary nature of the two techniques. Spectra from methacrylate copolymer samples, introduced with a freeze-dry technique, were compared with spectra obtained with the conventional introduction procedure (A195). Surface reori-
entation of polymer chains occurred to a depth of ∼4.5 nm. It was possible to determine compositions of copolymerized styrene and acrylonitrile mixtures using the N 1s/O 1s area ratios and sensitivity factors (A196). Agreement was obtained between XPS, 1H NMR, and elemental analyses. Sulfur, nitrogen, chlorine, and potassium contents were determined from a polypyrrole particle which had been sterically stabilized with poly(potassium 3-sulfopropyl methacrylate) to determine the location of the stabilizer (A197). A large S peak, small N and Cl peaks, and the absence of K led to the conclusion that the stabilizer covered the surface and KCl had been removed. The number of negatively charged binding sites on hair surfaces was determined by Ba2+ labeling (A198). After hair was exposed to a barium salt in an aqueous solution, the intensity of the Ba 3d peak was monitored and predominantly associated with organic sulfur oxide functional groups. Brain cell cytoplasm was analyzed using XPS after removal of the cell membrane top (A199). Differences in the spectra between cells with tops removed and those intact confirmed the removal of the top part of the cell membrane and made it possible for surface-sensitive techniques to probe the interior cell composition. The biomolecular composition of thin films of the enzyme glucose oxidase entrapped in a poly(pyrrole) film was determined by XPS (A200). Due to the complicated nature of the enzyme, characteristic spectra of the enzyme and the polymer were combined to determine the relative amount of the enzyme. A series of films prepared from mixtures of poly(perfluoropolyether) (PFPE) and poly(phenyl ether) (PPE) were examined using ARXPS (A201). An enhancement in the relative intensity of features associated with PFPE indicated the preferential ordering on PPE with a thickness of several nanometers. Valence and core-level spectra of eight polymers were calculated using model oligomers and deMon density function theories (A202). Closer agreement with experiment was obtained with these calculations than could be obtained with Koopmanns’s theory. C 1s BEs of N-, O-, F-, and Cl-containing polymers were predicted using ab initio MO calculations of model compounds (A203). After correction of all of the calculated positions by an amount equal to the difference between the predicted and experimental BEs (the sum of work function and other energy effects), good agreement between experimental and theoretical spectra was obtained. The BEs of oxygen-containing polymers also were determined from model compounds, although better correspondence was obtained with the generalized transition-state model using the deMon density function theory program than was obtained with Koopmanns’s theorem (A204). Films. A procedure for correcting percent composition results with overlayer attenuation was outlined (A205). The correction utilized the kinetic energy dependence of an element’s IMFP to calculate a reduced width that then allowed the determination of the corrected intensities for the substrate elements. The IMFP of photoelectrons from Ag was determined by total reflection XPS (A206). Correction for the contribution from the X-rays after a separate determination led to a value of 1 nm, which was one-half of the accepted value. Theoretical equations, taking into account the finite escape depth of photoelectrons, were derived that predicted how XPS intensities varied with a threedimensional phase transformation (A207). In agreement with
experiment, it was shown that the photoemission intensity was related to the square root of the fraction from the untransformed bulk phase. The BE of Zn 2p3/2, Sr 3d5/2, and S 2p3/2 peaks changed with composition of ZnxSr1-xS thin films (A208). Both X-ray diffraction results and BE shifts indicated phase changes at about x ) 0.3 and 0.85. A simplified Kerkof-Moulijn model for predicting crystal sizes of catalysts with a homogeneously distributed supported phase was proposed (A209). Using XPS signals to determine the atomic amounts as if it were a homogeneous mixture, the crystal size could be determined as long as the IMFPs through the support and supported material were known. A spherical model was proposed to predict signal intensities from supported metal catalysts (A210). Particles were assumed to be equally sized and spherical in shape, with only surface particles contributing to the XPS signal. Monte Carlo modeling along with atomic force microscopy (AFM) studies of rough aluminum substrates was used to analyze the effect of surface roughness on the error in film thickness determinations from intensity measurements (A211). This led to a roughness parameter that was directly determined from AFM measurements and correlated with errors in the thickness determination; XPS analysis at the magic angle of ∼35° minimized the errors. Photoelectron angular distributions from polycrystalline materials were calculated using Boltzmann transport and Monte Carlo models (A212). At the magic angle geometry, the agreement between the methods was very good, and in the worst instances the differences were about 15%. Further modeling studies of surface roughness effects on overlayer thickness determinations revealed that the true magic angle was probably ∼45° (A213). At this angle, errors in the estimation of film thickness from intensity data are smaller than 10%. Simulated ARXPS data for three thin films with significantly different composition profiles were reproduced using two variations of the regularization method (A214). A smoothing procedure that automatically chose the correct parameters was critical to the successful application of this technique. Although the goodness of fit was not totally independent of the composition profile, it was generally satisfactory if there was a small amount of error (1%) associated with the data. Smooth composition profiles could be predicted from ARXPS results using a polynomial function of varying degree (A215). Unfortunately, experimental errors in intensity measurement could not exceed about 0.3%, which is lower than is usually possible. It was proposed that modifications in the procedure would make it possible to use data with a higher experimental error. A method for depth profiling of films that extend over only a few atomic layers for two-component systems using ARXPS was outlined (A216). The model, containing three adjustable parameters, was applied to a Pt-Ni alloy as well as segregated sulfur in iron. Theoretical procedures for estimating the elemental ratio of a two-component film from XPS depth profiles were outlined (A217). Approximations that reflect alterations in the profile as a result of ion interactions also were considered. Relationships necessary to estimate concentration profiles from inelastic backgrounds were derived as a standard optimization problem (A218). The relationships between the various parameters indicated their relative importance; the standard errors of model parameters for two simple models were presented. ReAnalytical Chemistry, Vol. 70, No. 12, June 15, 1998
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duced error in determining the composition of nanometer-thick films was obtained by accounting for the relative position in the film from the shape of the inelastic loss structure (A219). Theoretical peak shapes agreed with experiment for a layered Ni/ Au/Ni structure. An algorithm, based upon a cyclical simplex algorithm, was developed for nondestructive elemental depth profiling using ARXPS (A220). The method was applicable to a wide range of chemical systems with an experimental error as high as 10%. QUASES, recently developed software, was used to determine the growth and in-depth distribution of the films of Cu, Ag, Pt, and Au on Si (A221). The method involves analysis of the inelastic background of XP spectra. This method also was applied to epitaxially grown films of graphite(0001) on Ni(111) (A222). Analysis of the inelastic background using the Tougaard method led to the conclusion that the first layer was a complete monolayer, but subsequent growth was of the Stranski-Krastanov type. The method was reviewed also, and a few application examples were discussed (A223). Smaller errors in the concentration depth profile are obtained when the shape of the peak is considered relative to the simple use of the peak area. The intensity of the Auger peak relative to that of the XPS peak was examined after background subtraction by the Tougaard method (A224). Larger than predicted intensities were attributed to a contribution from a Coster-Kronig transition. A general formalism that accounts for both elastic and inelastic scattering of electrons in elemental solids was described (A225). Satisfactory agreement was obtained between computed results from this method and results obtained using Monte Carlo simulations. Elastic scattering of electrons in Mg/Cd alloys was studied using Monte Carlo techniques (A226). A nonlinear dependence of Mg 2s signal intensity with Mg relative amount was calculated when elastic scattering was taken into account; this was greatest at small emission angles. A method was discussed for treating elastic scattering in the analysis of the ARXPS intensities (A227). The possible use of the method for determining relative amounts in depth profiles was discussed (A228). A procedure was outlined to perform quantitative analysis of the relative amounts of Fe°, Fe2+, and Fe3+ as well as to determine their in-depth distribution (A229). The method used a standard to determine line shape and position of each chemical state and a Tougaard background, and it then summed the contribution from each with appropriate fitting parameters. Nucleation and growth of thin films of Au on polypyrrolle was studied using a combination of XPS and scanning tunneling microscopy (STM) to decide if XPS was capable of determining the growth mode of nanometer-size gold crystals (A230). Intensity ratios of the gold peaks at various coverages indicated droplike film formation. Assuming a constant contact angle with the surface, nucleation densities were determined; these were in good agreement with those obtained from STM. PdO/Pd particle sizes and oxide distributions on SiO2/Si(100) were determined from standard XP spectra using a model that assumed the particles were spherical (A231). Particle sizes from this method agreed with those obtained from AFM. A blue shift in the plasmon loss structure of Si was dependent upon the mass of silicon deposited on Al2O3 (A232). This shift was attributed to a possible quantum confinement effect that was due to the nanosize of the Si particles. Depth profiles using the N 1s peak to monitor nitrogen (implanted 238R
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with a low-energy ion source) were compared with Monte Carlo simulations (A233). Discrepancies in the results were attributed to atomic mixing and preferential sputtering. Self-assembled films of compounds containing isolated doubled bonds on finely dispersed Fe nanoparticles contained a C 1s peak that was attributed to the formation of conjugated double bonds on the surface (A234). The intensity ratio of the Fe 2p and Fe 3p spectra gave an estimate of the organic layer’s film thickness. Surface acidities of anodized titanium and passivated stainless steel were determined by measurement of the intensity and position of the N 1s signal from the adsorption of 1,2-diaminoethane (A235). The existence of Bro¨nsted acidic sites was correlated with the presence of hydroxyl groups on the surface of the titanium, but a like correlation for stainless steel could not be made. The surface electronegativity or anionic activity was studied by XPS analysis of Ni adsorbed on single-crystal oxides (A236). The appearance of peaks associated with Ni2+ in the Ni 2p spectrum was linked to the presence of anionic point defects. The progress of ion-induced modification with organosilane self-assembled monolayers was followed by observing changes in the N 1s and C 1s signals (A237). Electrodeless metalization was possible on this surface after >50% loss in the N 1s signal. Thin films of Ti were deposited onto self-assembled end-functional alkanethiol layers on Au (A238). The extent of clustering of Ti on the surface during film growth was related to the reactivity of the end-functional group. X-ray as well as ion-induced decomposition of PTFE-coated gas diffusion electrodes was faster than that of PTFE foils (A239). A buildup of graphitic carbon was observed, which pointed to the necessity of using standards when studying by depth profiling after fuel cell operation. The O 1s signals were employed to determine the thickness of a PFPE film adsorbed on SiO2 (A240). Contaminants precluded use of the C 1s peaks, and some X-ray-induced changes in the layer thickness were noted. Surface composition of ex situ-prepared films of cetyltrimethylammonium bromide (CTAB) on silica plates, as determined from XPS intensities, varied with the CTAB concentration, and the coverage could be determined without knowledge of the IMFP (A241). CTAB was found to be a suitable coverage standard below the critical micelle concentration (cmc) since the amount of C and N increased with CTAB concentration and no bromine was observed until solution concentrations were greater than the cmc. Bulk analysis by XPS of AlN after sputtering resulted in substantial errors in the quantification of the amount of C and O (A242). Carbon appeared to accumulate as a hydrocarbon from the vacuum system after ion milling. Large changes in the valence band of Cu nanoparticles (deposited on Si(100) by laser ablation) were observed during Ar+ sputtering (A243). This was attributed to changes associated with the size and geometry of the particles. It was shown that these changes were not due to silicide formation. Monitoring the O 1s spectra during oxidation of Ni to NiO indicated the moderately rapid growth of an oxide 3-5 atomic layers thick (A244). The pressure dependence of increased oxidation rates with Ar+ bombardment were suggestive of an increase in diffusivity. Selective sputtering of oxygen was avoided during the depth profiling of the chemical states of SrTiO3/Si interface (A245). Using an O ion beam at a 70° angle produced minimal Ti reduction. A broad O 1s peak from lead zirconium
titanate films grown by laser ablation was observed (A246). Line broadening was attributed to the hybridization of Zr 3d and Ti 3d orbitals with the O 2p orbital. Shake-up features of the Fe 2p spectrum along with peak-fitting results of the Fe 2p and O 1s spectra provided a means to determine the Fe2+/Fe3+ ratio from thin oxide films on polycrystalline Fe foil (A247). It was shown that this ratio was strongly influenced by the exposure time to oxygen. Surface segregation of Sb in doped TiO2 was observed by combined ARXPS and depth profiling analysis (A248). Sb was enriched in both the surface and bulk regions. Compositional changes in GaAs(100) after various etching procedures were studied using a combination of ARXPS and low-energy ionscattering spectroscopy (A249). Differences in the depth distributions were found that required a combination of techniques to understand. Tungsten oxide films on SiO2/Si surfaces were characterized by ARXPS and depth profiling as model-supported catalysts (A250). Compared to those of high surface area materials, line widths were narrower since charging was not observed and W was highly dispersed on this surface. Instrumentation. Recent developments in the instrumentation for production of spatially resolved XP spectra were outlined with a resolution of approximately 5-30 µm presently being observed (A251). Possible future improvements in resolution were also discussed. The geometrical aberration and beam shape of an ellipsoidal diffractor was simulated and compared with experimental results (A252). An observed beam size under 10 µm was close to the simulation findings, showing that the diffractor was nearly ideal. X-rays were produced by focusing a Nd:YAG laser on an Al target that produced a plasma (A253). The source had an energy width of 2 eV and spatial resolution of 20 µm and was useful in examining the laser ablation effects on a silicon surface, although the S/N ratio was quite low. A laser-produced plasma from a Ta sheet produced X-rays that were monochromatized using a Schwarzschild mirror and was combined with a time-of-flight electron analyzer to produce an electron spectrometer (A254). Si 2p and LVV electrons were detected with a bandwidth of 15 eV; the number of electrons detected per pulse was close to the theoretical value. An XP spectrometer based upon two independent toroidal analyzers was described (A255). Although it was designed to measure the angular distributions of low-energy electrons (1-50 eV) from photodouble ionization of atoms and molecules, the novel complex multielement electron lenses could be applied to other types of electron spectrometers. A spin- and angle-resolved photoelectron spectrometer was built that consisted of an electron energy analyzer, lens system, and electron spin detector (A256). The experimental performance of the instrument was tested with O/Cs/GaAs(100). The cause of the X-ray-induced reduction of CuO to Cu2O was studied (A257). Thermal, X-ray flux, and slow electron effects from a nonmonochromatic X-ray source were studied; the slow electrons were the most important. A bias of -250 V on the sample decreased the reduction by 80-90%. Spectra of electrochemically oxidized carbon fibers using achromatic and monochromatic X-rays were compared to evaluate the relative performance of these two sources for carbon fiber analysis (A258). Decomposition of the fiber was drastically reduced, and narrower line widths facilitated chemical state identification with the
monochromatic source. A monochromatic Si KR (hν ) 1739.9 eV) X-ray source with a total instrumental resolution of 0.35 eV for the Ag 3d core level was developed (A259). This source was somewhat more useful for deeper core levels than employing an Al KR source; however, the signal intensity for the Ag 3d level was substantially lower than that obtained with a monochromatized Al KR source. Charging problems associated with biomaterials were overcome using a filament (coaxial to the lens system) to generate low-energy (2-3 eV) electrons which were injected into the magnetic field and focused onto the sample (A260). The combined use of magnetic and electrostatic fields provided a very narrow energy range for the compensating electrons. AUGER ELECTRON SPECTROSCOPY Auger electron spectroscopy (AES) by electron beam interaction with a solid surface is employed widely for both elemental and chemical analyses of the near-surface region. Also, efforts continue to make the technique more quantitative. Several general reviews of AES appeared during the review period (B1B3). Various specialized topics in AES were published concerning the history (B4), spin-polarized Auger electrons (B5), metallurgical research (B6), grain boundaries in steels (B7), semiconductors (B8), radiation damage in insulators (B9), electrochemical oxide film formation (B10), spin-polarized Auger electrons (B11), and positron annihilation AES (B12). Line Shapes. Investigations of Auger line shapes from both experimental and theoretical perspectives have continued, and reviews on this topic were published (B13, B14). Line shapes often can be used to “fingerprint” certain species and provide an understanding of chemical bonding. The 3-eV split in the KVV transition for Li adsorbed on Cu was analyzed using cluster calculations (B15). The source of the lower energy peak was assigned to a diffuse bonding orbital and a Cu 3d orbital, while the higher energy peak was ascribed to two diffuse bonding orbitals. The XAES line shape for diamond-like carbon was described as a mixture of graphite and diamond line shapes, and the relative amounts of sp2 and sp3 bonding could be found (B16). If electron excitation is used, the energy separation must be relatively small, so that some of the energy loss features will not be lost. A background removal method based on the Bethe equation that uses a fast Fourier transform (FFT) with an energy loss spectrum was described for diamond and graphite (B17). This allows normalization of different spectra by considering detector response, analyzer transmission, etc. Changes in the C KVV line shapes of diamond films were found when the electron beam was focused to approximately 0.2 µm, compared to an analysis area of 8 × 10 µm (B18). Hydrogen desorption from the surface was suggested for the observed differences. Small changes in the C KVV line shape for hydrogen-terminated diamond were observed with electron beam exposure; the effects were reversible (B19). Preliminary calculations of the DOS agreed with the observations. The extent of sp2 bonding in several diamond films was studied by examining small changes in the Auger line shapes and energy loss spectra (B20). Also, from Fourier transformations of extended energy loss data, nearest-neighbor distances were found. By proportional addition of standard spectra of diamond and graphite, the rate of change from diamond to graphite caused by sputtering was monitored (B21). The procedure also was Analytical Chemistry, Vol. 70, No. 12, June 15, 1998
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employed to study the interfacial region of Ti deposited on a diamond surface. XAES KVV spectra of diamond and poly(ethylene) were found to be very similar and somewhat different from those of HOPG (B22). Most of the variations were on the low-energy part of the spectra and were ascribed to sp2 and sp3 features. Large differences in the relative intensity of the C KVV/C 1s (X-ray excited) peaks were observed for diamond, graphite, and carbene (B23). These trends were correlated approximately with estimates made from intensity calculations that considered the number of valence electrons of the various materials. The use of second derivative C KVV spectra for some carbonaceous materials indicated that the s and p components of the valence band could be distinguished (B24). Effects due to contamination with carbon-containing species were minimal. Algebraic diagrammatic construction theory has been applied to calculate Auger spectra for several systems (B25). In most cases, the agreement was fair with experimental data for carbon-based compounds. Atomic overlay was included in the Cini-Sawatzky model for the analysis of the Si L23VV and KVV spectra (B26). This yielded improved fits over earlier work. Differences between electronand ion beam-excited Ti MVV spectra were observed (B27). The ion beam spectra were consistent with free atom data. Calculations were made for the sudden charge change Z f Z + 1 for Auger shake events using a hydrogenic model (B28). Shake-up, shake-down, and no shake probabilities were 90%, 9%, and 1%, respectively. The Si L23VV line shape from porous Si was compared with the line shape from cyrstalline Si (B29). From theoretical calculations, differences in the spectra were attributed to Si atoms bonded to H and H2. Principle component analysis (PCA) determinations of K deposited on Si(100) and GaAs(110) indicated two and three states on these substrates, respectively (B30). For both systems, it was suggested that two states were related to the size of the deposited K particles. The third state was ascribed to the reactivity of the substrate. A reevaluation of the V L23M45M45 transition line shape suggested that earlier data may have been incorrect due to surface contamination (B31). A two-hole process predominates with screening playing a small role for this transition. A shift of
