Application of Cr Kα X‐ray photoelectron spectroscopy system to overlayer thickness determination

A laboratory hard X‐ray photoelectron spectroscopy (HXPS) system at 5.4‐keV excitation energy was used to measure the angle dependence of a silicon oxide overlayer on a Si(0 0 1) substrate with overlayer thickness ranging from 4 to 25 nm. The thickness values of the SiO₂ overlayers were determined by utilizing a focused monochromatized Cr Kα source and a high‐energy hemispherical analyzer with an angle‐resolved wide acceptance angle objective lens. The modulation of the photoemission intensity due to photoelectron diffraction, which deteriorates high‐precision thickness determination, was suppressed significantly by continuous sample rotation around the sample's normal during the measurements. The resultant thickness values very well agree with those determined by ellipsometry in the same sample set. To demonstrate merits of the large information depth measurements, profiling of a wedged SiO₂ layer buried in a gate stack model structure with Ir (8 nm) and HfO₂ (2 nm) overlayers was performed. Copyright © 2011 John Wiley & Sons, Ltd.     

Quantitative surface analysis by Auger and x-ray photoelectron spectroscopy

Recent developments in quantitative surface analysis by Auger (AES) and x-ray photoelectron (XPS) spectroscopies are reviewed and problems relating to a more accurate quantitative interpretation of AES/XPS experimental data are discussed. Special attention is paid to consideration of elementary physical processes involved and influence of multiple scattering effects on signal line intensities. In particular, the major features of core-shell ionization by electron impact, Auger transitions and photoionization are considered qualitatively and rigorous approaches used to calculate the respective transition probabilities are analysed. It is shown that, in amorphous and polycrystalline targets, incoherent scattering of primary and signal Auger and photoelectrons can be described by solving analytically a kinetic equation with appropriate boundary conditions. The analytical results for the angular and energy distribution, the mean escape depth, and the escape probability as a function of depth of origin of signal electrons as well as that for the backscattering factor in AES are in good agreement with the corresponding Mote Carlo simulation data. Methods for inelastic background subtraction, surface composition determination and depth-profile reconstructions by angle-resolved AES/XPS are discussed. Examples of novel techniques based on x-ray induced photoemission are considered.     

Quantification of hard X-ray photoelectron spectroscopy: Calculating relative sensitivity factors for 1.5- to 10-keV photons in any instrument geometry

A method for the rapid determination of theoretical relative sensitivity factors (RSFs) for hard X-ray photoelectron spectroscopy (HAXPES) instruments of any type and photon energy has been developed. We develop empirical functions to describe discrete theoretically calculated values for photoemission cross sections and asymmetry parameters across the photon energy range from 1.5 to 10 keV for all elements from lithium to californium. The formulae describing these parameters, in conjunction with similar practical estimates for inelastic mean free paths, allow the calculation of a full set of theoretical sensitivity factors for a given X-ray photon energy, X-ray polarisation and instrument geometry. We show that the anticipated errors on these RSFs are less than the typical errors generated by extracting X-ray photoelectron spectroscopy (XPS) intensities from the spectra and thus enable adequate quantification for any XPS/HAXPES experiment up to 10 keV. A spreadsheet implementation of this method is provided in the supporting information, along with example RSFs for existing commercial instruments.     

Two-dimensional angle-resolved photoelectron spectroscopy using display analyzer—Atomic orbital analysis and characterization of valence band

The information obtained by two-dimensional angle-resolved photoelectron spectroscopy in UPS (ultraviolet photoelectron spectroscopy) region is described. A display-type spherical mirror analyzer can measure wide-angle angular distribution of photoelectrons of one particular kinetic energy (binding energy) without changing the angles of incident light and the sample. The shape of the cross section of valence band, especially of the Fermi surface can be observed directly on the screen. Three-dimensional energy band and Fermi surface are obtained by scanning the binding energy of two-dimensional band mapping. In the case of linearly polarized light excitation the symmetry relation in the photoelectron excitation process can also be displayed as “angular distribution from atomic orbital ADAO”, which is used to distinguish the atomic orbitals constituting the energy band. An example is shown for the atomic orbital analysis of Cu Fermi surface at each k point. It was successfully revealed that the Cu 4p orbitals are aligned with their axes pointing outwards. Another important angular distribution is the “photoemission structure factor PSF”, which originates from the interference among photoelectron waves from individual atoms. PSF determines the intensity inequivalency between Brillouin zones and reveals the bonding character of the energy band.     

Monte‐Carlo simulation of x‐ray photoelectron emission from silver

Silver 3d x‐ray photoelectron spectroscopy (XPS) spectra were simulated with the Monte‐Carlo method using an effective energy‐loss function that was derived from a reflected electron energy‐loss spectroscopy (REELS) analysis based on an extended Landau approach. After confirming that Monte‐Carlo simulation based on the use of the effective energy‐loss function can successfully describe the experimental REELS spectrum and Ag 3d XPS spectrum, we applied Monte‐Carlo simulation to predict the angular distribution of Ag 3d x‐ray photoelectrons for different x‐ray incidence angles and different photoelectron take‐off angles. The experimental photoelectron emission microscope that we are constructing was confirmed as being close to the optimum configuration, in which the x‐ray incident angle as measured from the surface normal direction is 74° and the photoelectron take‐off angle is set normal to the surface. The depth distribution functions of the Ag 3d X‐ray photoelectrons for different energy windows suggest that the photoelectron emission microscope will exhibit greater surface sensitivity for narrower photoelectron energy windows. Copyright © 2003 John Wiley & Sons, Ltd.     

Quantitative surface analysis by X-ray photoelectron spectroscopy and X-ray excited auger electron spectroscopy

It is shown that X-ray excited KLL Auger electron spectra allow it to describe measured signal strengths similarly to X-ray photoelectron signals, thus offering valuable information on the quantitative surface composition of a solid sample. The principal equation and corresponding fundamental parameters are discussed. As a result Auger spectra of C, N, O, F, and Na can be easily used in a multiline approach for quantitative analysis. LMM and MNN spectra give rise to more problems, due to their more complicated structure, uncertainties with regard to the background and the influence of Coster-Kronig transitions. These problems are overcome by the use of empirical ratios of the strongest lines of 2p/LMM or 3d/MNN. Since these ratios are independent of sample composition, they allow it to transform the Auger signal into the corresponding photoelectron signal, provided that a standard sample has been measured. Thus a true additional information is obtained and moreover difficulties in cases of photoelectron spectra with overlapping lines from other chemical elements can be overcome.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Site-sensitive X-ray photoelectron spectroscopy of Fe3O4 by photoelectron diffraction

Fe 2p core-level photoelectron spectra of magnetite were measured using soft X-ray and hard X-ray, and its emission angle dependence was investigated. The photoelectron diffraction pattern from different atomic sites differs because the atomic arrangement surrounding each site is different. By selecting the forward-focusing-peak (FFP) directions characteristic to each atomic site and measuring the kinetic energy dependence of the FFP intensities at the Fe 2p core-level range, we succeed in detecting the variation of the peak intensity of Fe 2p core-level spectra at different emission directions. This result, consistent with recent results, suggests that the lower-binding-energy peak of the Fe 2p core-level spectrum may be assigned as the B site component.     

Evaluation of elastic‐scattering cross sections for electrons and positrons over a wide energy range

Quantification of surface‐ and bulk‐analytical methods, e.g. Auger‐electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS), electron‐probe microanalysis (EPMA), and analytical electron microscopy (AEM), requires knowledge of reliable elastic‐scattering cross sections for describing electron transport in solids. Cross sections for elastic scattering of electrons and positrons by atoms, ions, and molecules can be calculated with the recently developed code ELSEPA (Elastic Scattering of Electrons and Positrons by Atoms) for kinetic energies of the projectile from 10 eV to 50 eV. These calculations can be made after appropriate selection of the basic input parameters: electron‐density distribution, a model for the nuclear‐charge distribution, and a model for the electron‐exchange potential (the latter option applies only to scattering of electrons). Additionally, the correlation‐polarization potential and an imaginary absorption potential can be considered in the calculations. We report comparisons of calculated differential elastic‐scattering cross sections (DCSs) for silicon and gold at selected energies (500 eV, 5 keV, 30 keV) relevant to AES, XPS, EPMA, and AEM, and at 100 MeV as a limiting case. The DCSs for electrons and positrons differ considerably, particularly for medium‐ and high‐atomic‐number elements and for kinetic energies below about 5 keV. The DCSs for positrons are always monotonically decreasing functions of the scattering angle, while the DCSs for electrons have a diffraction‐like structure with several minima and maxima. A significant influence of the electron‐exchange correction is observed at 500 eV. The correlation‐polarization correction is significant for small scattering angles at 500 eV, while the absorption correction is important at energies below about 10 keV. Copyright © 2005 John Wiley & Sons, Ltd.     

Electronic Structure and Interface Properties of a Model Molecule for Organic Solar Cells

We study the electronic structure of 4,7‐bis(5‐methylthiophen‐2‐yl)benzo[c][1,2,5]thiadiazole (MTBT) and its interface properties with gold using X‐ray photoemission spectroscopy (XPS), valence‐band ultraviolet photoemission spectroscopy (UPS), X‐ray absorption spectroscopy (XAS), as well as resonant photoemission (ResPES). MTBT can be regarded as a model molecule for PCPDTBT, a promising candidate for efficient bulk heterojunction solar cells. Almost no contribution of sulfur and only a weak contribution of nitrogen to the HOMO level is found. At the interface with gold, a strong chemical interaction between the sulfur of the benzothiadiazole and gold occurs, which may have consequences for interface properties in devices.     

DEPTH RESOLUTION IN PHOTOELECTRON MICROSCOPY OF ORGANIC SURFACES. THE PHOTOELECTRIC EFFECT OF PHTHALOCYANINE THIN FILMS

Abstract— The application of photoelectron microscopy as a general method of imaging organic and biological surfaces requires a knowledge of the photoelectric effect of very thin organic films. In this study, low magnification images of a 7 Å thick pattern of copper phthalocyanine were obtained, demonstrating that it is possible to visualize a monolayer of organic compound in photoelectron microscopy. Relative photoelectron currents were measured for a series of copper phthalocyanine films ranging in thickness up to 1900 Å. The relative photoelectron currents were independent of temperature (90–298°K), suggesting that electron-electron and not electron-phonon scattering is the dominant mechanism. The photoelectric properties measured are determined primarily by the large organic ring structure and not the central metal atom, as evidenced by the fact that substitution of metal-free phthalocyanine for copper phthalocyanine did not substantially alter the values of observed photoelectron currents. An analysis of the data indicates the depth resolution is 15 ű 5 Å, and equals the electron mean free path. This very good depth resolution is a result of the low kinetic energy associated with electrons released by irradiation near the photoemission threshold.     

Quantitative analysis of complex amino acids and RGD peptides by X‐ray photoelectron spectroscopy (XPS)

The C 1 s, N 1 s, and O 1 s core level binding energies (BEs) of the functional groups in amino acids (glycine, aspartic acid, glutamic acid, arginine, and histidine) with varied side‐chains and cell‐binding RGD‐based peptides have been determined and characterized by X‐ray photoelectron spectroscopy with a monochromatic Al K_α source. The zwitterionic nature of the amino acids in the solid state is unequivocally evident from the N 1 s signals of the protonated amine groups and the C 1 s signature of carboxylate groups. Significant adventitious carbon contamination is evident for all samples but can be quantitatively accounted for. No intrinsic differences in the XP spectra are evident between two polymorphs (α and γ) of glycine, indicating that the crystallographic differences have a minor influence on the core level BEs for this system. The two nitrogen centers in the imidazole group of histidine exhibit an N 1 s BE shift that is in line with previously reported data for theophylline and aqueous imidazole solutions, while the nitrogen and carbon chemical shifts reflect the unusual guanidinium chemical environment in arginine. It is shown that the complex envelopes of C 1 s and O 1 s photoemission spectra for short‐chain peptides can be analyzed quantitatively by reference to the less complex XP spectra of the constituent amino acids, provided the peptides are of high enough purity. The distinctive N 1 s photoemission from the amide linkages provides an indicator of peptide formation even in the presence of common impurities, and variations in the relative intensities of N 1 s were found to be diagnostic for each of the three peptides investigated (RGD, RGDS, and RGDSC). Copyright © 2013 John Wiley & Sons, Ltd.     

Intramolecular band mapping of poly(p-phenylene) via UV photoelectron spectroscopy of finite polyphenyls

Ultraviolet photoelectron spectra were measured of solid sexiphenyl with synchrotron radiation and of gaseous polyphenyls from biphenyl to sexiphenyl with a Hel light source. The similarity of the spectrum of the solid with the XPS spectrum of poly(p-phenylene) (PPP) shows the usefulness of sexiphenyl as a model compound of PPP. Examination of the fine structure observed in the low binding-energy region clearly shows how the electronic structure of the p-phenylenes evolves from that of benzene including the effects of deeper levels and of the non-planarity of the molecular geometry. The experimental E = E(k) energy band-dispersion relation of a PPP chain can be deduced by giving each energy level of the oligomers an appropriate k value. An extrapolation for the total bandwidth and the threshold photoemission energy of solid PPP yields 3.9₅ and 5.6₅ eV. respectively.     

Simulating the photoelectron spectra of rare-gas clusters

Motivated by the recent experiments of the Swedish group [M. Tchaplyguine, R. R. Marinho, M. Gisselbrecht et al., J. Chem. Phys. 120, 345 (2004)], we simulate the photoelectron spectra of pure xenon and argon clusters. The clusters are modeled using molecular dynamics with Hartree-Fock-dispersion type pair potentials while the spectrum is calculated as the sum of final state energy shifts of the atoms ionized within the cluster relative to the isolated gas phase ion. A self-consistent polarization formalism is used. Since signal electrons must travel through the cluster to reach the detector, we have accounted for the attenuation of the signal intensity by integrating an exponentially decaying scattering expression over the geometry of the cluster. Several different approaches to determining the required electron mean free paths (as a function of electron kinetic energy) are considered. Our simulated spectra are compared to the experimental results.     

Structural study of self-assembled monolayers of ferrocenylalkanethiols on gold by angleresolved X-ray photoelectron spectroscopy

An angle-resolved X-ray photoelectron spectroscopic study has been performed on structures of self-assembling systems, viz ferrocenylthiols on a gold (111) crystal. The angular dependence of the intensities of photoemission reveals that ferrocenyl groups are on the outermost layer, separated from the gold substrate by hydrocarbon chains of the thiol groups.     

Nondestructive compositional depth profiling using variable‐kinetic energy hard X‐ray photoelectron spectroscopy and maximum entropy regularization

We discuss the calculation of nondestructive compositional depth profiles from regularization of variable kinetic energy hard X‐ray photoelectron spectroscopy (VKE‐XPS) data, adapting techniques developed for angle‐resolved XPS. Simulated TiO₂/Si film structures are analyzed to demonstrate the applicability of regularization techniques to the VKE‐XPS data and to determine the optimum choice of regularization function and the number of data points. We find that using a maximum entropy‐like method, when the initial model/prior thickness is similar to the simulated film thickness, provides the best results for cases where prior knowledge of the sample exists. For the simple structures analyzed, we find that only five kinetic energy spectra are necessary to provide a good fit to the data, although in general, the number of spectra will depend on the sample structure and noisiness of the data. The maximum entropy‐like algorithm is then applied to two physical films of TiO₂ deposited on Si. Results suggest interfacial intermixing. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.     

Angle- and spin-resolved photoelectron spectroscopy of thepπ outer valence shell of HBr molecules

Using circularly polarized synchrotron radiation, the photoionization of HBr molecules was studied by angle- and spin-resolved photoelectron spectroscopy in the photon energy range from 11.7 eV to 21 eV. For photoelectrons corresponding to the final ionic states HBr⁺ X ²Π_3/2(v=0) andX ²Π_1/2(v=0), the energy dependence of the dynamical photoionization parameters was measured and compared with ab initio calculations for HBr⁺ by Raseev et al. and RRPA calculations for Kr⁺ by Huang et al., This comparison indicates that, for energies above the electronic autoionization region, photoemission from the outer valence orbital exhibits distinct atomic behavior. By combining the experimental data for the cross section σ and the spin polarization parameter A, sums of partial cross section contributions to σ were determined and analyzed to obtain specific information on the outgoing partial electron waves. Furthermore, the validity of the so-called non-relativistic relationships for the dynamical photoionization parameters was tested as a function of equal photon and kinetic photoelectron energy, respectively.     

Concepts for chemical state analysis at constant probing depth by lab-based XPS/HAXPES combining soft and hard X-ray sources

The greater information depth provided in hard X-ray photoelectron spectroscopy (HAXPES) enables nondestructive analyses of the chemistry and electronic structure of buried interfaces. Moreover, for industrially relevant elements like Al, Si, and Ti, the combined access to the Al 1s, Si 1s, or Ti 1s photoelectron line and its associated Al KLL, Si KLL, or Ti KLL Auger transition, as required for local chemical state analysis on the basis of the Auger parameter, is only possible with hard X-rays. Until now, such photoemission studies were only possible at synchrotron facilities. Recently, however, the first commercial XPS/HAXPES systems, equipped with both soft and hard X-ray sources, have entered the market, providing unique opportunities for monitoring the local chemical state of all constituent ions in functional oxides at different probing depths, in a routine laboratory environment. Bulk-sensitive shallow core levels can be excited using either the hard or soft X-ray source, whereas more surface-sensitive deep core-level photoelectron lines and associated Auger transitions can be measured using the hard X-ray source. As demonstrated for thin Al₂O₃, SiO₂, and TiO₂ films, the local chemical state of the constituting ions in the oxide may even be probed at near-constant probing depth by careful selection of sets of photoelectron and Auger lines, as excited with the combined soft and hard X-ray sources. We highlight the potential of lab-based HAXPES for the research on functional oxides and also discuss relevant technical details regarding the calibration of the kinetic binding energy scale.     

Peak shape analysis of Ag 3d core‐level X‐ray photoelectron spectra of Au@Ag core‐shell nanoparticles using an asymmetric Gaussian–Lorentzian mixed function

This article reports on the peak shape analysis of X‐ray photoelectron spectra of gold‐silver core‐shell (Au@Ag) nanoparticles (NPs) using an asymmetric Gaussian–Lorentzian mixed function. Unlike Ag NPs, Au@Ag NPs have no oxide peak and show asymmetric line shape with a high energy tail in Ag 3d core‐level spectra. A monotonic increase in the Ag 3d binding energy and a decrease in the degree of asymmetry with increasing the Ag shell thickness were observed supporting the occurrence of charge transfer from Au core to Ag shell. Copyright © 2012 John Wiley & Sons, Ltd.     

Study of the degradation of plasma‐oxidized polystyrene by time‐ and angle‐resolved x‐ray photoelectron spectroscopy

Angle‐resolved x‐ray photoelectron spectroscopy (ARXPS) measurements were made, in repeated sequences employing Al and Mg x‐ray sources alternately, on a polystyrene sample that had been exposed to an oxygen plasma. It was observed that oxygen was lost from the sample over a period of 5 h and 40 min. The ARXPS data sets were corrected for the time displacement between consecutive measurements at different photoemission angles and fitted with three simple models in order to extract oxygen concentration–depth profiles, consistent with the data, as a function of time. The oxygen depth profiles were found to evolve in a consistent manner, indicating both a loss of average oxygen content and thickness in the ‘oxidized polymer layer’. Copyright © 2003 John Wiley & Sons, Ltd.     

Hydrogen bonds in liquid water studied by photoelectron spectroscopy

The authors report on photoelectron emission spectroscopy measurements of the oxygen 1s orbital of liquid water, using a liquid microjet in ultrahigh vacuum. By suitably changing the soft x-ray photon energy, within 600-1200 eV, the electron probing depth can be considerably altered as to either predominantly access the surface or predominantly bulk water molecules. The absolute probing depth in liquid water was inferred from the evolution of the O1s signal and from comparison with aqueous salt solution. The presence of two distinctive components in the core-level photoelectron spectrum, with significantly different binding energies, is revealed. The dominant contribution, at a vertical binding energy of 538.1 eV, was found in bulk and surface sensitive spectra. A weaker component at 536.6 eV binding energy appears to be present only in bulk water. Hartree-Fock calculations of O1s binding energies in different geometric arrangements of the water network are presented to rationalize the experimental distribution of O1s electron binding energies.