Continuous X-ray-induced Auger electron spectroscopy

Bremsstrahlung emission from a tungsten anode has been used, for the first time, to obtain X-ray-induced Auger electron spectra (CXAES) in a conventional X-ray photoelectron spectrometer. Auger KLL signal and background intensities obtained from a silicon—aluminium sample have been compared with those obtained un der the same experimental conditions but using a standard aluminium anode. For such an aluminium anode, the respective contributions of the bremsstrahlung and the characteristic radiation to the C_KVV intensity have also been established.     

X-ray photoelectron and X-ray Auger electron spectroscopy studies of heavy ion irradiated C60 films

The influence of 200 MeV Au ion irradiation on the surface properties of polycrystalline fullerene films has been investigated. The X-ray photoelectron and X-ray Auger electron spectroscopies are employed to study the ion-induced modification of the fullerene, near the surface region. The shift of C 1s core level and decrease in intensity of shake-up satellite were used to investigate the structural changes (like sp² to sp³ conversion) and reduction of π electrons, respectively, under heavy ion irradiation. Further, X-ray Auger electron spectroscopy was employed to investigate hybridization conversion qualitatively as a function of ion fluence.     

AES: energy calibration of electron spectrometers. IV. A re-evaluation of the reference energies

Reference kinetic energies for Auger electrons were published by the National Physical Laboratory (NPL) in 1990. These energies are traceable via measurements of X-ray photoelectron binding energies, referenced to the Fermi level position in the spectrum, and values for the X-ray energies involved. Revised values of both the Fermi level position and the X-ray energies have now been obtained. Use of these new values leads to an increase of the recommended reference kinetic energies for AES by 0.07 eV from the previous values.     

Surface composition and depth concentration profile of platinum/tin alloys from combined X-ray photoelectron and Auger spectroscopic data

Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (X-PS) have been combined to study surface enrichment of two platinum/tin alloys, viz. PtSn and Pt₃Sn, effected by reduction in hydrogen at 550°C. By making use of the different information depths of AES and X-PS for this alloy system, as determined by the corresponding mean free paths of inelastic scattering of the emitted characteristic electrons, a rather detailed picture has been obtained of the composition of the topmost atomic layers in these alloys. It is shown that both alloys exhibit phenomena of surface enrichment and depletion of the underlying layers over distances of the order of 10 Å.     

X-ray photoionization cross sections for quantitative analysis

The relative photoemission intensities from subshells of 51 elements and simple compounds have been measured using X-ray photoelectron spectroscopy. The chemical composition of each specimen surface was monitored by Auger electron spectroscopy and contamination was minimized by Ar⁺ ion bombardment. Experimental photoelectron cross sections derived for MgKα (1254 eV) radiation were compared with theoretical Hartree-Slater subshell photoionization cross sections calculated by Scofield. Good correlation (r = 0.96) between theory and experiment was observed over two orders of magnitude for most of the elements studied. Relative cross sections for subshells of a given element are also consistent with theoretical values, with several notable exceptions. The results indicate that quantitative analysis of ESCA spectra is possible using the Scofield cross sections.     

Auger and direct electron spectra in X-ray photoelectron studies of zinc,zinc oxide,gallium and gallium oxide

Auger and direct electron spectra from Zn, ZnO, Ga and Ga₂O₃ have been studied with X-ray photoelectron spectroscopy (ESCA). The chemical shift between zinc electron binding energies in Zn and ZnO is very small, whereas the zinc Auger electron signals are separated by 4.3 eV. In gallium, the oxide and metal signals are separated by 1.9 eV, but the Auger electron energy shift is three times as large. Thus the Auger signals are more sensitive to the chemical environment than the direct electron signals, which is the same relation as earlier observed for copper and copper oxides.     

Photoelectron diffraction effects in XPS angular distributions from GaAs(110) and Ge(110) single crystals

Angular distribution measurements of XPS intensities have been made for various spectral lines from GaAs(110) and Ge(110) single-crystal surfaces. Observed angular distribution curves (ADC's) showed steep intensity variations and sharp peaks due to X-ray photoelectron diffraction (XPED) phenomena. The effects of the type of transition process (photoelectron or Auger), electron kinetic energy and crystal structure on the XPED patterns were examined. Considerably different ADC patterns were observed for high-energy photoelectrons and Auger electrons and for low-energy photoelectrons. ADC's for Ga 3d, As 3d and Ge 3d showed almost the same patterns for scans of the type [110] → [100] → [1$\text{1}$0], but they showed substantially different patterns for [110] → [11$\text{1}$] → [00$\text{1}$] scans. These features correspond well with the structural characteristics of GaAs and Ge crystals. A discussion of the applicability of XPS angular distribution measurements to the geometric analysis of crystal surfaces is presented.     

Investigation of ageing characteristics and identification of surface chemical changes on SrGa2S4:Ce display phosphor under electron beam bombardment

Cathodoluminescent ageing characteristics of SrGa₂S₄:Ce³⁺ under prolonged electron beam bombardment was studied and the data are presented. The cathodoluminescent intensity with an increasing Coulomb loading was observed to degrade under different primary electron beam voltages. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to monitor the surface chemical changes during electron beam bombardment and after the degradation process. Auger peak to peak heights monitored during the ageing process suggest a loss in S and C and an initial increase in oxygen concentration on the surface. XPS results indicate the formation of a SrO overlayer due to electron stimulated surface chemical reactions (ESSCRs).     

Fuchs-Kliewer phonon spectrum of Co3O4(110) single crystal surfaces by high resolution electron energy loss spectroscopy

The Fuchs-Kliewer phonon spectrum of single crystal Co₃O₄(110) has been analyzed by high resolution electron energy loss spectroscopy (HREELS) and the four fundamental phonon losses have been identified at 26.8, 47.5, 71.1 and 84.7 meV (216, 383, 573 and 683 cm⁻¹). This is the first HREELS study reported for an intrinsic spinel single-crystal surface with primary focus on the Fuchs-Kliewer phonon structure. The Co₃O₄ crystal is first characterized by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED), which establish the composition, cleanliness, and order of the (110) surface. Electron scattering is then used to obtain a series of well-resolved Fuchs-Kliewer phonon spectra over 2.25-14.25 eV incident electron energy range. The variation in phonon intensity with primary beam energy is shown to agree with that predicted by dielectric theory.     

The characterisation of iron-containing intermetallic phases within industrially cast aluminium

⁵⁷Fe conversion electron Mössbauer spectroscopy has been used to investigate the intermetallic phases near the surface of a D.C. cast aluminium ingot. The CEMS data is used with SAAES (selected area Auger electron spectroscopy) and SAXPS (selected area X-ray photoelectron spectroscopy) data to propose a model of the surface region above the grain boundaries.     

Electron spectroscopic studies of the adsorption of methanol on zinc oxide powder

X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and Auger electron spectroscopy (AES) have been used to study the adsorption of methanol on ZnO powder. The He(II) spectrum of electron-beam-eleaned ZnO was similar to that reported by Rubloff .¹ for the ZnO (11̄ 0 0) surface. The spectrum of methanol condensed at ?150°C matched that of gas-phase methanol. At low methanol coverages, difference spectra showed no increase in the methanol O(2p) lone pair binding energy reported to be indicative of chemisorption. A residual carbon and oxygen surface component on the ZnO powder may have prevented chemisorption. XPS measurements were found to be more useful in observing this layer than was AES. A decrease in the ZnO valence band intensity near the valence band maximum indicated that a strong interaction with the substrate had occurred.In this system the valence bands of the adsorbate and substrate overlap, and the true position of the methanol O(2p) lone pair orbital is obscured by the effects of the substrate band modification. Also, for insulators or semiconductors, a change in the measured position of the valence band maximum relative to the Fermi level may occur upon adsorption. Since the Zn(3d) core level should not be affected, it was used as an energy reference for the present work. The problems of energy referencing in UPS studies of adsorption on semiconductors are discussed.     

Observation of resonance recombination lines in electron excited auger spectra of Gd

Combined measurements of electron excited N_4,5 Auger spectra and photoelectron emission on clean and oxidized Gd lead to a distinction between Auger lines originating from 4d → continuum and 4d → 4? resonance excitations. Several Auger structures are identified as due to the direct recombination of 4d⁹4?⁸ states with the 4f and valence electrons. The shape of the most prominent Auger line for oxidized Gd agrees perfectly with the Fano profile of the 4? photoemission intensity.     

Electron spectroscopy of graphite,graphite oxide and amorphous carbon

Core-level and valence-band photoelectron spectra and Auger spectra for the basal plane of highly ordered graphite, the edge plane of highly ordered graphite, the basal plane of graphite oxide, and for the basal plane of disordered graphite are compared in an effort to determine spectral features that may be used to identify these chemical species in carbon-rich specimens. The photoelectron spectra were recorded using 1253.6 eV X-ray excitation. The Auger spectra were obtained using both X-ray (1253.6 eV) and electron (3 keV) excitation. The differential X-ray excited C-KVV spectra were the most useful in distinguishing the different types of carbon. In particular, the plasmon structure and the energy separation between the two major excursions were very sensitive to changes in the surface chemistry and structure. Changes in peak position and peak width observed in the valence band and C(1s) photoelectron spectra were also very helpful in distinguishing the pristine, structurally damaged, and the oxidized graphite surfaces. Much of the structural and chemical information apparent in the X-ray excited C-KVV spectra was lost upon electron beam exposure. Differences in the C-KVV Auger lineshapes for X-ray and electron excitation were attributed to both structural and chemical changes induced by electron beam exposure.     

Preparation and structural properties of YBCO films grown on GaN/c-sapphire hexagonal substrate

Epitaxial YBCO thin films have been grown on hexagonal GaN/c-sapphire substrates using DC magnetron sputtering and pulsed laser deposition. An MgO buffer layer has been inserted between the substrate and the YBCO film as a diffusion barrier. X-ray diffraction analysis indicates a c-axis oriented growth of the YBCO films. Φ-scan shows surprisingly twelve maxima. Transmission electron microscopy analyses confirm an epitaxial growth of the YBCO blocks with a superposition of three a-b YBCO planes rotated by 120° to each other. Auger electron spectroscopy and X-ray photoelectron spectroscopy reveal no surface contamination with Ga even if a maximum substrate temperature of 700 °C is applied.     

Internal photoemission and plasmon gain in solid and thin films of Al

Weak features in the electron spectrum of Al excited by bremsstrahlung radiation from a Cu anode have been studied using a recently developed multidetector. A feature at ～1410 eV which has been identified as the Al 2p internal photoelectron line was found to have an intensity 4.2 × 10^?3 times that of the Al KL_2,3L_2,3:¹D₂ Auger line, in agreement with a simple theoretical treatment. The identification of this feature is confirmed by the observation of an ～67% decrease in its intensity in spectra obtained from clean Al films in the thickness range 3.1–34.0 nm. The intensity of a plasmon gain peak at ～1404 eV is found to be independent of thickness for films of thickness greater than 7.5 nm.     

The alignment of spectrometers and quantitative measurements in X-ray photoelectron spectroscopy

The alignment of the sample in X-ray photoelectron spectrometers is usually made to optimize the spectral intensities. There are two important classes of spectrometer: (i) those in which the analyser acceptance area is independent of the analysed electron kinetic energy; and (ii) those in which this area varies. Model experiments show how an example of a VG ESCALAB II conforms to class (i) whereas an example of an SSI X-probe is of class (ii) and shows an analyser acceptance area which depends approximately inversely on the emitted electron kinetic energy. This latter result means that the SSI X-probe spectrometer must be aligned for the electrons of the highest kinetic energy (smallest analyser acceptance area). A misalignment of 0.1 mm in the sample height can cause a 10% change in the relative intensities between 0 and 1000 eV binding energies. This dependence of the analyser acceptance area with energy is an effect likely to be common in the advanced electron optical systems of modern electron spectrometers and should be understood in order to use such spectrometers effectively. Such dependencies should be determined by analysts for their own instruments in the operating mode that is used for conducting work in which the repeatability of intensity measurements is important.     

Interpretation of Auger satellites in Co,Ni, and Cu compounds

X-ray photoelectron spectra of the 2p levels of Co, Ni, and Cu compounds are examined concurrently with their L₃M_4,5M_4,5 Auger spectra. A correlation is established between the presence or absence of Auger satellites with the presence or absence of photoelectron shake-up satellites for Co and Ni compounds. The correlation is less clear for cupric compounds. We propose the mechanism of Auger shake-up as a plausible interpretation for the observed behavior of these Auger satellites.     

PL and CL degradation and characteristics of SrAl2O4: Eu,Dy phosphors

Strontium aluminate phosphors are ideal for luminescent infrastructure materials. Their brightness and persistent glow time are much higher than previously used sulphide phosphors. Strontium aluminates prepared by the sol–gel and combustion methods are compared with commercially available strontium aluminate. High luminescent efficient SrAl₂O₄:Eu²⁺,Dy³⁺ pulsed laser deposited (PLD) thin films were also produced using the commercially available powder. Photoluminescence (PL) degradation studies showed that the phosphor intensity decreased about 20% over a period of 2 weeks under ultraviolet (UV) irradiation. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) showed that cathodoluminescence (CL) degradation is due to the formation of SrO due to electron stimulated surface reactions. The light output mechanism of the phosphor is also discussed in more detail.     

Account of photoelectron elastic determination of overlayer thickness,in-depth profiling,escape depth,attenuation coefficients and intensities in surface systems

An analytical connection is found between parameters of Monte Carlo and transport theory methods to account for elastic electron scattering in solids. A simple analytical method is proposed for the intensity calculations in X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) with account of elastic scattering. The method is applied for solution of many problems in XPS including determination of the overlayer thickness, correction factors for the in-depth profiling, escape depth, attenuation lengths, intensities in layered systems.     

Design of an electron spectrometer for automated photoelectron diffractogram imaging over π steradians

An electron spectrometer is presented which has been designed and built for the fast automated acquisition of photoelectron diffraction data over π steradians above single crystal surfaces. The principle features include the use of externally-selectable stainless steel channel arrays in front of the electron optics for high accuracy definition of the analyzer acceptance angle, a five channeltron detection system allowing the determination of the photocurrent from a given photoemission core level or Auger transition without changing the analyzer energy, a sample holder with heating and cooling capabilities which has been designed for very high (< 0.1°) angular control around both the polar and azimuthal rotation axes, and an automated data acquisition system to perform the angular scanning, collect the data, and represent it in the form of photoelectron ‘diffractograms’. These data provide a chemically specific visualization of interatomic directions in the first few atomic layers of the system of interest and can be used to image intra-molecular bonds of molecules deposited on surfaces, follow the epitaxial growth of one substance on the ordered surface of another, or study interdiffusion at interfaces.