G.G. Kleiman

X-ray excited Auger studies of metals and alloys

Abstract Auger spectroscopy has played a fundamental role in the development of modern surface science. Its characteristics of speed and surface sensitivity permit, for example, its wide application to problems of chemical identification and concentration analysis. Basic to many of these applications is the assumption of “finger printing”, or the insensitivity of the Auger lineshape to the chemical environment. Verification of the validity of this assumption requires understanding of the lineshape. We present a review of work on high resolution Auger lineshapes of metals and alloys. By appealing to basic physical arguments, we explain, for example, the nature of the process, the types of Auger spectra (i.e., “band-like” versus “quasi-atomic”), current theories and their relation to excited state electronic structure, some connections with photoemission, and the present level of understanding of available Auger data. Among the consequences of this understanding which we will discuss is the possibility of using the Auger spectrum of a noble metal constituent of an alloy to determine some important, previously unmeasured quantities: the relative Fermi energy and dipole barrier in an alloy with respect to those in the pure noble metal.

High resolution study of the M45N67N67 and M45N45N67 Auger transitions in the 5d series

The authors present the results of high resolution measurements of bremsstrahlung excited Auger electron spectra in the 5d series. The spectra provide an experimental test of atomic models. The measured effective Coulomb interaction between the final state holes displays a jump in value between Pt and Au, analogous to that between Ni and Cu in the 3d series. This similarity supports the validity of a quasi-atomic model of screening for the 5d metals.

X-ray photoemission lineshapes and energies in PdxCu1-x

Measurements of core and valence level X-ray photoemission spectra are reported for a series of ten PdxCu1-x alloys with varying compositions. The core level binding energy shifts display a distinct dichotomy in behaviour at x approximately=0.4. For x 0.4, the Pd shifts are consistent with a Fermi level different from that of pure Pd. The systematics of the Pd core level lineshapes are shown to be in accordance with this interpretation.

Energy shifts and electronic structure changes in alloys: an unfulfilled promise?

Abstract The sensitivity of XPS and Auger core level energies to the chemical environment promises a direct method for the determination of valence electronic changes. In alloys involving noble metals, the seemingly simple relations between the energy shifts and the d- and sp-electron occupations is complicated by a number of factors: they involve the relative Fermi energy, a bulk quantity not accessible to direct measurement; they involve potential parameters whose values in the solid are not well known; and relaxation, or screening, of the final state must take into account the difference of the d- and sp-electrons. In short, we seem to have too many unknown quantities for the two shifts to yield the desired electronic changes. In recent years, considerable progress has been achieved in overcoming these difficlties. The relative Fermi energy has been determined in a number of alloys, through consideration of experimental systematics and the different d- and sp-screening. Determination of the atomic potential parameters indicates an important dependence upon the valence electron configuration, indicating a previously unanticipated, non-linear, dependence of the energy shifts upon the valence occupation changes. Finally, the average charge transfer in the vicinity of the ionized atom has been derived in a linear coupling model. Here, we present a critical analysis of the XPS and Auger shifts for these alloys in view of these developments and indicate to what extent the resulting occupation changes reflect an internal consistency.

Core hole screening and electronic structure information

Abstract Understanding the nature of core hole screening is fundamental for interpretation of the results of electron spectroscopies of metals and alloys. The idea of complete screening of the core hole explains a large amount of XPS and high resolution Auger (XAES) data. Here, we address the question of the extent to which the complete screening model can be used to extract quantitative information about the electronic structure of metals and their alloys from measured Auger energies. Starting from general considerations, analysis of Auger parameter data illuminates the screening process throughout the 4d and 5d series, including the consequences of closing the d valence shell; the observed systematics are faithfully reproduced, quantitatively as well as qualitatively, in the 3d series as well; and parameters of a simple model consistent with complete screening which are extracted from the Auger parameter data are applicable to analysis of XPS and Auger shifts in alloys. Similar treatment of core level Auger shift data of the noble metal component in bimetallic alloys indicates the relation between the measured shifts, the relative Fermi energy (a usually unknown quantity involved in binding energy shifts) and the charge transfer. Consideration of various alloys (AuPd, CuPd, AgPd, AuCu, AuAg, and CuPt) implies a very small charge transfer and a unique determination of the relative Fermi energy. Use of noble metals as “impurities” in trimetallic alloys bears out these conclusions regarding charge transfer and Fermi energies.

L3M4,5M4,5 Auger transitions and valence hole screening of the 4d metals

High kinetic energy (2000–3000 eV) L3M4,5M4,5 Auger transitions (excited by Bremsstrahlung) are reported for the 4d series. The valence electron contribution to the measured Auger parameters was extracted from the data by exploiting systematics predicted theoretically. The systematic dependence on atomic number clearly manifests the effect of the nature of the screening charge and agrees remarkably well with independent theoretical calculations of metallic renormalization. Corresponding valence contributions for the 3d and 5d metals exhibit similar screening systematics.

LMM and LMN Auger spectra of Nb, Mo, Pd and Ag: an investigation by X-ray excited Auger electron spectroscopy

The authors present high-resolution measurements of several of the LMM and LMN Auger spectra of Nb, Mo, Pd and Ag excited by bremsstrahlung. Because of their high energy (2.0-3.5 keV), these spectra have not been reported in the literature. Assignments of the peaks are made based upon energy and intensity considerations.

Hafnium silicide formation on Si(100) upon annealing

High dielectric constant materials, such as HfO{sub 2}, have been extensively studied as alternatives to SiO{sub 2} in new generations of Si based devices. Hf silicate/silicide formation has been reported in almost all literature studies of Hf based oxides on Si, using different methods of preparation. A silicate interface resembles close to the traditional Si/SiO{sub 2}. The silicate very likely forms a very sharp interface between the Si substrate and the metal oxide, and would be suitable for device applications. However, the thermal instability of the interfacial silicate/oxide film leads to silicidation, causing a dramatic loss of the gate oxide integrity. Despite the importance of the Hf silicide surface and interface with Si, only a few studies of this surface are present in the literature, and a structural determination of the surface has not been reported. This paper reports a study of the Hf silicide formation upon annealing by using a combination of XPS, LEED, and x-ray photoelectron diffraction (XPD) analyses. Our results clearly indicate the formation of a unique ordered Hf silicide phase (HfSi{sub 2}), which starts to crystallize when the annealing temperature is higher than 550 deg. C.

ELECTROCHROMISM IN MOOX FILMS CHARACTERIZED BY X-RAY ELECTRON SPECTROSCOPY

Abstract The ability of molybdenum oxide thin films to reversibly exchange lithium ions (with an electrolyte) and electrons (with an external circuit) has been explored in optical electrochromic devices. The as-grown films are either transparent or slightly blue and show a deep blue color upon Li+/e− insertion. The thin film electronic band and crystallographic structure govern the kinetics of insertion reaction and the charge capacity of the material, as well as the optical behavior of the system. In this work, molybdenum oxide thin films were produced by r.f. reactive sputtering of a metallic molybdenum target in an O2–Ar atmosphere. Thin films with distinct compositions, were obtained by varying the oxygen flow (φ) during deposition and maintaining all other deposition parameters constant. Lithium intercalation was promoted by electrochemical means, from an aprotic, Li+-containing electrolyte. The films of distinct compositions were analyzed by X-ray photoelectron spectroscopy in order to investigate the relation between MoO3 reduction and the electrochromic effect reported in the literature [1] . Data obtained on high oxygen flow samples show narrow Mo 3d lines, with binding energies characteristic of the 6+ state. Lowering the oxygen flow results in very broad 3d lines, indicating the presence of Mo6+, 5+ and 4+. This broadening of the 3d lines is also observed upon Li intercalation in high flow samples both on as-grown films and on films intercalated with Li+/e−.

STUDY OF AMORPHOUS GERMANIUM-NITROGEN ALLOYS THROUGH X-RAY PHOTOELECTRON AND AUGER ELECTRON SPECTROSCOPIES

In this work, experimentally determined values of electron spectroscopic shifts induced by nitrogen in Ge core levels of substoichiometric amorphous germanium‐nitrogen (a‐GeN) alloys are discussed and presented. X‐ray photoelectron spectroscopy (XPS) and x‐ray excited Auger electron spectroscopy (XAES) are employed to study the behavior of the Ge 3d and LMM spectra, respectively, and combined the corresponding XPS and Auger core levels shifts to determine Δα’, the modified Auger parameter shift, which is exempt from problems inherent in the interpretation of XPS and XAES shifts. It is demonstrated how one can use Δα’ to reliably estimate ΔnGe, the change in Ge valence charge in the alloys, and how one can calibrate XPS shifts of Si and Ge based alloys in terms of approximate values of ΔnGe.