Extended Energy Loss Fine Structure Research Articles

Structural studies of amorphous titanium diboride thin films by extended x-ray-absorption fine-structure and extended electron-energy-loss fine-structure techniques.

The local atomic structure of amorphous titanium diboride thin films, prepared by electron-beam vaporization (EBV) of the crystalline compound onto liquid-nitrogen-cooled substrates, was studied using extended x-ray-absorption fine-structure (EXAFS) and extended energy-loss fine-structure (EXELFS) techniques. From a comparison of the extended fine-structure spectra of the amorphous films with corresponding spectra of crystalline titanium diboride, accurate information was derived on the nature of the local structure, or short-range order, and on the coordination numbers, interatomic distances, and nanostructural atomic disorder in amorphous ${\mathrm{TiB}}_{2}$. A relaxation of the interatomic spacing and a reduction of coordination number for the nearest-neighbor atoms was inferred for the amorphous state. Local prismatic coordination with random 90\ifmmode^\circ\else\textdegree\fi{} rotations about prismatic planes is proposed as a likely atomic structure consistent with the data for the amorphous form. Finally, EXAFS and EXELFS were employed to examine in detail the structural changes induced in amorphous ${\mathrm{TiB}}_{2}$ by variations in the EBV deposition parameters, and to determine a set of optimized parameters for the EBV deposition of a ${\mathrm{TiB}}_{2}$ stable amorphous phase.

The Analysis of Surface Extended Energy Loss Fine Structure of Zinc Oxide

The SEELFS (Surface Extended Energy Loss Fine Structure) of zinc oxide was observed. We obtained a nearest-neighbor atomic spacing of 0.202±0.001 nm (l=2) by correcting the phase shift which was dependent on the final angular momentum of l. This value is about 0.007 nm longer than the one of bulk.

Evaluation of amorphous and crystalline SiC by extended energy loss fine structure (EXELFS) analysis

Crystalline as well as amorphous forms of silicon carbide are materials which have important uses in many areas of materials science including structural ceramics, coatings, and semiconductors. The characterization of SiC by the use of the EXELFS technique yields radial distribution functions (RDF) which are sensitive to the structure and composition within near neighbor distances of either C or Si atoms. In this way a quantitative analysis of possible variations in “structure” can be made. EXELFS analyses of crystalline SiC are especially important in order to evaluate the method when comparison with results from the amorphous state are to be made. The EXELFS technique has previously been applied to study both crystalline as well as amorphous materials.Crystalline as well as amorphous SiC was evaluated. One specimen was a commercially available SiC single crystal. Two other specimens utilized a similar SiC starting material which had been made amorphous either by ion implantation with chromium or by irradiation from 300 kV electrons.

AES and EELFS Studies of Initial Stages of Growth of GaAs/InAs/GaAs Heterostructures.

ABSTRACTWe report on a systematic study of the composition and structure of GaAs/InAs/GaAs quantum wells using Auger Electron Spectroscopy (AES), Extended Energy Loss Fine Structure (EELFS), and Reflection High Energy Electron Diffraction (RHEED) techniques. Double heterostructures with InAs thickness ranging from 2 to 10 monolayers, capped by 2 to 10 monolayers of GaAs, were grown by MBE using a variety of techniques, including those employing sequential, interrupted, and delayed shutter timing sequences. AES peak ratios are compared with model calculations to monitor compositional development of the multilayers. The AES results are correlated with RHEED measurements to determine MBE growth parameters for optimal control of the stoichiometry and surface morphology. EELFS was used to monitor strain in the buried InAs layers. The AES results show departure from smooth laminar growth of layers of stoichiometric InAs on GaAs at temperatures below 420 C and above 470 C. AES results on the quantum well structures suggest floating InAs layers on top of the GaAs and/or facet formation in the GaAs layers. The EELFS results, when compared to bulk InAs, indicate the presence of strain in the buried InAs quantum well.

The characterization of amorphous Al2O3by extended energy loss fine structure (EXELFS) analysis

Amorphous phases have been shown to influence properties of many metallic, semiconducting, and ceramic materials. However, analysis of these phases presents a difficult challenge since most conventional characterization techniques provide only limited qualitative information. Extended Energy Loss Fine Structure (EXELFS) analysis is a technique which is sensitive to both structure and composition within near neighbor distances of specific types of atoms in both crystalline and amorphous materials. The fine structure, which is present in the form of modulations on the core loss edges of electron energy loss spectra, results from diffraction of ejected core electrons by neighboring atoms. The ability to obtain EXELFS measurements with an analytical electron microscope allows direct observation of changes in composition, microstructure, and local atomic environment at a spatial resolution (<100 nm) not attainable by other techniques.Recently an investigation of the effects of ion implantation on the microstructure and properties of A12O3has revealed differences in the response of various forms of amorphous A12O3to post-implantation annealing.

Local structure of graphite by EELFS spectroscopy: Influence of multiple plasmons and orientational dependence

The application of the extended energy-loss fine structure (EELFS) technique in reflection mode to study the clean surface of graphite is reported. The possible effect of multiple plasmons on the extended features above the carbon K edge has been taken into account and removed by a deconvolution procedure applied to the energy-loss spectra. This procedure is particularly necessary in order to obtain reliable lattice parameters whenever the multiple plasmon contribution dominates the energy-loss spectrum close to the elastic peak and when high-energy electron beams are used. The radial distribution functions F(R) deduced from the extended fine structure for graphite are in good agreement with energy-loss results obtained in transmission mode confirming the validity of the dipole approximation for the scattering cross section also in reflection mode. Finally, we show the orientational sensitivity of the EELFS technique to probe the bond length direction with respect to the momentum transfer q̄ by using different primary energies.

Theory of extended energy loss fine structure in the reflection mode

An analytic expression for the current detected in an extended energy loss fine structure experiment in the reflection mode is derived, which takes into account the diffraction by the atoms of the crystal of the two electrons participating in the excitation process. Within a single-scattering approximation, the authors show that the signal contains two oscillating factors, one of the EXAFS type and the other, which has been neglected in all studies until now, similar to a photo-electron diffraction signal. They perform a model computation in order to show the experimental conditions in which an EXAFS-type analysis still applies and the precision in the distances that may be expected. They also propose a new type of experiment in which the detection is angularly resolved and which would allow a bond angle determination in addition to the distance determination, under specified excitation conditions.

Structure determination of graphitic carbon on Cu(110) by means of surface extended energy loss fine structure

Extended energy loss fine structures (EELFS) have been detected in the reflection mode above the K edge of graphitic carbon chemisorbed on a copper (110) single crystal. To explain the features of the radial distribution function of the EELFS signal a model is proposed for the geometrical structure of graphitic C on Cu(110) which reproduces well the experimental data. According to the model the graphitic carbon overlayer is positioned 1.275 Å above the Cu(110) surface and the bond length between carbon atoms in the basal plane of the hexagonal lattice turns out to be 1.47 Å.

Structural characterization of surfaces by extended energy-loss fine-structure spectroscopy

We present recent structural investigations by extended energy-loss fine-structure (EELFS) measurements. The EELFS technique relies on the detection of the fine-structure losses above a core edge in the reflection mode. We demonstrate the possibility of extracting the radial distribution F(R) around each investigated atom. This is obtained by applying to the experimental EELFS data the Fourier transformation and the theoretical analysis used for extended x-ray absorption fine structure (EXAFS) spectroscopy. The aim of this paper is to answer the principal questions addressed by the EELFS technique in the last few years (dipole approximation, phase shift, backscattering amplitude transferability, and quantitative structural parameters, etc.). This is obtained by a systematic investigation on different elements around different core levels and by comparing our experimental spectra with EXAFS spectra. Technological application of the surface-EELFS technique for interface formation studies and clusters lattice parameter determination will be presented and discussed.

Local structure of Ni 2Si

The local structure around the Ni atom in Ni 2Si has been studied by measuring and analyzing the Extended Energy Loss Fine Structure above the Ni M 2,3 edge. The radial distribution function we found is characterized by the efficiency of the only nearest neighbours Si atoms. We also obtained the phase shift of the Ni-Si pair and the backscattering amplitude.

Core and valence excitations in Ni 2Si

We present an electron energy loss analysis of Ni 2Si around the NiM 2,3 and NiL 2,3 edges and in the range 0–30 eV (valence excitations). An intensity reduction of the Ni L 2,3 edge of the silicide is observed and interpreted in terms of a depletion of d metal electrons at the Fermi level E F. The reduction in the Ni M 2,3 Fano line shape with respect to that for pure nickel is also described in a similar way. The near-edge region gives information on the partial density of states in the region above E F and is related to the structures observed in the valence band range. Particular attention was paid to the study of the Ni 2Si local structure obtained from the analysis of the extended energy loss fine structures beyond the Ni M 2,3 level.

Contributions of s- and d-final states to the M 2, 3 extended energy loss fine structure in copper

Extended energy loss fine structures have been detected in the reflection mode above the M 2, 3 edge of a copper (1 1 0) single crystal. The radial distribution function (RDF) obtained by performing the Fourier transform of the EELFS signal shows a dependence of the nearest-neighbour distance peak on the integration range of the wave vector k. For k between 2.9 Å -1 and 9 Å -1 the RDF peaks at r= 2.12 ± 0.02 A ̊ as reported by other groups. However, by integrating over a wider range of wave vectors between 2.9 Å -1 and 12 Å -1 we have obtained a peak at r = 2.21 ± 0.02 A ̊ . This can be explained as arising from significant contributions from both the d- and s-symmetric final states.

Extended energy loss fine structures (EELFS): A new structural probe for surfaces and interfaces

Electron energy loss spectra above M 2,3 and L 2,3 core levels of transition metals are presented. The analysis of the structures up to several hundred electronvolts above these edges shows strict analogies with EXAFS oscillations. The comparison with synchrotron radiation X-ray absorption results, above the same edges, shows that accurate radial distribution functions can be obtained from EELFS data using the same Fourier transform and formalism. The EELFS measurements are performed in the reflection mode using a conventional electron analyzer with a beam current of a few μA. Important surface phenomena can be investigated. In this paper I shall present some recent progress of the EELFS technique for the study of metallic clusters and of a monolyer of Cobalt on Si(111).

Extended Energy Loss Fine Structures (EELFS): A new structural probe for surfaces and interfaces

Extended Energy Loss Fine Structures (EELFS): A new structural probe for surfaces and interfaces

Extended energy loss fine structure measurement above shallow and deep core levels of 3d transition metals

The authors present a systematic investigation of surface structure of 3d transition metals through electron energy loss fine structure (EELFS) measurements, above the edges of shallow and deep core levels in the reflection mode. Fine structures above these edges, similar to those measured in X-ray absorption spectra and transmission energy loss spectra, extend for several hundred eV from the threshold. From the Fourier analysis of these features they obtained the radial distribution function around the excited atom. They discuss the EELFS analysis (Fourier transformations, backscattering amplitudes and phase shifts) above the M2,3 edges of Cu, Ni, Co, Fe and Ti metals. The experimental NiM2.3 phase shift is compared with recent theoretical calculations of shallow core levels and the discrepancy observed is ascribed to possible Fano-interference relaxation effects. They present a direct comparison between the EELFS spectrum of the TiL2.3 edge with the EXAFS spectrum obtained with synchrotron radiation. This comparison gives the most convincing evidence that the dipole (q equivalent to 0 AA-1, Delta l=+or-1) is the predominant term in the inelastic cross-section even when low primary beam energies are used and high energy loss are involved. Finally the surface sensitivity of the present technique is shown through the modification of surface-EELFS features above the FeM2,3 edge during the early stages of oxidation.

Extended energy loss fine structure in reflection electron energy loss spectra of Cu and Ni

Reflection electron energy loss spectra of the surfaces of polycrystalline Cu, Ni and Al have been recorded with impact energies ranging from 300 to 2400 eV. Weak structures observed in the M- and L-shell continua of Cu and Ni are interpreted as extended energy loss fine structure (EXELFS), the electron scattering analog of EXAFS. The nearest neighbour spacings derived from the M 23 EXELFS are 0.2 to 0.3 Å less than the values for bulk Cu or Ni. This difference is attributed at least in part to the inaccuracy of calculated phase shift corrections rather than a surface contraction of this magnitude. Complications to reflection EXELFS from multiple plasmon losses are demonstrated in the aluminum L 23 spectrum.

Structural study of iron and carbidic iron by surface extended energy loss spectroscopy

We have applied the Extended Energy Loss Fine Structure (EELFS) technique for studying the radial distribution function of iron before and after the formation of a surface carbidic iron layer. The experiment is performed in the reflection mode with a conventional CMA electron analyzer. The energy range scanned is 50–500 eV. The Fourier analysis of the structures measured above the Fe M 2,3 edge and the C K edge gives structural information around different absorbing sites on surfaces which lack long range order. The structural results are compared with the local structure of bulk Fe 3C measured with the EXAFS technique. We have found that each carbon on the surface is coordinated to six nearest neighbours iron atoms in a trigonal prism coordination as in the crystalline iron carbide.

Surface extended energy loss fine structures above the K-edge of oxygen on Al

Surface Extended Energy Loss Fine Structures (SEELFS) have been detected above the K-edge of oxygen adsorbed on an Al sample. This is the first evidence that the local geometry of adsorbates on metal surfaces can be investigated with the electron energy loss technique in the reflection mode. This experimental technique is highly surface sensitive and accurate bond lengths can be obtained. The oxidation model implied by our results for the OAl system is in agreement with data in the literature.

Some Aspects of Exelfs Analysis in the Electron Microscope

Recently, the applications area of analytical electron microscopy has been extended to include the study of Extended Energy Loss Fine Structure (EXELFS). Modulations past an ionization edge in the energy loss spectrum (EXELFS), contain atomic fine structure information similar to Extended X-ray Absorbtion Fine Structure (EXAFS). At low momentum transfer the main contribution to these modulations comes from interference effects between the outgoing excited inner shell electron waves and electron waves backscattered from the surrounding atoms. The ability to obtain atomic fine structure information (such as interatomic distances) combined with the spatial resolution of an electron microscope is unique and makes EXELFS an important microanalytical technique.