J C Le Bosse

N-K ELNES study of anisotropy effects in hexagonal AlN.

Anisotropic effects in hexagonal aluminium nitride have been studied by electron energy‐loss spectroscopy (EELS) in the N‐K energy loss near edge structure (ELNES). Experimental data acquired with different collection angles and with a nearly parallel incident electron beam aligned along the c‐axis have been compared to simulations based on ab initio calculations. The extraction of intrinsic parallel I‖ (with momentum transfer q ‖ c axis) and perpendicular I⊥ (with momentum transfer q ⊥ c axis) components has been performed directly from the experimental spectra. This has been done according to their description as linear combinations of I‖ and I⊥, with adequate weights deduced from a geometrical model of anisotropic behaviour.

Subthreshold effects in VLEED and the non-isotropy of absorption potential in metal

Abstract Sharp structures observed in LEED specular intensities on the (111) surfaces of fcc metals appear close to the emergence of new diffracted beams inside the crystal. It is shown that they issue from the energy band structure of the metal. A LEED calculation of these structures leads to a good agreement with experiment only if we suppose that the damping of a plane wave in the metal depends on the direction of its wave vector. In this way, the absorption potential is no more isotropic. In our case, the new emerging waves which propagates in directions parallel to the densely packed (111) surfaces are less damped than waves which propagate in other directions.

Interaction energy between two atoms chemisorbed at a transition metal surface

A detailed investigation of the interaction energy between two identical monovalent atoms chemisorbed at a transition metal surface is presented in terms of a tight-binding model. The direct interaction between the adatoms is explicitly included in the model. Information on the diadatom chemisorption process is obtained from an algebraical investigation of the limiting cases corresponding to weak and strong adatom-metal couplings. The intermediate coupling domain is studied through a computation carried out in the case of the (100) surface of an s-band simple cubic crystal with or without steps.

A general picture of threshold effects in LEED

The authors reinvestigate the interpretation of fine structures associated with beam emergence threshold in LEED. This work is based upon a new expansion of the total amplitude matrix of the crystal in a power series of the off-diagonal part of the bulk amplitude matrix. They clearly show that the pure resonance picture is incorrect, even in the favourable case where the fine structures fall inside a forbidden gap. Their alternative picture is that the dominant mechanism is an interference between the waves directly reflected at the barrier or at the bulk, and the waves associated with processes including on metal-metal reflection at the barrier. This interpretation, which proved to be adequate in their previous study of Al(001) and Ni(001), holds equally well for the fine structure measured by Adnot and Carette (1977) on W(001) in the presence of a band-gap. As a consequence the authors discuss why McRaes method (1967) to extract structural information from a 2D surface band structure cannot be used.

A method for comparing measured and calculated VLEED fine structures

Abstract In this paper, we present a method for comparing measured and calculated VLEED fine structures for the determination of surface barrier shapes. This method amounts to compare — via a metric distance — a discrete set of experimental data with the corresponding set obtained from calculation. For this comparison we choose the location of maxima and minima of the rapid intensity oscillations associated with threshold effects. The distance between experiment and theory essentially depends on the height, the coulomb tail location and the shape of the surface barrier. On the other hand, this distance is not sensitive to the absorption potential inside the metal nor to its decrease into vacuum. Contrarily to other works devoted to this problem, where the comparison between experiment and theory was carried out profile by profile, we must emphasize that in our method this comparison simultaneously involves several profiles obtained at various incidence conditions. Our method is tested out in the case of a data set including five profiles obtained on W(110). We adopt here the modified image barrier model of Jennings and Read. Although a good overal agreement is obtained with a barrier height of 15 eV and a barrier image origin of −1.41 A, small discrepancies subsist when we examine each profile separately. The presumed origin of these discrepancies is discussed and some improvements are proposed in the description of surface barrier shapes.

Threshold effects in LEED: resonance or interference effects?

Much work has been done on threshold effects in LEED assuming their resonance origin. The authors use here a perturbation expansion to separate the contributions of interference and resonance effects in LEED profiles. They show, on five typical examples, that oscillations near thresholds have a clear interference origin. It appears that it is difficult to find situations for which resonance conditions are actually satisfied. This fact has important consequences on the information that one can extract from analysis of threshold effects.

Simulation of electron energy loss near-edge structure at the Al and N K edges and Al L23 edge in cubic aluminium nitride

Calculations of electron energy near edge structures (ELNES) are compared with experimental data obtained in a high-resolution transmission electron microscope. This study concerns small precipitates of aluminium nitride in low carbon steel. The ELNES technique allows to clearly establish that these precipitates crystallize in a cubic rather than in a hexagonal crystallographic cell. The influence on simulated spectra of different parameters are investigated: the size of the atomic shell and its relation with the electron inelastic mean free path. We also examine the influence of the core hole and the sensitivity to cell parameters. We particularly examine the Al L(23) near edge structure and features relating to the different transition channels (A(1g), E(g) and T(2g)). Results of a multiple scattering and band structure calculations using ICXANES and WIEN97 codes, respectively, are compared in the region from 0 to 30 eV above the edge onsets. Both calculations are in a rather good agreement.

Effect of the variation of the inner potential of the crystal surface in VLEED

In very low-energy electron diffraction (VLEED), the fine structures appearing below the emergence thresholds in I00(E) are usually used to test surface barrier models. The authors reinvestigate this problem, in particular for the case of a loosely packed surface. Two surface barrier models are tested in the case of a very-low-energy intensity profile (E <or approximately=5 eV) obtained on the W(001) surface for theta =45 degrees along the (10) azimuth: the modified image barrier introduced by Jennings and Read (1974) and a new saturated image barrier. Neither of these models yields a good fit of experimental data as long as one assumes that the barrier joins the bulk inner potential at the top layer edge continuously or with a potential step. Without challenging the validity of the muffin-tin approximation inside the first layer, it can be suspected that the top-layer potential is less attractive than any bulk-layer one. it is found that for both barrier models, shifting the top-layer inner potential upwards leads to a significant improvement of the fit. It is also found that the modified image barrier tends to exhibit a non-physical region of constant potential potential near the top layer and as a consequence the saturated image barrier appears to be more reliable. Finally this work illustrates the fact, that, especially for loosely packed surfaces, the surface barrier concept must not be restricted to the vacuum region but has to be extended inside the crystal region where the bulk potential is modified by the presence of the surface.

Dynamical effects of the surface potential barrier in very low energy electron diffraction

Abstract On the basis of algebraical results examplified with exact computations, we investigate the dynamical effects associated with the presence of the surface potential barrier in very low energy electron diffraction (VLEED). These effects essentially have three origins: (i) the diverging phase of the reflection amplitude from metal back to metal leads to rapid oscillations of intensity located below emergence thresholds, the maxima of which are arranged in a Rydberg-like series. This effect, wrongly attributed to a resonance process, is actually due to a simple interference mechanism, (ii) The singularity near zero of the transmission amplitude of the barrier from metal to vacuum leads to a discontinuity in the current of non-specular beams at their emergence threshold, (iii) The interference between the waves directly reflected at the surface barrier and at the atomic planes generally induces large modifications of the intensity profiles spreading out on the whole VLEED energy range. Thus the approximation consisting in neglecting the reflection at the surface barrier is inadequate. In conclusion, the above barrier effects must be properly included in any quantitative analysis of VLEED spectra.

Interpretation of a threshold effect on Al(001) in terms of the discontinuity of non-specular beam intensities at grazing emergence

Abstract A widely known threshold effect in very low energy electron diffraction (VLEED) is the appearance of rapid oscillations of diffracted intensities just before the emergence of one or several diffracted beams. Their maxima and minima are arranged in a Rydberg-like series due to the coulombic long range behaviour of the surface potential barrier. Another curious consequence of this behaviour is the discontinuity of non-specular beam intensities at their emergence threshold. Contrarily to the prediction of the classical LEED theory, non-specular beams appear in vacuum with a non-zero current. Although this discontinuity has already been theoretically predicted, it has never been noticed in measured LEED spectra. However, measurements of current transmitted from crystal to ground performed by Henrich on Al(001) near the normal incidence show the existence of such a discontinuity. At about 18.5 eV, four new diffracted beams emerge from the crystal and thus the backscattered electron discontinuity induces a discontinuity in the transmitted current. In this paper, VLEED spectra on Al(001) near normal incidence are reinvestigated in details. In order to take into account the energy spread of the incident beam, calculated VLEED spectra are convoluted with a gaussian distribution. In this way, they can be compared with experimental spectra. The rapid Rydberg-like oscillations are completely smoothed and the discontinuity effects subsist alone in convoluted spectra. These effects are actually large at normal incidence for which four beams simultaneously emerge. In accordance with Henrichs observations, they disappear as soon as the incident beam deviates from normal incidence.