C. Bonnelle

Instrument for research on interfaces and surfaces

We describe an instrument designed for studying the electronic structure of bulk, surface, and deep solid–solid interface. The analysis is made by soft‐x‐ray emission spectroscopy induced by electron bombardment. The target is placed under ultrahigh vacuum and can be prepared and treated in situ. High resolution is achieved both as concerns the photon energy and the electron‐beam energy. Tests have been made in the dispersive mode and in the characteristic isochromat mode. In both cases experimental resolution is in good agreement with the expected one.

F+ and F centers in α-Al2O3 by electron-induced x-ray emission spectroscopy and cathodoluminescence

Study of an α-Al2O3 single crystal by electron-induced x-ray emission spectroscopy and cathodoluminescence is reported. The relative intensities of optical emissions due to F+ and F centers have been determined as a function of the parameters of the electron beam and the annealing of the sample. It is shown that the F+ centers, i.e., the oxygen vacancies with one trapped electron, are predominant when the density of the incident electron beam increases. Similar variation is observed when the electron energy varies from 1 to 4 keV. From the comparison between x-ray and optical spectra, the F+ centers are determined to be stable defects in the bulk of the sample.

Investigation of an Mo/SiO2 interface by electron‐induced x‐ray emission spectroscopy

We have studied the solid/solid interface between Mo and SiO 2 films deposited, respectively, by magnetron d.c. sputtering and plasma-enhanced chemical vapour deposition (PECVD). The sample depth profile was characterized by SIMS. We used electron-induced x-ray emission spectroscopy to characterize the interface from the valence Si 3p and O 2p spectral densities, which are very sensitive to the physicochemical environment, and the Si 2p → Is transition, which is sensitive to the oxidation degree of the silicon atoms. By gradually decreasing the emissive thickness, which is a function of the primary electron energy, we have shown the presence of an interfacial compound 2-4 nm thick that is not seen by SIMS. We suggest that this compound is an Mo-Si-O mixed oxide in which the silicon atoms have an oxidation degree equal or close to that of silica.

Study of the Interface between Polyacrylonitrile Thin Films and a Nickel Cathode: Ni 3d States Analysed by EXES

The interaction between a nickel substrate and thin polyacrylonitrile films is studied by electron-induced x-ray emission spectroscopy (EXES). The analysis of the metal 3d distribution at the interface suggests that a direct interaction takes place between the metallic sites and the monomer molecules when the metal is cathodically polarized. The existence of bonds due to mixing of Ni 3d and C 2p states is shown. In contrast, there is no detectable interaction in the absence of polarization of the metallic surface.

Radiative Decay of Al 1s Core Hole in Monocrystalline AlAs

The 3p density of states of Al in AlAs has been analysed by soft x-ray emission spectroscopy. It is found that the density of states calculated by a self-consistent FLAPW method is in good agreement with experiment. Electron-electron correlations lead to the formation of an excitonic electron-hole pair. As a result of direct recombination, radiative decay is observed as a very faint structure. This is the first evidence for a core excitonic emission in a semiconductor.

Study of the adhesion between a-CH films and TA6V substrates by electron-induced X-ray emission spectroscopy (EXES)

Abstract The Ti 3d valence states have been analyzed by electron-induced soft X-ray emission spectroscopy (EXES) in the TA6V bulk alloy and at a-CH/TA6V and a-SiO x C y H/TA6V interfaces. These states stem from metallic titanium in both cases. In contrast, EXES analysis shows that aluminum is in an oxide environment. Both aluminium compounds have a different behaviour depending on the presence of a-SiO x C y H in contact with the alloy. Adherence of a-CH films on the TA6V alloy can be explained from physicochemical interactions.

Electron trapping in α‐alumina observed by electron‐induced x‐ray emission

Radiative recombination from states located in the band gap of single‐crystal α‐alumina has been observed by electron‐induced x‐ray emission spectrometry. The variation of intensity as a function of the incident electron beam current has been determined. From this variation, we show that trapping of thermalized incident electrons takes place in the defect sites associated with the observed states. From the binding energies of the states, the sites have been identified as oxygen vacancies.

Computer‐controlled Cauchois‐type x‐ray spectrometer

A laboratory x‐ray spectrometer designed for routine analysis in the 15–60‐keV spectral range is described. It consists of a 40‐cm bent‐crystal transmission spectrometer in the Cauchois geometry, controlled by a microcomputer. The choice of the crystal analyzer and of the detection system is discussed. The instrument is well suited for large spectral range x‐ray absorption and emission spectroscopy (XAS, XES) and x‐ray source diagnostics.

Importance of the mixing of valence states on adhesion at solid/solid interfaces

Abstract We show, by means of two different examples, that good adhesion between solids can be achieved if mixing exists between the valence states of both materials present at the interface. We use X-ray emission spectroscopy induced by electrons. For a metal/ceramic interface (AuPd/Al 2 O 3 ), by comparing spectra from samples obtained by sputtering in the presence or absence of oxygen, we observe that the mixing between valence states increases with the mechanical and thermal qualities of the multilayer. For a polymer/metal interface [polyacrylonitrile (PAN)/Ni] we compare samples with the polymer deposited on the metallic substrate either by cathodic electropolymerization or by simply dipping the metal into a PAN solution. In the former case it is not possible to completely dissolve the polymer layer, and we observe Ni3d-C 2p mixing at the interface. In the other case, no mixing of valence states is seen and the polymer layer can be completely dissolved.

Correlation between electronic and atomic structures in Ag‐Ni multilayers

Ag3Ni3 and Ag4Ni4 multilayers have been analyzed by electron induced x‐ray emission spectroscopy. Changes observed on the density of valence states in the Ni layers, with respect to the bulk, are correlated with the variation of the lattice parameter deduced from x‐ray diffraction experiments. In the Ag layers no change in the density of valence states is observed and the lattice parameter is almost unchanged. These behaviors are explained by an electronic effect due to the low dimensionality of the multilayers.