M. Romeo

Effect of surface treatments, photon and electron impacts on the ceria 3d core level

Abstract We have investigated the effect of surface treatments and damage induced on the Ce3d core level of ceria samples by the X-ray power density, the exposure time to the X-ray beam and the sample charge effect. We show that a CeO2 surface requires passivation through a mild oxidant exposure such as CO2 to yield a pure Ce(IV)3d spectrum. Thermal “in vacuo” treatments lead to partial chemical reduction of the surface. Damage induced by the X-ray beam and sample charging produces similar effects. We have studied in more detail the effect of sample charging. We mainly assign these Ce3d core level changes to a photon-stimulated oxygen desorption (PSD) via an Auger interatomic decay. However, owing to the partially reversible character of this effect and to main modifications of the Ce3d final states obtained through hybridization of the Ce4f-O2p states U′ and U″ (V′ and V″, respectively), we propose alternative pathways based on intra-atomic rearrangements or on oxygen migration towards interstitial sites of the fluorite lattice.

Broad distribution effects in sums of lognormal random variables

Abstract:The lognormal distribution describing, e.g., exponentials of Gaussian random variables is one of the most common statistical distributions in physics. It can exhibit features of broad distributions that imply qualitative departure from the usual statistical scaling associated to narrow distributions. Approximate formulae are derived for the typical sums of lognormal random variables. The validity of these formulae is numerically checked and the physical consequences, e.g., for the current flowing through small tunnel junctions, are pointed out.

Surface smoothing induced by epitaxial Si capping of rough and strained Ge or Si1−xGex morphologies: a RHEED and TEM study

The same Si/Ge/Si or Si/Si1−xGex/Si structures grown at 400°C on Si(0 0 1) are compared, either in real time by reflection high-energy electron diffraction (RHEED) or on the final product by transmission electron microscopy (TEM). This allows us to follow interface morphology variations during Si re-growth upon Ge containing layers of various Ge thicknesses or alloy x fractions. As shown by the passage from spotty to streaky RHEED patterns and by specular beam intensity oscillation evolutions, the surfaces roughen systematically during strained Ge or Si1−xGex (SiGe) growth and smooth rapidly during subsequent growth of 4 to 6 Si monolayers, at least in the elastic hut-clustering islanding range with {1 0 5} facets. With the help of TEM examinations, a coherent picture may be proposed for these surface smoothing observations: (i) A dominant mechanism in form of a quick Si surface diffusion occurring initially on the Ge-strained surfaces. It ensures a heteregeneous Si accumulation towards the places of minimized misfit, i.e., in the troughs of the Ge or SiGe morphologies. (ii) A slower Ge diffusion (as occuring on Si) depleting the emerging island crests and contributing to an overall Ge surface termination (Ge surface segregation) and to a complementary island smoothing. The latter mechanism, only important at low growth kinetics, favours the formation of alloyed interfaces as a by-product of the island smoothing and lateral intermixing. At high Si growth kinetics the former mechanism prevails leading to better preserved island morphologies and final interfaces appearing chemically more abrupt but less flat.

Photoemission study of Pt adlayers on Ni(111)

Abstract The growth of Pt on Ni(111) was followed up to 1.7 monolayers by photoelectron spectroscopy. During the growth, the binding energy of the Pt 4f peaks remained almost constant and close to that of platinum bulk. This reveals that a high Pt-Ni interaction is attained at least in the submonolayer range. The direct consequence of this observation is that the Pt-Pt bond is weakened and that a distorted layer is formed. We show that this is consistent with results from other authors concerning Pt/Ni interfaces as well as Pt-Ni alloys.

Statistical properties of currents flowing through tunnel junctions

Abstract This paper presents an overview of the statistical properties arising from the broadness of the distribution of tunnel currents in metal–insulator–metal junctions. Experimental current inhomogeneities can be modelled by a lognormal distribution and the size dependence of the tunnel current is modified at small sizes by the effect of broad distributions.

Surface characterization of microwave-assisted chemically vapour deposited carbon deposits on silicon and transition metal substrates

Carbon films deposited on silicon and transition metal (iron, cobalt, nickel) substrates were prepared by a microwave-assisted chemical vapour deposition process. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) in the scanned mode and Raman spectroscopy were used to characterize these deposits. From the analysis of the C KVV fine structure we can conclude that the carbon film is diamond like or diamond on silicon and graphite-like on cobalt and nickel. On iron two forms, graphitic and probably amorphous, coexist. In addition, scanning Auger microscopy (SAM) provides chemical images of the deposits, which allows the areas covered or uncovered by the carbon film to be displayed. XPS, AES and SAM in combination with Raman spectroscopy are thus efficient and complementary tools to determine both (i) the nature of the carbon deposits and (ii) the adhesion relationship between the substrate and the films, which are two main challenges in the preparation of hard protective coatings.

Diamond nucleation and growth at the early stages on Si(100) monitored by electron spectroscopies

Diamond growth on Si(100) is studied by scanning Auger microscopy (SAM), Auger electron spectroscopy (AES), electron loss spectroscopy (ELS) and X-ray photoelectron spectroscopy (XPS) focussing on the early stages of nucleation and growth. The low nucleation density allows us to monitor the evolution of both the substrate and the deposit intensities during the deposition process. Using the structural and chemical imaging probe provided by scanning Auger electron microscopy, we localize areas formed by carbon, only of diamond-type, and featureless areas with C, Si and O. The carbon nature on these last domains is initially a complex mixture of amorphous carbon, carbide and diamond. The amorphous deposit rapidly disappears (30 min), but the occurrence of surface carbide is still detected even after 120 min of growth. The surface carbon content, recorded both by AES focussed on the featureless domains and by XPS, initially drops. This is interpreted by the formation of silicon carbide embedding or removing the amorphous carbon layer. We identify three different sites of nucleation, which are (i) diamond seeds left by the substrate pretreatment; (ii) amorphous carbon at the very beginning of the nucleation process and (iii) silicon carbide. Then the diamond growth and the carbide formation are competitive processes, limited by (i) the reactive content in the gas phase and (ii) diffusion and etching of the silicon.

Kinetics of diamond growth on Si(100) substrate monitored by electron spectroscopy

The nucleation and growth of diamond on Si(100) were studied by X-ray photoelectron spectroscopy. We focus on the early stages of growth. The main results of this study are that the diamond growth needs an induction period before starting. The surface carbon content drops initially, which is interpreted as the diffusion of silicon to form carbide and/or carbon removal.

Local lattice distortions in spherical carbon nanoparticles as studied by HRTEM image analysis.

The study of lattice distortions in structures with spherical or cylindrical geometry is of growing interest in the field of carbon nanoparticles (onions, nanotubes, etc.). We report an image analysis procedure entirely performed in reciprocal space which provides a global map of the inter-shell distances in carbon nanoparticles. This procedure is applied to carbon nanoparticles with a size of 100 nm that are generated under CVD conditions and exhibit positive as well as negative curvature of the basal lattice planes. These nanoparticles are subjected to intense electron irradiation under the beam of a high-voltage electron microscope with an acceleration voltage of 1.25 MeV. We observe a compression in their centre and a dilation of the outer shells. The reciprocal-space analysis of the high-resolution electron microscopy images opens the way to investigate the stability and equilibrium structure of carbon nanoparticles and to conclude on the formation mechanism.

HRTEM study of strained Si/Ge multilayers

Deformation in Si/Ge strained multilayers has been characterised by HRTEM. The HRTEM strain profile determined by image treatment has been used to compare two growth techniques: Hot wire assisted gas source and ultra high vacuum molecular beam epitaxy. We have shown that for the first technique, germanium layers are highly strained. This is probably due to the incorporation of atomic hydrogen which would prevent relaxation by stacking faults formation.