Epitaxial refractory-metal buer layers with a chemical gradient for adjustable lattice parameter and controlled chemical interface

We have developed and characterized the structure and composition of nanometers-thick solid-solution epitaxial layers of (V,Nb) on sapphire (1120), displaying a continuous lateral gradient of composition from one to another pure element. Further covered with an ultrathin pseudomorphic layer of W, these provide a template for the fast combinatorial investigation of any growth or physical property depending of strain

Etude de la formation et de l'oxydation de couches minces d'alliages Al-Ni après dépôt d'Al sur un monocristal de Ni(111)

Cette étude porte sur la formation et l oxydation de couches minces d alliages Al-Ni après dépôt d Al, à 130 K, sur un monocristal de Ni(111), sous UHV. Leur composition, leur structure, en surface et en volume, et leur relation d épitaxie ont été déterminées in situ par analyse par faisceau d ions, diffraction d électrons lents et spectroscopie Auger, en fonction de la quantité d'Al déposée et de la température de recuit. La formation d une couche ordonnée et épitaxiée de Ni3Al(111) est suivie, au-delà d une épaisseur critique d Al de 3,8 nm, par celle d une couche ordonnée et relaxée de NiAl(110). La cinétique de formation de ces couches alliées est complexe et correspond probablement à une croissance hétérogène. En oxydant à 300 K puis en recuisant à 1000 K ces alliages minces on obtient une couche ultramince (épaisse d'environ 5 Å) d'oxyde d aluminium épitaxiée sur Ni(111), de stoechiométrie voisine d'Al2O3.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Growth of Ni-Al alloys on Ni(111), from Al deposits of various thicknesses: (II) Formation of NiAl over a Ni3Al interfacial layer

International audienceThis paper describes the second part of a study devoted to the growth of thin Ni-Al alloys after deposition of Al on Ni(111). In the previous paper [S. Le Pevedic, D. Schmaus, C. Cohen, Surf. Sci. 600 (2006) 565] we have described the results obtained for ultra-thin Al deposits, leading, after annealing at 750 K, to an epitaxial layer of Ni3Al(111). In the present paper we show that this regime is only observed for Al deposits smaller than 8 X 10(15) Al/cm(2) and we describe the results obtained for Al deposits exceeding this critical thickness, up to 200 x 10(15) Al/cm(2). Al deposition was performed at low temperature (around 130 K) and the alloying process was followed in situ during subsequent annealing, by Auger electron spectroscopy, low energy electron diffraction and ion beam analysis-channeling measurements, in an ultra-high vacuum chamber connected to a Van de Graaff accelerator. We evidence the formation, after annealing at 750 K, of a crystallographically and chemically well-ordered NiAl(110) layer (whose thickness depends on the deposited Al amount), over a Ni3Al ``interfacial'' layer (whose thickness-about 18 (111) planes-is independent of rhe deposited Al amount). The NiAl over-layer is composed of three variants, at 120 degrees from each other in the surface plane, in relation with the respective symmetries of NiAl(l 10) and Ni3Al(111). The NiAl layer is relaxed (the lattice parameters of cc-B2 NiAl and fcc-Ll(2) Ni3Al differ markedly), and we have determined its epitaxial relationship. In the case of the thickest alloyed layer formed the results concerning the structure of the NiAl layer have been confirmed and refined by ex situ X-ray diffraction and information on its grain size has been obtained by ex situ Atomic Force Microscopy. The kinetics of the alloying process is complex. It corresponds to an heterogeneous growth leading, above the thin NiAl interfacial layer, to a mixture of At and NiAl over the whole Al film, up to the surface. The atomic diffusion is very limited in the NiAl phase that forms, and thus the progressive enrichment in Ni of the At film, i.e. of the mean Ni concentration, becomes slower and slower. As a consequence, alloying is observed to take place in a very broad temperature range between 300 K and 700 K. For annealing temperatures above 800 K, the alloyed layer is decomposed, Al atoms diffusing in the bulk of the substrate. (c) 2006 Elsevier B.V. All rights reserved

Growth of Ni-Al alloys on Ni(111): (I) Formation of epitaxial Ni3Al from ultra-thin Al deposits

International audienceIn this paper we describe the alloying process of ultra-thin Al layers (below 8 x 10(15) Al/cm(2)) deposited on Ni(1 1 1). For this purpose Auger electron spectroscopy, low energy electron diffraction, and ion beam analysis-channelling measurements have been performed in situ in an ultra-high vacuum chamber. Al deposits formed at low temperature (about 130 K) are strained defective crystalline layers retaining the substrate orientation. Alloying takes place, with very progressive Ni enrichment, in a very broad temperature range between 250 K and 570 K. This feature shows that diffusion of the alloy species is more and more difficult when the Ni concentration increases. At 570 K a crystallographically and chemically ordered Ni3Al phase is formed, and its order continuously improves upon annealing, up to 750 K. We have shown by ion beam methods that this alloy is three-dimensional, extending up to 16 (1 11) planes for the thickest deposits. The Ni3Al phase can also be obtained directly by Al deposition at 750 K, but its crystalline quality is lower and the layer is probably formed of grains elongated along (11 - 2) directions. The Al content of the thin Ni3Al layers formed mostly dissolves in the bulk above 800 K. However a small amount of Al remains segregated at the Ni crystal surface. (c) 2005 Elsevier B.V. All rights reserved

Formation of a well-ordered ultrathin aluminum oxide film on Ni(111): Determination of its thickness, composition and structure

International audienceA well-ordered ultrathin alumina film on top of Ni(1 1 1) has been obtained by an oxidation at 300 K of a thin Ni3Al(1 1 1) layer epitaxially grown on Ni(1 1 1) and subsequent annealing at 1000 K. The formation of this film was studied by Rutherford backscattering spectrometry (RBS) under channeling conditions, nuclear reaction analysis (NRA), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The absolute amounts of both types of atoms participating to the oxide film are, respectively, (2.3 +/- 0.3) x 10(15)Al/cm(2) and (3.3 +/- 0.3) x 10(15) O/cm(2). Hence it appears that this film, with a stoichiometry very close to Al2O3, has an oxygen content corresponding nearly to two compact planes of oxygen in bulk crystalline alumina (about 3.0 x 10(15) O/cm(2)). A commensurate (5 root 3 x 5 root 3)R30 degrees superstructure with a lattice parameter of 2.16 mn, can be deduced from the LEED pattern. This superstructure differs from the one observed by other authors for alumina films (of similar thickness and composition) formed on bulk-Ni3Al(1 1 1), a surface with symmetries and interatomic distances comparable to that of Ni(1 1 1). This difference in structure is most probably connected to the absence, in our case, of any Al atom not strongly bound to 0 atoms at the interface between the ordered alumina film and the Ni substrate: the Al atoms not involved in the oxide film have diffused deeply in the bulk of the Ni substrate during the high temperature annealing stage needed for alumina ordering. (C) 2007 Elsevier B.V. All rights reserved

Non-isotropic surface diffusion of Pb on Cu(110): a molecular dynamics study

By using numerical simulation we have studied the diffusion of isolated lead atoms on Cu(110). The calculations rely on a phenomenological many-body potential derived in the framework of the second-moment approximation of the tight-binding method, with parameters fitted on the physical properties of the bulk crystals of copper and lead and to the copper–lead phase diagram. Static calculations, at T=0 K, provide the energy and relaxed atomic positions of the equilibrium and saddle-point configurations of various possible diffusion mechanisms. In spite of the large miscibility gap present in the lead–copper phase diagram, we find that insertion of a lead adatom into the uppermost copper surface layer is a thermodynamically favoured process. Molecular dynamics calculations show that inserted lead atoms diffuse via an exchange mechanism with copper adatoms and via jumps in adatom position along the open [10] direction. These results confirm previously published experimental observations. They also confirm the validity of a statistical model that was developed to account for these observations. The quantity governing the variation of diffusion anisotropy with temperature is the difference Er−Ej between the activation energies for the insertion of a lead atom in the copper plane and for its jumps in adatom position. The value of this difference, as determined in the static simulations, compares very well with what can be deduced from experimental observations. The agreement is also very good concerning the value of the main diffusion barrier, which is the energy associated with the de-insertion of a lead atom. Simulations performed in the temperature range 400–700 K show that multiple jumps occur frequently. Their frequency increases with temperature, thus leading to lead diffusion that is more anisotropic and more steeply dependent on temperature than could be expected from the static calculations

Surface diffusion of Pb on Cu(110) at low coverage: competition between exchange and jump

We have studied the surface diffusion of Pb on Cu(110) at low coverage by Rutherford backscattering spectrometry (RBS) in the 500–800 K temperature range. The spread, after annealing, of a deposit achieved at room temperature is measured along the and [001] directions, providing the corresponding diffusion coefficients. The activation energy is found to be close to 0.6 eV along both directions: the ratio of the two diffusion coefficients is nearly constant and equal to 2.4 in the temperature range studied. These results are interpreted in the frame of a model which takes into account the fact that, at low coverages, Pb atoms form a disordered surface alloy above room temperature. Thus, for a Pb atom, an elementary diffusion event is a multi-process consisting successively in a de-insertion (exchange with a Cu adatom), some jumps along the ‘open' direction, and a reinsertion in the surface plane (exchange with a Cu surface plane atom). The model predicts that, at low temperature, the diffusion becomes much more anisotropic as Pb adatoms undergo a great number of jumps before their insertion. This behaviour also accounts for the formation of two-dimensional Pb clusters which have been observed on various Cu surfaces by scanning tunneling microscopy experiments on low temperature deposits

Macroscopic and mesoscopic surface diffusion from a deposit formed by a Stranski-Krastanov type of growth: Pb on Cu(100) at above one layer coverage

Under ultrahigh-vacuum conditions, we have studied the diffusion from Pb deposits on Cu(100), at above one-layer coverage, in the 250–625 K temperature range. The growth mode is Stranski-Krastanov, and the deposits consist of thick three-dimensional Pb islands which form above a dense Pb single layer. This latter layer has a two-domain structure which “melts” around TM=520 K. In the 475–625 K temperature range, we have measured the spread of the deposits in the mm range, using in situ Rutherford backscattering analysis. To study diffusion at lower temperature, on a smaller scale, the Pb layer between the islands has been removed by sputtering around 150 K. We have then studied, in the 250–380 K temperature range, the kinetics of its reformation by using in situ Auger spectroscopy. In these experiments the Pb diffusion is followed over distances of the order of the spacing between Pb islands, i.e., about 1 μm. Our measurements demonstrate that the diffusing species is a Pb adatom moving above the dense Pb layer, leading to its spread with a quasiconstant Pb concentration, the adatom source being the Pb islands. We have analyzed our results via numerical integration of a diffusion equation with a concentration-dependent diffusion coefficient, using the finite-difference method. This analysis provides the activation energy ET governing the process. ET is the sum of two characteristic energies ES and Ed. ES is the formation energy of Pb adatoms from the islands, and Ed the activation energy for the motion of these adatoms on the Pb layer. Ed is markedly higher below TM than above, indicating that when the Pb layer is structured, the diffusion is probably limited by the crossing of domain boundaries

Temperature dependence of the atomic relaxations and vibrations on a stepped surface: a molecular dynamics study of Cu(1,1,19)

We have studied the relaxations and vibrations of atoms near the surface of a (1,1,19) copper crystal. For this purpose, we have performed molecular dynamics simulations using a semi-empirical many-body potential derived from tight binding models. The total displacement field can be described as the sum of a mean surface relaxation and a specific contribution of the steps, which is maximal for step edge atoms (0.08 Å) and corner atoms (0.06 Å). Whereas step edge atoms relax towards the inner terrace and towards the bulk, corner atoms relax in the opposite direction, leading to vortex-like structures in the displacement field. We demonstrate that, as predicted by continuous elasticity, the displacement field induced by steps is equivalent to the one created by a line of dipoles on a flat surface. In the particular case studied here, the equivalent dipole density is 3.3×10−10 N. The specific relaxations of kink atoms have been calculated. We have also studied the variation of the relaxations as a function of temperature (T). A strong effect is predicted for inner terrace atoms: when increasing T, the contraction of the first interplanar distance, with respect to the bulk value, progressively cancels and turns to an expansion at high T. This is not the case for the specific contraction of step edge atoms that is nearly temperature independent. This latter behaviour is related to very strong longitudinal correlation between vibrations of the step edge atom and of its nearest neighbour inside the terrace. In the same time, whereas the vibrations of inner terrace atoms are found to be isotropic, the ones of step edge atoms are anisotropic, with a larger component along the direction parallel to the terrace plane and perpendicular to the step edge, the other components being the same as for inner terrace atoms

Observation of an eg-derived metallic band at the Cs/SrTiO3 interface by polarization-dependent photoemission spectroscopy

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