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

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

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

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Combined x‐ray reflectivity and grazing incidence x‐ray fluorescence study of Ta/Cr/Pt thin film stacks

International audienceThe Ta/Cr/Pt three‐layer system can be used as a planar x‐ray waveguide, that is to say it can guide an x‐ray beam inside its chromium layer. This property comes from the difference in density and hence in optical index between the two “heavy” or cladding tantalum and platinum layers and the “light” or guiding chromium layer. The waveguide will be efficient provided the layers are a few nanometers thick and that the interfaces are as sharp as possible. To control the quality of the stack, we combine grazing incidence x‐ray fluorescence (GIXRF) and x‐ray reflectivity (XRR) measurements on a series of Ta/Cr/Pt samples, whose only difference is the thickness of the Cr layer. The three considered samples have been deposited by magnetron sputtering and their designed structures are: Ta (8 nm)/Cr (5, 10, and 15 nm)/Pt (14 nm)/Si substrate. The combination of XRR and GIXRF tightens constraints on the parameters used to simulate the stack: thickness, roughness, composition, and density of the layers and their interlayers. For each sample we used six GIXRF curves obtained from three different characteristic x‐ray lines (Ta Lα, Cr Kα, and Pt Lβ 2,15 ) excited at three different incident photons energies (6.25, 10, and 12 keV) as well as one XRR curve obtained at 6.25 keV. The XRR‐GIXRF combined analysis demonstrates that the Ta/Cr/Pt structure is too simplistic and that it is necessary to introduce some interlayers at the top and bottom of the stacks to obtain a reliable agreement between the experimental and simulated GIXRF and XRR curves

Kossel diffraction observed with X-ray color camera during PIXE of nano-scale periodic multilayer

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Kossel diffraction observed with X-ray color camera during PIXE of nano-scale periodic multilayer

International audienceBy combining Kossel diffraction with particle induced X-ray emission, we have developed a new methodology to analyze nano-scale thin films. We report the Kossel diffraction generated by irradiating Pd/Y based nano-scale periodic multilayers with 2 MeV protons. The intensity of characteristic Pd Lα X-ray emission is measured as a function of the detection angle (grazing exit). An oscillation of its intensity is observed when the detection angle varies around the Bragg angle, which corresponds to the energy of the emission and the period of the multilayer. Use of the X-ray color camera enables the whole setup to be fixed so that no angular scan is required, greatly simplifying the experimental condition. From the features of the Kossel curves, we are able to deduce that nitrided Pd/Y multilayers exhibit much less layer intermixing than the non-nitrided multilayers. The experimental results show that it is possible to distinguish by the shape of Kossel curves of multilayers with B 4 C barrier layers located in different interfaces. This demonstrates that Kossel diffraction is structural sensitive