123 research outputs found
The SrTiO displacive transition revisited by Coherent X-ray Diffraction
We present a Coherent X-ray Diffraction study of the antiferrodistortive
displacive transition of SrTiO, a prototypical example of a phase
transition for which the critical fluctuations exhibit two length scales and
two time scales. From the microbeam x-ray coherent diffraction patterns, we
show that the broad (short-length scale) and the narrow (long-length scale)
components can be spatially disentangled, due to 100 m-scale spatial
variations of the latter. Moreover, both components exhibit a speckle pattern,
which is static on a 10 mn time-scale. This gives evidence that the
narrow component corresponds to static ordered domains. We interpret the
speckles in the broad component as due to a very slow dynamical process,
corresponding to the well-known \emph{central} peak seen in inelastic neutron
scattering.Comment: 4 pages, 3 figures, accepted in PR
Surface composition of BaTiO3/SrTiO3(001) films grown by atomic oxygen plasma assisted molecular beam epitaxy
We have investigated the growth of BaTiO3 thin films deposited on pure and 1%
Nb-doped SrTiO3(001) single crystals using atomic oxygen assisted molecular
beam epitaxy (AO-MBE) and dedicated Ba and Ti Knudsen cells. Thicknesses up to
30 nm were investigated for various layer compositions. We demonstrate 2D
growth and epitaxial single crystalline BaTiO3 layers up to 10 nm before
additional 3D features appear; lattice parameter relaxation occurs during the
first few nanometers and is completed at {\guillemotright}10 nm. The presence
of a Ba oxide rich top layer that probably favors 2D growth is evidenced for
well crystallized layers. We show that the Ba oxide rich top layer can be
removed by chemical etching. The present work stresses the importance of
stoichiometry and surface composition of BaTiO3 layers, especially in view of
their integration in devices.Comment: In press in J. Appl. Phy
The influence of alloying on the phase formation sequence of ultra-thin nickel silicide films and on the inheritance of texture
The controlled formation of silicide materials is an ongoing challenge to facilitate the electrical contact of Si-based transistors. Due to the ongoing miniaturisation of the transistor, the silicide is trending to ever-thinner thickness's. The corresponding increase in surface-to-volume ratio emphasises the importance of low-energetic interfaces. Intriguingly, the thickness reduction of nickel silicides results in an abrupt change in phase sequence. This paper investigates the sequence of the silicides phases and their preferential orientation with respect to the Si(001) substrate, for both "thin" (i.e., 9 nm) and "ultra-thin" (i.e., 3 nm) Ni films. Furthermore, as the addition of ternary elements is often considered in order to tailor the silicides' properties, additives of Al, Co, and Pt are also included in this study. Our results show that the first silicide formed is epitaxial theta-Ni2Si, regardless of initial thickness or alloyed composition. The transformations towards subsequent silicides are changed through the additive elements, which can be understood through solubility arguments and classical nucleation theory. The crystalline alignment of the formed silicides with the substrate significantly differs through alloying. The observed textures of sequential silicides could be linked through texture inheritance. Our study illustrates the nucleation of a new phase drive to reduce the interfacial energy at the silicide-substrate interface as well as at the interface with the silicide which is being consumed for these sub-10 nm thin films
Direct strain and elastic energy evaluation in rolled-up semiconductor tubes by x-ray micro-diffraction
We depict the use of x-ray diffraction as a tool to directly probe the strain
status in rolled-up semiconductor tubes. By employing continuum elasticity
theory and a simple model we are able to simulate quantitatively the strain
relaxation in perfect crystalline III-V semiconductor bi- and multilayers as
well as in rolled-up layers with dislocations. The reduction in the local
elastic energy is evaluated for each case. Limitations of the technique and
theoretical model are discussed in detail.Comment: 32 pages (single column), 9 figures, 39 reference
Formation and preferential orientation of Au-free Al/Ti-based ohmic contacts on different hexagonal nitride-based heterostructures
Wide-bandgap nitride semiconductors are currently in development for high-power electronic applications. Compositional layered heterostructures of such nitrides result in a high polarization field at the interface, enabling a higher electron mobility, a higher power density, and a higher conversion efficiency. Further optimization of such GaN-based high-electron-mobility transistors can be achieved by evolving from a top AlxGa1âxN barrier toward AlN or even InyAl1âyN. An ongoing challenge in using such hexagonal nitride semiconductors is the formation of a low-resistive, Au-free, ohmic contact far below 1Ωmm. In this paper, we investigate the formation of ohmic contacts by TiâAlâTiN-based metalization as a function of different annealing temperatures (up to 950âC), TiâAl ratios (from 15 up to 35âat.â%) and nitride barrier composition (AlxGa1âxN, GaN, AlN, and InyAl1âyN). Contacts processed on AlxGa1âx/GaN, and AlN/GaN heterostructures result in low contact resistance of, respectively, 0.30 and 0.55Ωmm, whereas the same contact stack on InyAl1âyN results in resistance values of 1.7Ωmm. The observed solid-phase reaction of such TiâAlâTiN stacks were found to be identical for all investigated barrier compositions (e.g., AlxGa1âxN , GaN, AlN, and InyAl1âyN), including the preferential grain alignment to the epitaxial nitride layer. The best performing ohmic contacts are formed when the bottom Ti-layer is totally consumed and when an epitaxially-aligned metal layer is present, either epitaxial Al (for a contact which is relatively Al-rich and annealed to a temperature below 660âC) or ternary Ti2AlN (for a relatively Ti-rich contact annealed up to 850âC). The observation that the solid-phase reaction is identical on all investigated nitrides suggests that a further decrease of the contact resistance will be largely dependent on an optimization of the nitride barriers themselves
Microsecond time-resolved X-ray diffraction for the investigation of fatigue behavior during ultrasonic fatigue loading
International audienceA new method based on time-resolved X-ray diffraction is proposed in order to measure the elastic strain and stress during ultrasonic fatigue loading experiments. Pure Cu was chosen as an example material for the experiments using a 20â
kHz ultrasonic fatigue machine mounted on the six-circle diffractometer available at the DiffAbs beamline on the SOLEIL synchrotron facility in France. A two-dimensional hybrid pixel X-ray detector (XPAD3.2) was triggered by the strain gage signal in a synchronous data acquisition scheme (pumpâprobe-like). The method enables studying loading cycles with a period of 50â
”s, achieving a temporal resolution of 1â
”s. This allows a precise reconstruction of the diffraction patterns during the loading cycles. From the diffraction patterns, the position of the peaks, their shifts and their respective broadening can be deduced. The diffraction peak shift allows the elastic lattice strain to be estimated with a resolution of âŒ10â5. Stress is calculated by the self-consistent scale-transition model through which the elastic response of the material is estimated. The amplitudes of the cyclic stresses range from 40 to 120â
MPa and vary linearly with respect to the displacement applied by the ultrasonic machine. Moreover, the experimental results highlight an increase of the diffraction peak broadening with the number of applied cycles
Imaging the displacement field within epitaxial nanostructures by coherent diffraction: a feasibility study
International audienceWe investigate the feasibility of applying coherent diffraction imaging to highly strained epitaxial nanocrystals using finite-element simulations of SiGe islands as input in standard phase retrieval algorithms. We discuss the specific problems arising from both epitaxial and highly strained systems and we propose different methods to overcome these difficulties. Finally, we describe a coherent microdiffraction experimental setup using extremely focused x-ray beams to perform experiments on individual nanostructures
Trace elements discriminate between tissues in highly weathered fossils
Palaeontologists assess the affinities of fossils using either morphology-based phylogenetic analyses, possibly enhanced by the use of advanced imaging techniques, or the identification of remnants or derivatives of fossil organic molecules with high taxonomic specificity (âbiomarkersâ). However, these approaches are often of little use for the majority of fossils whose original morphology and chemistry have been severely altered or completely lost during decay, diagenesis and modern weathering. Here we show that the inorganic incorporation of trace elements during fossilization and diagenesis can be used to assess the affinity of highly altered fossils, constituting a powerful tool overlooked so far. This is illustrated by the study of a wide range of animals from the Early Ordovician Fezouata Shale (Tremadocian, Morocco) using synchrotron X-ray fluorescence major-to-trace elemental mapping. Although all fossils studied here have turned into iron oxides, spectral analyses reveal that their different tissue types (i.e. biomineralised, sclerotised, cuticularised, and internal tissues) can be distinguished on the basis of their trace element inventories. The resulting elemental classes and distributions allowed us to identify an enigmatic, highly weathered organism as a new stem euarthropod preserving remains of its nervous system
An investigation by EXAFS of local atomic structure in an Mg-Nd alloy after processing by high-pressure torsion and ageing.
The local atomic structure of an Mg-1.44Nd (wt.%) alloy was investigated after solution annealing, high-pressure torsion (HPT) processing up to 1 and 10 turns and ageing at 250 °C for 5 h using X-ray absorption fine structure (XAFS) measurements at the Nd LIII-edge. The results show that HPT processing has no effect on the atomic structure around Nd atoms compared to the unprocessed state, whereas ageing at 250 °C for 5 h induces a significant modification in the coordination number and interatomic distances around the Nd atoms. These variations are analyzed based on the correlations between precipitation, defects and atomic mobility of the chemical species
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