Formation and properties of nanometer-thick platinum silicide layers

Platinum silicide films are widely used in silicon devices for ohmic and Schottky contacts. It has been demonstrated in the recent years that Schottky barriers employing ultra-thin platinum silicide films (thickness < 10 nm) are useful for photodetection in the near infrared. We have studied the formation of thin platinum silicide films and their electrical properties as a function of the annealing temperature in presence of an interfacial native silicon oxide layer and with plasma etching damage on the Si substrate surface. The thickness of the interfacial oxide layer was varied as well as the depth of the plasma etching damage. The interest of these investigations lies in understanding the role played by the imperfections due to the two usual cleaning processes, namely incomplete SiO2 chemical removal and Si damace due to plasma etching bombardment, on the final silicide film characteristics. The quality and reproducibility of platinum silicide films in routine fabrication depends strongly of the process control, where the substrate cleanliness plays a crucial role. The native silicon oxide can be present at the silicon substrate surface due to an incomplete chemical etching or due to the rapid re-growth of the oxide after the chemical etching. It is well accepted that this interfacial oxide layer can cause problems for Pt-contact metallization, although the characteristics of the resulting platinum silicide film as a function of the oxide thickness and the annealing temperature are not well known. On the other hand, to guarantee the complete removal of the native silicon oxide mainly in the delicate ultra-thin silicide films fabrication, a plasma etching bombardment is performed on the substrate surface prior to metal deposition. This can introduce a certain amount of damage in the substrate surface which can influence the Pt-Si reaction as well as the morphology, crystallography and electrical characteristics of these films. We prepared samples using different chemical etching procedures to obtain complete and incomplete native oxide layer removal. Plasma etching on the substrate surface was performed for various duration and using a range of discharge powers to obtain different plasma etching damage depth. The silicidation reaction was performed at different annealing temperatures. Five characterization techniques were used to study these samples: Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Photoelectron Microscopy (PEM), Internal Photoemission (IPE) and Four-Point Probe Measurements. Morphological, compositional, structural and crystallographic information is obtained from the first three techniques. This is subsequently correlated to the electrical properties of the films obtained using the last two techniques. We observe that silicide films with interfacial oxide layer up to 2.2 nm-thick, annealed at an appropriate temperature, are equivalent to oxide-free samples for application in device metallization. Resistivity measurements show the same minimum value of 30 μΩcm, for samples with and without interfacial oxide in the 350 – 550 °C annealing temperature range. The role played by the interfacial native oxide layer is observed during the early stages of Pt – Si formation. In the absence of the interface oxide, platinum silicide can easily form even at low temperatures (T < 200 °C). If the interfacial oxide is present, the reactant interdiffusion proceeds through the oxide pinholes; the Pt-Si reaction rate and the phase formation mostly depend on the pinhole density and diameter. In the 350 – 550 °C annealing temperature range, silicide films with and without interfacial oxide layer reach a stable, homogeneous and quite similar structure, consisting of epitaxial and polycrystalline PtSi grains and a very flat silicide/Si interface. This annealing temperature range gives crystallographic and morphologic conditions in which silicide films with and without interfacial oxide layer show the best transport properties. Silicide films with an interfacial oxide show a continuous Pt2Si sub-layer on the top. It is observed from 350 °C and remains unchanged up to annealing temperatures as high as 600 – 650 °C. The existence of this sub-layer can guarantee the film continuity up to 700 °C, even if PtSi grains below it are already epitaxial and isolated from each other. In oxide-free samples this layer does not exist and the transformation to an island-type film is observed after annealing at 550 – 600 °C. The presence of the Pt2Si layer is beneficial and even desirable for preserving the transport properties. A few nanometers of plasma etching damage on the substrate surface are beneficial to ultra-thin (3 – 5 nm-thick) silicide films. Plasma damage contributes to slowing down the beginning of the island formation and extends the temperature range in which platinum silicide films maintain their continuity and the transport properties. Plasma damage can slightly increase the resistivity of films in the 4 – 5 nm thickness range. On the other hand, the plasma etching damage depth in the range mentioned above can help films thinner than 3.5 nm to improve their transport properties

Assemblages bois-fer et biocorrosion : étude des sulfures de fer formés en conditions anoxiques dans des bois d'épaves

International audienceLes conséquences de la corrosion des renforts de fer dans les assemblages de bois peuvent compromettre la durabilité d'une structure. Si le bois est gorgé d'eau, les conditions à la surface du métal deviennent anoxiques. La présence de fer et l'activité microbiologique favorisée par la présence de matière organique font des systèmes composites bois-fer des milieux propices à l'apparition de sulfures de fer. Or ces derniers se transforment soit en milieux anaérobies sulfurés, soit lorsque l'oxygène diffuse dans le bois, soit lors d'une remise à l'air de la structure, avec pour conséquence de contribuer à la dégradation du bois. Afin de mieux comprendre les mécanismes impliqués, nous étudions des bois d'épaves archéologiques. Cet article sera illustré par des exemples d'époques récentes (18 è s. 19 è s.) et antiques et par une approche analytique multi-techniques originale. La mackinawite, la greigite et la pyrite ont été identifiées et il semble que la nature des sulfures de fer présents soit liée à l'âge du vestige conformément aux schémas proposés dans la littérature d'évolution de ces composés. La greigite est intéressante à considérer. Seule phase détectée quel que soit l’âge, elle peut s’avérer un bon marqueur de la biocorrosion du fer dans les assemblages bois-fer

A Review of Hydride Precipitates in Titanium and Zirconium Alloys: Precipitation, Dissolution and Crystallographic Orientation Relationships

This work proposes a review of recent results on the formation and dissolution of hydrides in HCP alloys (Ti and Zr alloys) correlated to the nature of crystallographic hydride phases and their ORs. The crystallographic coherence observed between the surface hydride layer and the substrate is very important for many applications as for biomaterials devices. Five particular orientation relationships (OR) were identified between titanium/zirconium hydride precipitates and the oc-Ti and a-Zr substrates. In addition, the nature of hydrides have a large implication on the ductility, the strain hardening, and the local plastic strain accommodation in the Ti alloys. Our studies using XDR, TEM and SEM-EBSD have been demonstrating that the nature of the hydride phase precipitates depends on the hydrogen content. DSC has been used to obtain the hydride dissolution and precipitation energy values at the bulk scale, whose difference can be associated to misfit dislocations. Local in-situ TEM dissolution observations show the depinning of part of misfit dislocations during dissolution process. Hydride reprecipitation is thus possible only if hydrogen is not driven away during heating by misfit dislocations depinning

Pierre et carrières dans la Saintonge antique : identification, usages et diffusion

Dans la cité des Santons, dès la conquête romaine, des constructions ont été édifiées sur un bassin sédimentaire offrant aux bâtisseurs un éventail de pierres calcaires de qualité. Une recherche élargie à l’ensemble du territoire a consisté à faire le lien entre extraction et mise en œuvre dans le bâti. Pour ce faire, les roches des principales carrières ont été identifiées chimiquement sous la forme de référentiels auxquels les blocs d’architecture mis au jour par les fouilles ont pu être comparés afin d’en connaître la provenance. La microscopie électronique à balayage (MEB) équipée d’un analyseur EDS (energy dispersive spectroscopy) pour l’analyse chimique multi-élémentaire des grains piégés par la sédimentation marine est l’outil principal de cette identification qui permet d’appréhender les circuits de distribution de la pierre. Le présent article fait le point de quinze années de recherche et d’analyses.This article reviews archaeological and archaeometric research on stone and quarries of the Charente basin in southwestern France, a geographical area corresponding to the ancient civitas of the Santones. The study, which is ongoing and began over fifteen years ago, aims to identify the ancient quarries and to better understand the subsequent distribution of building stone. The briefly described geomorphological history, underscores the Pyrenean orogeny, a major event which transformed the Aquitaine basin of the secondary era into the current sedimentary plateau. Compressed into vast anticlinal and synclinal undulations, cut into by rivers, notably the Charente, it offered stonemasons a wide range of high-quality limestone, suitable for sculptural and architectural purposes, as well as all other construction needs, whether ashlar or rubble. Sedimentary formations ranging from the Jurassic to the Cretaceous are the ones most frequently found along lengthy stretches on either side of the Saintonge anticline axis. Turonian limestone was particularly appreciated in this context because of its fine grain and white color. Knowledge of ancient quarries and the individuals who exploited them requires a specific archaeological practice, focused on the reading of quarry faces and floors, in order to reveal the strategies implemented toward block extraction. This particular approach, however, implies the clearing of considerable quantities of ancient and modern waste, and it is therefore understandable that archaeology alone cannot identify all the quarries that punctuate the landscape and remain visible today. The Thénac (Charente-Maritime) quarry in Saintonge is the only one to have been the subject of programmed excavations and formally recognised as an ancient stone source, dated to the Julio-Claudian period. It was therefore necessary to use other methods to identify the stone from the large quarries in the Charente River basin, whatever the period of exploitation, and to create a sort of identification card for each of them. About fifteen quarries have been referenced throughout the area in question. How can we recognise the characteristics of the stone from a particular quarry that differentiate it from another, more or less distant one? And how can we establish that an architectural block encountered in an archaeological site comes from a particular quarry? These questions represent the main challenges of this research. The archaeologist cannot hope to identify the solution to these enigmas if they rely solely on limestone observation, using only the naked eye. Though undoubtedly better equipped in this endeavor, the geologist may still have difficulty distinguishing between limestone from two quarries located close to one another and attributable to the same geological stage. Thus, only the stonemason would be left with his intimate knowledge of the material, its lustre, or resistance to tools; yet this craftsman can only know the stone that they have worked themselves. It was therefore necessary to apply an original method, developed at the Université de La Rochelle by Professor Jean-Claude Mercier, based on the chemical nature of certain limestone components, and in particular by analysing the terrigenous grains trapped by marine sedimentation. These microscopic grains, transported from the surrounding land by the rivers, picked up by the currents, and deposited elsewhere depending on a variety of physical conditions, create a scenario in which the deposits at any given point are not the same as those at another location only slightly further away. This a basic principle of geographical discrimination, which is the basis for limestone recognition in each quarry and which the archaeometrist may be able to evaluate. In order to do so, they must methodically sample the quarry in question using vertically staggered samples, thus taking into consideration both the duration of the sedimentation and the time spent by the quarrymen exploiting the successive banks. Dissolution using hydrochloric acid allows the collection of these residual grains, which must then be prepared into thin sections. The multi-element analysis of these grains is carried out using a scanning electron microscope (SEM), the principles of which are described in this article. An emitter produces an electron probe that scans the six contiguous areas of the thin slide, which will cause secondary electrons to emerge from the surface of the sample, providing the topography of the grains, as well as backscattered electrons whose contrast is related to the atomic number of the chemical elements present. At the same time, the characteristic X-rays also emitted by the sample make it possible to determine the nature of the grains, the quantity and the identity of the chemical elements present on the surface by obtaining spectra and quantification tables. This technique is known as EDS (energy dispersive spectroscopy). Scanning electron microscope analysis reveals a mass of grains composed mainly of pure silica, with a minority of other more remarkable grains, wherein silica has combined with elements such as aluminium, potassium and iron. Other grains containing elements such as titanium or zirconium also occasionally appear independently. The list of these elements is not exhaustive. It remains for the archaeometrist to count, sort, and assign them false colours, as well as to arrange them on individualised layers with the help of image processing software in order to establish discriminating statistical groups of mineral species. This will ultimately provide a singular graphic synthesis: the quarry reference frame. Any archaeological sample, tested using this process in the hopes of confirming a specific quarry as its place of origin, must, statistically speaking, possess the same discriminating characteristics and fit into the ranges of the graphic synthesis of the quarry in question. Occasionally, the concordance is more or less precise, and in this case, we speak of assured, probable, possible, or unknown provenance. This method, which accompanies and reinforces the geological identification of a given limestone, also has the significant advantage of being reversible. If an archaeologically well-defined block is analysed using this method and its discriminatory characteristics relate to a quarry that is referenced, but whose original exploitation has not yet been documented, then it can be affirmed that this quarry was exploited in Antiquity. This article describes the conditions for use of local stone and rubble, and focuses on the main quarries exploited in Saintonge during Antiquity: Thénac, Saint-Vaize, Crazannes, Pons en Charente-Maritime, and Marcamps en Gironde. The major role played by the quarries of Thénac, Saint-Vaize and Pons in monumental construction is evident in the early days of Roman colonisation. The settlements of Saintes and Barzan (Charente-Maritime) present recurrent evidence of this. The conditions of exploitation and transport are described and the commercial areas established by maps and tables that correlate production and work sites. The general observation is that Saintonge is self-sufficient in building stone, apart from marble. It seems that the city’s context is a geographically ideal area for the trade of a material whose price increases considerably with the length of the journey, especially if it is by land. Saintes benefited greatly from the placid waters of the Charente, used to bring in stone from Saint-Vaize in abundance. The port town of Barzan, on the edge of the Gironde estuary, relied on a strategically placed road, specially created to link it to Saintes, its capital, with the Thénac quarry conveniently located along the route. Finally, this study reveals a rubble trade that is generally under studied, as is the case for the quarries of Marcamps, which supplied the second phase of the reconstruction of Barzan. To be viable, this trade benefited from the duration and abundance of a market facilitated by the downstream current of the estuary. Was Saintonge stone exported far beyond its territory, as the historian Camille Jullian speculated? To answer this question, archaeologists working on stone supply outside of this region and who suspect a possible origin in Charente should familiarise themselves with our diagnostic methods. Uncertainties and shortcomings remain in terms of an inventory and assessment which, by their very nature, can never truly be completed

Characterization of the colloidal products of pentacarbonyliron oxidation

Pentacarbonyliron was oxidized with H2O2, in organic solvents, to give colloidal sols. The aqueous-ethanolic sol is highly stable and undergoes thermally-reversible coagulation. Its solid phase was found to be a non-crystalline Fe (III) hydroxoacetate which is transformed to α-Fe2O3 when heated to 300°C. Iron-bound acetate groups are assumed to have a major role in the sol stability, by preserving the amorphous solid phase. Dry hydroxoacetate particles were heated under vacuum; scanning electron microscopy revealed that these particles coalesce and grow, as in a sintering process but at low temperatures (100-250°). © 1987

Assemblages bois-fer et biocorrosion : étude des sulfures de fer formés en conditions anoxiques dans des bois d’épaves

Les conséquences de la corrosion des renforts de fer dans les assemblages de bois peuvent compromettre la durabilité d’une structure. Si le bois est gorgé d’eau, les conditions à la surface du métal deviennent anoxiques. La présence de fer et l’activité microbiologique favorisée par la présence de matière organique font des systèmes composites bois-fer des milieux propices à l’apparition de sulfures de fer. Or ces derniers se transforment soit en milieux anaérobies sulfurés, soit lorsque l’oxygène diffuse dans le bois, soit lors d’une remise à l’air de la structure, avec pour conséquence de contribuer à la dégradation du bois. Afin de mieux comprendre les mécanismes impliqués, nous étudions des bois d’épaves archéologiques. Cet article sera illustré par des exemples d’époques récentes (18ème s. 19ème s.) et antiques et par une approche analytique multi-techniques originale. La mackinawite, la greigite et la pyrite ont été identifiées et il semble que la nature des sulfures de fer présents soit liée à l’âge du vestige conformément aux schémas proposés dans la littérature d’évolution de ces composés. La greigite est intéressante à considérer. Seule phase détectée quel que soit l’âge, elle peut s’avérer un bon marqueur de la biocorrosion du fer dans les assemblages bois-fer

The structure of titanate nanobelts used as seeds for the nucleation of hydroxyapatite at the surface of titanium implants

International audienceThe sequence of steps of a chemical treatment having as its goal the induce of nucleation and the growth of hydroxyl carbonated apatite (HCA) at the surface of titanium implants was studied by scanning and transmission electron microscopy in cross-section. In the first step, an acid etching forms a rough titanium hydride layer, which remains unchanged after subsequent treatments. In the second step, soaking in an NaOH solution induces the growth of nanobelt tangles of nanocrystallized, monoclinic sodium titanate. In the third step, soaking in simulated body fluid transforms sodium titanate into calcium titanate by ion-exchange in the monoclinic structure. HCA then grows and embodies the tangled structure. The interfaces between the different layers seem to be strong enough to prevent interfacial decohesion. Finally, the role of the titanate structure in the nucleation process of HCA is discussed

Biomimetic apatite coatings - Carbonate substitution and preferred growth orientation

Biomimetic apatite coatings were obtained by soaking chemically treated titanium in SBF with different HCO3- concentration. XRD, FTIR and Raman analyses were used to characterize phase composition and degree of carbonate substitution. The microstructure, elemental composition and preferred alignment of biomimetically precipitated crystallites were characterized by cross-sectional TEM analyses. According to XRD, the phase composition of precipitated coatings on chemically pre-treated titanium after exposure to SBF was identified as hydroxy carbonated apatite (HCA). A preferred c-axis orientation of the deposited crystals can be supposed due to the high relative peak intensities of the (0 0 2) diffraction line at 2 theta = 26 degrees compared to the 100% intensity peak of the (2 1 1) plane at 2 theta = 32 degrees. The crystallite size in direction of the c-axis of HCA decreased from 26 nm in SBF5 with a HCO3- concentration of 5 mmol/l to 19 nm in SBF27 with a HCO3- concentration of 27 mmol/l. Cross-sectional TEM analyses revealed that all distances correspond exactly to the hexagonal structure of hydroxyapatite. The HCO3- content in SBF also influences the composition of precipitated calcium phosphates. Biomimetic apatites were shown to have a general formula of Ca10-x-yMgy(HPO4)(x-z)(CO3)(z)(PO4)(6-x)(OH)(2-x-w)(CO3)(w/2). According to MR and Raman analyses, it can be supposed that as long as the HCO3- concentration in the testing solutions is below 20 mmol/l, only B-type HCA (0 < z < 1; w = 0) precipitates. At higher HCO3- concentration, it can be assumed that AB-type HCA (z = 1;0 < w < 1) is formed. (C) 2007 Elsevier B.V. All rights reserved

La pierre antique à Saintes : provenances, usages et pratiques du tournage

National audienc