Growth Kinetics and Characterization of Chromia Scales Formed on Ni–30Cr Alloy in Impure Argon at 700 °C

Oxidation of Metals - The oxidation of a Ni–30Cr alloy at 700 °C in impure argon was studied in order to provide new elements of understanding on chromia scale growth in low...     

Chromia Scale Thermally Grown on Pure Chromium Under Controlled <Emphasis Type="Italic">p</Emphasis>(O<Subscript>2</Subscript>) Atmosphere: II—Spallation Investigation Using Photoelectrochemical Techniques at a Microscale

Pure chromium oxidized at 900 °C at low oxygen partial pressure (10^?12 atm) gives duplex Cr₂O₃ scale with an internal part made of equiaxed grains and exhibiting an n-type conduction, and an external part made of columnar grains and exhibiting a p-type conduction. Spalled regions occurring during cooling have been studied with photoelectrochemical techniques at a microscale. New information in the form of a specific image (structural quality image) could be obtained and revealed a level of structural defect density in the internal chromia subscale higher than that measured in the non-spalled region. The results complement the spallation scenario proposed in part I of this work.     

Growth and characterization of gold catalyzed SiGe nanowires and alternative metal-catalyzed Si nanowires

The growth of semiconductor (SC) nanowires (NW) by CVD using Au-catalyzed VLS process has been widely studied over the past few years. Among others SC, it is possible to grow pure Si or SiGe NW thanks to these techniques. Nevertheless, Au could deteriorate the electric properties of SC and the use of other metal catalysts will be mandatory if NW are to be designed for innovating electronic. First, this article's focus will be on SiGe NW's growth using Au catalyst. The authors managed to grow SiGe NW between 350 and 400°C. Ge concentration (x) in Si1-xGex NW has been successfully varied by modifying the gas flow ratio: R = GeH4/(SiH4 + GeH4). Characterization (by Raman spectroscopy and XRD) revealed concentrations varying from 0.2 to 0.46 on NW grown at 375°C, with R varying from 0.05 to 0.15. Second, the results of Si NW growths by CVD using alternatives catalysts such as platinum-, palladium- and nickel-silicides are presented. This study, carried out on a LPCVD furnace, aimed at defining Si NW growth conditions when using such catalysts. Since the growth temperatures investigated are lower than the eutectic temperatures of these Si-metal alloys, VSS growth is expected and observed. Different temperatures and HCl flow rates have been tested with the aim of minimizing 2D growth which induces an important tapering of the NW. Finally, mechanical characterization of single NW has been carried out using an AFM method developed at the LTM. It consists in measuring the deflection of an AFM tip while performing approach-retract curves at various positions along the length of a cantilevered NW. This approach allows the measurement of as-grown single NW's Young modulus and spring constant, and alleviates uncertainties inherent in single point measurement.     

Carburization of Si microwires by chemical vapour deposition

We report the elaboration of silicon carbide (SiC) nanostructures thanks to the carburization of silicon microwires (MWs) under methane at high temperature. The produced SiC nanostructures display a tubular shape and are polycrystalline. The as-prepared silicon carbide microtubes (MTs) were characterized and studied by scanning electron microscopy (SEM), dual focused ion beam-scanning electron microscope (FIB-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The formation of microtubes can be explained by the out-diffusion of Si through the SiC during the carburization process.     

About the control of semiconducting properties of chromia: investigation using photoelectrochemistry and orientation mapping in a TEM

Abstract

The semiconducting properties of chromia, studied by photoelectrochemistry (PEC), are varied by oxidizing pure Cr as a function of temperature and oxygen activity. At 800 °C and a p(O2) of 10-14 atm, a single n-chromia is observed, while at 900 °C and a p(O2) of 10-12 atm a n- & p-layer is obtained. For intermediate conditions, an insulating stoichiometric Cr2O3 is identified at 850 °C for a p(O2) of 10-13 atm. The TEM investigation reveals a duplex morphology for every scales: an equiaxed (resp. columnar) morphology has been developed in the internal (resp. external) part. Between these two subscales, a textured chromia layer has been identified as the first layer to form. Finally, the association of TEM and PEC techniques permits the identification of major point defects. It is revealed that the morphology is only linked to the growth direction: anionic (resp. cationic) growth leads to equiaxed (resp. columnar) grains.     

Growth of one-dimensional Si/SiGe heterostructures by thermal CVD

The first results on a simple new process for the direct fabrication of one-dimensional superlattices using common CVD chambers are presented. The experiments were carried out in a 200?mm industrial Centura reactor (Applied Materials). Low dimensionality and superlattices allow a significant increase in the figure of merit of thermoelectrics by controlling the transport of phonons and electrons. The monocrystalline nanowires produced according to this process are both one-dimensional and present heterostructures, with very thin layers (40?nm) of Si and SiGe. Concentrations up to 30?at.% Ge were obtained in the SiGe parts. Complementary techniques including transmission electronic microscopy (TEM), selected area electron diffraction (SAED), energy dispersive x-ray spectroscopy (EDS), scanning transmission electron microscopy (STEM) in bright field and high angle annular dark field (HAADF STEM), and energy-filtered transmission electron microscopy (EF-TEM) were used to characterize the nanoheterostructures.     

Duplex n- and p-Type Chromia Grown on Pure Chromium: A Photoelectrochemical and Microscopic Study

Chromia grown on pure chromium at 900 °C for 30 min at an oxygen partial pressure p(O₂) of 10^?12 atm has been characterized using photoelectrochemical and electron microscopy techniques. This study reveals a duplex scale: n-chromia with equiaxis morphology in the internal part (~650 nm thick) and p-chromia with columnar morphology in the external part (~900 nm thick). Grain orientation maps also revealed the presence of a c-oriented chromia layer at the interface between the n- and the p-subscales. This 〈 0001 〉 textured layer was identified as the first-grown chromia layer. It means that internal n-equiaxis chromia grew by anionic transport governed by oxygen vacancy diffusion, whereas external p-columnar chromia layer grew outwards and was controlled by chromium vacancy diffusion.     

Chromia Scale Thermally Grown on Pure Chromium Under Controlled <Emphasis Type="Italic">p</Emphasis>(O<Subscript>2</Subscript>) Atmosphere: I. Spallation Investigation Using Photoelectrochemical Techniques at a Mesoscale

Chromia scales isothermally grown on pure chromium at 900 °C and a p(O₂) of 10^?12 atm during 30 min exhibit n- and p-type conduction associated with a duplex morphology with an internal equiaxed subscale for an inward (anionic) growth and an external columnar subscale for an outward (cationic) growth. After oxidation exposure, spalled regions in the oxide scale can be observed and have been studied with photoelectrochemical techniques at a mesoscale (probe diameter in the range of 50 µm). Owing to the semiconducting properties of each subscale (bandgap and conduction type), a scenario of spallation is proposed and clarifies when and where the oxide scale spallation occurs.     

A generic approach for vertical integration of nanowires

We report on the collective integration technology of vertically aligned nanowires (NWs). Si?and ZnO NWs have been used in order to develop a generic technological process. Both mineral and organic planarizations of the as-grown nanowires have been achieved. Chemical vapour deposition (CVD) oxides, spin on glass (SOG), and polymer have been investigated as filling materials. Polishing and/or etching of the composite structures have been set up so as to obtain a suitable morphology for the top and bottom electrical contacts. Electrical and optical characterizations of the integrated NWs have been performed. Contacts ohmicity has been demonstrated and specific contact resistances have been reported. The photoconducting properties of polymer-integrated ZnO NWs have also been investigated in the UV-visible range through collective electrical contacts. A small increase of the resistivity in the ZnO NWs under sub-bandgap illumination has been observed and discussed. A comparison of the photoluminescence (PL) spectra at 300?K of the as-grown and SOG-integrated ZnO nanowires has shown no significant impact of the integration process on the crystal quality of the NWs.     

The Role of Oxygen Partial Pressure on the Nature of the Oxide Scale on a NiCr Model Alloy

Oxidation of Metals - The high-temperature oxidation behavior of a binary Ni-30Cr model alloy has been studied at 1050&nbsp;°C at three oxygen partial pressures p(O2):...