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

4 pagesInternational audienceThe 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

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

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 oxygen partial pressure atmosphere (10−5 atm). Oxidation tests were carried out during 30 min to 50 h in a thermogravimetric analysis system using a symmetrical balance with in situ monitoring of the oxygen partial pressure. The oxidation kinetics were determined as parabolic with an estimated stationary parabolic constant value of 10−15 cm2 s−1, after a transient stage of about 3 h. The oxide scale was identified as a pure chromia layer by TEM and XPS characterisations. After 50 h at 700 °C, the scale thickness estimated by TEM cross section observation was about 100 nm. A slightly thicker and more porous oxide scale was observed above the alloy’s grain boundaries. The metal/oxide interface also exhibited a deeper recession towards the substrate above the alloy’s grain boundaries. The orientation of chromia grains was determined by TKD (transmission Kikuchi diffraction). A strong preference was noted for the orientation perpendicular to the surface, along the direction of the corundum structure. Finally, the type of semiconduction was determined for the scales formed after 7 h and 50 h of oxidation. For the shorter oxidation time, the chromia scale exhibited an n-type semiconduction, whereas for the longer exposure, both n-type and insulating semiconduction were identified

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

Rutile-structured TiO 2 deposited by plasma enhanced atomic layer deposition using tetrakis(dimethylamino)titanium precursor on in-situ oxidized Ru electrode

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Croissance de monocristaux massifs de carbure de silicium cubique

Le carbure de silicium cubique (3C-SiC) est un matériau d'intérêt pour des applications électroniques fonctionnant à puissance et fréquences élevées. En raison de son état métastable, l'élaboration de ce matériau demeure problématique. Ces travaux portent sur la croissance de monocristaux massifs de 3C-SiC à partir de substrats hexagonaux de SiC. Le procédé de croissance choisi est une technique de sublimation alimentée en continu (CF-PVT). Le mécanisme de germination du 3C-SiC a tout d'abord été étudié à haute température 100C). Cette première étude a permis de cerner les conditions expérimentales requises pour la germination et la sélection d'une orientation cubique. Si la phase hexagonale est totalement éliminée pendant les premiers stades de croissance, il est possible d'élaborer un véritable monocristal massif de 3C-SiC. Les caractérisations structurales multi-échelles ont permis d'dentifier les différents défauts structuraux présents dans des monocristaux de 3C-SiC élaborés par CF-PVT.Cubic silicon carbide (3C-SiC) is a material of great interest for high power and high frequencies electronic devices. Despite its high potential, availability of 3C wafers is still a challenging issue as no one succeeded in 3C-SiC bulk growth. This study deals with the bulk growth of 3C-SiC single crystals from hexagonal SiC substrates. The CF-PVT (Continuous Feed-Physical Vapor Transport) is used to grow such crystals. This process consists of a continuously fed sublimation technique. Firstly, 3C-SiC nucleation from hexagonal substrates has been extensively studied at high temperature (1900C). Experimental conditions for nucleation and selection of a cubic orientation have been determined. If the hexagonal phase is eliminated during the first growth steps, it has been demonstrated that bulk 3C-SiC single crystals can be obtained. The different structural defects generated in such single crystals grown by CF-PVT have been identified through a muIti-scale characterization.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

High-Temperature Nucleation of Cubic Silicon Carbide on (0001) Hexagonal-SiC Nominal Surfaces

International audienceThe development of 3C-SiC crystals from oriented hexagonal seed has always suffered from systematic twinning that appears during the nucleation step of the layer. To investigate the possibility to reduce or eliminate the incoherent twin boundaries at high temp. (for conditions close to bulk growth ones), we conducted an exptl. study on 3C-SiC nucleation. A mechanism for the selection of one 3C-SiC orientation among the two possible is proposed. It is based on a strong interaction between the a-SiC substrate steps and the anisotropic lateral expansion of the b-SiC domains. This model is confirmed by cross-sectional high resoln. transmission microscopy observations of the a-b interface. The mechanism is discussed with respect to the surface polarity (Si or C faces), the miscut angle, and the substrate polytype

Synthèse chimique et assemblage sous champ électrique de nanocristaux semiconducteurs anisotropes

Cette thèse a eu pour objectif la synthèse de nanocristaux de semi-conducteur de forme allongée et leur assemblage contrôlé entre des électrodes. L'alignement de ces nanorods à grande échelle permet d'obtenir des matériaux ayant des propriétés optiques et électriques intrinsèquement anisotropes. Nous avons synthétisé des nanorods de CdSe dans une large gamme de rapports de forme (de 1,6 à 10), et des hétérostructures cœur sphérique de CdSe et coquille allongée de CdS. Nous avons utilisé les techniques de spectroscopie UV-vis et PL pour caractériser les propriétés optiques des nanorods ; leur morphologie et leur structure ont été étudiées par MET et diffiaction des rayons X. La mise en œuvre d'un dispositif pour réaliser l'assemblage en parallèle de plusieurs électrodes a permis d'implémenter une technique rapide et reproductible. Bien que l'application d'un champ électrique seul permet de guider les nanorods entre les électrodes, le contrôle de l'assemblage et leur alignement est réalisé grâce à l'application simultanée d'un champ électrique et d'une excitation ultraviolette pendant l'évaporation d'une goutte de solution colloïdale. Différents paramètres expérimentaux ont été explorés pour identifier leurs influences sur les assemblages. Ces derniers ont ensuite été caractérisés par destechniques de microscopie (électronique à balayage et optique), révélant un degré d'alignement considérable des nanorods de CdSe selon les lignes de champ électrique quand il y a l'action synergique d'un champ électrique et d'une source UV.This work is devoted to the synthesis and controlled assembly of semiconductor nanorod between electrodes. The alignment of nanorods on a macroscopic scale makes it possible to prepare a novel class of materials with intrinsically anisotropic optical and electric properties. To integrate nanorods in useful devices, low-cost assembly methods have to be developed. ln this perspective, four sizes of rod-shaped CdSe and a new cIass of asymmetric core@shell system (CdSe@CdS) were prepared by chemical synthesis. Spectroscopic (UV-vis and PL) studies were performed in order to characterise the optical properties of these nanorods, while their morphology and their structure were investigated by transmission eIectron microscopy (TEM) and by X-ray difftaction (XRD). Then, a simple set-up was engineered to perform the assembly. The alignment was achieved by the application of an external electric field during the solvent evaporation after drop-cast deposition across lithographically pre-patterned metal electrodes (Ti/Au) on silicon oxide substrates. Experimental parameters were tested in order to evaluate their impact on the structure of the assembly. Their morphology was investigated by scanning electron microscopy (SEM) and by optical microscopy techniques. These analyses revealed a strong enhancement ofNR's alignment parallel to the field lines, when an UV excitation (365 nm) is ilPplied in addition to the electric field.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF