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

Silicon nanowires as negative electrode for lithium-ion microbatteries

International audienceThe increasingly demand on secondary batteries with higher specific energy densities requires the replace- ment of the actual electrode materials. With a very high theoretical capacity (4200 mAh g−1 ) at low voltage, silicon is presented as a very interesting potential candidate as negative electrode for lithium-ion micro- batteries. For the first time, the electrochemical lithium alloying/de-alloying process is proven to occur, respectively, at 0.15 V/0.45 V vs. Li+ /Li with Si nanowires (SiNWs, 200-300 nm in diameter) synthesized by chemical vapour deposition. This new three-dimensional architecture material is well suited to accom- modate the expected large volume expansion due to the reversible formation of Li-Si alloys. At present, stable capacity over ten to twenty cycles is demonstrated. The storage capacity is shown to increase with the growth temperature by a factor 3 as the temperature varies from 525 to 575 ◦ C. These results, showing an attractive working potential and large storage capacities, open up a new promising field of research

Conductive-probe atomic force microscopy characterization of silicon nanowire

The electrical conduction properties of lateral and vertical silicon nanowires (SiNWs) were investigated using a conductive-probe atomic force microscopy (AFM). Horizontal SiNWs, which were synthesized by the in-plane solid-liquid-solid technique, are randomly deployed into an undoped hydrogenated amorphous silicon layer. Local current mapping shows that the wires have internal microstructures. The local current-voltage measurements on these horizontal wires reveal a power law behavior indicating several transport regimes based on space-charge limited conduction which can be assisted by traps in the high-bias regime (> 1 V). Vertical phosphorus-doped SiNWs were grown by chemical vapor deposition using a gold catalyst-driving vapor-liquid-solid process on higly n-type silicon substrates. The effect of phosphorus doping on the local contact resistance between the AFM tip and the SiNW was put in evidence, and the SiNWs resistivity was estimated

Fabrication et étude physique de dispositifs électroniques à nanotubes de carbone

Les nanofibres et nanotubes de carbone constituent des matériaux très prometteurs pour les applications microélectronique, stockage ou encore récupération d'énergie, en termes de caractéristiques électriques. Néanmoins leur intégration industrielle à court terme n'est toujours pas d'actualité, à cause de divers verrous technologiques et en particulier celui de la résistance de contact. Ces travaux de thèse se sont intéressés à la problématique du contact électrique entre électrode et nanotubes, lorsque ceux-ci sont intégrés dans des dispositifs par CVD et que le contact se forme durant la croissance. La réalisation et la caractérisation de dispositifs à nanofibres dans un premier temps a permis de mettre en évidence l'importance de la rugosité et de l'oxydation de surface de l'électrode en termes de qualité de contact. Par la suite la fabrication et les caractérisations électrique et physique de dispositifs à nanotubes essentiellement mono-paroi nous ont permis de constater que des barrières de potentiel sont également présentes aux interfaces, liées à la présence d'oxydes natifs ou issus de réactions thermodynamiquement favorables. La modélisation des caractéristiques I-V suggère également l'importance de la cristallinité des électrodes sur les paramètres des barrières. Enfin, nous avons positionné nos résultats par rapport à l'état de l'art, et discuté sur la base de nos travaux et de certains issus de la littérature des différents points-clés permettant de réduire les résistances de contact.Carbon nanofibers and nanotubes are very promising materials for microelectronic, energy storage or harvesting applications, regarding their electrical characteristics. Nevertheless their industrial integration in the short term is not about to happen, because of several technologic issues and more particularly the one concerning contact resistance. This work deals with this problem of contact resistance between an electrode and a carbon nanotube, while integrated and self-connected during the growth by CVD. Firstly the manufacturing and characterization of nanofibers devices allow to emphasis the importance of roughness and surface oxidation of the electrode for the contact quality. Fabrication, electrical and physical characterization of single wall carbon nanotubes devices then permit us to evidence the presence of potential barriers at the interfaces, due to the presence of native or thermodynamically created oxides. Modelling of I-V curves also suggests the importance of the crystallinity of the electrodes on the barrier parameters. Finally, we compared our results to those from literature, and discussed the different key points allowing to decrease contact resistances.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Scanning thermal microscopy of individual silicon nanowires

Thermal imaging of individual silicon nanowires (Si NWs) is carried out by a scanning thermal microscopy (SThM) technique. The vertically aligned 1.7 (micro)m long Si NWs are fabricated combining nanosphere lithography and metal-induced wet chemical etching. A thermal model for the SThM probe is then presented with two steps: a model out of contact which enables a calibration of the probe, and a model in contact to extract thermal parameters from the sample under study. Using this model and the experimental thermal images, we finally determine a mean value of the tip-to-sample thermal contact resistance and a mean value of the Si NWs thermal conductivity. No significant thermal conductivity reduction in comparison with bulk Si is observed for Si NWs with diameters ranging from 200 to 380 nm. However, the technique presented here is currently the only one available to perform thermal measurements simultaneously on an assembly of individual one-dimensional nanostructures. It enables to save time and to make a statistical processing of the thermal data in order to deduce a reliable mean thermal conductivity, even when the tip-to-sample thermal contact resistance cannot be considered neither negligible in comparison with the Si NW intrinsic thermal resistance nor constant from one Si NW to another

Thermal exchange radius measurement: Application to nanowire thermal imaging

International audienceIn scanning thermal microscopy (SThM) techniques, the thermal exchange radius between tip and sample is a crucial parameter. Indeed, it limits the lateral spatial resolution but, in addition, an accurate value of this parameter is necessary for a precise identification of thermal properties. But until now, the thermal exchange radius is usually estimated but not measured. This paper presents an experimental procedure, based on the 3ω-SThM method, to measure its value. We apply this procedure to evaluate the thermal exchange radius of two commercial probes: the well-known Wollaston one and a new probe constituted of a palladium film on a SiO2 substrate. Finally, presenting silicon nanowire images, we clearly demonstrate that this new probe can reach a spatial resolution better than 100 nm whereas the Wollaston probe hardly reaches a submicronic spatial resolution

Si and SiGe Nanowires: Fabrication Process and Thermal Conductivity Measurement by 3ω-Scanning Thermal Microscopy

We have grown various samples of Si and SiGe nanowires (NWs), either by a classical vapor?liquid?solid (VLS) process or by chemical etching, to measure their thermal conductivity and thus evaluate their efficiency for thermoelectrics applications. To do so, we have chosen a 3ω-Scanning Thermal Microscopy (SThM) imaging technique which is until now the only method able to perform topographical and thermal measurements simultaneously on an assembly of individual NWs, leading to a statistical value of their thermal conductivity. A size effect is clearly observed on Si NWs: 50 nm diameter NWs offer a reduced thermal conductivity in comparison with 200 nm diameter or even larger NWs. On the contrary, the thermal conductivity of SiGe NWs is widely reduced in comparison with the SiGe bulk value, even for large diameters, bigger than Si NWs ones. We discuss our results, comparing them with thermal conductivity values from the literature obtained by other measurement methods or models.Etude de l'amplification de la Conduction Thermique dans des Réseaux de Nanoparticule

Si and SiGe Nanowires: Fabrication Process and Thermal Conductivity Measurement by 3ω-Scanning Thermal Microscopy

We have grown various samples of Si and SiGe nanowires (NWs), either by a classical vapor?liquid?solid (VLS) process or by chemical etching, to measure their thermal conductivity and thus evaluate their efficiency for thermoelectrics applications. To do so, we have chosen a 3ω-Scanning Thermal Microscopy (SThM) imaging technique which is until now the only method able to perform topographical and thermal measurements simultaneously on an assembly of individual NWs, leading to a statistical value of their thermal conductivity. A size effect is clearly observed on Si NWs: 50 nm diameter NWs offer a reduced thermal conductivity in comparison with 200 nm diameter or even larger NWs. On the contrary, the thermal conductivity of SiGe NWs is widely reduced in comparison with the SiGe bulk value, even for large diameters, bigger than Si NWs ones. We discuss our results, comparing them with thermal conductivity values from the literature obtained by other measurement methods or models.Etude de l'amplification de la Conduction Thermique dans des Réseaux de Nanoparticule

New carbon nanotubes growth process in a closed microfabricated channel for liquid chromatography application

International audienceThe present paper describes a new process for the production of chromatographic columns made out of silicon, containing a micropillar array nanostructured with carbon nanotubes, and suitable for use in the reversed phase mode. According to this original collective process, a nickel layer sputtered on the whole wafer is used at the same time to realize the bonding process of the silicon cap cover, as well as to catalyze the carbon nanotubes growth on the sidewalls of the micropillars in the closed microsystem. The development of the bonding process via the formation of nickel silicide and the optimization of the growth of a homogenous layer of carbon nanotubes along the channel length are described. Preliminary chromatographic experiments with a mixture of two amines demonstrate the possibility to implement, in the hydrodynamic pumping mode, reproducible and efficient chromatographic separations with such a carbon nanotubes based stationary phase

Growth parameters and shape specific synthesis of silicon nanowires by the VLS method

International audienceIn this paper the effect of varying temperature, pressure and chemical precursors on the vapour–liquid–solid (VLS) growth of silicon nanowires (Si NWs) have been investigated. Some aspects of nucleation and growth mechanisms are discussed. Control on Si NW morphology by varying the choice of gaseous precursor (silane or dichlorosilane) at elevated temperatures is reported