Controlling nanowire growth through electric field-induced deformation of the catalyst droplet.

Semiconductor nanowires with precisely controlled structure, and hence well-defined electronic and optical properties, can be grown by self-assembly using the vapour-liquid-solid process. The structure and chemical composition of the growing nanowire is typically determined by global parameters such as source gas pressure, gas composition and growth temperature. Here we describe a more local approach to the control of nanowire structure. We apply an electric field during growth to control nanowire diameter and growth direction. Growth experiments carried out while imaging within an in situ transmission electron microscope show that the electric field modifies growth by changing the shape, position and contact angle of the catalytic droplet. This droplet engineering can be used to modify nanowires into three dimensional structures, relevant to a range of applications, and also to measure the droplet surface tension, important for quantitative development of strategies to control nanowire growth.European Research Council (Grant ID: 279342)This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/ncomms1227

Growth Dynamics of Gallium Nanodroplets Driven by Thermally Activated Surface Diffusion

International audienceThe growth of catalytic liquid metal nanodroplets on flat substrates is essential for many technological applications. However, the detailed nucleation and growth dynamics of these nanodroplets remains unclear. Here, using in situ Transmission Electron Microscopy (TEM) imaging, we track in real-time the growth of individual Ga nanodroplets from a beam of Ga vapor. We show that the nucleation and growth are driven by thermally-activated surface diffusion of Ga adatoms, with the diffusion activation energy of E_D=95±10 meV on a SiNx surface. More importantly, our analysis shows that Ga-dimers serve as the critical nucleation clusters and that the nanodroplet growth follows a power-law of form R(t)∝〖e^(〖-E〗_D⁄(k_B T)) (t-t_0 )〗^(1⁄2). These insights into the growth dynamics of metallic nanodroplets are essential for tailoring their size and density for their application in self-catalyzed growth of nanomaterials

A Simple Process for the Fabrication of Thermoelectric Silicon and Manganese Silicide Phases by Thin Film Solid Phase Reaction (SPR) of Mn/Si (100)

International audienceIn this paper, we followed the formation sequences of the different manganese silicides by performing simple annealing at different temperatures of the deposited manganese (Mn) on silicon (Si) substrate. Phase change and transition temperatures have been reported. In situ x-ray diffraction (XRD) is used in two different chambers and the results obtained are presented in 3D figures. The surface roughness is first obtained by atomic force microscopy (AFM) and then by scanning electron microscopy (SEM) characterization. The latter is also used for the detection of the different superimposed layers after the formation of the different silicides obtained in the solid phase reaction (SPR). Focus ion beam (FIB) cuts were used to analyze the different layers obtained during silicide formation at these temperatures. In this study, we have obtained Mn15Si26 for high temperatures (above 850°C). Finally, an epitaxy of HMS (Mn15Si26) is obtained at high temperatures with a simple annealing process

Nucleation and lateral growth kinetics of the NiSi phase at the epitaxial θ-Ni2Si/Si interface

International audienceThe first stages of the growth of the NiSi phase at the expense of θ-Ni2Si have been studied mainly by in-situ XRD measurements and atom probe tomography (APT) analysis. In-situ XRD isothermal annealing at different temperatures were performed on several samples in order to monitor the phase formation sequence, the time at which NiSi phase begin to form and its growth kinetics. These results show that while the phase formation sequence is the same, the time for the beginning of formation of NiSi varies from one sample to the other under the same isothermal temperature and experimental conditions. Comparing these findings with nucleation and growth models, the growth of the NiSi phase at the expense of θ-Ni2Si is controlled by nucleation compared to diffusion in the case of δ-Ni2Si as the first phase. The kinetics for the nucleation and lateral growth of the NiSi phase were deduced and the implications for the formation of this phase and for contacts are discussed

Growth Dynamics of Ga Nanodroplets on 2D Substrate

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Selective Wet Etching of Silicon Germanium in Composite Vertical Nanowires

International audienceSilicon germanium (SixGe1–x or SiGe) is an important semiconductor material for the fabrication of nanowire-based gate-all-around transistors in the next-generation logic and memory devices. During the fabrication process, SiGe can be used either as a sacrificial layer to form suspended horizontal Si nanowires or, because of its higher carrier mobility, as a possible channel material that replaces Si in both horizontal and vertical nanowires. In both cases, there is a pressing need to understand and develop nanoscale etching processes that enable controlled and selective removal of SiGe with respect to Si. Here, we developed and tested solution-based selective etching processes for SiGe in composite (SiNx/Si0.75Ge0.25/Si) vertical nanowires. The etching solutions were formed by mixing acetic acid (CH3COOH), hydrogen peroxide (H2O2), and hydrofluoric acid (HF). Here, CH3COOH and H2O2 react to form highly oxidizing peracetic acid (PAA or CH3 CO3H). The hydrofluoric acid serves both as a catalyst for PAA formation and as an etchant for oxidized SiGe. Our study shows that an increase in any of the two oxidizer (H2O2 and PAA) concentrations increases the etch rate, and the fastest etch rate of SiGe is associated with the highest PAA concentration. Moreover, using in situ liquid-phase TEM imaging, we tested the stability of nanowires during wet etching and identified the SiGe/Si interface to be the weakest plane; we found that once the diameter of the 160-nm-tall Si0.75Ge0.25 nanowire reaches ∼15 nm during the etching, the nanowire breaks at or very close to this interface. Our study provides important insight into the details of the nanoscale wet etching of SiGe and some of the associated failure modes that are becoming extremely relevant for the fabrication processes as the size of the transistors shrink with every new device generation

Analysis of superconducting silicon epilayers by atom probe tomography: composition and evaporation field

International audienceThree dimensional distributions of boron atoms incorporated into crystalline silicon (3-9 at.% of boron) well above the solubility limit are measured by atom probe tomography (APT). Samples have been prepared either by gas immersion laser doping (GILD) or by implantation followed by laser annealing (Pulsed Laser Induced Epitaxy: PLIE). GILD and PLIE silicon samples show superconducting properties at low temperatures due to the achieved their high doping level achieved. In both cases, boron atoms are found to be randomly distributed throughout the silicon as revealed by statistical distribution analysis. No clusters or precipitates are detected, which may be related to the high recrystallization rate of the Si:B alloy. A sharp 2D interface between the doped silicon region and the undoped substrate is also observed, characterizing a Si:B/Si epitaxy. Finally, the variation of the evaporation field is investigated by considering either the silicon charge state ratio or the variation of the total applied voltage during the analysis of the Si:B layer and silicon