Laurence Latu-Romain

Stress-induced leakage current and trap generation in HfO2 thin films

Stress-induced leakage current (SILC) is studied in 10 nm HfO2 metal-insulator-metal capacitors. Three regimes are observed in the current-time characteristics, namely, (1) an absorption current, (2) a quasi linear increase of current with time (SILC), and (3) thermal breakdown. Magnitude of SILC is strongly correlated to the nature of the cathode (being large for TiN and weak for Pt and Au), showing that SILC is governed by electron injection. Recovery is observed when short-circuiting the samples, pointing out that SILC is a reversible phenomenon. Desorption current and SILC are not correlated, which indicates that different defects control the absorption current and SILC. SILC is ascribed to the generation of oxygen vacancies upon hot electron injection, while recovery is ascribed to the recombination of oxygen ions with vacancies. In the SILC regime, the current varies as Ktn (n = 1.15 at room temperature). Bias and temperature dependence of K and n are studied. The dependence of K on bias can be desc...

Controlled growth of SiGe nanowires by addition of HCl in the gas phase

Growth of Si, Ge, and, thus, SiGe nanowires (NWs) by catalyzed chemical vapor deposition proceeds at different process conditions, preventing easy realization of axial multijunctions interesting for device realization. In this paper, we propose a common process to obtain both Si, Ge, and alloyed NWs simply by adding HCl in the gas phase. It is demonstrated that addition of HCl during the growth improves the structural quality of the SiGe NWs, avoids the tapering of NWs by decreasing the uncatalyzed growth, increases the Ge fraction of the SiGe alloy NWs, and decreases the growth rate. A qualitative model based on the experimental results is proposed to explain the role of HCl during the growth. This model can be more generally applied to explain the tendency observed in the literature concerning the growth of SiGe alloyed NWs without HCl. It is based on a competition between adsorption, decomposition, and incorporation of Si and Ge in the catalyst. This competition is mainly regulated by the gas phase com...

X-ray diffuse scattering from stacking faults in thick 3C-SiC single crystals

Stacking faults in thick (001)- and (111)-oriented 3C-SiC single crystals are studied by high resolution x-ray diffraction. The authors demonstrate that the analysis of the diffuse scattering intensity distribution can be used as a nondestructive means to accurately determine the densities of Shockley-type stacking faults. The diffuse scattering intensity is simulated with a scattering model based on a difference-equation description of faulting in fcc materials. It is shown that the (001) SiC crystals exhibit an anisotropic fault distribution, whereas the (111) SiC crystals exhibit an isotropic fault distribution, in excellent quantitative agreement with transmission electron microscopy observations.

Large Area DPB Free (111) β-SiC Thick Layer Grown on (0001) α-SiC Nominal Surfaces by the CF-PVT Method

Thick (111) oriented β-SiC layers have been grown by hetero-epitaxy on a (0001) a-SiC substrate with the Continuous Feed-Physical Vapour Transport (CF-PVT) method. The growth rate was 68 µm/h at a pressure of 2 torr and a temperature of 1950°C. The nucleation step of the β-SiC layer during the heating up of the process was studied in order to manage first the a to b heteropolytypic transition and second the selection of the b-SiC orientation. With a adapted seeding stage, we grew a 0.4mm thick layer almost free of Double Positioning Boundaries on a 30mm diameter sample. First observations of the layer by cross-polarised optical Microscopy are presented both in planar view and in cross section geometry.

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 articles focus will be on SiGe NWs growth using Au catalyst. The authors managed to grow SiGe NW between 350 and 400°C. Ge concentration (x) in Si1-xGexNW 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 NWs Young modulus and spring constant, and alleviates uncertainties inherent in single point measurement.

Silicon carbide nanotubes growth: an original approach

Because of their unique properties, silicon carbide nanotubes (SiC-NTs) have aroused particular research interest. In this letter, a new approach to fabricate SiC-NTs via the carburization of Si-NWs is presented. By controlling the pressure during the carburization process, out-diffusion of Si through the SiC layer can be monitored. Finally, 3C-SiC-NTs with faceted {2?0?0} sidewall planes are obtained with an excellent crystalline quality. The external diameter is about 300?nm (nearly the same than the etched Si-NWs used) and the thickness of the sidewalls is about 40?100?nm. The crystalline quality as well as the good reproducibility of the process may lead to various applications in physics, chemistry, energy storage and biology.

Silicon carbide based one-dimensional nanostructure growth: towards electronics and biology perspectives

One-dimensional (1D) nanostructures such as nanowires or nanotubes have attracted great interest in fundamental research as well as potential breakthrough applications. Among many materials, silicon carbide (SiC) has very interesting physical, chemical and electronic properties. This is why silicon carbide based 1D nanostructures, which combine excellent intrinsic properties with low dimensionality, have great potential. In this topical review, the growth of SiC 1D nanostructures is addressed as well as the potential applications of these peculiar nano-objects. This subject is first introduced by the interest in this material and by a summing up of the state of the art of SiC nanowire growth. In the second part, Si–SiC core–shell nanowire synthesis is described, followed by the growth of SiC nanotubes. In particular, these two kinds of nanostructures can be obtained via Si nanowire carburization. The third part is dedicated to the control of the synthesis from Si–SiC core–shell nanowires to SiC nanotubes using this original technique. Then, an alternative top-down approach to synthesize SiC 1D nanostructures is described. Finally, preliminary results towards integration for biology, energy and electronics are provided.

Characterization of Bulk 3C-SiC Single Crystals Grown on 4H-SiC by the CF-PVT Method

Because of the formation of DPB (Double Positioning Boundary) when starting from a hexagonal <0001> seed, DPB-free 3C-SiC single crystals have never been reported up to now. In a recent work we showed that, using adapted nucleation conditions, one could grow thick 3C-SiC single crystal almost free of DPB [1]. In this work we present the results of a multi-scale investigation of such crystals. Using birefringence microscopy, EBSD and HR-TEM, we find evidence of a continuous improvement of the crystal quality with increasing thickness in the most defected area, at the sample periphery. On the contrary, in the large DPB-free area, the SF density remains rather constant from the interface to the surface. The LTPL spectra collected at 5K on the upper part of samples present a nice resolution of multiple bound exciton features (up to m=5) which clearly shows the high (electronic) quality of our 3C-SiC material.

Possible connection between nodule development and presence of niobium and/or titanium during short time thermal oxidation of AISI 441 stainless steel in wet atmosphere

Abstract AISI 441 ferritic stainless steel is a good candidate for metallic interconnects of solid oxide fuel cells (SOFCs). In this alloy, the minor elements Ti and Nb are used to stabilise the ferritic structure but their influence on steel durability is not well understood. This study focuses on the early stages of oxidation (24 h) at 800°C of AISI 441 under 5%H2O in O2 following the cathodic SOFCs conditions. The typical duplex oxide scale, composed of a (Mn,Cr)3O4 spinel top layer and a Cr2O3 rich sublayer is observed, with oxide nodules growing in places. These objects, in the micrometre range in size, are studied by FIB tomography. The analyses reveal a complex structure and a development strongly linked to the presence of niobium and/or titanium compound(s) in the subjacent substrate.

Control of the interfacial abruptness of Au-catalyzed Si-Si1−xGex heterostructured nanowires grown by vapor–liquid–solid

Axial Si-Si1−xGex heterostructured nanowires were grown by Au-catalyzed vapor–liquid–solid method. In this work, the authors examine the changes in growth parameters on the interfacial-abruptness of Si-Si1−xGex heterointerfaces in nanowires. The authors have investigated the effect of temperature drop, pressure change, and growth stop on the droplet stability which in turn modifies nanowire morphology and interfacial abruptness. The authors found that Si/Si1−xGex heterointerface is relatively sharp while Si1−xGex/Si is much broader. They demonstrate that a short growth stop is a good way to minimize reservoir effect resulting in small interfacial abruptness value. Our observations reveal that Si/Si1−xGex interfacial abruptness is 20 ± 5 nm irrespective of the nanowire diameter while interfacial abruptness for Si1−xGex/Si is linearly dependent on nanowire diameter.