Christophe Girardeaux

Silicene on Ag(111) : domains and local defects of the observed superstructures

On Ag(111) substrate, the growth of a silicene layer (the equivalent of graphene for silicon) gives rise to four main superstructures due to the epitaxy between the silicene layer (honeycomb structure) and the Ag(111) substrate. Depending on both the substrate temperature and the deposition rate, the following superstructures were observed: (4×4), (× )R13.9°, (2× 2)R30° and (× )R19.1°. Each one corresponds to a specific rotation of the silicene layer with respect to the silver substrate. In this paper we show and discuss for each superstructure all the expected equivalent domains due to the symmetry properties of both the silicene honeycomb structure and the trigonal silver surface. Finally, we show, from STM images of the (2× 2)R30° superstructure, the role played by periodic local defects on the average direction of the superstructures.

Transition from anomalous kinetics toward Fickian diffusion for Si dissolution into amorphous Ge

Over the last years, several experimental and theoretical studies of diffusion kinetics on the nanoscale have shown that the time evolution (x∝tkc) differs from the classical Fickian law (kc=0.5). However, all work was based on crystalline samples or models, so far. In this letter, we report on the diffusion kinetics of a thin amorphous Si layer into amorphous Ge to account for the rising importance of amorphous materials in nanodevices. Employing surface sensitive techniques, the initial kc was found at 0.7±0.1. Moreover, after some monolayers of Si dissolved into the Ge, kc changes to the generally expected classical Fickian law with kc=0.5.

Subnanometric Si film reactive diffusion on Ni

The dissolution of 3–5 Si ML on Ni has been studied using in situ ultrahigh vacuum Auger electron spectroscopy (AES). The AES signal shows delays and kinetic changes in the dissolution process. These observations, combined with atomistic kinetic Monte Carlo simulations, considering an fcc Ni–Si nonregular solid solution, show that the AES signal should correspond to a successive apparition of the Ni-silicides present in the Ni–Si phase diagram at low temperatures, starting with the Si-richer phase.

Surface-interface exploration of Mg deposited on Si(100) and oxidation effect on interfacial layer

Using scanning tunneling microscopy and spectroscopy, Auger electron spectroscopy, and low energy electron diffraction, we have studied the growth of Mg deposited on Si(100)-(2 x 1). Coverage from 0.05 monolayer (ML) to 3 ML was investigated at room temperature. The growth mode of the magnesium is a two steps process. At very low coverage, there is formation of an amorphous ultrathin silicide layer with a band gap of 0.74 eV, followed by a layer-by-layer growth of Mg on top of this silicide layer. Topographic images reveal that each metallic Mg layer is formed by 2D islands coalescence process on top of the silicide interfacial layer. During oxidation of the Mg monolayer, the interfacial silicide layer acts as diffusion barrier for the oxygen atoms with a decomposition of the silicide film to a magnesium oxide as function of O2 exposure.

Experimental Evidence of Iron Segregation in Copper Grain Boundaries as Deduced from Type B Diffusion Measurements

Grain-boundary heterodiffusion of iron in pure copper and self diffusion of iron in copper–0.091at% iron were measured by the serial sectioning technique in the Harrison B-regime. The penetration profiles corresponding to iron heterodiffusion in pure copper show a strong positive curvature far beyond the (Dvt)1/2 depth . This peculiar shape, which does not exist for self diffusion in the solid solution, proves the presence of a strong non linear grain-boundary segregation of iron in copper in spite of the respective surface energies of these metals. This segregation is linked to the size effect which is, as predicted by numerical simulation, the main driving force for grainboundary segregation.

Determination of Grain Boundary Diffusion Coefficients in C-Regime by Hwang-Balluffi Method: Silver Diffusion in Pd

Diffusion controlled processes play a crucial role in the degradation of technical materials. At low temperatures the most significant of them is the diffusion along grain boundaries. In thin film geometry one of the best methods for determining the grain boundary (GB) diffusion coefficient of an impurity element is the Hwang-Balluffi method, in which a surface sensitive technique is used to follow the surface accumulation kinetics. Results of grain boundary diffusion measurements, carried out in our laboratory by this method in three different materials systems (Ag/Pd, Ag/Cu and Au/Ni) are reviewed. In case of Ag diffusion along Pd GBs the surface accumulation was followed by AES method. The data points can be well fitted by an Arrhenius function with an activation energy Q=0.99eV

Contact angles of liquid metals on quasicrystals

Wetting with ?m-sized Pb droplets on thin polycrystalline films of decagonal Al13Co4 is reported. The films were prepared under high vacuum conditions in order to have Pb droplets lying on a clean surface. The method used is sequential deposition and annealing of specific stackings of Al and Co layers of nanometric thicknesses. A 300?nm thick Pb slab was then deposited on top of the films and dewetting experiments were followed in situ in a scanning Auger microprobe. The contact angle between the Pb droplet and the surface of the film is measured to be 49? ? 7?. Further investigation performed by cross section transmission electron microscopy allows us to better characterize the interface. Taking into account the rugosity of the film, it is concluded that there is partial wetting of the film, which corresponds to a smaller contact angle. The comparison with other results obtained either with pure metals or with a cubic AlCo compound leads to the conclusion that the wetting behaviour of Pb on the surface of a decagonal compound is close to that of a metal with a high melting point and not significantly different from that of a crystalline compound with a small unit cell.

Sequential phase formation during reactive diffusion of a nanometric-thick Mn film on Ge (111)

In recent years, spintronics whose principle is based on controlling the spin of electrons in semiconductor layers is presented as a complementary or even an alternative solution for production of logic devices in microelectronics. It relies on the fact that electric current in a magnetic layer can be spin polarized. Manufacture of such components is based on the use of materials or heterostructures whose electronic properties depend on their magnetic state. The magnetic Mn-Ge system is interesting because of its possible development at high Curie temperature and its integration on Si substrate. Among all of the Mn-Ge phase compounds of the diagram, Mn5Ge3 seems the most interesting one for spintronics applications: it is a stable and ferromagnetic phase at room temperature. In this paper, we present first results of the study, by Reflection High Energy Electron Diffraction (RHEED), X-ray diffraction (XRD) and Atomic Force Microscopy (AFM), of the sequence of formation of the MnxGey phases in the case of reaction of a nanometric-thick Mn film (200nm) deposited by MBE on Ge (111).

Unusual Behaviour of the Dissolutions Kinetics of one Monolayer of Si in Cu(001)

We present an experimental study by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) of the dissolution of about one monolayer of silicon previously deposited at room temperature on Cu (001). The isochronal dissolution has been recorded in the temperature range [50-320°C] (annealing rate 1.5°C/min). The plateau observed in the kinetics dissolution for temperatures between 95°C and 240°C, reveals the formation of an intermetallic two dimensional superficial phase thermally stable in this range of temperature. On the plateau, LEED patterns show the formation of a (5x3) superstructure. Above 255°C, we observe a very fast dissolution of the surface alloy characteristic of a first-order surface transition. Isothermal dissolutions kinetics have been recorded above and under the surface transition temperature (250°C and 270°C). From these measurements, we have evaluated bulk diffusion coefficients of Si in Cu assuming a local equilibrium. The diffusion coefficients measured within this hypothesis at 250°C and 270°C are respectively higher and lower than those extrapolated from high temperature measurements for Ge in Cu.

Wetting of decagonal Al13Co4 and cubic AlCo thin films by liquid Pb

Wetting of micron-sized Pb droplets on thin polycrystalline films of decagonal Al13Co4 and cubic crystalline AlCo phases is reported. The sample preparation is crucial to having Pb droplets lying on a clean surface. Decagonal and cubic films were prepared under high vacuum conditions, by sequential deposition and annealing of specific stackings of Al and Co layers of nanometre thicknesses. A 300 nm thick Pb slab was then deposited on the top of the films. Dewetting experiments were performed in situ in a scanning Auger microprobe where the surface chemistry can be monitored: the Pb slab dewets into droplets. The wetting of Pb on both substrates was found to be similar with contact angles around 45°.