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The propagation of frost in an assembly of supercooled dew droplets takes place by the formation of ice protrusions that bridge ice particles and still-liquid droplets. In this work, we develop a Kinetic Monte Carlo (KMC) model to study the formation kinetics of the ice protrusions. The KMC simulations reproduce well the experimental results reported in the literature. The elongation speed of the ice protrusions does not depend on the droplet size but increases when the interdroplet distance decreases, the temperature increases, or the substrate wettability increases. While 2D diffusion of the water molecules on the substrate surface is sufficient to explain the process kinetics, high 3D (vapor) water-molecule concentration can lead to the development of 3D lateral branches on the ice protrusions. A 1D analytical model based on the water-molecule concentration gradient between a droplet and a nearby ice particle reproduces well the simulation results and highlights the relation between the protrusion elongation kinetics and parameters like the interdroplet distance, the water diffusivity, and the concentration gradient. The bridge-formation time has a quadratic dependence on the droplet-ice distance. Comparing the simulations, the analytical model, and the experimental results of the literature, we conclude that the propagation of frost on a flat substrate in an assembly of supercooled dew droplets with interdroplet spacing larger than about 1 μm is limited by water-molecule diffusivity.

International audience; Second-harmonic generation (SHG) is a nonlinear optical method allowing the study of the local structure, symmetry, and ferroic order in noncentrosymmetric materials such as ferroelectrics. The combination of SHG microscopy with local polarization analysis is particularly efficient for deriving the local polarization orientation. This, however, entails the use of tedious and time-consuming modeling methods of nonlinear optical emission. Moreover, extracting the complex domain structures often observed in thin films requires a pixel-by-pixel analysis and the fitting of numerous polar plots to ascribe a polarization angle to each pixel. Here, the domain structure of GeTe films is studied using SHG polarimetry assisted by machine learning. The method is applied to two film thicknesses: A thick film containing large domains visible in SHG images, and a thin film in which the domains' size is below the SHG resolution limit. Machine learning-assisted methods show that both samples exhibit four domain variants of the same type. This result is confirmed in the case of the thick film, both by the manual pixel-by-pixel analysis and by using piezoresponse force microscopy. The proposed approach foreshows new prospects for optical studies by enabling enhanced sensitivity and high throughput analysis.

International audience; In the quest for materials for ferroelectrics-based spintronics with a large spin-orbit coupling, it is essential to carefully control the ferroelectric domains structure, their spatial organization and the domain wall type. Here, we perform the growth of GeTe thin lms on Si by molecular beam epitaxy in a large thickness range. We show that the volume fraction along with the size of the ferroelectric nanodomains can be controlled by nely adjusting the deposition thickness and temperature. We evidence the formation of 71 •-type domain walls and in situ measurements during thermal cycling show the hysteretic appearance and decay of ferroelectric domains. In combination with a detailed analysis of the GeTe/Si interface, we demonstrate that the interfacial mist dislocations formed during the growth plays a key role in the stability of the ferroelectric nanodomains.

International audience; Au deposition on Si(110) at temperature higher than 633 K leads to the formation of elongated liquid nano-droplets. In this work we show that under an electric eld, the droplets move along the [1-10] axis towards the positive electrode by electromigration. During the motion, they dissolve the substrate forming a hole underneath and a nanowire behind them. We develop a model highlighting that the driving force of these processes is bound to the respect (i) of the Au-Si bulk chemical equilibrium and (ii) of the correct balance of surface/interfacial energies. The motion takes place by means of sudden jumps with length of the order of a few µm and by small readjustments of the order of some nm of the droplet front and back edges. With respect to experiments without electric eld, electromigration changes the droplet motion by widening the droplet dissolution front and by facilitating droplet jumps.

The passage of an electric current in a material can cause a biased mass transport at its surface. This migration phenomenon is intimately related to the microscopic details of atomic processes of diffusion and attachment/detachment at step edges. Using low-energy electron microscopy, we have examined in operando under an electric current the migration of $\mathrm{Si}(111)\text{\ensuremath{-}}1\ifmmode\times\else\texttimes\fi{}1$ advacancy islands confined on $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ terraces. The islands move opposite to the current direction, with velocity increasing with the radius. The effective valence of Si adatoms is $2.8\ifmmode\pm\else\textpm\fi{}0.5$ and the kinetic length of attachment-detachment is about 500 nm. The analysis of the island's shape reveals that the electric current significantly biases the kinetic rate of mass transfers at step edges modifying the overall island's shape.

We report a combined scanning tunneling microscopy and x-ray magnetic circular dichroism study to investigate the structural properties and magnetic behavior of a Co ultrathin film composed of dimer nanolines. These Co nanolines, \ensuremath{\sim}6 nm in length, are grown on a Si nanotemplate composed of nanoribbons self-organized on Ag(110). The first two Co layers present a weak magnetic response while upper Co layers exhibit an enhanced magnetization. Orbital and spin moments are experimentally determined. We show that an in-plane magnetization is favored and the magnetic anisotropy energy associated with the directions parallel and perpendicular to the nanolines are measured. The Co ultrathin film is shown to behave as a superparamagnetic system composed of one-dimensional segments containing each \ensuremath{\sim}170 ferromagnetically coupled Co atoms, with a blocking temperature estimated to be between 20 and 40 K. Our set of experiments allows for a comprehensive description of the magnetic behavior of the Co nanoline ultrathin film grown on a functionalized metallic substrate.

We report on the observation of a coupling between the dewetting of a Si layer on $$\hbox {SiO}_{{2}}$$ induced by surface/interface energy minimization and the etching between both materials due to the $$\hbox {Si}+\hbox {SiO}_{2}\longrightarrow 2$$ $$\hbox {SiO}^\uparrow _g$$ reaction. In the limit of a thin $$\hbox {SiO}_{{2}}$$ layer ( $$\le 10 \,\hbox {nm}$$ ) sandwiched between a Si layer and a Si handle wafer, the front of Si dewetting and the front of $$\hbox {SiO}_{{2}}$$ etching coexist in a narrow region. The interplay between both phenomena gives rise to specific morphologies. We show that extended Si fingers formed by dewetting are stabilized with respect to Rayleigh–Plateau-type instability over tenth of microns thanks to a localized etching of the $$\hbox {SiO}_{{2}}$$ layer. The breakup of this structure occurs abruptly by an unzipping process combining dewetting and etching phenomena. We also put in evidence that Si rings are created with a thin $$\hbox {SiO}_{{2}}$$ layer in the center. These processes are thermally activated with an activation energy of $$2.4\pm 0.5 \,\hbox {eV}$$ and $$4.0\pm 0.5 \,\hbox {eV}$$ , respectively, for dewetting and the etching reaction. All these results highlight the respective roles of wetting and etching in Si/ $$\hbox {SiO}_{{2}}$$ /Si system dynamics and could be a stepping stone for the development of advanced processes based on Silicon-On-Insulator technology.

International audience; The capability of controlling the motion of nanoobjects on a surface would open perspectives in nanofabrication. Here, we show the proof of concept that the displacement of a 2D nanostructure on a surface can be controlled by means of two perpendicular electric fields. With a specifically designed sample holder, we displace a 2D negative island on Si(001) along a close loop, in a low-energy electron microscope. Our technique could be applied to other systems to assemble nanomaterials.

Nucleation, growth and coalescence of 3D Au-Ge droplets on Ge(1 1 1) surface have been studied in real time by low energy electron microscopy. The surface diffusion energy barrier of Au on Ge(1 1 1) is estimated as 1.8 ± 0.2 eV from the dependence of the number of droplets per surface unit with temperature and Au deposition rate. At long time scale it is also shown that the droplets have a diffusive motion that results in dynamic coalescence events. From the decrease of the droplet number, the size-dependence of the diffusion coefficient of the droplets is shown to decay as R - 1.9 ± 0.5 where R is the droplet radius. This result indicates that the spontaneous motion of the droplets is controlled by an interfacial mass-transport phenomenon.

International audience; Nanostructures on surfaces can be displaced by applying an electric eld or electric currents through a material. This induced mass transport is referred to as electromigration. In this article we show that the anisotropy of diusion may control the direction of motion of electromigrating nanostructures. For this purpose we study in situ and in real time, by Low Energy Electron Microscopy, the motion of 2D one-atom thick islands or one-atom deep holes on a highly anisotropic surface (reconstructed Si(100)). Based on experiments and Kinetic Monte Carlo simulations, we propose a simple analytical model that explains most of the observations. In particular, the direction of motion of the nanostructures depends on the diusion anisotropy and does not necessarily coincide with the electric eld direction. This work opens a way for the manipulation of 2D nanostructures by means of an electric eld on anisotropic surfaces.

The deposition of Au on Ge(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Au above the eutectic temperature results in the formation of AuGe liquid droplets that reach the liquidus composition by digging a hole in the Ge substrate. The combination of low-energy electron microscopy and atomic force microscopy measurements shows that AuGe droplets randomly migrate or electromigrate under an applied electric current dragging their underneath hole. The droplet motion is due to a mass transport phenomenon based on Ge dissolution at the droplet front and Ge crystallization at its rear. At high temperature the mass transport is limited by attachment or detachment at the solid-liquid interface and the activation energy is $1.05\ifmmode\pm\else\textpm\fi{}0.3\text{ }\text{ }\mathrm{eV}$. At low temperature the effective activation energy increases as a function of the droplet radius. This behavior is attributed to the nucleation of 2D layers at the faceted liquid-solid interface.

The surface diffusion energy of Au on the 3 × 3 Au–Si(111) reconstructed surface is determined from low energy electron microscopy experiments. We have used two methods, one based on the nucleation of Au particles, the other one on the island growth by Ostwald ripening. The two methods give Ed = 1.10 ± 0.34 eV and Ed = 1.56 ± 0.31 eV respectively and with a weighted mean we obtain Ed = 1.3 ± 0.2 eV. We suggest that this high activation energy could be due to a mechanism of diffusion by exchange.

The quantum Hall effect provides a universal standard for electrical resistance that is theoretically based on only the Planck constant h and the electron charge e. Currently, this standard is implemented in GaAs/AlGaAs, but graphene's electronic properties have given hope for a more practical device. Here, we demonstrate that the experimental conditions necessary for the operation of devices made of high-quality graphene grown by chemical vapour deposition on silicon carbide can be extended and significantly relaxed compared with those for state-of-the-art GaAs/AlGaAs devices. In particular, the Hall resistance can be accurately quantized to within 1 × 10−9 over a 10 T wide range of magnetic flux density, down to 3.5 T, at a temperature of up to 10 K or with a current of up to 0.5 mA. This experimental simplification highlights the great potential of graphene in the development of user-friendly and versatile quantum standards that are compatible with broader industrial uses beyond those in national metrology institutes. Furthermore, the measured agreement of the quantized Hall resistance in graphene and GaAs/AlGaAs, with an ultimate uncertainty of 8.2 × 10−11, supports the universality of the quantum Hall effect. This also provides evidence of the relation of the quantized Hall resistance with h and e, which is crucial for the new Systeme International d'unites to be based on fixing such fundamental constants of nature. Large-area graphene devices synthesized by chemical vapour deposition are used to develop electrical resistance standards, based on the quantum Hall effect, with state-of-the-art accuracy and under an extended range of experimental conditions of magnetic field, temperature and current.

International audience; We demonstrate the kinetically controlled growth of one-dimensional Co nanomagnets with a high lateral order on a nanopatterned Ag(110) surface. First, self-organized Si nanoribbons are formed upon submonolayer condensation of Si on the anisotropic Ag(110) surface. Depending on the growth temperature, individual or regular arrays (with a pitch of 2 nm) of Si nanoribbons can be grown. Next, the Si/Ag(110) system is used as a novel one-dimensional Si template to guide the growth of Co dimer nanolines on top of the Si nanoribbons, taking advantage of the fact that the thermally activated process of Co diffusion into the Si layer is efficiently hindered at 220 K. Magnetic characterization of the Co nanolines using X-ray magnetic circular dichroism reveals that the first atomic Co layer directly adsorbed onto the Si nanoribbons presents a weak magnetic response. However, the second Co layer exhibits an enhanced magnetization, strongly suggesting a ferromagnetic ordering with an in-plane easy axis of magnetization, which is perpendicular to the Co nanolines.

article i nfo By Low Energy Electron Microscopy, we show that the spontaneous motion of gold droplets on silicon (111) is chemically driven: the droplets tend to dissolve silicon monoatomic steps to reach the temperature-dependent Au-Si equilibrium stoichiometry. According to the droplet size, the motion details are different. In the first stages of Au deposition small droplets nucleate at steps and move continuously on single terraces. The droplets tempo- rarily pin at each step they meet during their motion. During pinning, the growing droplets become supersatu- rated in Au. They depin from the steps when a notch nucleate on the upper step. Then the droplets climb up and locally dissolve the Si steps, leaving behind them deep tracks formed by notched steps. Measurements of the dissolution rate and the displacement lengths enable us to describe quantitatively the motion mechanism, also in terms of anisotropy of Si dissolution kinetics. Scaling laws for the droplet position as a function of time are proposed: x ∝ t n

Fundamental interest for 2D electron gas (2DEG) systems has been recently renewed with the advent of 2D materials and their potential high-impact applications in optoelectronics. Here, we investigate a 2DEG created by the electron transfer from a Ag adatom gas deposited on a Si(111) $$\sqrt{{\bf{3}}}{\boldsymbol{\times }}\sqrt{{\bf{3}}}$$ 3 × 3 -Ag surface to an electronic surface state. Using low-energy electron microscopy (LEEM), we measure the Ag adatom gas concentration and the 2DEG-induced charge transfer. We demonstrate a linear dependence of the surface work function change on the Ag adatom gas concentration. A breakdown of the linear relationship is induced by the occurrence of the Ag adatom gas superstructure identified as Si(111) $$\sqrt{{\bf{21}}}{\boldsymbol{\times }}\sqrt{{\bf{21}}}$$ 21 × 21 -Ag only observed below room temperature. We evidence below room temperature a confinement of the 2DEG on atomic terraces characterised by spatial inhomogeneities of the 2DEG-induced charge transfer along with temporal fluctuations. These variations mirror the Ag adatom gas concentration changes induced by the growth of 3D Ag islands and the occurrence of an Ehrlich-Schwoebel diffusion barrier of 155 ± 10 meV.

International audience; Owing to its ability to produce an assembly of nanoislands with controllable size and locations, the solid state dewetting of patterned films has recently received great attention. A simple Kinetic Monte Carlo model based on two reduced energetic parameters allows one to reproduce experimental observations of the dewetting morphological evolution of patterned films of Si(001) on SiO2 (or SOI for Silicon-on-Insulator) with various pattern designs. Thus, it is now possible to use KMC to drive further experiments and to optimize the pattern shapes to reach a desired dewetted structure. Comparisons between KMC simulations and dewetting experiments, at least for wire-shaped patterns, show that the prevailing dewetting mechanism depends on the wire width.

Spontaneous dewetting of solid thin films proceeds by edge retraction of film edges and/or by heterogeneous void growth. Classical 1D and 2D continuous models of the evolution of a dewetting film, based on surface diffusion mechanisms, predict that in the long-time limit dewetting obeys universal scaling laws. In this paper, we review 1D and 2D predictions and recent experimental results. For this purpose, using Si(001)/SiO2 and Ge(001)/SiO2 single-crystalline thin films in different geometries, we have been able to compare theoretical predictions to experimental results obtained by combining in situ LEEM and ex situ AFM measurements. For dewetting from film edges, experimental results partially differ from continuous models predictions. More precisely, because of the crystallographic anisotropy: (i) the facetted edges remain stable during dewetting (they simply recede at constant shape) while poorly or un-facetted edges are unstable (they recede by finger formation); (ii) rim formation, induced by mass-conservation condition, proceeds in a layer-by-layer mode and is limited by 2D nucleation properties on the top of the rim; (iii) the island generation mechanism differs from the mass shedding behaviour predicted by 1D models. For dewetting mechanisms involving void growth, different behaviours are reported and discussed. For thin Si(001)/SiO2 films, the corners of the opening square-shaped voids lead to a local destabilisation of the growing voids. For thin Ge(001)/SiO2 films, the side of the voids invariably turns instable and forms tip dendrites whose branch density depends on the temperature and the initial film thickness. Finally, ultra-thin films, more sensitive to local fluctuations, dewet in a fractal geometry.

We have grown graphene films on 6H-SiC(0001) using propane CVD and evidenced the strong impact of the hydrogen/argon mixture used as the carrier gas on the graphene/SiC interface and on the orientation of graphene layers. By studying a set of samples grown with different hydrogen/argon mixture using Raman spectroscopy and grazing incidence X-ray diffraction, we evidence the links between graphene/SiC interface and strain in graphene.

In semiconductor industry, the continuous scaling reduction of CMOS components improves devices performances and reduces manufacturing cost. However, the electrostatic control of the channel must be very accurate. To address this requirement, Fully-Depleted Silicon-On-Insulator (FD-SOI) technology, relying on the use of ultra-thin SOI films (less than 10 nm), is a promising option for the technical nodes beyond 14nm. This option requires the realization of Raised Source & Drain (RSD) by epitaxial Chemical-Vapor Deposition (CVD) of silicon alloys [1, 2]. The quality of the RSD epitaxy depends on the cleanliness of the surface before the deposition. For standard SOI technology, the state of the art generally consists in a high temperature treatment of the surface (>1000°C) in presence of hydrogen. However, such a thermal budget cannot be used for ultra-thin SOI films that dewets and agglomerates into 3D islands in this temperature range [3, 4]. Therefore, for ultra-thin SOI films, an efficient low temperature surface preparation process is required to avoid improper growth of Si or SiGe:B ( resp. SiC:P) layers, used as RSD material for pFET (resp. nFET). This works focus on the surface preparation prior to the low-temperature epitaxy of Si0.7Ge0.3:B layers on full-sheet and patterned wafers. In a first stage different processes of surface preparations are evaluated including standard wet cleans (HF-last, HF-RCA), plasma-based clean (SiCoNiTM setup [5, 6]) and their combination. For SiGe channels, the so-called HFRCA + SiCoNi process without any subsequent baking is recommended to reach clean surfaces [7]. In a second stage, we show that the Si1-xGex growth rate increases linearly when increasing the B2H6 boron-precursor gas flow. However the quality of the SiGe:B epitaxial films is degraded for high boron doping (≈1021at.cm-3) with B accumulation at the interface (Fig. 1). We report on the conditions necessary to reach high boron concentrations while maintaining a uniform boron concentration by incorporating carbon (≈1%) in the RSD deposition step. A wide set of characterization tools are used to assess the sample quality in terms of defect density, chemical analysis, roughness and surface morphology: AFM, SEM, TEM, Spectrometric Ellipsometry (SE), EDX, XPS, Secondary Ion Mass Spectroscopy (SIMS), Model Based Infrared Reflectometry (MBIR), Atom Probe Tomography (APT), Spectroscopy Infrared Reflectometry (SIR) and Low-Energy Electron Microscopy (LEEM). Keywords: Si, SiGe, SiCoNiTM Preclean, FD-SOI, SiGe:B, WET clean, RSD References: [1] N. Loubet et al. Ultra-Low resisitivity in-situ phosphorus doped Si and SiC epitaxy for source/drain formation in advanced 20 nm n-type field effect transistor devices, Thin Solid Films 520 (2012) 3149-3159. [2] J.M et al. Very Low Temperature (Cyclic) Deposition / Etch of In Situ Boron-doped SiGe Raised Sources and Drains. ECS Journal of Solid State Science and Technology, 3 (11) P382-P390 (2014). [3] F.Cheynis et al. Dynamics and instability of solid-state dewetting. Comptes Rendus Physique 14 578-589 (2013). [4] D.Dutartre et al. Defect free Stranski Krastanov growth of strained Si1-xGex layers on Si, Journal of Crystal Growth 142, p. 78-86 (1994). [5] H Yang, X Lu, CT Kao, M Chang - US Patent 7,780,793, (2010). [6] R Arghavani, CT Kao, X Lu - US Patent 7,955,510, (2011). [7] M.Labrot et al. Low thermal budget for Si and SiGe surface preparation for FD-SOI technology. Applied Surface Science 371, p. 436–446 (2016). Figure 1

Conference on Precision Electromagnetic Measurements (CPEM), Natl Res Council, Ottawa, CANADA, JUL 10-15, 2016; International audience; We report on measurements in large quantum Hall devices, made of high-quality graphene grown by propane/hydrogen chemical vapor deposition on SiC. These devices, having all the properties of an ideal quantum electrical resistance standard, surpass state-of-the-art GaAs/AlGaAs devices by considerable margins in their operational conditions. The Hall resistance can be found accurately quantized within one part in 10(9) over a 10-T range of magnetic fields with a lower bound at 3.5 T, temperatures as high as 10 K, or currents as high as 0.5 mA. This simplification sets the superiority of graphene for accessible and low-cost primary resistance standards.

International audience; In the past decade there have been many theoretical and experimental efforts to study the mechanisms of solid state dewetting, that means the spontaneous agglomeration of a thin solid film on a substrate into an assembly of 3D islands. The dewetting studies of solid films on solid substrates have not yet reached the degree of maturity achieved for liquids but there is now enough experimental data to consider the possibility of a future ``dewetting engineering''. By dewetting engineering we mean all the ways to tune and/or control the kinetics of dewetting as well as the morphology of the final dewetted state. The ultimate goal is to avoid dewetting when it complicates the fabrication of thin film-based devices or to use it for the spontaneous production of an assembly of nanoscaled islands on solid substrates. For this purpose we review the different parameters that influence the dewetting then illustrate how the dewetted state may be tuned by varying the thickness of the film, the annealing temperature, or the state of strain in the film. Moreover, adsorbed or absorbed species (by deposition or ionic impingement/ion bombardment) may modify the surface properties of the film or the mobility properties of the contact line film/substrate and thus the dewetting properties. Anisotropic properties of the film may also be used to initiate the dewetting from perfectly oriented edge fronts, leading to highly ordered 3D islands. New approaches using substrate pre-patterning or film patterning are very promising to achieve the dewetting engineering. Ideal systems for studying solid state dewetting are single crystalline films deposited or bonded on amorphous substrates, so that, among the numerous dewetting systems reported in the literature, ultra-thin crystalline silicon-on-insulator (SOl) film (a Si film bonded on an amorphous SiO2 substrate) is considered as a model system for studying how to control solid state dewetting. Other systems, as Ni epitaxially grown on MgO, are also used to illustrate the different approaches for a ``dewetting engineering''. (C) 2016 Elsevier B.V. All rights reserved.

International audience; Ultra thin Silicon films of Silicon-on-Insulator technology are metastable and thus cannot be submitted to high temperature treatments that may roughen or disrupt the film during the set of technological steps required for device fabrication. This paper concerns the development of an efficient low temperature cleaning process of Si and SiGe surfaces that enables a subsequent good-quality epitaxy of raised source and drain. For this purpose wet-clean, plasma-clean and several combinations of both are used. We thus propose two effective surface cleaning processes with low thermal budget optimized for FD-SOI technology. (C) 2016 Elsevier B.V. All rights reserved.

International audience; The thermal decomposition of Si dioxide layers formed by wet chemical treatment on Si(001) has been studied by low-energy electron microscopy. Independent nucleations of voids occur into the Si oxide layers that open by reaction at the void periphery. Depending on the voids, the reaction rates exhibit large differences via the occurrence of a nonlinear growth of the void radius. This non-steady state regime is attributed to the accumulation of defects and silicon hydroxyl species at the SiO2/Si interface that enhances the silicon oxide decomposition at the void periphery. (C) 2016 AIP Publishing LLC.

We study by grazing incidence x-ray diffraction the strain field induced by periodic double steps on a Si(1 1 15) surface that is a vicinal of a Si(001) surface misoriented by $5.{4}^{\ensuremath{\circ}}$ towards the $\ensuremath{\langle}110\ensuremath{\rangle}$ direction. The best fit of the experimental structure factors is reached on the basis of the rebonded ${D}_{B}$ step edge model and the displacement field is well characterized assuming that steps are described by parallel rows of extended buried elastic dipoles. The dipole characteristics are the dipole position with respect to the step edge, the dipole amplitude ($2.0\ifmmode\pm\else\textpm\fi{}0.5$ nN), and the lever arm $\mathrm{\ensuremath{\Omega}}=5.{3}^{\ensuremath{\circ}}$ and force $\mathrm{\ensuremath{\Phi}}=3.{7}^{\ensuremath{\circ}}$ orientations. We show that the dipole is dominated by a large stretch component localized between the lower and the upper corners of the step, which we assign to the presence of the rebonded atom at the step.

International audience; A micromagnetic study of epitaxial micron-sized iron dots is reported through the analysis of Fresnel contrast in Lorentz Microscopy. Their use is reviewed and developed through analysis of various magnetic structures in such dots. Simple Landau configuration is used to investigate various aspects of asymmetric Bloch domain walls. The experimental width of such a complex wall is first derived and its value is discussed with the help of micromagnetic simulations. Combination of these two approaches enables us to define what is really extracted when estimating asymmetric wall width in Lorentz Microscopy. Moreover, quantitative data on the magnetization inside the dot is retrieved using phase retrieval as well as new informations on the degrees of freedom of such walls. Finally, it is shown how the existence and the propagation of a surface vortex can be characterized and monitored. This demonstrates the ability to reach a magnetic sensitivity a priori hidden in Fresnel contrast, based on an original image treatment and backed-up by the evaluation of contrasts obtained from micromagnetic simulations.

We investigated with XMCD-PEEM magnetic imaging the magnetization reversal processes of N\'eel caps inside Bloch walls in self-assembled, micron-sized Fe(110) dots with flux-closure magnetic state. In most cases the magnetic-dependent processes are symmetric in field, as expected. However, some dots show pronounced asymmetric behaviors. Micromagnetic simulations suggest that the geometrical features (and their asymmetry) of the dots strongly affect the switching mechanism of the N\'eel caps., Comment: Proceeding for MMM-Intermag 2010 (Washington)

Single-crystalline Fe dots self-assembled under ultra high vacuum (UHV) are used as a model system to discuss micromagnetic properties of sub-micron size magnetic dots. Landau and diamond states were identified by magnetic force microscopy (MFM) and reproduced by simulations. To understand mechanisms involved in the magnetization reversal, the in-plane angular variations of nucleation and annihilation fields of a multi-domain magnetic single dot with a micro-SQUID were studied. Resume. Des ˆolots de fer auto-assembles obtenus sous ultravide (UHV) sont utilises comme systeme modele pourles proprietes micromagnetiques de plots sub-microniques. LesLandau et losange onte identifies et reproduits par simulations. Pour comprendre les mecanismes de renverse- ment de l'aimantation, les variations angulaires des champs de nucleation et d'annihilation d'un plot unique multi-domaines onteees par microsquid.

We report on a new mechanism of nanowire formation: during Au deposition on Si(110) substrates, Au–Si droplets grow, move spontaneously, and fabricate a Si nanowire behind them in the absence of Si external flux. Nanowires are formed by Si dissolved from the substrate at the advancing front of the droplets and transported backward to the crystallization front. The droplet shape is determined by the Si etching anisotropy. The nanowire formation can be tuned by changing experimental parameters like substrate temperature and Au deposition rate.

Monte Carlo algorithms are used to simulate the morphologies adopted by polymer chains in a polymer-blend film in the limits where the chains are mutually attractive (homophilic regime) and mutually repulsive (heterophilic regime) and then to simulate the drift transport of charges through the polymer chains. In the homophilic regime, chains aggregate into tangled domains resulting in a relatively high percolation threshold, a high density of configurational trap states, and slow, dispersive charge transport. In the heterophilic regime at the same polymer volume fraction, chains self-organize into a lacework pattern resulting in a low percolation threshold and efficient, trap-free charge transport. For homophilic morphologies interchain hopping is rate-limiting and mobility is insensitive to chain length, whereas for heterophilic morphologies intrachain transport is important and mobility increases with increasing chain length. The morphologies are used in simulations of photocurrent quantum efficiency for donor-acceptor blend photodiodes, which show that the effects of morphology on charge pair generation and recombination compete with the effect on transport, such that the optimum blend composition is sensitive to both morphology and recombination rate. We conclude that it is essential to consider the connectivity of and morphology adopted by polymer chains in the optimization of materials for organic solar cells.

International audience; Since a few years there have been many efforts for studying solid state dewetting that means the spontaneous agglomeration of a metastable film into an assembly of 3D crystallites [1–3]. Since the size of the so-formed 3D crystallites depends on the initial thickness of the film [3–6], solid state dewetting of ultra-thin films is a promising method for producing nanodots [7, 8]. Many studies thus focused on the effect of the film thickness on the dewetting morphologies. Generally thicker films dewet by void nucleation and opening process forming then well-organized 3D islands [1–3]. In contrast to this thinner films tend to form more complex structures [9–14]. For instance Kinetic Monte Carlo simulations have shown that a 3 ML-thick film dewets by void nucleation whereas a 1 ML-thick film dewets by forming a labyrinthine pattern of bilayer islands [10, 11]. In the literature the specific behavior of the thinner films has been attributed to barrierless void nucleation [15], spinodal dewetting (enhanced sensitivity to thickness or thermal fluctuations) [16], short range effect of the wetting potential [17, 18] or local stresses [14, 19]. Recently we investigated dewetting properties of Silicon-on-Insulator (SOI) samples capped by a chemically-prepared oxide layer. We thus reported that films thicker than 8 nm dewet in four steps and break down into self-organized 3D crystals whereas films thinner than 6 nm dewet by labyrinthine formation [9]. In this paper we show that this thin film effect is actually due to a subtle interplay between film deoxidation and film dewetting. Indeed the temperature at which a film dewets (T dew (h)) depends on its thickness h whereas the deoxidation temperature (T deox) is a constant. We thus show that if T dew (h) > T deox SOI dewetting occurs by void opening and leads to 3D crystals whereas if T dew (h) < T deox SOI dewetting leads to labyrinthine morphologies. In other words, the dewetting of the thicker films does not depend on the oxide cap (that decomposes before dewetting and thus only depends on the surface energy gain due to the disappearance of the Si surface with respect to the uncovered bare substrate) while the dewetting of the thinner films depends on the deoxidation (since only the clean deoxidized parts of the silicon film can dewet). We use LEEM to investigate in-situ and in real time the decomposition of the oxide as well as the dewetting of the SOI films. We define the deoxidation temperature T deox as the temperature at which LEEM enables to see void opening in the oxide cap layer with subsequent silicon film appearance. In a similar manner, the dewet-ting temperature T dew is defined as the temperature at which the amorphous SiO 2 substrate starts becoming visible by retraction of the silicon film. Obviously both temperatures T deox and T dew do not correspond to phase transitions and thus depend on the observation means. However since measured in the same experimental conditions they can be compared without any ambiguity. The SOI samples used in the experiments (12 nm Si on top of 25 nm SiO 2) were prepared as described elsewhere [20]. By means of repeated etching and oxidation cycles a stepwise thinning of the samples is possible to produce films of a certain thickness with a very high accuracy ending with a final oxidation step. Since this protective oxide layer is produced with the same wet chemical oxidation for each Si film thickness the thickness of the oxide is the same for all samples. Ellipsometry measurements have proven that the oxide thickness is constant with approximately 1 nm. FIG. 1. Temperature at which the initial deoxidation (black) and dewetting (red) was observed depending on the Si film thickness. Dewetting velocities are in the range of 1 to 8 nm s. The data point at 21.6 nm was obtained using a SOI sample of 22 nm Si on top of 150 nm SiO2. In Figure 1 are thus reported T deox and T dew as a function of the Silicon film thickness. As expected, the decomposition temperature of the capping oxide does not depend on the thickness of the silicon film and is rather constant at T deox =760 • C whereas T dew decreases as h n with n = 0.08 ± 0.01 comparable to [21, 22] where exponents n = 0.15 ± 0.01 have been reported. In the following we will report dewetting morphologies

The structural, optical, and transport properties of graphene grown by chemical vapor deposition (CVD) of propane under hydrogen on the Si face of SiC substrates have been investigated. We show that little changes in temperature during the growth can trigger the passivation of the SiC surface by hydrogen. Depending on the growth condition, hole or electron doping can be achieved, down to a few ${10}^{11}$ cm${}^{\ensuremath{-}2}$. When the growth temperature is high ($T\ensuremath{\approx}1500--1550{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C), we obtain electron-doped graphene monolayers lying on a buffer layer. When the growth temperature is slightly lowered ($T\ensuremath{\approx}1450--1500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C), hole-doped graphene layers are obtained, lying on a hydrogen-passivated SiC surface, as confirmed by the enhancement of the mobility (of the order of 4500 cm${}^{2}/$Vs) and the persistence of weak localization almost up to room temperature (250 K). The high homogeneity of this graphene allows the observation of the half-integer quantum Hall effect, typical of graphene, at the centimeter scale in the best cases. The influence of the SiC steps on the transport properties is discussed.

The dewetting properties of $\mathrm{Ge}/{\mathrm{SiO}}_{2}$ have been studied by low-energy electron microscopy and grazing incidence small-angle x-ray scattering in two temperature ranges characterized by the presence or the absence of {15 3 23} facets on the dewetting fronts. Thanks to a comparison with the $\mathrm{Si}/{\mathrm{SiO}}_{2}$ system, we show that the {15 3 23} facets: (i) play a role in the stabilization properties of Ge dewetting fronts, (ii) lead to a rotation of ${45}^{\ensuremath{\circ}}$ of the Ge fingers with respect to the Si fingers, and (iii) increase the Ge fingers' stability delaying the formation of a three-dimensional Ge islands with respect to Si for the benefit of the formation of Ge nanowires. Studying the dewetting kinetics enables estimating the activation energy for Ge dewetting to $2.7\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. The weak energetics differences between Si and Ge systems are sufficient to change the dewetting morphologies from a squared-void opening for $\mathrm{Si}/{\mathrm{SiO}}_{2}$ to multibranch dendrites for Ge with specific consequences on the relative dewetting velocities of the $\mathrm{Si}/{\mathrm{SiO}}_{2}$ and $\mathrm{Ge}/{\mathrm{SiO}}_{2}$ systems.

International audience; We report here in situ measurements of the evolution of the Ag(110) surface during Si growth, using scanning tunneling microscopy and grazing incidence x-ray diffraction. We provide compelling evidence of an Ag(110) surface reconstruction associated with the release of Ag atoms induced by the growth of Si nanoribbons. Our results are in agreement with a missing row reconstruction of the Ag layer underneath the nanoribbons. This challenges the current understanding of the Si growth on nonreconstructed Ag(110), interpreted within the framework of silicene models.

We report on the anisotropy of solid state dewetting of straight fronts of Si(001)/SiO${}_{2}$ obtained by electron beam lithography. The $\ensuremath{\langle}110\ensuremath{\rangle}$ front is stable. It recedes with formation of a faceted rim that thickens in a layer-by-layer mode kinetically limited by 2D nucleation on the top facet of the rim. The front position and the height of the rim respectively obey $\ensuremath{\sim}{t}^{0.37\ifmmode\pm\else\textpm\fi{}0.03}$ and $\ensuremath{\sim}{t}^{0.38\ifmmode\pm\else\textpm\fi{}0.02}$ power laws. The $\ensuremath{\langle}100\ensuremath{\rangle}$ front is unstable. It breaks down then recedes at constant velocity. The $\ensuremath{\langle}100\ensuremath{\rangle}$ front instability is characterized by the formation of void fingers whose tips retract at constant shape with a kinetics limited by the mass transfer from the void tip to the Si fingers. The conditions of stability of a straight front are discussed in the light of recent theoretical models. New perspectives for controlling solid dewetting are opened.

Using low-energy electron microscopy (LEEM), we have measured, in real time, the dewetting of single-crystal Si(001) thin films on amorphous silicon dioxide substrates, which transforms the two-dimensional (2D) thin film into three-dimensional (3D) compact Si nanocrystals. The dewetting scenario has been reported by Bussmann et al. [New J. Phys. 13, 043017 (2011)]. Analytic 2D and 3D models based on simple approximate geometries of the dewetting front have been developed to analyze LEEM measurements. They enable us to estimate the driving force for dewetting ${E}_{s}\ensuremath{\sim}14$ eV/nm${}^{2}$. Starting from a Si-film thickness dependent effective dewetting activation barrier, a single Si(001) surface self-diffusion energy of ${E}_{a}=2.0\ifmmode\pm\else\textpm\fi{}0.2$ eV is derived. First nanoisland-formation dynamics measurements are discussed. Finally, grazing incidence small-angle x-ray scattering (GISAXS) is used to characterize the structure and the morphology of the Si nanocrystals created by the dewetting process.

International audience; Dimensionality cross-over is a classical topic in physics. Surprisingly it has not been searched in micromagnetism, which deals with objects such as domain walls (2D) and vortices (1D). We predict by simulation a second-order transition between these two objects, with the wall length as the Landau parameter. This was conrmed experimentally based on micron-sized ux-closure dots.

We report on a new approach, entirely based on electron-beam lithography technique, to contact electrically, in a four-probe scheme, single nanostructures obtained by self-assembly. In our procedure, nanostructures of interest are localised and contacted in the same fabrication step. This technique has been developed to study the field-induced reversal of an internal component of an asymmetric Bloch domain wall observed in elongated structures such as Fe(110) dots. We have focused on the control, using an external magnetic field, of the magnetisation orientation within N\'eel caps that terminate the domain wall at both interfaces. Preliminary magneto-transport measurements are discussed demonstrating that single Fe(110) dots have been contacted., Comment: 5 pages

International audience; We report a detailed magnetic study of a new type of self-organized nanowires disclosed briefly previously [B. Borca et al., Appl. Phys. Lett. 90, 142507 (2007)]. The templates, prepared on sapphire wafers in a kinetically-limited regime, consist of uniaxially-grooved W(110) surfaces, with a lateral period here tuned to 15nm. Fe deposition leads to the formation of (110) 7 nm-wide wires located at the bottom of the grooves. The effect of capping layers (Mo, Pd, Au, Al) and underlayers (Mo, W) on the magnetic anisotropy of the wires was studied. Significant discrepancies with figures known for thin flat films are evidenced and discussed in terms of step anisotropy and strain-dependent surface anisotropy. Demagnetizing coeffcients of cylinders with a triangular isosceles cross-section have also been calculated, to estimate the contribution of dipolar anisotropy. Finally, the dependence of magnetic anisotropy with the interface element was used to tune the blocking temperature of the wires, here from 50K to 200 K.

While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called N\'eel caps. We demonstrated the controlled switching of these caps under a magnetic field, occurring via the propagation of a surface vortex. For this we considered flux-closure states in elongated micron-sized dots, so that only the central domain wall can be addressed, while domains remain unaffected., Comment: 4 pages, 3 figures

International audience; X-ray photoelectron emission microscopy in combination with x-ray magnetic circular dichroism is used to investigate the influence of an applied magnetic field on Néel caps (i.e., surface terminations of asymmetric Bloch walls). Self-assembled micron-sized Fe(110) dots displaying a moderate distribution of size and aspect ratios serve as model objects. Investigations of remanent states after application of an applied field along the direction of Néel-cap magnetization give clear evidence for the magnetization reversal of the Néel caps around 120 mT, with a ±20 mT dispersion. No clear correlation could be found between the value of the reversal field and geometrical features of the dots.

We have revisited the epitaxial growth modes of Fe on W(110) and Mo(110), and propose an overview or our contribution to the field. We show that the Stranski-Krastanov growth mode, recognized for a long time in these systems, is in fact characterized by a bimodal distribution of islands for growth temperature in the range 250-700°C. We observe firstly compact islands whose shape is determined by Wulff-Kaischev's theorem, secondly thin and flat islands that display a preferred height, ie independant from nominal thickness and deposition procedure (1.4nm for Mo, and 5.5nm for W on the average). We used this effect to fabricate self-organized arrays of nanometers-thick stripes by step decoration. Self-assembled nano-ties are also obtained for nucleation of the flat islands on Mo at fairly high temperature, ie 800°C. Finally, using interfacial layers and solid solutions we separate two effects on the preferred height, first that of the interfacial energy, second that of the continuously-varying lattice parameter of the growth surface., Comment: 49 pages. Invited topical review for J. Phys.: Condens. Matter

This paper presents investigations of magnetization configuration evolution during in-situ magnetic processes in materials exhibiting planar and perpendicular magnetic anisotropy. Transmission electron microscopy has been used to perform magnetic imaging. Fresnel contrasts in Lorentz Transmission Electron Microscopy (LTEM) and phase retrieval methods such as Transport of Intensity Equation (TIE) solving or electron holography have been implemented. These techniques are sensitive to magnetic induction perpendicular to the electron beam and can give access to a spatially resolved (resolution better than 10 nm) mapping of magnetic induction distribution and could be extended to dynamical studies during in-situ observation. Thin foils of FePd alloys with a strong perpendicular magnetic anisotropy (PMA) and self-assembled Fe dots are presented. Both are studied during magnetization processes exhibiting the capacities of in-situ magnetic imaging in a TEM.

We study the nano-pits formed during the etching of a SiO2 film by reactive Si islands at T≈1000 °C. Combining low energy electron microscopy, atomic force microscopy, kinetic Monte Carlo simulations, and an analytic model based on reaction and diffusion at the solid interface, we show that the shape of the nanopits depend on the ratio R/xs with R the Si island radius and xs the oxygen diffusion-length at the Si/SiO2 interface. For small R/xs, nanopits exhibit a single-well V-shape, while a double-well W-shape is found for larger R/xs. The analysis of the transition reveals that xs∼60 nm at T≈1000 °C.

Using low-energy electron microscopy movies, we have measured the dewetting dynamics of single-crystal Si(001) thin films on SiO2 substrates. During annealing (T > 700 degrees C), voids open in the Si, exposing the oxide. The voids grow, evolving Si fingers that subsequently break apart into self-organized three-dimensional (3D) Si nanocrystals. A kinetic Monte Carlo model incorporating surface and interfacial free energies reproduces all the salient features of the morphological evolution. The dewetting dynamics is described using an analytic surface-diffusion-based model. We demonstrate quantitatively that Si dewetting from SiO2 is mediated by surface-diffusion driven by surface free-energy minimization.