A significant progress toward next-generation electronic devices that combine memory and processing functions could involve the electrical manipulation of charge carrier spin textures in semiconductors. In this context, GeTe has recently emerged as a promising ferroelectric Rashba semiconductor, exhibiting a giant spin splitting in its band structure. This remarkable property stems from the inversion symmetry breaking induced by its ferroelectric polarization. Here, we address the control of the domain structure of GeTe thin films grown on miscut silicon substrates. We show that the domain structure of the GeTe thin films is strongly influenced by the miscut direction with respect to the nominal Si(111) substrate. Considering miscut in the [ 1 1 2 ] direction, highly crystallized GeTe films are grown without twins, and the domain structure exhibits a reduction in domain size parallel to the step edges. In the case of [ 11 2 ] miscut direction, we evidence that thin films exhibit a predominance of twins and a complex ferroelectric structure that is affected by a large density of interfacial defects. Our results also show that the miscut direction plays a key role in the growth morphology of the GeTe thin film. All these results support the view that atomic steps on silicon substrates have a profound effect on the structure and growth morphology of GeTe thin films as well as on the domain structure via local stress relaxation mechanisms.

Using spin- and angle-resolved photoemission spectroscopy combined with density functional theory calculations, we investigate the electronic structure of alpha-GeTe grown on a Si(111)-Sb substrate in the ultrathin limit. The full band structure, including the Fermi surface, and spin texture characterizations are presented, with a focus on a film thickness of 5 nm. Well-defined spin-polarized quantum well bulk states are evidenced. This confirms the high quality of elaborated ultrathin films and allows precise determination of the thickness. The Rashba splitting and the in-plane spin texture of GeTe bulk bands are shown to be fully preserved on the Si(111)-Sb substrate. This suggests the persistence of inversion symmetry breaking and ferroelectric properties in the ultrathin limit of GeTe films. Our results demonstrate that device downscaling can be pursued in the context of all-electrically controlled spin-orbitronics applications.

The motion of microdroplets under thermal gradients, also known as thermomigration (TM), plays a critical role in a wide range of surface phenomena and technological applications. In this work, we study the TM behavior of Au-Ge micro-droplets on Ge(111) surfaces and show that the droplet motion leads to the formation of atomically flat terraces, which are essential for thin film epitaxy. Experimental observations are combined with a microscopic model to investigate droplet motion and the formation of flat atomic terraces. Our results show that, at low temperatures, droplet velocity increases with size, but this size dependence diminishes as the temperature rises. The microscopic model we develop is based on thermally activated detachment, diffusion, and attachment of solute (Ge) atoms across the droplet, driven by a temperature-dependent concentration difference between the leading edge and the trailing edge. The model reproduces the experimental trends. Our results provide insights into the fundamental mechanisms of droplet TM and suggest ways to control surface morphology.

\/ \/ Ag deposition on Si(111)3 x3Ag surface is studied in the temperature range 220-470 K using low-energy electron microscopy (LEEM). Above a critical Ag coverage, Ag-3D islands nucleate and exhibit growth shape morphologies characterized by assemblies of Ag microcrystals that evolve from elongated morphologies at low temperature to rounded contours at higher temperature. From real-time Ag deposition monitoring and Ag-3D island surface density measurements, we determine a critical size for a stable Ag-3D island of 7 +/- 2 Ag atoms, a Ag adatom formation energy by detachment from a 3D island of 0.053 +/- 0.005 eV and an activation barrier \/ \/ for the diffusion of Ag adatom on the Si(111)3 x3Ag surface of 0.32 +/- 0.05 eV.

Nuclear fusion, as a carbon-free energy source, is actively pursued by the international community. However, the irradiation by helium (He) of the walls facing the plasma can lead to important damages at the surface and deeper in the bulk, affecting the properties and life span of the materials. To address the effects of He irradiation, we have studied in real time, during He bombardment at 1273 K, the nucleation and growth of He bubbles nearby the surface of tungsten by grazing incidence small angle X-ray scattering (GISAXS). We could disentangle the surface and bubble contributions on the measured GISAXS patterns by adjusting the X-ray incident angle below and above the critical angle of total external reflection. The occurrence of tilted diffuse scattering rods proves the presence of facetted bubbles buried inside the tungsten matrix. The time evolution of the X-ray diffuse scattering has been used to evaluate the growth kinetics. A close comparison with analytical modelling of the growth process suggests a growth kinetics dominated by the migration-coalescence of bubbles during which the bubble motion is hindered by the nucleation of ledges at the bubble facet surface.

In a thermal gradient, surface nanostructures have been experimentally observed to move due to thermomigration. However, analytical models that describe the thermomigration force acting on surfaces are still controversial. In this work, we start from a thermodynamic approach based on the Massieu function, which is used to describe thermomigration of single adatoms, to develop an expression for the velocity of thermomigrating 2D holes. The model can be simplified in two limiting cases: (i) When the hole motion is limited by adspecies diffusion, the velocity is independent from the hole size (as in our experiments). (ii) If the hole motion is limited by the attachment or detachment of species to or from steps, then the velocity is proportional to the hole width. We have studied the thermomigration of 2D monatomic deep holes on Si(100) using low energy electron microscopy. From the velocity measurements taken at different temperatures, we find, using our model, that the sum of the migration energy and the adatom creation energy is 1.95 +/- 0.16 eV. This value is consistent with those found by other authors, reinforcing the validity of our thermomigration model.

We demonstrate that the step-train orientation of a vicinal surface plays an important role in solid-state dewetting.Focusing on SOI(111), we observe the formation of hexagonal voids in the silicon film due to dewetting.These voids are surrounded by an asymmetric rim, which is wider on the higher-terrace side compared to the lower-terrace side.Our findings also reveal a layer-by-layer growth of the rim around the dewetting voids during the early stages, before the formation of branched structures.Dewetting fingers develop faster and are thinner in the step-down direction compared to the step-up direction.Kinetic Monte Carlo (KMC) simulations confirm our experimental observations and provide insights into the atomic details underlying rim and finger formation.During the initial stages of dewetting, the nucleation of a new rim layer is not required: Atoms leaving the dewetting void are captured by the adjacent upper step, leading to the growth of the upper terrace that thus surrounds the dewetting void.The nucleation of a new rim layer only becomes necessary when the distance between the edge of the void and the upper terrace increases.The energetics involved in the dewetting kinetics of SOI(111) are consistent with those of SOI(100).

Ferroelectric Rashba semiconductors (FERSCs) such as alpha-GeTe are promising candidates for energy-efficient information technologies exploiting spin-orbit coupling (SOC) and are termed spin-orbitronics. In this work, the thickness limit of the Rashba-SOC effect in alpha-GeTe films is investigated. We demonstrate, using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations performed on pristine GeTe, that down to 1 nm, GeTe(111) films on a Sb-covered Si(111) substrate continuously exhibit a giant Rashba effect characterized, at 1 nm, by a constant of 5.2 +/- 0.5 eV& Aring;. X-ray photoemission spectroscopy (XPS) allows the understanding of the persistence of the Rashba effect by evidencing a compensation of Ge vacancy defects resulting from the insertion of Sb interfacial atoms in the early stage growth of the GeTe(111) films.

Ferroelectric germanium telluride is under active consideration for spintronic and thermoelectric applications. The control of the ferroelectric domain walls is a key issue to optimize the electronic and thermal properties of GeTe thin films. Domain walls properties are usually driven by the mechanical and electrostatic compatibility conditions of twin domains. However, in dense ferroelectric domain structures these compatibility conditions are hardly fulfilled everywhere. In particular intersection of domains may result in complex lattice relaxations and polarization textures. In this study, we have fabricated GeTe thin films on silicon substrate and elucidated the intersections of $a$-type domains using 3D reciprocal space maps, scanning tunneling microscopy, and second-harmonic microscopy. We demonstrate the presence of complex structural reorganizations that manifest by the formation of charged domain walls, large lattice rotations, and enhanced stretching of the rhombohedral lattice.

Epitaxial growth of WTe2 offers significant advantages, including the production of high-quality films, possible long-range in-plane ordering, and precise control over layer thicknesses. However, the mean island size of WTe2 grown by molecular beam epitaxy (MBE) in the literature is only a few tens of nanometers, which is not suitable for the implementation of devices at large lateral scales. Here we report the growth of T-d -WTe2 ultrathin films by MBE on monolayer (ML) graphene, reaching a mean flake size of similar or equal to 110 nm, which is, on overage, more than three times larger than previous results. WTe2 films thicker than 5 nm have been successfully synthesized and exhibit the expected Td phase atomic structure. We rationalize the epitaxial growth of T-d-WTe2 and propose a simple model to estimate the mean flake size as a function of growth parameters that can be applied to other transition metal dichalcogenides (TMDCs). Based on nucleation theory and the Kolmogorov-Johnson-Meh-Avrami (KJMA) equation, our analytical model supports experimental data showing a critical coverage of 0.13 ML above which WTe2 nucleation becomes negligible. The quality of monolayer WTe2 films is demonstrated by electronic band structure analysis using angle-resolved photoemission spectroscopy (ARPES), which is in agreement with first-principles calculations performed on free-standing WTe2 and previous reports. We found electron pockets at the Fermi level, indicating a n-type doping of WTe2 with an electron density of n = 2.0 +/- 0.5 x 10(12) cm(-2) for each electron pocket.

Epitaxial growth of WTe2 offers significant advantages, including the production of high-quality films, possible long-range in-plane ordering, and precise control over layer thicknesses. However, the mean island size of WTe2 grown by molecular beam epitaxy (MBE) in the literature is only a few tens of nanometers, which is not suitable for the implementation of devices at large lateral scales. Here we report the growth of Td -WTe2 ultrathin films by MBE on monolayer (ML) graphene, reaching a mean flake size of ≃110 nm, which is, on overage, more than three times larger than previous results. WTe2 films thicker than 5 nm have been successfully synthesized and exhibit the expected Td phase atomic structure. We rationalize the epitaxial growth of Td-WTe2 and propose a simple model to estimate the mean flake size as a function of growth parameters that can be applied to other transition metal dichalcogenides (TMDCs). Based on nucleation theory and the Kolmogorov-Johnson-Meh-Avrami (KJMA) equation, our analytical model supports experimental data showing a critical coverage of 0.13 ML above which WTe2 nucleation becomes negligible. The quality of monolayer WTe2 films is demonstrated by electronic band structure analysis using angle-resolved photoemission spectroscopy (ARPES), which is in agreement with first-principles calculations performed on free-standing WTe2 and previous reports. We found electron pockets at the Fermi level, indicating a n-type doping of WTe2 with an electron density of n = 2.0 ± 0.5 × 1012 cm-2 for each electron pocket.

This paper discusses the conditions of spontaneous appearance of a local (positive or negative) protuberance on a solid surface in equilibrium with a uid and submitted to an epitaxial stress. For this purpose we simply modify the classical thermodynamic approach of the nucleation by introducing surface elastic forces. To catch the essential of the physics and obtain analytical expressions we use strong assumptions (weak slopes approximation, isotropic material, asymptotic developments...). Such a pedagogical approach might help to get a better understanding of many stressed-induced mechanisms as the Stranski Krastanov growth mode, the equilibrium roughening transition or some adsorption phenomenons in close-to-equilibrium conditions.

The advent of germanium telluride as a promising ferroelectric Rashba semiconductor for spintronic applications requires the growth of nanometer-thick films of high crystalline quality. In this study, we have elucidated the initial growth stages of GeTe on Si(111)-Sb by scanning tunneling microscopy and low energy electron diffraction. We demonstrate the presence of an initial 0.35-nm-thick GeTe buffer layer followed by the 2D growth of GeTe via Frank-Read sources of atomic steps. As shown by core level spectroscopy, Sb is acting as a surfactant during growth up to a 5-nm-thick film. X-ray diffraction, transmission electron microscopy, and low energy electron microscopy evidence that numerous mirror domains and in-plane misorientations appear early in the growth process and are gradually buried at the film/substrate interface. The use of a miscut Si substrate close to Si(111) allows suppressing these defects from the early beginning of growth.

The evolution of the tungsten microstructure and properties under helium irradiation could cause problems for both operational and safety reasons in fusion reactors. In particular, the presence of helium bubbles, formed mainly in the subsurface area, modifies the mechanical and retention properties of radioactive elements. In that context we have investigated the shape of helium bubbles in tungsten induced by helium irradiation under well-defined experimental conditions. We have used single crystals with controlled bombardment energy, flux, fluence and post-irradiation annealing at high temperature under ultra-high vacuum in order to get closer to the thermodynamic equilibrium of the helium bubbles. The bubble shape has been characterized by a combination of TEM and 3D reciprocal space map by in-situ GISAXS. We have shown the presence of facetted bubbles exposing {110} and {100} facets of similar size. Using Wulff theorem and experimental results from both techniques, we derive a surface energy ratio of γ100γ110= 1.03 ± 0.03.

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.

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.

We studied the diffusion of 3D nanovoids in a bcc solid by kinetic Monte Carlo simulations. The diffusion coefficient as a function of the void size increases, reaches a maximum, and then decreases. The first increase is particularly interesting, as the diffusion of clusters is generally considered a decreasing function of the cluster size. We attribute this behavior to a curvature-dependent energy barrier for mass transport. We propose an analytical modeling of the void diffusion coefficient that reproduces the simulation data over the whole size range. In addition, for low temperatures and small sizes, the void diffusion coefficient vs size displays valleys, i.e., regions where the diffusion coefficient is smaller than the general trend. This behavior cannot be explained with analytical developments and is due to the formation of compact shapes for certain magic void sizes. In these shapes, the atoms at the void surface are strongly bound, displace less, and thus also void diffusion is slower.

A key development toward new electronic devices integrating memory and processing capabilities could be based on the electric control of the spin texture of charge carriers in semiconductors. In that respect, GeTe has been recently recognized as a promising ferroelectric Rashba semiconductor, with giant spin splitting of the band structure, due to the inversion symmetry breaking arising from ferroelectric polarization. Here, we address the temperature dependence of the ferroelectric structure of GeTe thin films grown on Si(111). We demonstrate the hysteretic behavior of the ferroelectric domain density upon heating/cooling cycles by low energy electron microscopy. This behavior is associated with an abnormal evolution of the GeTe lattice parameter as shown by x-ray diffraction. We explain these thermomechanical phenomena by a large difference of thermal expansion coefficients between the film and the substrate and to the pinning of the GeTe/Si interface. The accumulated elastic energy by the GeTe thin film during sample cooling is released by the formation of a-nanodomains with in-plane ferroelectric polarization components.

Thermal gradients in nanomaterials can cause surface mass transport phenomena. However, the atomic fluxes are challenging to quantify and the underlying atomic mechanisms are complex. Using low energy electron microscopy we have examined in operando, under a thermal gradient of 10^{4}  K/m, the thermomigration of supercooled Si(111)-1×1 advacancy islands. The islands move in the direction of the thermal gradient at 0.26±0.06  nm/s. This reveals that the adatoms move toward the cold region and the effective force exerted on Si adatoms is 1.4±0.4×10^{-8}  eV/nm. We quantify the heat of transport of Si atoms Q^{*}=1.2±0.4  eV and show that it corresponds to the combined effects of adatom creation at step edges and adatom diffusion on atomically flat terraces.

We have observed the motion of two-dimensional phases of Au on Ge(111) under an electric bias by in operando low energy electron microscopy. Electromigration of Au results in a complex dynamics that depends on the nature of the involved mobile phases: two-dimensional adatom gas or two-dimensional islands. We show that the Au adatoms move at the surface in the direction opposite to the electric current. The wind force induced by the electron flow is measured: the effective valence of Au (Z* = -82 +/- 15) is directly deduced from the coverage profile of a Au adatom gas spatially retained by a strong Erlich-Schwoebel barrier at a downhill step edge. The velocity of two-dimensional Au islands versus island size reveals a mass transport by terrace diffusion inside the islands. The energy barrier for diffusion above 820 K is 1.16 +/- 0.08 eV and it strongly increases up to 3.1 +/- 0.6 eV below. We attribute this change of regime to a modification of diffusing species from single Au atoms at high temperature to Au clusters at low temperature. The strong shape fluctuations of the 2D islands is consistent with a nearly vanishing line tension of 1.2 +/- 0.4 meV/nm at 800 K.