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Single and few layers of the two-dimensional (2D) semimetal ZrTe2 are grown by molecular beam epitaxy on InAs(111)/Si(111) substrates. Excellent rotational commensurability, van der Waals gap at the interface and moire pattern are observed indicating good registry between the ZrTe2 epilayer and the substrate through weak van der Waals forces. The electronic band structure imaged by angle resolved photoelectron spectroscopy shows that valence and conduction bands cross at the Fermi level exhibiting abrupt linear dispersions. The latter indicates massless Dirac Fermions which are maintained down to the 2D limit suggesting that single-layer ZrTe2 could be considered as the electronic analogue of graphene.

We investigate by in situ X-ray diffraction the early stage of GaN nanowire growth on a single layer of polycrystalline graphene transferred onto a Si(100)/SiO2 carrier substrate. The experiment wa...

The moir\'e pattern created by the epitaxy of a graphene sheet on an iridium substrate can be used as a template for the growth of 2D atomic or cluster arrays. We observed for the first time a coherent organization of hard magnetic preformed FePt nanoparticles on the 2D lattice of graphene on Ir(111). Nanoparticles of 2 nm diameter have been mass selected in a gas phase and deposited with low energy on the hexagonal moir\'e pattern. Their morphology and organization have been investigated using grazing incidence small angle x-ray scattering, while their magnetic properties have been studied by x-ray magnetic circular dichroism, both pointing to a FePt cluster-graphene surface specific interaction. The spatial coherence of the nanoparticles is preserved upon annealing up to $700\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ where the hard magnetic phase of FePt is obtained.

International audience; Despite a large number of studies [2,3] over the years since the first discovery [7] and a couple of comprehensive reviews [8,9] the actual mechanism for PLD/CDW formation is still under debate. The most recent experimental [10-13] and theoretical [14] works focus on the large area growth of the CDW phase [13] the thickness dependence , and the possible unconventional behavior in the ultimate 2D limit of a single layer TiSe 2. [10-12,14] On the other hand, the other Ti dichalcogenides namely TiS 2 and TiTe 2 did not show any clear evidence until very recently when a CDW state was reported only for 1 monolayer (ML)-thin TiTe 2 at temperatures lower than 92 K. [15] It is surprising that the CDW in TiTe 2 was found to be totally suppressed for films thicker than 1 ML, [15] unlike the case of other TMDs where 1 ML and bulk-like films both make the transition to a CDW at nearly the same temperature. The interest about TiTe 2 is continuously increasing in view of theoretical predictions [16] and more recent experimental evidence [17] about pressure induced topological phase transitions in TiTe 2. The possibility to also manipulate superconduc-tivity by external pressure as predicted [18] and more recently evidenced [19] in bulk TiTe 2 creates the prospect to explore the emergence of topological superconductivity in this material. In the latter work [19] it has been shown that under nonhydro-static pressure, a CDW-like state with estimated transition temperature above room temperature (RT) appears in bulk TiTe 2 at around 0.5-1.8 GPa. These results call for a re-examination of the possibility to obtain a CDW in multilayer TiTe 2 and indeed at RT with good potential for real world applications utilizing the properties of the CDW state. These applications include a voltage-controlled oscillator device operating at room temperature , [20] fast electronic resistance switching for nonvolatile memories, [21,22] and field-effect transistor devices potentially suitable for implementation of non-Boolean logic. [23] In this paper it is shown that multilayer films (50 ML ≈ 32 nm), as well as single layer TiTe 2 epitaxially grown on InAs(111)/ Si(111) substrates by molecular beam epitaxy exhibit, in ambient pressure conditions, a CDW distortion at room temperature which is sustained up to higher temperatures, at least 400 °C, as evidenced by reflection high energy electron diffrac-tion (RHEED) (Figure S1, Supporting Information). The results are explained in terms of anisotropic strain imposed by the substrate. The group IVB 2D transition metal dichalcogenides are considered to be stable in the high symmetry trigonal octahedral structure due to the lack of unpaired d-electrons on the metal site. It is found that multilayer epitaxial TiTe 2 is an exception adopting a commensurate 2 × 2 × 2 charge density wave (CDW) structure at room temperature with an ABA type of stacking as evidenced by direct lattice imaging and reciprocal space mapping. The CDW is stabilized by highly anisotropic strain imposed by the substrate with an out-off-plane compression which reduces the interlayer van der Waals gap increasing the coupling between TiTe 2 layers.

Based on previous photoemission findings, the influence of newly observed aluminum surface segregation at the low (110), (100) and (111) index surfaces of the A 2 body-centered Fe 0.85 Al 0.15 random alloy is explored. Scanning Tun-neling Microscopy, Low-Energy Electron Diffraction and Grazing-Incidence X-Ray Diffraction are combined to explore surface structures and topogra-phies. The formation of a surface composition close to the B 2 Fe 0.5 Al 0.5 occurs with the appearance of (i) a long-range pseudo-hexagonal incommen-surate superstructure with a periodicity of ∼ 18A on the (110) surface, (ii) a (1 × 1) termination on the (100) surface and (iii) a intense vicinal faceting of the (111) surface in the form of triangular pits.

International audience; Van der Waals heterojunctions composed of graphene and transition metal dichalcogenides have gain much attention because of the possibility to control and tailor band structure, promising applications in two-dimensional optoelectronics and electronics. In this report, we characterized the van der Waals heterojunction MoSe$_2$/few-layer graphene with a high-quality interface using cutting-edge surface techniques scaling from atomic to microscopic range. These surface analyses gave us a complete picture of the atomic structure and electronic properties of the heterojunction. In particular, we found two important results: the commensurability between the MoSe$_2$ and few-layer graphene lattices and a band-gap opening in the few-layer graphene. The band gap is as large as 250 meV, and we ascribed it to an interface charge transfer that results in an electronic depletion in the few-layer graphene. This conclusion is well supported by electron spectroscopy data and density functional theory calculations. The commensurability between the MoSe$_2$ and graphene lattices as well as the band-gap opening clearly show that the interlayer interaction goes beyond the simple van der Waals interaction. Hence, stacking two-dimensional materials in van der Waals heterojunctions enables us to tailor the atomic and electronic properties of individual layers. It also permits the introduction of a band gap in few-layer graphene by interface charge transfer.

International audience; The observation of atomic ordering and signatures of defects in self-assembled Ge islands using x-ray diffraction techniques have been previously treated as unrelated subjects. However, mutual understanding can be achieved when both subjects are studied in a common frame. Here, we report on measurements and analysis of both defects and atomic ordering in Si1-xGex islands epitaxially grown on Si(001) substrates as a function of growth temperature. By using x-ray diffraction and mapping around a bulk forbidden reflection, defect sizes, and in-plane spacing between nearby dislocations are extracted and related to the composition of the islands. The results fit well with an independent determination using selective wet chemical etching and atomic force microscopy measurements. Moreover, the temperature dependence of the ordered domain size is discussed. Although both atomic ordering and defect formation take place independently in the system, it is found that the relaxation provided by the onset of defects does not affect the formation of ordered domains, recently pointed out to be stabilized by strain and surface equilibrium on islands facets. (C) 2016 AIP Publishing LLC.

The most accessible way to grow semiconductor nanowires remains so far the use of gold nanoclusters on a (1 1 1) semiconductor surface, for instance Si(1 1 1). Precise knowledge of the morphology of gold nanoparticles obtained by growth or dewetting on Si(1 1 1) is essential to better control the growth of nanowires. Both formation routes were investigated in situ by grazing incidence small-angle x-ray scattering. During the growth at 300 °C, Au atoms first nucleate on the wetting layer and give rise to islands having the shape of truncated spheres with an average contact angle of 38° for deposits of 1.4 and 2 monolayers. Dewetting is obtained by annealing of a 2 monolayer room temperature deposit, leading to the formation of particles. The average contact angle increases from 53° to 73° from 270 °C to 360 °C. Moreover, an analysis of the island height suggests that an AuSi alloy with a thickness of approximately 1.5 nm could be present at the top of the monocrystalline islands in the solid state before the eutectic temperature. The liquid–solid interface tension of the AuSi alloy droplet (1.0 J m−2) and the work of adhesion (1.1 J m−2) are deduced at the eutectic temperature.

The growth of Ge islands on a pit-patterned Si(001) template is investigated in situ, combining grazing incidence diffraction, multiple wavelength anomalous diffraction, and small angle scattering. This allows monitoring in situ the detailed structural and morphological evolutions of the pits, of the wetting-layer and of the nucleated islands on the pit-patterned Si(001) substrate. It is shown that after Si regrowth, the Si substrate displays {107} and {1 1 11} facets. During the very first stages of Ge growth, the preliminary facets of the Si substrate are energetically unfavourable, and the pit facets break up into a rather complex pattern of {10n} and {11m} facets with n > 7 and m > 11. At 5 and 6 ML, intensity rods from {105} and {113}-type facets appear in the GISAXS images revealing the formation of pyramids and domes, respectively. The degree of ordering, the shape, strain and composition of the islands are characterized during the growth process to provide a detailed evolution of their structure and morphology.

International audience; The direct observation at room temperature (RT) of the noncentrosym-metric orthorhombic topological Weyl semimetal phase in epitaxial thin films of MoTe 2 grown on InAs(111)/Si(111) substrates by molecular beam epitaxy (MBE) is reported. The orthorhombic phase is typically found at lower temperatures but its observation at RT in this work is attributed to the enlarged lattice parameters, influenced by the substrate, which stabilize an interlayer antibonding state compatible with the orthorhombic stacking. First-principles calculations predict eight type II Weyl nodes which are located below (but near) the Fermi energy making them accessible to charge transport and creating the prospect for practical applications exploiting the nontrivial topological properties. The orthorhombic phase coexists with an unconventional triclinic layer stacking which is different than the monoclinic or orthorhombic structures but it is centrosymmetric and topologically trivial.

We investigate the morphology of islands obtained by epitaxial growth of Ge on Si(0 0 1). During a low growth rate or during annealing and depending of the quench method, we observe the formation of a new type of superdomes, which exhibit additional {15 3 23} facets, an increase of their {0 0 1} and {1 0 5} facet area on their top and a smaller aspect ratio compared to known superdomes. This flat-top superdome shape results from the complex evolution of domes and superdomes: coalescence, intermixing and ripening. Dome or superdome coalescence which results in the newly observed shape appears as a dominant pathway towards dislocation nucleation for the investigated temperature range.

We have studied the nano-patterning of CoO film induced by misfit dislocation network at the interface with the Ag(001) substrate. Grazing incidence diffraction (GIXD), X-ray photoemission spectroscopy (XPS) and low energy electron diffraction (LEED) have been used to characterize chemistry and structure of the CoO layers. The XPS spectrum of the Co 2p core level permitted to establish the stoichiometric growth of CoO. The structure of the CoO film together with the absorption sites of cobalt and oxygen atoms was determined, thanks to GIXD measurements. Moreover we have followed the evolution of the in-plane lattice constant of the CoO as a function of the film thickness. It turns out that the CoO film growth starts with the same in-plane lattice constant of the Ag(001) substrate up to 3-4 ML; afterwards the in-plane parameter of CoO steadily increases before reaching a stable value of 2.98 angstrom at 23 ML. During the relaxation process, at about 8 ML of film thickness, we observe the formation of a buried misfit dislocation network. These dislocations, that have a period of 9.2 nm for a film thickness of 23 ML, induce mosaicity in the CoO film which then appears as a regular distribution of tilted domains. (C) 2006 Elsevier B.V. All rights reserved.

International audience; The structure of a graphene monolayer on Ir(111) has been investigated {\it in situ} in the growth chamber by surface x-ray diffraction including the specular rod, which allows disentangling the effect of the sample roughness from that of the nanorippling of graphene and iridium along the moiré-like pattern between graphene and Ir(111). Accordingly we are able to provide precise estimates of the undulation associated with this nanorippling, which is small in this weakly interacting graphene/metal system and thus proved difficult to assess in the past. The nanoripplings of graphene and iridium are found in phase, i.e. the in-plane position of their height maxima coincide, but the amplitude of the height modulation is much larger for graphene (\(0.379 \pm 0.044\) \AA) than, {\it e.g.}, for the topmost Ir layer (\(0.017 \pm 0.002\) \AA). The average graphene-Ir distance is found to be \(3.38 \pm 0.04\) \AA.

The conditions of formation of the Au-induced Si-(6×6) reconstruction, its stability, as well as its atomic structure are studied experimentally using grazing incidence x-ray diffraction techniques. This reconstruction is found to form at 680 K when cooling down eutectic droplets previously obtained by the dewetting of thin gold films deposited at room temperature on a Si(111) substrate. The quality of the reconstruction is found to depend on the annealing temperature. The formation of the (6×6) reconstruction at low temperature after it has been destroyed by ion bombardment, together with the recovery of the Si surface, give evidence of its high stability and highlight the surfactant properties of gold atoms. The determination of the Si(111)-(6 × 6)Au atomic structure is performed using quantitative surface x-ray diffraction with an existing complex model proposed in the literature as a starting structure. The resulting gold structure is found to be 1 ML thick and consists in two domains related by a mirror. They are composed of trimer and pentagonal units with special sites presenting a partial occupancy of 0.5. Our experimental data set does not provide enough accuracy to determine the positions of the Si atoms of the substrate, but ab initio calculations tend to confirm that they are only slightly displaced from the bulk position.

5 pages, 2 figures, 1 table.-- PACS 61.10.Eq, 61.10.Ht, 61.10.Nz, 61.46.+ w, 68.65.Cd, 68.65.Hb, We report on a general method that takes advantage of the full capability of anomalous diffraction and can be applied to the challenging case of small size embedded nanostructures. We study in situ and ex situ GaN Quantum Dots (QDs) grown by Plasma Assisted Molecular Beam Epitaxy (PAMBE), and encapsulated by an AlN epilayer. We investigate the QD strain and composition that are related to size, morphology, and cap layer thickness by means of anomalous diffraction in grazing incidence. The X-ray energy is tuned across the Ga K-edge where the Ga atoms scattering power is strongly modified and diffraction becomes chemically selective, giving direct information on composition. Quantitative analysis of the oscillatory extended region above the edge gives information on composition and out-of-plane strain of the dots.

International audience; Compositional atomic ordering is a crucial issue in the epitaxial growth of nanoparticles and thin films. Here, we report on a method based on x-ray diffuse scattering close to basis forbidden Bragg reflections to infer the lateral location, the symmetry, and the thermal stability of ordered domains in GeSi dome-shaped islands on Si(001) after growth and during annealing. We observe that atomic ordering does not disappear after annealing, demonstrating that it is a resilient metastable phenomenon. (C) 2015 AIP Publishing LLC.

The structure of ultrathin silicon layers obtained by molecular hydrophobic bonding is investigated. The twist and tilt angles between the two crystals are accurately controlled. The buried Si|Si interface is observed by transmission electron microscopy and by grazing incidence X-ray techniques. For low twist angle values (/spl psi/

Grazing-incidence X-ray diffraction data are combined with energy-minimization calculations to analyse the atomic structure of the Al-rich (3sqrt{3} x 3sqrt{3})R 30 deg reconstructed surface of sapphire alpha-Al_2O_3(0001). The experiments on the BM32 beamline of the ESRF provide the non-integer-order diffraction intensities and, after Fourier transform, an incomplete Patterson map. The computer simulations are implemented to obtain structural information from this map. In the simulations, the interactions between the Al overlayer atoms were described with the Sutton-Chen potential and the interactions between the overlayer and the sapphire substrate with a laterally modulated Lennard-Jones potential. We have shown that the hexagonal reconstructed unit cell is composed of triangles where the two layers of Al adatoms are FCC(111) ordered whereas between the triangles the stacking is FCC(001).

We report various experiments that cast some light on the growth process of self-organized dots and vertical Co pillars on Au(1 1 1). We first report grazing incidence small angle X-ray scattering (GISAXS) on Co dots and films, that allowed us to perform a crystallographic study of the order in the self-organized array, and also yielded a statistical view of the coalescence process. We then report scanning tunneling microscopy (STM) and spectroscopy (STS) images performed during the course of vertical growth. Details about the atomic growth process and the possible influence of the growth technique used are reported.

International audience

The structural properties of Ge islands grown by molecular beam epitaxy on SrTiO 3 (001) substrates are investigated. We report on the effect of the temperature on the epitaxial relationship and morphology of the Ge islands. By combining X-ray diffraction measurements and atomic force microscopy we evidence a correlation between the island size and shape, the structural properties of the interface and the sample texture. In particular, we show that the growth temperature affects the nature of the interface between the semiconductor and the perovskite oxide and thus the island orientation, that evolves from fully (001) oriented to a mixture of (111) and (001) oriented islands.

A detailed structural analysis of the $\mathrm{NiO}(111)\ensuremath{-}p(2\ifmmode\times\else\texttimes\fi{}2)$ surface by surface x-ray diffraction is presented and previous interpretations are revisited. The surface adopts the octopolar reconstruction after air annealing and a spinel-like configuration after UHV annealing under low oxygen pressure corresponding to a reduced state. The transformation rate is determined and discussed. Both configurations and their transformation allow drawing a coherent scheme of the behavior of this polar oxide surface.

We present a structural investigation of the polar (111) surface of CoO single crystals by grazing incidence X-ray diffraction. The surface is found to be always covered by a ∼50 A-thick spinel Co3O4 layer that was quantitatively characterized. Attempts to remove this layer by UHV annealings or Ar+ bombardments lead to the formation of metallic Co clusters on the surface that transform again in Co3O4 by annealing under oxygen.

The experimental results obtained by X-ray photoelectron spectroscopy (XPS), in-situ grazing incidence X-ray diffraction (GIXD) and grazing incidence small angle X-ray scattering (GISAXS) on the growth of silver thin films on MgO(001) surfaces have been interpreted in terms of a general model which reproduce all the data for a wide thickness range. Silver films grow along a three-dimensional Volmer–Weber growth mode. Clusters are pictured as truncated pyramids whose shape is described through parameters that vary continuously during growth, thus evidencing an evolution, which cannot be taken into account by simpler models. The corresponding growth laws for the clusters were quantitatively determined.

The structure and morphology of Ag deposits grown at room temperature on high-quality MgO(001) surfaces have been investigated in situ, from 0.2 to 300 equivalent monolayers (ML) of Ag deposited. Surface x-ray diffraction and grazing incidence small angle x-ray scattering parallel and perpendicular to the surface were combined. Nucleation, growth, and coalescence of islands are found from 0.2 ML. The average in-plane width, height, and in-plane separation of growing islands are deduced and are found to reproduce well the Mg $1s$ x-ray photoemission spectroscopy spectrum previously reported by other authors. The height over width ratio of the islands is $\ensuremath{\sim}0.37\ifmmode\pm\else\textpm\fi{}0.05$ at all stages of the deposit. Ag grows in cube-on-cube epitaxy with respect to the MgO(001) substrate. A very unusual evolution of the state of strain in Ag with increasing amount of Ag deposited is observed. Below 4--6 ML (island width smaller than 90 \AA{}), the small Ag islands are coherent with the MgO. Below 1 ML (island width smaller than 35 \AA{}), they have their bulk lattice parameter, and between 1 and 4 ML they become more and more strained by the MgO substrate, with an average lattice parameter intermediate between those of Ag and of MgO. Around 4--6 ML, the islands reach a critical size and misfit dislocations are introduced at the edges. Above 30 ML, the film is almost continuous, and the interfacial misfit dislocations reorder to form a square network, oriented along 〈110〉 directions. Stacking faults appear in Ag at this stage. A small amount of twinned Ag also starts to grow around 4 ML. This unusual evolution of the strain in the Ag islands and the following introduction of misfit dislocations are interpreted on the basis of a one-dimensional Frenkel-Kontorova model involving a very weak Ag-MgO interaction and a weak corrugation of the interatomic potential. Quantitative measurements and analysis of the MgO crystal truncation rods (CTR's) during growth were shown to provide different structural parameters of the interface that are important for theoretical calculation, especially the epitaxial site, above oxygen atoms of the substrate, and the interfacial distance $(2.52\ifmmode\pm\else\textpm\fi{}0.1\AA{}).$ The origin of the interference along the CTR's is discussed according to the strain state of the epitaxial Ag.

AlN layers were grown by molecular beam epitaxy on GaN single crystals (grown at high-hydrostatic pressure). The layers were examined in situ using RHEED, high-resolution X-ray reflectivity and X-ray diffraction at grazing incidence conditions. It was found that layers grown on single-crystal GaN had very smooth surfaces (3–5 A RMS roughness). A 100 A thick AlN layer was almost strained, while a 1500 A thick one was partially relaxed. The obtained results, which differ from previously published for AlN epitaxy on GaN on sapphire, are the first concerning lattice relaxation of AlN grown on GaN single crystals. The measurements performed for the strained AlN layer enabled us to determine the Poisson ratio for this compound ν =0.20±0.02.

The behaviour of the NiO(111) surface has been investigated by grazing incidence X-ray scattering (GIXS). We show on single crystals with an improved crystalline quality that NiO(111) is always p(2×2) reconstructed. After air annealing the structure is close to the predicted octopolar reconstruction; after in situ annealing and oxidation it is very different. The transformation of the surface is continuous with temperature but seems to be only based on two possible atomic configurations.

The growth of Pd on MgO(001) has been investigated by grazing incidence X-ray scattering. Deposits in the range of 0.2 to 183 monolayers (ML) were analyzed in situ, in UHV. The growth proceeds by nucleation and growth of islands from 0.2 ML. The coalescence starts around 18 ML and is complete at 35 ML. Pd is in cube-on-cube epitaxy with respect to MgO(001), and has a lattice parameter close to that of bulk Pd. The epitaxial site, above oxygen ions of the substrate, and the interfacial distance (2.22±0.02 A for 1 ML of Pd deposited) were determined for the first time as a function of the amount of Pd deposited, by a quantitative analysis of the substrate crystal truncation rods.

The growth of Pd on MgO(001) has been investigated by grazing incidence X-ray scattering. Deposits in the range 0.2 to 183 monolayers (ML) were analyzed in situ, in UHV. The growth proceeds by nucleation and islands growth from 0.2 ML. The coalescence starts at 18 ML and is complete at 35 ML. Pd is in cube on cube epitaxy with respect to the MgO(001) substrate, and has a lattice parameter close to that of bulk Pd. The islands are coherent with the substrate up to ∼5 ML of Pd deposited. Above 5 ML, interfacial misfit dislocations are introduced at the edges of the islands. Above 35 ML, the interfacial misfit edge dislocations reorder to form a square network. The dislocation lines are oriented along 〈110〉 directions, and the Burger's vectors are of the 1/2〈110〉 type.

The misfit dislocation network at the Ag/MgO(001) interface has been investigated by grazing incidence x-ray diffraction on Ag films of different thicknesses, between 100 and 1500 \AA{} deposited at room temperature by molecular beam epitaxy on MgO(001) surfaces with different step densities. They were analyzed after growth and after annealing at increasing temperatures. A square network of edge dislocations is found to release the 3% lattice misfit between Ag and MgO. The dislocation lines are found oriented along 〈110〉 directions, with $a/2[110]$ Burgers vector. Quantitative analysis shows that the substrate is deformed according to the elasticity theory. The as-grown Ag films contain stacking faults and twins, in amounts that increase with increasing substrate surface step density. Annealing the film at low temperature 300 \ifmmode^\circ\else\textdegree\fi{}C eliminates the stacking faults. Annealing at higher temperatures induces a recrystallization of the Ag epilayers, as well as an improved ordering of the dislocation network. For increasing temperatures, the period of the dislocation network instantaneously adapts to the actual lattice parameter misfit. After cooling, a large residual deformation is found in Ag, which is smaller when the substrate step density is larger. This is interpreted in terms of an energetic barrier for the nucleation of misfit dislocations.

The behaviour of the NiO(111) surface has been investigated by grazing incident X-ray scattering. After air annealing and prior to any in-situ treatment, the surface is unreconstructed. We discuss the stability of this polar surface and show that NiO(111) surfaces of good crytalline quality can be prepared and investigated by grazing incident X-ray scattering. The (1×1) surface is shown to be stabilized by the adsorption of light elements.

The structure of MgO(001) single crystal surfaces that are of very high crystalline quality, extremely flat, clean and stoichiometric, was investigated by grazing incidence X-ray scattering. A new procedure for preparing these surfaces is described, which includes high-temperature annealing in air followed by high-temperature ion bombardment, and annealing under partial oxygen pressure. The relaxation, −0.56±0.35%, and rumpling, 1.07±0.5%, were determined with a much better accuracy than in any previous experimental study. These values are compared with those given in previous experimental and theoretical studies. The atomic structure of the Ca-segregated MgO(001) surface was also investigated. The most likely structure is the theoretically predicted ( 2 × 2 ) R45° reconstruction. Atomic displacements within this unit cell were determined.

We have investigated the unreconstructed (0001) surface structure of sapphire (α-Al2O3) by Grazing Incidence X-ray Scattering. Modulations along the crystal truncation rods were analyzed in order to determine the chemical nature of the terminating plane, and the structural relaxations of the first few atomic planes below the surface. The most likely model yields a single Al layer termination with relaxations of the first four planes of -51%, +16%, -29% and +20% respectively. These results compare well with the most recent theoretical calculations on this surface.

The growth of Ag on MgO(001) was investigated from sub monolayer up to 600 Å coverage. A new method for the preparation of very flat, clean and stoichiometric MgO(001) surfaces is described. The growth is found to be intrinsic, with negligible influence of surface defects. Twins appear at the very first stages of deposition while stacking faults form during the coalescence. A detailed mechanism is proposed for the elastic and plastic misfit relaxation processes. The introduction of a new dislocation within an island is, whatever its size, preceded by a strong deformation of its edges. Plastic relaxation is shown to be a continuous process that starts well before coalescence and the appearance of an interfacial dislocation network.

Experimental determinations of the atomic structure of insulating oxide surfaces and metal/oxide interfaces are scarce, because surface science techniques are often limited by the insulating character of the substrate. Grazing incidence X-ray scattering (GIXS), which is not subject to charge effects, can provide very precise information on the atomic structure of oxide surfaces: roughness, relaxation and reconstruction. It is also well adapted to analyze the atomic structure, the registry, the misfit relaxation, elastic or plastic, the growth mode and the morphology of metal/oxide interfaces during their growth, performed in situ. GIXS also allows the analysis of thin films and buried interfaces, in a non-destructive way, yielding the epitaxial relationships, and, by variation of the grazing incidence angle, the lattice parameter relaxation along the growth direction. On semi-coherent interfaces, the existence of an ordered network of interfacial misfit dislocations can be demonstrated, its Burger's vector determined, its ordering during in situ annealing cycles followed, and sometimes even its atomic structure can be addressed. Careful analysis during growth allows the modeling of the dislocation nucleation process. This review emphasizes the new information that GIXS can bring to oxide surfaces and metal/oxide interfaces by comparison with other surface science techniques. The principles of X-ray diffraction by surfaces and interfaces are recalled, together with the advantages and properties of grazing angles. The specific experimental requirements are discussed. Recent results are presented on the determination of the atomic structure of relaxed or reconstructed oxide surfaces. A description of results obtained during the in situ growth of metal on oxide surfaces is also given, as well as investigations of thick metal films on oxide surfaces, with lattice parameter misfit relaxed by an array of dislocations. Recent work performed on oxide thin films having important physical properties such as superconductivity or magnetism is also briefly reviewed. The strengths and limitations of the technique, such as the need for single crystals and surfaces of high crystalline quality are discussed. Finally, an outlook of future prospects in the field is given, such as the study of more complex oxide surfaces, vicinal surfaces, reactive metal/oxide interfaces, metal oxidation processes, the use of surfactants to promote wetting of a metal deposited on an oxide surface or the study of oxide/liquid interfaces in a non-UHV environment.

The story of some not yet solved problems is presented, putting emphasis on the respective role of instrumentation, ab initio theories, phenomenology and simulations.

We report an in situ grazing incidence X-ray scattering study of the NiO(111) single crystal surface. It evidences a relatively high stability of this surface. After an air anneal at 1300 K it is flat and almost not reconstructed. Annealing at 860 K in ultra-high vacuum leads to two domains: one covered by epitaxial and relaxed metallic Ni clusters associated with a highly ordered interfacial supercell on the coincidence site lattice, presumably a misfit dislocation network and one exhibiting a p(2 × 2) reconstruction. After oxidation the metallic Ni transforms again into NiO(111) and only a p(2 × 2) reconstructed flat surface remains. In these conditions macroscopic (100) facetting was never observed. The general behaviour of the NiO(111) surface is discussed and compared with other polar and non-polar oxide surfaces, with emphasis on the specific properties which could explain its stability.

International audience; The effects of the temperature on the structure of a single layer of graphene on Ir(111) have been investigated in situ in the growth chamber by grazing incidence x-ray diffraction between 10 and 1300 K. In addition, the effect of two growth temperatures has been studied. The graphene lattice parameter of the main, nonrotated phase displays a characteristic hysteresis whose lower and upper branches correspond to phases in which integer numbers (m,n) Ir of the Ir lattice parameters match integer numbers (p,q) Gr of the graphene lattice parameters, i.e., commensurate phases, (19,0) Ir × (21,0) Gr and (9,0) Ir × (10,0) Gr , respectively. With a higher growth temperature, graphene presents, in addition to a main nonrotated, incommensurate phase, domains with a small rotation (2.37 •) relative to the substrate, corresponding to a (7,2) Ir × (8,2) Gr commensurate phase. Despite the weak interaction between graphene and iridium, the thermal expansion coefficient of graphene is positive at all temperatures, even at liquid helium temperature contrary to free-standing graphene. It is also close to the iridium thermal expansion because of the tendency to form commensurate phases, which creates a strong link between the graphene and substrate lattices and decreases the elastic energy.

We report here an interface accommodation mechanism observed by using in situ grazing incidence X-ray diffraction in the very early stages of Ge epitaxial growth on SrTiO3. This mechanism leads to interface-localized misfit accommodation and involves two regimes: very early dislocation emergence followed by a damped collective oscillatory lattice parameter evolution. We show that this behavior is compatible with the simplest nonlinear Frenkel-Kontorova model assuming the weak elastic-chain/substrate interaction.

The structure and morphology of the Ag/MgO(001) interface have been investigated in situ by grazing incidence X-ray diffraction during the first stages of Ag deposition by molecular beam epitaxy. At the very beginning, Ag is perfectly registered, consisting of wide, one monolayer high platelets that cover half of the surface. The growth then proceeds in the form of islands, with Ag relaxation on surfaces. Finally, plastic relaxation occurs through a misfit dislocation network. The epitaxial site (on top of oxygen atoms) and interfacial distance are experimentally determined for the first time. Comparison with previous theoretical results on this model metal/insulator interface shows that the most recent ab initio calculations predict the correct interfacial structure.

We investigate the fine structure of graphene on iridium, which is a model for graphene weakly interacting with a transition-metal substrate. Even the highest-quality epitaxial graphene displays tiny imperfections, i.e., small biaxial strains of $\ensuremath{\sim}$0.3$%$, rotations of $\ensuremath{\sim}$0.5${}^{\ensuremath{\circ}}$, and shears over distances of $\ensuremath{\sim}$100 nm, and is found incommensurate, as revealed by x-ray diffraction and scanning tunneling microscopy. These structural variations are mostly induced by the increase of the lattice parameter mismatch when cooling the sample from the graphene preparation temperature to the measurement temperature. Although graphene weakly interacts with iridium, its thermal expansion is found to be positive, contrary to free-standing graphene. The structure of graphene and its variations is very sensitive to the preparation conditions. All these effects are consistent with initial growth and subsequent pinning of graphene at steps.

International audience; Using in situ x-ray diffraction, epitaxial relationships have been measured for different Au deposits on Si(111) surfaces, under ultrahigh vacuum conditions, from room temperature to the eutectic temperature (T-e) of theAuSi binary system. Epitaxies perpendicular to the substrate have been studied for different gold coverages (2, 5, 7, and 20 monolayers). The Au(111) parallel to Si(111) out-of-plane epitaxy was found to be dominant and to present three different in-plane orientation relationships evolving with increasing temperature. These epitaxial variations are induced by the variation of the lattice-parameter misfit with temperature due to differing thermal expansion of the two materials. The temperature dependence of the epitaxial relationship is explained using the coincidence site lattice theory and a study of the "matching quality" at the interface. In addition, ab initio calculations have been performed to estimate the stability of the different configurations and compared to the experimental results

Plastic relaxation and formation of defects are crucial issues in the epitaxial growth of nanoparticles and thin films. Indeed, defects generate local stress in the crystalline lattice, which affects their surroundings and may lead to undesired effects such as reduced charge-carrier lifetime or nonradiative recombinations. Here, we use a nondestructive method based on x-ray diffuse scattering close to forbidden reflections to identify the defect types with a high sensitivity and quantify their average size and strain field. Combined with transmission electron microscopy, it offers opportunities to track both ensemble average and single defects inside three-dimensional structures. These techniques have been applied to partially embedded and high-Ge-content (x{sub Ge}=0.87{+-}0.06) dots selectively grown in 20-nm-sized pits on Si(001) surfaces through openings in a SiO{sub 2} template. The stress in the 20-nm-wide Ge islands is relaxed not only by interfacial dislocations but also by microtwins and/or stacking faults located at the interface, proving the importance of {l_brace}111{r_brace} planes and twinning in the relaxation process of nanometer-size Ge dots.

The 2 \ifmmode\times\else\texttimes\fi{} $N$ reconstruction of the Ge/Si(001) wetting layer has been investigated by surface x-ray diffraction. At a substrate temperature of $670 {}^{ˆ}\mathrm{C}$, the average $N$ periodicity decreases from $N$ $=$ 11.5 to 8 with an increasing Ge coverage from one to three monolayers (ML). The top layer consists of asymmetric dimers with a bond length in the range of 2.50--2.60 \AA{} and a buckling angle in the range of 9.4\ifmmode^\circ\else\textdegree\fi{}--15.6\ifmmode^\circ\else\textdegree\fi{}, depending on the Ge coverage. The obtained dimer bond lengths are similar to those calculated for alternating asymmetric mixed dimers. Intermixing of Ge with Si is found down to the sixth (eighth) layer for 2 (from 3 to 5) ML coverage. For 2 ML coverage, a quantitative surface x-ray diffraction data set has been measured. It is analyzed using a model describing the atomic structure and Ge occupation probability with a limited set of parameters to bypass the intrinsic lack of appreciable reflections of the 2 \ifmmode\times\else\texttimes\fi{} $N$ ($N$ $=$ 9) reconstruction. The Ge occupation probability varies periodically along the $N$ direction, having its minimum value below the dimer vacancy lines. In addition, a more direct calculation of the Lorentz and detector acceptance corrections is given for rocking and radial scans.

The growth and annealing behavior of strongly twinned homoepitaxial films on Ir(111) has been investigated by scanning tunneling microscopy, low energy electron diffraction and surface X-ray diffraction. In situ surface X-ray diffraction during and after film growth turned out to be an efficient tool for the determination of twin fractions in multilayer films and to uncover the nature of side twin boundaries. The annealing of the twin structures is shown to take place in a two step process, reducing first the length of the boundaries between differently stacked areas and only then the twins themselves. A model for the structure of the side twin boundaries is proposed which is consistent with both the scanning tunneling microscopy and surface X-ray diffraction data., 13 pages, 11 figures

The molecular-beam epitaxy growth of AlN/GaN nanowire superlattices has been studied by using a combination of in situ x-ray diffraction experiments, high-resolution electron-microscopy analysis and theoretical calculations performed in a valence force field approach. It is found that the nanowire superlattices are in elastic equilibrium, in contrast with the two-dimensional case but in line with the predicted increase in the critical thickness in the nanowire geometry.

The kinetics of the growth of Ge superdomes and their facets on Si(001) surfaces are analyzed as a function of deposited Ge thickness for different growth temperatures and at a low growth rate by in situ grazing-incidence small-angle x-ray scattering in combination with in situ grazing-incidence x-ray diffraction. At a low growth rate, intermixing is found to be enhanced and superdomes are formed already at lower coverages than previously reported. In addition, we observe that at the dome-to-superdome transition, a large amount of material is transferred into dislocated islands, either by dome coalescence or by anomalous coarsening. Once dislocated islands are formed, island coalescence is a rare event and introduction of dislocations is preferred. The superdome growth is thus stabilized by the insertion of dislocations during growth.

The growth of Ag/MgO(001) was investigated at three temperatures, 300, 540, and 640 K, in situ, in ultrahigh vacuum, by grazing incidence small angle x-ray scattering (GISAXS). The samples prepared at 300 and 540 K were also examined ex situ by transmission electron microscopy (TEM). Nucleation, growth, and coalescence are studied via the evolution of the average particle diameter, height, interparticle distance, and size distributions. Power laws are given. Size-spacing correlations are evidenced by TEM and GISAXS. A heterogeneous nucleation of silver on the defects $[(7.5\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\text{defects}\text{ }{\text{cm}}^{\ensuremath{-}2}]$ of the MgO surface is evidenced. Above a silver coverage of approximately 2 nm, a dynamic coalescence regime takes place involving diffusion of the islands. A value of $0.76\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{J}\text{ }{\text{m}}^{\ensuremath{-}2}$ for the Ag/MgO(001) adhesion energy is derived by using a Wulff-Kaishew construction from the particle shape obtained by GISAXS. The diffusion energy of Ag on MgO(001) is estimated to be $0.05\ifmmode\pm\else\textpm\fi{}0.02\text{ }\text{eV}/\text{atom}$ via kinetic Monte Carlo simulations of the growth stage.

Nanoscience and nanotechnology are tremendously increasing fields of research that aim at producing, characterizing and understanding nanoobjects and assemblies of nanoobjects. Their new physical or chemical properties, which arise from confinement effects, intimately depend on their morphological properties, i.e. their shapes, their sizes and their spatial organization. This calls for dedicated morphological characterization tools, among which is the Grazing Incidence Small Angle X-Ray Scattering (GISAXS). This reciprocal space technique has emerged in the last two decades as a powerful tool that allows investigating in a non-destructive way the morphological properties from one to billions of nanoparticles, either on a surface, or embedded in a matrix, with sizes ranging from 1 nm to several microns. The advantages of the technique are that it is non-destructive; it yields statistical information averaged on a large number of nanoparticles; it allows probing both the surface or deep below it, by changing the incident angle of the X-ray beam; it can be used in very different sample environments, in particular in situ in the course of a given process such as growth, annealing, gas exposure; and it may be given chemical sensitivity by use of anomalous scattering. This report presents a review of the GISAXS technique, from experimental issues to the theories underlying the data analysis, with a wealth of examples. The physical morphological information contained in GISAXS data and its analysis are presented in simple terms, introducing the notions of particle form factor and interference function, together with the different cases encountered according to the size/shape dispersion. The theoretical background of X-ray diffuse scattering under grazing incidence is presented in a general way, and then applied to the particular case of grazing incidence small angle X-ray scattering from assemblies of particles either on a substrate, or buried below it. Most of the GISAXS measurements published to date are reported, covering the fields of ex situ studies of embedded metallic nanoparticles, granular multilayered systems, implanted systems, embedded or stacked or deposited semi-conductor nanostructures, porous materials and copolymer thin films. A special emphasis is brought on in situ experiments, performed either in ultra-high vacuum during nanoparticle growth by molecular beam epitaxy, or in gas-reactors during catalytic reactions. This covers a very broad field, from (i) the 3D island (Volmer–Weber) growth of metals on oxides surfaces to (ii) the organized growth of metals on surfaces that are nanopatterned either by surface reconstruction or by underlying dislocation networks or by deposit-induced nanofacetting, to (iii) the in situ investigation of the self-organized Stranski–Krastanow hetero-epitaxial growth of semi-conductor quantum dots on semi-conductor surfaces, or (iv) the in situ surface nanopatterning by ion bombardment. Many examples are discussed in detail, to illustrate the large diversity of systems and morphologies that can be addressed as well as the different analysis issues and the conclusions of the technique in terms of growth mode.