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International audience; A number of renormalization schemes for improving the convergence of multiple scattering series expansions are investigated. Numerical tests on a small Cu(111) cluster demonstrate their effectiveness, for example increasing the rate of convergence by up to a factor 2 or by transforming a divergent series into a convergent one. These techniques can greatly facilitate multiple scattering calculations, especially for spectroscopies such as photoelectron diffraction, Auger electron diffraction, low energy electron diffraction , where an electron propagates with a kinetic energy of hundreds of eV in a cluster of hundreds of atoms.

International audience; In this paper, we present the first steps of a process toward the development of MoS2/Si heterojunctions photovoltaics, using 2D 2H-MoS2, whose natural abundance and tunable bandgap make it suitable for such application. A focus is made here on the optimization of the MoS2 material and its deposition process, through preliminary optical and structural characterizations of thin 2H-MoS2 layers deposited on 80nm SiO2 on top of Si (001) substrates. Our investigations revealed oxidation of the MoS2 layers, and limited longitudinal crystallite size, which may strongly affect the band lineup between MoS2 and Si, and thus, the performance of the solar cell.

Producción Científica, The luminescence of SrTiO3 depends on the sample type, either doped or stoichiometric, as grown or treated, the excitation conditions, and temperature. The origin of the luminescence emissions, blue, green, and infrared, remains controversial. In particular, the role played by defects, mainly oxygen vacancies, impurities, and self-trapped holes and electrons on the different emissions are far to be elucidated. We present a cathodoluminescence (CL) and photoluminescence (PL) study of undoped and Nb-doped samples. The different excitation conditions of CL and PL permit to distinguish the luminescence emission from the bulk (CL) and from a surface skin region (PL). Significant differences between both techniques are seen for the undoped sample, while the Nb-doped sample presents less differences, highlighting the role played by the surface defects and the doped electrons. The study is complemented by the temperature dependence of the luminescence spectra and the emission due to defects generated by plastic deformation., Junta de Castilla y León (project VA283P18), Ministerio de Economía, Industria y Competitividad (project ENE2017-89561-C4-3-R)

Hybrid materials taking advantage of the different physical properties of materials are highly attractive for numerous applications in today's science and technology. Here, it is demonstrated that epitaxial bi‐domain III–V/Si are hybrid structures, composed of bulk photo‐active semiconductors with 2D topological semi‐metallic vertical inclusions, endowed with ambipolar properties. By combining structural, transport, and photoelectrochemical characterizations with first‐principle calculations, it is shown that the bi‐domain III–V/Si materials are able within the same layer to absorb light efficiently, separate laterally the photo‐generated carriers, transfer them to semimetal singularities, and ease extraction of both electrons and holes vertically, leading to efficient carrier collection. Besides, the original topological properties of the 2D semi‐metallic inclusions are also discussed. This comb‐like heterostructure not only merges the superior optical properties of semiconductors with good transport properties of metallic materials, but also combines the high efficiency and tunability afforded by III–V inorganic bulk materials with the flexible management of nano‐scale charge carriers usually offered by blends of organic materials. Physical properties of these novel hybrid heterostructures can be of great interest for energy harvesting, photonic, electronic or computing devices., Here, it is demonstrated that epitaxial bi‐domain III–V/Si are hybrid structures, composed of bulk photo‐active semiconductors with 2D topological semi metallic vertical inclusions, endowed with ambipolar properties. Operating III–V/Si photoelectrodes confirm that this hybrid material can by itself photogenerate and laterally separate carriers, which are efficiently extracted afterward from the photoelectric device.

The mechanisms governing the formation of Schottky barriers at graphene/hydrogen-passivated silicon interfaces where the graphene plays the role of a two-dimensional (2D) metal electrode have been investigated by means of x-ray photoemission spectroscopy and density functional theory (DFT) calculations. To control the graphene work function without altering either the structure or the band dispersion of graphene we used a method that consists in depositing small amounts of gold forming clusters on the graphene/hydrogen-passivated silicon system under an ultra-high-vacuum environment. We observe from experimental measurements that the Fermi level is mainly free from pinning at the graphene/hydrogen-silicon interface whereas for a semi-infinite metal on silicon the Fermi level is almost fully pinned. This alleviation of the Fermi level pinning observed with the graphene layer is explained by DFT calculations showing that the graphene and the semiconductor are decoupled and that the metal-induced gap states (MIGS) density at the silicon midgap at the interface is very low ($l5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.16em}{0ex}}\mathrm{states}/(\mathrm{eV}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2})$]. The important conclusion that stems from the DFT results analysis is that the low MIGS density at the semiconductor midgap is related to the 2D nature of the graphene layer. More precisely, the MIGS density is low owing to the lack of propagating states perpendicular to the graphene layer. This finding brings important information to understand the mechanisms that govern the formation and the electronic properties of Schottky barriers at 2D-metal/three-dimensional-semiconductor interfaces.

Schottky barrier formation at metal/insulating oxide interfaces relies on complex mechanisms which are difficult to unravel. We propose a detailed numerical study of the atomic, magnetic, and electronic properties of the $\mathrm{Fe}/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}(001)$ interface, in which we focused our discussion on different parameters which can affect the Schottky barrier height (SBH). The interface termination appears to be the most critical aspect to be controlled for this interface: While an ideal $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated interface would guarantee a $n$-type barrier of about 1.2--1.6 eV, the presence of a SrO termination can drastically decrease its value down to few meV. The oxidation state of the interface is also an important criterium to maintain a high barrier value. Oxygen vacancies are always cited as the source of a deterioration of the SBH. For a $\mathrm{Ti}{\mathrm{O}}_{2}$-terminated interface, we found that in their most stable position, i.e., in the interface layer, the oxygen vacancies do not affect the value of the SBH; when moving some atomic layers away from the interface, the SBH on the contrary decreases regularly. We propose that oxidizing the interface would allow us to improve the $n$-type SBH by healing the oxygen vacancies and forming an interfacial FeO layer, which seems favorable to the formation of a higher SBH.

International audience; Ultrathin MgO films grown on Ag(001) have been investigated using X-ray and ultraviolet photoemission spectroscopies for oxide films successively exposed to Mg and O-2 flux. Studying work functions and layer-resolved Auger shifts allows us to keep track of band profiles from the oxide surface to the interface and reveal the charge-transfer mechanisms underlying the controlled creation of Mg-induced surface color centers and the catalytic enhancement of O-2 activation. Our results demonstrate that one can intimately probe the catalytic properties of metal-supported ultrathin oxide films by studying the electronic band alignment at interfaces.

International audience; The interface resistance at metal/semiconductor junctions has been a key issue for decades. The control of this resistance is dependent on the possibility to tune the Schottky barrier height. However, Fermi level pinning in these systems forbids a total control over interface resistance. The introduction of 2D crystals between semiconductor surfaces and metals may be an interesting route towards this goal. In this work, we study the influence of the introduction of a graphene monolayer between a metal and silicon on the Schottky barrier height. We used X-ray photoemission spectroscopy to rule out the presence of oxides at the interface, the absence of pinning of the Fermi level and the strong reduction of the Schottky barrier height. We then performed a multiscale transport analysis to determine the transport mechanism. The consistency in the measured barrier height at different scales confirms the good quality of our junctions and the role of graphene in the drastic reduction of the barrier height.

International audience; Due to the difficulty of growing high-quality semiconductors on ferromagnetic metals, the study of spin diffusion transport in Si was limited to lateral geometry devices. In this work, by using an ultrahigh-vacuum wafer-bonding technique, we have successfully fabricated metal–semiconductor–metal CoFeB/MgO/Si/Pt vertical structures. We hereby demonstrate pure spin-current injection and transport in the perpendicular current flow geometry over a distance larger than 2 μm in n-type Si at room temperature. In those experiments, a pure propagating spin current is generated via ferromagnetic resonance spin pumping and converted into a measurable voltage by using the inverse spin Hall effect occurring in the top Pt layer. A systematic study varying both Si and MgO thicknesses reveals the important role played by the localized states at the MgO–Si interface for the spin-current generation. Proximity effects involving indirect exchange interactions between the ferromagnet and the MgO–Si interface states appears to be a prerequisite to establishing the necessary out-of-equilibrium spin population in Si under the spin-pumping action.

International audience; Fermi level pinning at metal/semiconductor interfaces forbids a total control over the Schottky barrier height. 2D materials may be an interesting route to circumvent this problem. As they weakly interact with their substrate through Van der Waals forces, deposition of 2D materials avoids the formation of the large density of state at the semiconductor interface often responsible for Fermi level pinning. Here, we demonstrate the possibility to alleviate Fermi-level pinning and reduce the Schottky barrier height by the association of surface passivation of germanium with the deposition of 2D graphene

We have investigated the interface formation at room temperature between Fe and ${\mathrm{TiO}}_{2}$-terminated ${\mathrm{SrTiO}}_{3}(001)$ surface using x-ray photoelectron spectroscopy. Oxygen vacancies within the ${\mathrm{SrTiO}}_{3}$ lattice in the first planes beneath the $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_{3}$ interface are induced by the Fe deposition. Through a detailed analysis of the Fe $2p$, Sr $3d$, and Ti $2p$ core-level line shapes we propose a quantitative description of the impact of the vacancies on the electronic properties of the $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_{3}$ system. While for an abrupt $\mathrm{Fe}\text{/}{\mathrm{SrTiO}}_{3}$ junction the Schottky barrier height for electrons is expected to be about 1 eV, we find that the presence of oxygen vacancies leads to a much lower barrier height value of 0.05 eV. The deposition of a fraction of Fe monolayer also pushes the surface conduction band edge of the ${\mathrm{SrTiO}}_{3}$ below the Fermi level in favor of the formation of a surface electron accumulation layer. This change in the band bending stems from the incorporation of oxygen vacancies in the near-surface region of ${\mathrm{SrTiO}}_{3}(001)$. We deduce the conduction band profile as well as the carrier density in the accumulation layer as a function of the surface potential by solving the one-dimensional Poisson equation within the modified Thomas-Fermi approximation. Owing to the electric-field dependence of the dielectric permittivity, the ${\mathrm{SrTiO}}_{3}$ with oxygen vacancies at the surface shows original electronic properties. In particular, our simulations reveal that variations of a few percent of the vacancies concentration at the surface can cause changes of several tenths of an eV in the band bending that can lead to important lateral surface inhomogeneities for the potential. We also find through our modeling that the defect states density related to oxygen vacancies at the ${\mathrm{SrTiO}}_{3}$ surface cannot exceed, at room temperature, a critical value of $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{13}/\mathrm{c}{\mathrm{m}}^{2}$.

We investigate the influence of the MgO growth process on the bias dependence of the electrical spin injection from a Co-Fe-B=MgO spin injector into a GaAs-based light-emitting diode (spin LED). With this aim, textured MgO tunnel barriers are fabricated either by sputtering or molecular-beam-epitaxy (MBE) methods. For the given growth parameters used for the two techniques, we observe that the circular polarization of the electroluminescence emitted by spin LEDs is rather stable as a function of the injected current or applied bias for the samples with sputtered tunnel barriers, whereas the corresponding circular polarization decreases abruptly for tunnel barriers grown by MBE. We attribute these different behaviors to the different kinetic energies of the injected carriers linked to differing amplitudes of the parasitic hole current flowing from GaAs to Co-Fe-B in both cases.

International audience; The control of tunnel contact resistance is of primary importance for semiconductor-based spintronic devices. This control is hardly achieved with conventional oxide-based tunnel barriers due to deposition-induced interface states. Manipulation of single 2D atomic crystals (such as graphene sheets) weakly interacting with their substrate might represent an alternative and efficient way to design new heterostructures for a variety of different purposes including spin injection into semiconductors. In the present paper, we study by x-ray photoemission spectroscopy the band alignments and interface chemistry of iron-graphene-hydrogenated passivated silicon (001) surfaces for a low and a high n-doping concentration. We find that the hydrogen passivation of the Si(001) surface remains efficient even with a graphene sheet on the Si(001) surface. For both doping concentrations, the semiconductor is close to flat-band conditions which indicates that the Fermi level is unpinned on the semiconductor side of the Graphene/Si(001):H interface. When iron is deposited on the graphene/Si(001):H structures, the Schottky barrier height remains mainly unaffected by the metallic overlayer with a very low barrier height for electrons, a sought-after property in semiconductor based spintronic devices. Finally, we demonstrate that the graphene layer intercalated between the metal and semiconductor also serves as a protection against iron-silicide formation even at elevated temperatures preventing from the formation of a Si-based magnetic dead layer. © 2016 Author(s).

International audience; In the field of organic and molecular electronics at monolayer coverage, the need for abrupt and well-controlled top metal contacts is a key point. A general method which provides reliable molecular junctions with most metals remains to be found. In this paper we show that reliable molecular junctions Au/hexadecanethiol/n-GaAs(100) are obtained using buffer-layer-assisted growth (BLAG). They show in hot electron transport measurements at the nanoscale a tunnel regime through the organic monolayer with a full spatial uniformity. Using ballistic electron emission microscopy (BEEM) in the spectroscopic mode as well as photoemission and C(V)-transport measurements, we draw a coherent band alignment scheme of the whole heterostructure at the nanoscale and at the macroscopic scale. Through this study, the BLAG method appears as a general method that should work for contacting organic monolayers with most metals. © 2016 American Chemical Society.

Using reflection high-energy electron diffraction as well as ultraviolet and X-ray photoemission spectroscopy we have investigated the growth of epitaxial Fe ultrathin films onto Al 0.48 In 0.52 (001)-(2 x 4) surface. The Fe films grow in a body centered cubic (bcc) structure with epitaxial relationship Fe(001) //Al 0.48 In 0.52 As (001) . The analysis of the photoemission data demonstrates that Fe atoms react with the Al 0.48 In 0.52 As substrate. In and As atoms, liberated during the first stage of the growth, tend to segregate at the films surface while reacting Al atoms are accommodated in an interfacial alloy. The Fermi level pinning position at the Fe/Al 0.48 In 0.52 As (001) interface, determined from the photoemission results, is found 0.76 +/- 0.08eV below the conduction band minimum.

The Schottky barrier heights (SBH) at various metal-MgO interfaces (Mg, Al, Ni, Fe, Pd, and Ag) were studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). In order to determine the Schottky barrier height, the energy distance between the 02s core level of MgO and the MgO valence band maximum (VBM) is determined to be 18.0 ± 0.1 eV at first. The Schottky barrier heights variation with metal Pauling electronegativities is linear fitted with a slope parameter of 1.86 ± 0.15.

The surface structure obtained by deposition of a Ag monolayer on the ideal c(2 × 2) antiferromagnetic Mn monolayer on Ag(001) at 100 K and subsequent annealing at room temperature is determined by low energy electron diffraction. It is established that this system is actually a good realization of an inverted monolayer, i.e. a pseudomorphic Ag/Mn/Ag(001) structure that corresponds to a reversed composition of the two topmost layers with respect to the Mn overlayer. The Ag–Mn and Mn–Ag interlayer distances, d12 = 1.97 ± 0.015 Å and d23 = 1.97 ± 0.02 Å respectively, indicate only a fairly small contraction of ~ 3.5% (~ 1.5%) with respect to the ideal Ag bulk lattice (Mn monolayer on top) as compared to ~ 10% expected from atomic radii in bulk Mn and Ag. This clearly reveals a spectacular magnetovolume effect related to the high spin state of Mn in this two-dimensional structure.

We have measured the polar scans from a cubic GaN(001) single crystal by X-ray photoelectron diffraction (XPD) with high angular resolution. The intensities were recorded within (100), (110) and (110) substrate planes from Mg Ka excited N 1s and Ga 3d core levels. The data reveal unexpected shapes in forward focusing peaks for the N is XPD patterns. Along [001] and [111] directions a large splitting in the structures is observed. The observed patterns are reproduced very well by a multiple scattering spherical wave cluster calculation but the single scattering approach is sufficient to qualitatively explain the remarkable features in the N Is profiles. In particular we demonstrate that the splitting of the structures along some low index crystallographic directions is caused by the predominance of interference phenomena due to atoms located in specific positions close to the dense atomic chains. As a result of the smaller forward scattering amplitude of the N atoms as compared to that of the Ga atoms, the interference effects are strongly reinforced when there are Ga atoms close to N atomic chains. In the Ga 3d XPD patterns these interference effects also appear but forward scattering still prevails along low index directions.

We present a dynamical low-energy electron diffraction analysis of the atomic and magnetic structure obtained by deposition of 0.9 Mn monolayers on Ag(001) held at 100 K. We find that the film grows in an essentially flat ideal monoatomic layer with the Mn located in fourfold hollow sites of the Ag(001) substrate that undergoes no sizeable reconstruction of its topmost layers. The Mn-Ag interlayer distance ${d}_{12}=2.00\ifmmode\pm\else\textpm\fi{}0.03\AA{}$ is found to be much larger than \ensuremath{\sim}1.85 \AA{} expected from simple addition of atomic radii in elemental \ensuremath{\alpha}-Mn and face-centered-cubic (fcc) Ag. This is direct evidence of the strong reduction in Mn $3d$ contribution to cohesion due to the atomiclike high-spin state of the Mn in the monolayer structure. Taking into account the exchange scattering by means of a local von Barth-Hedin exchange-correlation potential, we demonstrate that the weak intensity of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure reflection intensities $I(E)$ observed at low energies only $(El~120\mathrm{eV})$ can be quite well explained in terms of a two-dimensional antiferromagnetic arrangement of the Mn moments. Nonmagnetic origins of these reflections such as out of plane (buckling) or in plane displacive reconstructions of the Mn monolayer as well as surface alloying with Ag or impurity related superstructures can be safely ruled out. Evidence of a $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic to $p(1\ifmmode\times\else\texttimes\fi{}1)$ paramagnetic phase transition is also presented.

Ultra-thin Mn films have been evaporated in ultra-high vacuum on a Ag(001) substrate kept at temperatures between 210 and 340 K. A c(2×2) LEED (low-energy electron diffraction) diagram is obtained due to the formation of a two-layer-thick superficial Mn–Ag alloy. The intensity and width of the half integer spots are investigated as a function of coverage, deposition temperature, and time elapsed after Mn deposition. The brightest and sharpest diagram is obtained for ∼1.5 monolayers deposited at room temperature, which corresponds to a surface plane made of c(2×2) domains formed by Mn and Ag atoms in a checkerboard arrangement, while the subsurface plane is saturated in Mn. Depending on the preparation conditions, various c(2×2) surface alloys may be obtained that mainly differ in the MnxAg1−x composition of the second atomic plane. A comparison of the time evolution of the LEED spot intensity with ion scattering spectroscopy data reveals that two different thermally activated phenomena determine the formation and nature of the c(2×2) superstructure: on the one hand, part of the deposited Mn atoms must exchange with Ag atoms from the substrate first plane, and on the other hand, the Mn and Ag atoms of the film surface plane diffuse laterally so that nucleation and growth of the c(2×2) domains take place.

We observe a complex shape of the Mn2p core level photoemission spectra for Mn in structures of reduced dimensionality such as an ideal monolayer on Ag(001). The Mn2p 3/2 and Mn2p 1/2 spectral intensity is found to split into doublets with separation about 4 and 5 eV, respectively. The data indicate an interpretation in terms of a poorly screened and a well screened final state of the Mn with a core hole. In the monolayer range these two final states have comparable intensities and dominant contributions from atomic 2p 5 3d 5 and 2p 5 3d 6 configurations, respectively. Their intensity ratio is found to be extremely sensitive to the Mn atomic environment and directly reflects the degree of the Mn3d localization, electron correlation and reduction in charge fluctuations. Additional fine structure is clearly visible in high-resolution data for both final state 3d electrons counts and can be assigned to direct and exchange Coulomb interactions of the unfilled core and 3d shells. Slow 3d-charge fluctuations also result in a remarkable three-peak structure of the Mn3s line.

We studied the magnetic properties of ultra-thin Mn films deposited on Ag (001) held at 80 K with soft X-ray absorption and magnetic circular dichroism. The observed shape and branching ratio of the Mn 2p absorption edge as a function of Mn coverage demonstrate that, up to \(\), the Mn adopts a stable high spin state similar to the Mn atom Hund's rule 6S5/2 ground state. Above this coverage a rapid transition from localized high spin to itinerant low spin behavior of the Mn 3d electrons is evidenced. Magnetic circular dichroism shows no sign of long range ferromagnetic order in these films at 80 K. The data, first confirm the large atomic-like local magnetic moment, and second are in line with the in-plane \(\) antiferromagnetic order, reported recently (Phys. Rev. B 57, 1141 (1998)), for Mn in the nearly ideal on-top Mn monolayer formed by 0.9 ML deposited at 80 K.

The determination of the atomic structure of a Mn monolayer deposited at room temperature on Ag(001) was achieved by using surface extended x-ray absorption fine structure. Keeping in mind the previous results of x-ray photoelectron diffraction, several structural models involving the two topmost layers of the Ag substrate were tested in ab initio polarization-dependent XAFS calculations. Among these models, the most consistent one with the experimental data is made of a mixture of two different local environments that correspond to two different structures: first an inverted layer of Mn atoms substituting Ag ones underneath the topmost plane in fcc Ag; second a ${\mathrm{Ag}}_{0.5}{\mathrm{Mn}}_{0.5}$ surface alloy confined to the two topmost layers of Ag substrate. This latter arrangement corresponds to the observed $c(2\ifmmode\times\else\texttimes\fi{}2)$ low-energy electron diffraction long-range order. Accurate values for the Mn-Ag and Mn-Mn first interatomic distances were derived from standard analysis and found to be $2.86\ifmmode\pm\else\textpm\fi{}0.02$ and $2.88\ifmmode\pm\else\textpm\fi{}0.02\AA{},$ respectively. This definitely establishes that, in this surface alloy, Mn adopts an unusually large atomic volume, essentially the same as Ag. This finding is consistent with a high spin state of the Mn similar to the one in dilute Ag-based alloys or in the atomic ${}^{6}{S}_{5/2}$ ground state.

Mn films with a thickness of 2–15 monolayers (ML) have been deposited, in ultra-high vacuum, on a Ag(001) single crystal and analyzed by low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). When a 12 ML film is deposited at room temperature (RT), a ∼3 ML-thick diffuse interface is formed, and Ag segregation occurs on top of an epitaxial body centered tetragonal (bct) Mn layer. If the same amount of Mn is deposited at 80 K in order to block interdiffusion, the film grows in a polycrystalline α-Mn phase. Annealing the sample at 300 K does not markedly change its structure. However, when a 3 ML template of Mn is first deposited on the substrate kept at 80 K, and then 9 ML at 300 K, the data show that the film is pseudomorphic and adopts an epitaxial bct phase, and that the system exhibits an abrupt interface.

The growth mode and electronic properties of ultrathin films of silver deposited at room temperature (RT) on a 9 monolayers (ML) pseudomorphic Mn film on a Ag(001) surface have been studied by means of low energy electron diffraction (LEED), X-ray photoelectron diffraction (XPD) and angle-resolved ultraviolet photoemission (ARUPS). In particular ARUPS was used to investigate the valence band structure. Initially the data show some Ag segregation on top of the Mn film grown at RT. In the 0.5–0.9 ML coverage range the Ag adlayer exhibits d-band spectral behavior characteristic of two-dimensional dispersion. Further evaporation of Ag results in the formation of Ag bilayer platelets, and > 3 ML deposition the system exhibits a three-dimensional electronic structure converging towards a bulk Ag(001)-like system. Thus the growth mode of Ag on the Mn film is found to be nearly layer by layer up to 2 ML.

We have carefully investigated the possibility of preparing a well-ordered $p(1\ifmmode\times\else\texttimes\fi{}1)$ two-dimensional Mn monolayer on Ag(001) by means of photoelectron diffraction. It is found that a flat monolayer (ML) with a good degree of perfection is actually achieved by deposition at low rates (typically 0.1--0.2 ML/min) on a substrate held at 80 K. Substrate temperatures higher than $\ensuremath{\sim}130\mathrm{K}$ invariably result in the exchange of Mn adatoms with Ag and the formation of a surface alloy. Valence-band photoemission indicates a giant atomiclike magnetic moment in the flat monolayer, essentially the same as in dilute Ag-based Mn alloys. Most interestingly, low-energy electron diffraction reveals a very sharp $p(1\ifmmode\times\else\texttimes\fi{}1)$ chemical cell pattern with weak but sizable $(\frac{1}{2},\frac{1}{2})$ extra spots visible up to about 100 eV and attributed to in-plane $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic order.

Mn films with a thickness of one monolayer have been deposited, in ultrahigh vacuum, on a Ag (100) single crystal at room temperature, and analyzed by X-ray photoelectron diffraction (XPD) and surface extended X-ray absorption fine structure (SEXAFS). The strong enhancement of the X-ray photoelectron Mn2p3/2 signal along the Ag[1100] direction indicates that a substantial fraction of the Mn atoms is located within the second topmost atomic layer. The SEXAFS data demonstrate that the Mn – Ag and Mn – Mn first neighbor distances are the same as the Ag – Ag distances in the Ag lattice; as a result, the atoms occupy Ag sites on the fcc lattice. Comparison of the experimental data with FEFF calculations indicates that a superficial Mn – Ag alloy is formed. If the film is mildly (330 K) annealed, the contrast in the XPD Mn 2p 3/2 modulations versus polar angle increases and reaches a maximum value of 60%. As confirmed by single scattering simulations, these results mean that atomic place exchange occurs between Mn and Ag atoms, and eventually the second atomic plane of the sample is constituted mainly by Mn atoms, whereas the first atomic layer is a pure Ag plane, i.e. a buried atomic Mn monolayer is formed.

Ultrathin Mn films deposited at room temperature (RT) in ultrahigh vacuum on Ag(100) were investigated using ion scattering spectroscopy (ISS), angle-resolved ultraviolet photoemission spectroscopy (ARUPS) and low-energy electron diffraction (LEED). Up to \ensuremath{\sim}1 ML, ISS and ARUPS definitely indicate that the Mn/Ag(100) interface is unstable at RT. We find that with time an Ag enrichment of the film surface takes place with eventually formation of a two-dimensional (2D) p(1\ifmmode\times\else\texttimes\fi{}1) Ag layer on top of a mixed Ag-Mn subsurface monolayer. Moreover a c(2\ifmmode\times\else\texttimes\fi{}2) LEED superstructure is observed in the 0.5\char21{}1-ML range immediately after deposition that also undergoes an evolution with increasing time and eventually disappears, leaving a p(1\ifmmode\times\else\texttimes\fi{}1) pattern, i.e., long-range c(2\ifmmode\times\else\texttimes\fi{}2) order is broken. Our observations clearly demonstrate a structural evolution of the film at RT over a period of several hours. The data suggest that in the initial stages of growth the instability of the interface is essentially due to an atomic place exchange of Mn atoms with substrate Ag atoms, both during and after deposition. Concomitantly, surface diffusion of Ag and Mn adatoms results in nucleation of 2D mixed c(2\ifmmode\times\else\texttimes\fi{}2) Ag-Mn and pure p(1\ifmmode\times\else\texttimes\fi{}1) Ag surface islands in relative amounts depending on Mn coverage and time of measurement. In particular, it is found that the c(2\ifmmode\times\else\texttimes\fi{}2) atomic order corresponds to a metastable surface structure of this kinetically hindered system formed during its evolution toward the more stable inverted layer configuration. This shows that kinetic factors play a major role in the growth of this metal on metal system near RT.

We have studied the structure of epitaxial Mn sandwiches made of ∼ 1 monolayer (ML) Ag/3 ML Mn/Ag(100) by using photoemission methods and low-energy electron diffraction (LEED). Epitaxial films of 3 ML Mn grown on Ag(100) at room temperature (RT) are found to have a body-centered-tetragonal (bct) structure. Yet, upon deposition of ∼ 1 ML Ag, remarkable changes occur in the Mn film. X-ray photoelectron diffraction data reveal that the Mn layer relaxes towards a face-centered-cubic (fcc) structure with the lattice parameter of Ag. This implies an exceptionally large atomic volume for a 3d transition metal as well as unusual electronic and magnetic properties. Actually, an evolution towards more atomic-like behaviour is clearly evidenced by valence band and Mn3s core level photoemission data.

By combining x-ray excited Auger electron diffraction experiments and multiple scattering calculations we reveal a layer-resolved shift for the Mg $K{L}_{23}{L}_{23}$ Auger transition in MgO ultrathin films (4--6 \AA{}) on Ag(001). This resolution is exploited to demonstrate the possibility of controlling Mg atom incorporation at the $\mathrm{MgO}/\mathrm{Ag}(001)$ interface by exposing the MgO films to a Mg flux. A substantial reduction of the $\mathrm{MgO}/\mathrm{Ag}(001)$ work function is observed during the exposition phase and reflects both band-offset variations at the interface and band bending effects in the oxide film.

The growth mode of Mn on Ag(100) in the monolayer range was studied using X-ray photoelectron diffraction (XPD), angle-resolved valence band photoemission and low energy electron diffraction. Our results show that MnAg intermixing takes place. The thickness of the intermixed films depends critically on the growth temperature. At room temperature the XPD data clearly indicate that Mn is located in the first and second topmost atomic layers. The growth at higher temperatures, i.e. for 1 ML deposited at 460 K, or 1 ML deposited at RT and then annealed at 460 K, results in enhanced interdiffusion and Ag segregation.

Thin Cu layers have been evaporated at room temperature on Cr films (~ 10 monolayers) deposited on a Cu(001) single crystal. The Crystallographic structure of these sandwiches is analysed by means of low-energy electron diffraction (LEED), angle-resolved ultra-violet photoemission spectroscopy (ARUPS) and X-ray photoemission spectroscopy. The data show that the attenuation of the Cr signal versus Cu deposition is much lower than expected for a uniform film growth. Moreover, the LEED pattern characteristic of the Cr film (made of Cr(110) domains with four different orientations) is visible for Cu coverages as high as 23 ML. This indicates a strong diffusion of the Cu atoms on the Cr film surface and the formation of Cu three-dimensional islands which present, according to ARUPS and X-ray photoelectron diffraction measurements, a highly disordered fcc Crystallographic structure. However the low reactivity of the sandwiches towards oxygen suggests that the whole Cr surface is wetted by at least 1 ML of Cu.

Thin Cr films (∼20 A) deposited in vacuum at 300 K on a Cu(001) single crystal have been annealed at various temperatures and analyzed by angle resolved ultraviolet photoemission, X-ray photoemission, X-ray photoelectron diffraction and low energy electron diffraction. The data show that Cu segregation occurs above 425 K, resulting in an atomic monolayer wetting the whole Cr film. At 775 K, the Cr overlayer collapses and the Cu(001) substrate surface becomes partly uncovered.

We have investigated the crystallographic structure of ultra-thin Mn films deposited at room temperature on a clean Ag(100) single crystal, using angle-resolved ultra-violet photoemission, X-ray photoelectron diffraction (XPD) and low-energy electron diffraction. The evolution of the Ag 4d valence band shows deviations from simple layer by layer growth. These results, combined with the XPD data, indicate that by ∼1–1.5 monolayers of Mn a two layers thick Mn-rich epitaxial MnAg alloy is formed that continues the face centered cubic lattice of the Ag substrate.

International audience; We demonstrate quantitative ballistic electron magnetic microscopy (BEMM) imaging of simple model Fe(001) nanostructures. We use in situ nanostencil shadow mask resistless patterning combined with molecular beam epitaxy deposition to prepare under ultra-high vacuum conditions nanostructured epitaxial Fe/Au/Fe/GaAs(001) spin-valves. In this epitaxial system, the magnetization of the bottom Fe/GaAs(001) electrode is parallel to the [110] direction, defining accurately the analysis direction for the BEMM experiments. The large hot-electron magnetoresistance of the Fe/Au/Fe/GaAs(001) epitaxial spin-valve allows us to image various stable magnetic configurations on the as-grown Fe(001) microstructures with a high sensitivity, even for small misalignments of both magnetic electrodes. The angular dependence of the hot-electron magnetocurrent is used to convert magnetization maps calculated by micromagnetic simulations into simulated BEMM images. The calculated BEMM images and magnetization rotation profiles show quantitative agreement with experiments and allow us to investigate the magnetic phase diagram of these model Fe(001) microstructures. Finally, magnetic domain reversals are observed under high current density pulses. This opens the way for further BEMM investigations of current-induced magnetization dynamics.

International audience; We present an experimental investigation of the interface electronic structure of thin MgO films epitaxially grown on Ag(001) by x-ray and ultraviolet photoemission spectroscopy as a function of the oxide growth conditions. It is shown that the Schottky barrier height at MgO/metal interface can be tuned over 0.7 eV by a modification of the oxygen partial pressure or the sample temperature. These experimental results are explained in the framework of the extended Schottky-Mott model and the MgO-induced polarization effect by Mg enrichment of the silver surface region.

International audience; The transport mechanisms through MgO ultrathin layers (0.5-1.2 nm) deposited on n-type doped GaAs(001) layers have been studied. In order to favor field emission (FE) across the junctions, a high doping concentration layer in vicinity of the semiconductor surfaces has been included. Varying doping concentration of the underlying GaAs layer we find that the dominant transport mechanism is either the variable-range hopping mechanism or a thermionic emission-like process instead of the FE process. The observation of such mechanisms can be explained by the fact that during the MgO deposition, defect states are introduced in the semiconductor band gap.

International audience; The electronic band structure and the work function of MgO thin films epitaxially grown on Ag(001) have been investigated using x-ray and ultraviolet photoelectron spectroscopy for various oxide thicknesses. The deposition of thin MgO films on Ag(001) induces a strong diminution in the metal work function. The p-type Schottky barrier height is constant at 3.85+/-0.10 eV above two MgO monolayers and the experimental value of the ionization potential is 7.15+/-0.15 eV. Our results are well consistent with the description of the Schottky barrier height in terms of the Schottky-Mott model corrected by an MgO-induced polarization effect.

International audience; We report on ballistic electron-emission spectroscopy on high-quality Au(110)/GaAs(001) and Fe(001)/GaAs(001) Schottky contacts. For the Au(110)/GaAs(001) interface, the ballistic current is characterized by a strong electron injection in the L valley of the GaAs conduction band. This remarkable spectroscopic feature is absent for the Fe(001)/GaAs(001) interface. These observations are explained by the different electronic structures in the two metal layers, assuming conservation of the electron transverse momentum at the metal/semiconductor epitaxial interfaces. Conversely, this comparative study suggests that the technique can be used for the analysis of local electronic states propagating in the metal films.

N. Tournerie,1 P. Schieffer,2 B. Lepine,2 C. Lallaizon,2 P. Turban,2 and G. Jezequel2 1Physique de la Matiere Condensee, Ecole Polytechnique, CNRS, 91128 Palaiseau, France 2Equipe de Physique des Surfaces et Interfaces, Institut de Physique de Rennes, UMR 6251, CNRS–Universite de Rennes 1, Campus de Beaulieu, Bât 11C, 35042 Rennes Cedex, France Received 4 July 2008; revised manuscript received 8 September 2008; published 1 October 2008

International audience; The spatially resolved electronic structure of the epitaxial Au/MgO/GaAs(001) tunnel junction has been studied by ballistic electron emission microscopy. The Schottky barrier height of Au on the MgO/GaAs heterostructure is determined to be 3.90 eV, in good agreement with spatially averaged x-ray photoelectron spectroscopy measurements. Locally, two well-defined conduction channels are observed for electrons energies of 2.5 and 3.8 eV, i.e., below the conduction band minimum of the oxide layer. These conduction channels are attributed to band of defect states in the band-gap of the tunnel barrier related to oxygen vacancies in the MgO layer. These defect states are responsible for the low barrier height measured on magnetic tunnel junctions with epitaxial MgO(001) tunnel barriers.

International audience; The electrical properties of Au/MgO/n-GaAs(001) tunnel structures have been investigated with capacitance-voltage and current-voltage measurements at room temperature with various MgO thicknesses between 0.5 and 6.0nm. For an oxide thickness higher than 2nm and for low bias voltages, the voltage essentially drops across the oxide and the structure progressively enters the high-current mode of operation with increasing reverse bias voltage, the property sought in spin injection devices. In this mode, we demonstrate that a large amount of charge accumulates at the MgO/GaAsinterface in interface traps located in the semiconductor band gap.

Structural properties of Mn films, with a thickness of one monolayer (ML), deposited on a Ag(100) substrate have been investigated both experimentally (photoemission and ion scattering spectroscopies) and theoretically (tight-binding linear muffin-tin orbital method). The magnetic structure of the films and the effect of magnetism on their relative stability have been investigated ab initio. We find that after Mn evaporation [at room temperature (RT)], a superficial MnAg alloy is formed. Mild annealing gives rise to the formation of an (almost) inverted Mn monolayer covered by a Ag plane, i.e., the second atomic plane of the sample is mainly constituted by Mn atoms, whereas the first atomic layer is almost a pure Ag plane. A complete inversion of the Mn ML can be obtained by direct sequential deposition of Mn and Ag at 80 K. Our calculations on the energetic stability of 1 ML of Mn on top of Ag(100) versus 1 ML of Mn covered by one Ag atomic plane, show that the second situation is energetically preferred. This is also true when this situation is compared to the formation of a 2-ML-thick MnAg alloy on Ag(100). We find that the inverted Mn monolayer tends to be ferromagnetic and that magnetism acts against interdiffusion.

The room temperature epitaxial growth of Fe films on the As-rich GaAs(001)-(2×4) surface is studied using x-ray photoelectron spectroscopy as well as reflection high-energy electron diffraction and photoelectron diffraction. Interdiffusion mechanisms take place between Fe and GaAs during the deposition of the first 4 ML (0.7nm) Fe. The authors find that an Fe-based substitutional alloy with a body-centered-cubic structure confined on several atomic planes and containing 30% of foreign species (Ga and As atoms) sits at the Fe∕GaAs(001) interface. This intermixed layer is then buried by an almost pure Fe layer.