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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 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.

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.

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; 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; 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.

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.

International audience; The electronic band structure in the epitaxial Fe∕MgO∕GaAs(001) tunnel junction has been studied by x-ray and ultraviolet photoelectron spectroscopy measurements. The Schottky barrier height (SBH) of Fe on MgO∕GaAs heterostructure is determined to be 3.3±0.1eV, which sets the Fe Fermi level at about 0.3eV above the GaAs valence band maximum. This SBH is also exactly the same as that measured from Fe on MgO monocrystal. After Fe deposition, no band bending change is observed in MgO and GaAs underlayers. On the contrary, Au and Al depositions led to clear variation of the band bending in both MgO and GaAs layers. This effect is analyzed as a fingerprint of defect states at the MgO∕GaAs interface.

We have probed the interface of a ferromagnetic/semiconductor (FM/SC) heterojunction by a combined high resolution photoemission spectroscopy and x-ray photoelectron diffraction study. Fe/ZnSe(001) is considered as an example of a very low reactivity interface system and it is expected to constitute large tunnel magnetoresistance devices. We focus on the interface atomic environment, on the microscopic processes of the interface formation and on the iron valence band. We show that the Fe contact with ZnSe induces a chemical conversion of the ZnSe outermost atomic layers. The main driving force that induces this rearrangement is the requirement for a stable Fe-Se bonding at the interface and a Se monolayer that floats at the Fe growth front. The released Zn atoms are incorporated in substitution in the Fe lattice position. This formation process is independent of the ZnSe surface termination (Zn or Se). The Fe valence-band evolution indicates that the d-states at the Fermi level show up even at submonolayer Fe coverage but that the Fe bulk character is only recovered above 10 monolayers. Indeed, the Fe Delta(1)-band states, theoretically predicted to dominate the tunneling conductance of Fe/ZnSe/Fe junctions, are strongly modified at the FM/SC interface.

The properties of MgO/Ag(001) ultrathin films with substitutional Mg atoms in the interface metal layer have been investigated by means of Auger electron diffraction experiments, ultraviolet photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL_23 L_23 Auger spectra and using multiple scattering calculations, we first determine the interlayer distances as well as the morphological parameters of the MgO/Ag(001) system with and without Mg atoms incorporated at the interface. We find that the Mg atoms incorporation drives a strong distortion of the interface layers and that its impact on the metal/oxide electronic structure is an important reduction of the work function (0.5 eV) related to band-offset variations at the interface. These experimental observations are in very good agreement with our DFT calculations which reproduce the induced lattice distortion and which reveal (through a Bader analysis) that the increase of the interface Mg concentration results in an electron transfer from Mg to Ag atoms of the metallic interface layer. Although the local lattice distortion appears as a consequence of the attractive (repulsive) Coulomb interaction between O2- ions of the MgO interface layer and the nearest positively (negatively) charged Mg (Ag) neighbors of the metallic interface layer, its effect on the work function reduction is only limited. Finally, an analysis of the induced work function changes in terms of charge transfer, rumpling, and electrostatic compression contributions is attempted and reveals that the metal/oxide work function changes induced by interface Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect., 13 pages, 8 figures

International audience; The hot-electron magnetotransport of epitaxial Fe/Au/Fe/GaAs(001) spin-valves is investigated by ballistic-electron magnetic microscopy. A magnetocurrent amplitude larger than 500% is observed at room temperature close to the Schottky barrier energy. Remarkably, this magnetocurrent is not significantly affected by the thickness reduction of ferromagnetic films, down to 5 atomic layers of the Fe(001) top electrode. This rather suggests a dominant interfacial spin-filtering effect. Finally, the magnetocurrent is strongly reduced when the effective mass of the semiconductor collector is increased. These observations are consistent with recent theoretical prediction of k-space spin-filtering effect in epitaxial spin-valves attached to a semiconducting lead.

The graphene / silicon interface is of interest to diverse fields such as photovoltaics, electronics and spin electronics to name a few. This thesis work, based on the combination of photoemission measurements, transport and DFT calculations, allows a better understanding of the electronic properties of this interface as well as of the mechanisms associated with the formation of the Schottky barrier. We show a de-anchoring of the closure level at the graphene/silicon or metal/graphene/silicon interface. This implies that the height of the Schottky barrier can be controlled by modifying the work of exiting the graphene. DFT calculations reveal that this non-anchoring of the closure level at the graphene/silicon interface describes a low density of states induced in the bandgap of silicon by graphene. We show that this phenomenon is mainly associated with the structure of the graphene band whose electrons close to the closing level are at the edge of the Brillouin zone, leading to a low evanescence length of electronic functions compared to classical metals. The removal of the anchoring of the closure level at the metal / silicon interface by addition to the interface of a graphene sheet has enabled us to obtain interesting metal/graphene/silicon structures for spin electronics., L’interface graphène/silicium présente un intérêt pour des domaines diverses et variés tels que le photovoltaïque, l’électronique et l’électronique de spin pour ne citer qu’eux. Ce travail de thèse, basé sur la combinaison de mesures de photoémission, de transports et de calculs DFT permet une meilleure compréhension des propriétés électroniques de cette interface ainsi que des mécanismes associés à la formation de la barrière Schottky. Nous mettons en évidence un désancrage du niveau de Fermi à l’interface graphène/silicium ou métal/graphène/silicium. Cela implique que la hauteur de la barrière Schottky peut être contrôlée par modification du travail de sortie du graphène. Les calculs DFT révèlent que ce non-ancrage du niveau de Fermi à l’interface graphène/silicium résulte d’une faible densité d’états induit dans la bande interdite du silicium par le graphène. Nous montrons que ce phénomène est principalement associé à la structure de bande du graphène dont les électrons proches du niveau de Fermi sont en bord de zone de Brillouin conduisant à une longueur d’évanescence des fonctions d’ondes électroniques faible comparée aux métaux classiques. La levée de l’ancrage du niveau de Fermi à l’interface métal/silicium par addition à l’interface d’un feuillet de graphène nous a permis d’obtenir des structures métal/graphène/silicium intéressantes pour l’électronique de spin.

Transition metal oxides are of great interest because of the wide range of properties they exhibit. They have potentially many technological applications in the field of electronics, especially based on the development of devices for information technology requiring contacting these oxides with metals. This thesis work, mainly based on photoemission spectroscopy, is a detailed study of the Fe/SrTiO₃ interface grown at room temperature in which we focus in particular on structural and electronic properties. We show that iron has an epitaxial growth with an island morphology and that films completely cover the substrate for the deposition of few atomic monolayers. We demonstrate that the metal reacts with the substrate during the formation of the interface which results in the presence of reduced titanium at the interface. We associate this reduction of titanium with the presence of oxygen vacancies at the Fe/SrTiO₃ interface. While the expected Schottky barrier height for a Fe/SrTiO₃(001) abrupt junction is about 1 eV for electrons, we show that the presence of oxygen vacancies at the interface lowers this Schottky barrier height to about 0.05 eV. The creation of oxygen vacancies during the deposition of fractions of iron monolayer on SrTiO₃ also leads to the metallization of the semiconductor surface. This mechanism is related to the creation of positively charged donor states associated with oxygen vacancies during deposition. To determine the band profile in the substrate, we solved the one-dimensional Poisson equation in a modified approach of the density functional theory taking into account the electron accumulation layer. By comparing these calculations with our photoemission results, we find that the surface potential has spatial inhomogeneities parallel to the surface., Les oxydes de métaux de transition sont d'un grand intérêt en raison du large éventail de propriétés qu'ils présentent. Ils ont potentiellement de nombreuses applications technologiques dans le domaine de l'électronique, reposant en partie sur le développement de dispositifs pour la technologie de l'information nécessitant de les contacter avec des métaux. Ce travail de thèse, basé principalement sur l'utilisation de la spectroscopie de photoémission, porte sur une étude détaillée de l'interface Fe/SrTiO₃ réalisée à température ambiante, dans laquelle nous nous sommes intéressés en particulier aux propriétés structurales et électroniques. Nous montrons que le fer est épitaxié, que sa croissance est en îlots et que les films couvrent entièrement le substrat pour le dépôt de quelques monocouches atomiques. Nous mettons en évidence une réaction du métal avec le substrat lors de la formation de l'interface qui se manifeste par la présence de titane réduit à l'interface. Nous associons cette réduction du titane à la présence de lacunes d'oxygène à l'interface Fe/SrTiO₃. Alors que la hauteur de barrière Schottky attendue pour une jonction abrupte Fe/SrTiO₃(001) est d'environ 1 eV pour les électrons, nous montrons que la présence des lacunes d'oxygène à l'interface abaisse la hauteur de barrière Schottky à environ 0,05 eV. La création de lacunes d'oxygène lors du dépôt de fractions de monocouche de fer sur le SrTiO₃ conduit aussi à une métallisation de la surface du semi-conducteur. Ce mécanisme est relié à la création d'états donneurs chargés positivement associés aux lacunes d'oxygène pendant le dépôt. Pour déterminer le profil de bande dans le substrat nous avons résolu l'équation de Poisson à une dimension dans une approche de la théorie de la fonctionnelle de la densité modifiée, en tenant compte de la couche d'accumulation d'électrons. Confrontant ces calculs avec nos résultats de photoémission nous trouvons que le potentiel de surface présente des inhomogénéités spatiales parallèlement à la surface.

During this thesis work, we studied by ballistic electron magnetic microscopy (BEMM) the hot electron magnetotransport properties of epitaxial Fe/Au/Fe/GaAs(001) heterostructures. In these experiments, hot electrons are injected from an STM tip through the metallic base. The measurement of the ballistic electron current collected at the back of the substrate under magnetic field gives access to the local magnetoconductance properties of the sample. The first part of this work consists in the study of a planar heterostructures. BEMM measurements on epitaxial Fe/Au/Fe/GaAs(001) samples demonstrate hot electron magnetocurrent as high as 500% at room temperature. This high magnetocurrent value is observed to be almost independent of the Fe layers thickness, and thus can not be explained by the spin asymmetry of the electron attenuation length in the iron layers. In this epitaxial heterostructure, the hot electron beam is mainly spin-polarized at the interfaces due to band structure effects. In the metallic thin films, electrons propagate as Bloch states. The electron wave function transmission at the interfaces should satisfy two selection rules: the transverse momentum (k║) of the electron wave vector and the symmetry of the electron wave function should be conserved. Above the Schottky barrier height, hot-electrons are collected in the Г valley of GaAs selecting thus only electrons with a transverse momentum (k║) close to zero. Among these k|| ≈ 0 states, conservation of the electron wave-function symmetry at the Fe/Au epitaxial interfaces additionally selects electrons with the Δ1 symmetry. These Δ1 states are fully spin-polarized and are responsible for the observed high magnetocurrent in these heterostructures. This analysis is experimentally confirmed by the observation of a correlation between the magnetocurrent value and the semiconductor effective mass. By increasing the semiconductor effective mass, we open the collimator which filters the electronic states around k║=0 and the magnetocurrent value decreases. To take advantage of the lateral resolution of the microscope and of its high sensitivity to magnetism, the second part of this work was devoted to the study of sub-micrometric iron structures prepared under UHV by evaporation through a nanostencil. In these structures, the modulation of the collected current by the local magnetic domain structure in the Fe dots allows magnetic imaging with a high spatial resolution. The experimental magnetocontrasts observed on these sub-micrometric Fe dots are in excellent agreement with BEMM current maps calculated from micromagnetic simulation results. This opens the way to a quantitative magnetic microscopy with high contrast and nanometric lateral resolution., Au cours de ces travaux de thèse, nous avons étudié par microscopie magnétique à émission d’électrons balistiques (BEMM) les propriétés de magnétotransport d’électrons chauds de la vanne de spin Fe/Au/Fe épitaxiée sur GaAs(001). Dans ces expériences, la pointe d’un microscope à effet tunnel (STM) injecte localement un courant d’électrons chauds à la surface de la vanne de spin. La mesure sous champ magnétique du courant d’électrons balistiques collecté à l’arrière de l’échantillon donne accès aux propriétés locales de magnétoconductance de l’échantillon. Nous avons dans un premier temps étudié les propriétés de magnétotransport de vannes de spin planaires. Les mesures BEMM démontrent un magnétocourant d’électrons chauds pouvant atteindre 500 % à température ambiante. Ces forts effets de magnétoconductance ne sont que très faiblement dépendants des épaisseurs des électrodes de fer et ne peuvent donc être dus à l’asymétrie en spin de la longueur d’atténuation des électrons chauds dans les couches de fer. Dans cette structure épitaxiée, la polarisation en spin du faisceau d’électrons chauds s’acquiert principalement aux interfaces via des effets de structure électronique. L’électron traversant les couches minces métalliques se propage comme un état de Bloch. Sa transmission aux différentes interfaces se fait en conservant d’une part la composante transverse k║ du vecteur d’onde électronique, et d’autre part, la symétrie de la fonction d’onde. Au-dessus de la barrière Schottky, les électrons chauds sont collectés dans la vallée Г du GaAs se projetant à l’interface dans la direction k║=0. Dans cette direction k║=0, la conservation de la symétrie de la fonction d’onde à l’interface Fe/Au conduit au filtrage des états de Bloch de symétrie Δ1 du fer. Ces états de symétrie Δ1, totalement polarisés en spin, sont responsables des forts magnétocourants d’électrons chauds observés. Cette analyse est confirmée expérimentalement par l’observation d’une corrélation entre amplitude du magnétocourant et masse effective du substrat semiconducteur. En augmentant la masse effective du semiconducteur, on ouvre le collimateur filtrant le courant d’électrons chauds autour de la direction k║=0, et le magnétocourant diminue sans modifier la vanne de spin. Dans un second temps, tirant partie de la résolution latérale du microscope et de sa sensibilité au magnétisme, des microstructures de fer préparées sous ultra-vide par évaporation à travers un masque (méthode du nanostencil) ont été étudiées. Dans ces structures, la modulation du courant collecté par la structure locale en domaines magnétiques a permis la réalisation d’images magnétiques avec une haute résolution spatiale. Les contrastes observés sur ces microstructures sont en excellent accord avec les images BEMM calculées à partir de simulations micromagnétiques ouvrant la voie à une microscopie magnétique quantitative à forte sensibilité et résolution latérale nanométrique.

Magnesium oxide ultrathin films grown on metal substrates have received considerable attention because of their technological importance in many research fields, such as spintronics and heterogeneous catalysis. The ability to tune and control the band alignment in magnetic tunnel junctions or the cluster/oxide systems in catalytic applications implies a detailed knowledge of the metal/oxide interface electronic structure. The first part of this thesis deals with a photoemission study of the interface electronic structure of ultrathin MgO films on Ag(001) as a function of the oxide thickness. It is shown that the Fermi-level pinning at the interface is essentially controlled by an interfacial dipole governed by an MgO-induced polarization effect. Next, this extended Schottky-Mott model was also invoked to explain the dependence of the Schottky barrier height on the oxide growth conditions. The lowering of the metal/oxide work function induced by the Mg enrichment of the Ag surface region during the growth was demonstrated. Finally, we present a joint experimental-theoretical study of a three monolayer's MgO/Ag(001) system. Combining x-ray excited Auger electron diffraction measurements and multiple-scattering simulations provide a layer-by-layer resolution of the MgKLL Auger electron emissions which has been used to demonstrate the ability to intercalate Mg atoms at the buried interface by post-growth interface engineering. The induced work function changes and large interlayer relaxation at the metal/oxide interface are evidenced and show an excellent agreement with density functional theory calculations., Présentes dans de nombreux dispositifs comme les jonctions tunnel magnétiques ou les systèmes cluster/oxyde utilisés en catalyse hétérogène, les interfaces métal/oxyde présentent des propriétés spécifiques venant du raccordement entre deux matériaux de structure atomique et surtout de structure électronique très différentes. Ce projet de recherche repose sur l'étude, par diverses techniques de spectroscopies de photoémission, de la géométrie et de la structure électronique aux interfaces MgO/Ag(001) pour des épaisseurs d'oxyde subnanométriques. Nous avons, tout d'abord, montré que la valeur de la hauteur de la barrière Schottky (SBH) à l'interface pouvait être décrite grâce au modèle de Schottky-Mott et d'un effet de polarisation induit par le MgO. Ensuite, une attention particulière a été portée sur la capacité à contrôler la hauteur de barrière Schottky à l'interface MgO/métal en modifiant les conditions de préparation/traitement de la couche d'oxyde. Enfin, l'étude par diffraction de photoélectrons de films minces de MgO dans un régime de très faible épaisseur a révélé des propriétés originales de l'interface MgO/métal quant aux propriétés de relaxation électronique, permettant ainsi d'obtenir une résolution couche par couche de l'interface MgO/Ag(001). Cette résolution expérimentale, confrontée à des calculs basés sur la théorie de la fonctionnelle de la densité (DFT), a été utilisée pour démontrer la possibilité d'intercaler des atomes de Mg à l'interface MgO/Ag(001). La variation de travail de sortie du système induite par la présence de ces impuretés est corrélée à la modification des effets de polarisation induits par le MgO.

We developed an ultra-high vacuum ballistic electron emission microscope (BEEM) dedicated to the study of the interface electronic properties of some III-V-based heterostructures for spintronics applications. First, we investigated the epitaxial Au(110)/GaAs(001) and Fe(001)/GaAs(001) Schottky contacts. A theoretical study demonstrates that coherent hot-electron transport through the metal layer associated with transverse momentum conservation at the metal/GaAs interface are responsible for the different BEEM spectroscopic fingerprints experimentally observed for these systems. Second, BEEM investigations on the MgO/GaAs(001) tunnel contact have revealed new conduction channels localized in the band gap of MgO. These channels are related to oxygen vacancies in the oxide layer, which strongly reduce the tunnel barrier height. Finally, hot-electron magnetoresistance in the Fe/Au/Fe/GaAs(001) spin-valve system allows the observation of magnetic domains and magnetic domain walls with nanometric lateral resolution., La microscopie à émission d'électrons balistiques (BEEM) permet l'étude locale des propriétés électroniques d'hétérostructures avec une résolution spatiale nanométrique. Au cours de ce travail de thèse, nous avons mis en oeuvre un microscope balistique sous ultravide, dédié à l'étude des propriétés électroniques d'interfaces d'hétérostructures à base de semiconducteurs III-V pertinentes pour des applications potentielles en électronique de spin. Dans un premier temps, le montage expérimental a été validé par l'étude du contact Schottky modèle Au/GaAs(001). Nous avons préalablement étudié par RHEED, STM et photoémission X la croissance épitaxiale de ce système. Une relation d'épitaxie originale Au(110)/GaAs(001) a été démontrée, la surface d'Au(110) présentant en outre une reconstruction c(2*2) liée à la ségrégation d'une demie monocouche de gallium. Les mesures de transport diffusif témoignent de la qualité du contact Schottky formé. Les mesures de spectroscopie BEEM conduisent à une hauteur de barrière Schottky locale en accord avec les mesures intégrées spatialement. Des signatures spectroscopiques marquées de l'injection d'électrons chauds dans les vallées Γ, X et L de la bande de conduction de GaAs ont également été mises en évidence. Ces résultats ont pu être interprétés théoriquement en prenant en compte les effets de structure de bande sur la propagation des électrons chauds dans la couche d'Au(110) (approches DFT-LDA, liaisons fortes), associés à la conservation de la composante parallèle du vecteur d'onde à l'interface Au(110)/GaAs(001). Afin de valider cette approche, une étude BEEM comparative a été menée sur un second système Schottky Fe(001)/GaAs(001). Pour ce système épitaxié « cube sur cube », les mesures de spectroscopie BEEM démontrent l'absence d'injection d'électrons chauds dans la vallée L du GaAs, en bon accord avec l'analyse théorique. Cette étude confirme la sensibilité du BEEM aux effets de structure électronique pour ces interfaces épitaxiées. Dans un second temps, nous nous sommes penchés sur l'étude de contacts tunnels MgO/GaAs(001), candidats potentiels pour l'injection tunnel de spin dans les semiconducteurs III-V. Les études BEEM démontrent une hauteur de barrière tunnel en accord avec les mesures de photoémission intégrées spatialement. Toutefois, localement, des canaux de conduction marqués sont observés pour des énergies électroniques spécifiques inférieures à la hauteur de barrière tunnel. Ces canaux de conduction sont attribués à des états de défauts localisés dans la bande interdite de l'isolant, associés à des lacunes d'oxygène dans MgO. Ces états de défauts sont responsables des faibles hauteurs de barrières tunnel reportées dans la littérature pour les dispositifs intégrant des barrières de MgO. Enfin, nous avons modifié le montage expérimental pour réaliser des mesures BEEM sous champ magnétique. Un électro-aimant a été installé sur la tête du microscope afin d'évoluer vers l'étude du transport d'électrons chauds dépendant du spin. Des études préliminaires sur des vannes de spin épitaxiées Fe/Au/Fe/GaAs(001) valident ce montage par l'observation de domaines et parois de domaines magnétiques.