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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.

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

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.

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.

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.

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 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 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; 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.

The electronic band structure at the interface of the MgO-GaAs(001) tunnel contact has been experimentally studied. X-ray photoelectron spectroscopy has been used to measure the valence-band offset at the MgO-GaAs(001) heterojunction interface. The valence-band offset ΔEV is determined to be 4.2±0.1eV. As a consequence, a nested “type-I” band alignment with a conduction-band offset of ΔEC=2.2±0.1eV is found. The accurate determination of the valence and conduction band offsets is important for the fundamental understanding of the tunnel spin injection in GaAs.

International audience; We show that in agreement with the ternary Fe–Ga–As phase diagram, the solid-state interdiffusions in epitaxial Fe/GaAs(0 0 1) heterostructures lead, at a temperature of approximately 500 °C, to the formation of thermodynamically stable Fe3GaAs/GaAs(0 0 1) contacts quite similar to the well-known silicide/Si ones. The Fe3GaAs films are made of grains epitaxial on GaAs with a well-defined interface. Their magnetic and electrical properties make Fe3GaAs on GaAs an attractive metallization scheme for future magnetoelectronic devices. The results we report concern (25 or 80 nm Fe)/GaAs(0 0 1) heterostructures annealed at 480 and 500 °C for 10 min and characterized ex situ by He+ Rutherford backscattering and ion channeling, X-ray diffraction, transmission electron microscopy and alternating gradient field magnetometry.

Fe films have been grown on cubic GaN(0 0 1) layers by molecular beam epitaxy. They were studied in situ by reflection high-energy electron diffraction, X-ray photoemission spectroscopy and X-ray photoelectron diffraction and ex situ by Rutherford backscattering spectrometry (RBS), X-ray diffraction and alternating gradient field magnetometry. Despite the large lattice mismatch between Fe and cubic GaN [Δa/a=(2aFe−aGaN)/aGaN=+26%], the growth of Fe on GaN was found to be epitaxial with the orientation relationship (0 0 1)[1 0 0]Fe∥(0 0 1)[1 1 0]GaN i.e. with a 45° rotation of the Fe unit cell with respect to the GaN one. The mosaic spread of the Fe film does not exceed 1°. In such conditions we observed that the magnetic hysteresis loops show a cubic anisotropy with the easy axes along 〈1 0 0〉, as for bulk bcc Fe.

The epitaxial growth of ultrathin Fe films on InAs(0 0 1)-(2×4) and -(4×2) surfaces at room temperature is studied using reflection high-energy electron diffraction as well as ultraviolet and X-ray photoelectron spectroscopy. The InAs(0 0 1)-(2×4) and -(4×2) surfaces are obtained either by ion bombardment and annealing or by molecular beam epitaxy. For the different surfaces a detailed analysis of the In 4d core-level line shape leads to the same conclusion. The deposition of Fe onto InAs(0 0 1) induces a substrate disruption and a surface segregation: in a first step In atoms are released in the metallic overlayer and with increasing the Fe coverage these In atoms continue to segregate at the surface. At the same time, some In atoms are trapped in the Fe overlayer. A small amount of As surface segregation is also observed. Finally, the position of the Fermi level (independent of the surface reconstruction and the preparation method) at the Fe/InAs(0 0 1) interfaces is found ∼0.2 eV above the conduction band minimum.

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.

Mn films with a thickness of one monolayer (ML) have been deposited in ultra-high vacuum, at room temperature (RT), on a Ag(1 0 0) single crystal. Photoemission and ion scattering spectroscopy data indicate that after Mn evaporation a substantial fraction of the Mn atoms is located within the second topmost atomic layer. If the film is left at RT or mildly (60 °C) annealed, the Mn atoms tend to exchange further with Ag atoms, increasing the Mn concentration in the second atomic plane. Eventually the second atomic plane of the sample is constituted mainly by Mn atoms, whereas the first atomic layer is almost a pure Ag plane, i.e. an inverted atomic Mn ML is formed. To directly compare experiment and theory, we have performed ab initio electronic band structure calculations on the energetic stability of 1 ML of Mn on top of Ag(1 0 0) versus 1 ML of Mn covered by one Ag atomic plane. The case of a 2 ML-thick MnAg alloy is also investigated. The relation between magnetism and stability of Mn films is discussed.