Sylvain Tricot

Layer-resolved study of Mg atom incorporation at the MgO/Ag(001) buried interface.

By combining x-ray excited Auger electron diffraction experiments and multiple scattering calculations we reveal a layer-resolved shift for the Mg KL23L23 Auger transition in MgO ultrathin films (4-6 Å) on Ag(001). This resolution is exploited to demonstrate the possibility of controlling Mg atom incorporation at the MgO/Ag(001) interface by exposing the MgO films to a Mg flux. A substantial reduction of the MgO/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.

Induced work function changes at Mg-doped MgO/Ag(001) interfaces: Combined Auger electron diffraction and density functional study

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, ultravio-let photoemission spectroscopy, and density functional theory (DFT) calculations. Exploiting the layer-by-layer resolution of the Mg KL23L23 Auger spectra and using multiple scattering calcula-tions we first determine the inter-layer 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 atom 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 O 2− 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 inter-face Mg atoms incorporation are essentially driven by the increase of the electrostatic compression effect.

Thermoelectric Properties of Highly-Crystallized Ge-Te-Se Glasses Doped with Cu/Bi

Chalcogenide semiconducting systems are of growing interest for mid-temperature range (~500 K) thermoelectric applications. In this work, Ge20Te77Se3 glasses were intentionally crystallized by doping with Cu and Bi. These effectively-crystallized materials of composition (Ge20Te77Se3)100−xMx (M = Cu or Bi; x = 5, 10, 15), obtained by vacuum-melting and quenching techniques, were found to have multiple crystalline phases and exhibit increased electrical conductivity due to excess hole concentration. These materials also have ultra-low thermal conductivity, especially the heavily-doped (Ge20Te77Se3)100−xBix (x = 10, 15) samples, which possess lattice thermal conductivity of ~0.7 Wm−1 K−1 at 525 K due to the assumable formation of nano-precipitates rich in Bi, which are effective phonon scatterers. Owing to their high metallic behavior, Cu-doped samples did not manifest as low thermal conductivity as Bi-doped samples. The exceptionally low thermal conductivity of the Bi-doped materials did not, alone, significantly enhance the thermoelectric figure of merit, zT. The attempt to improve the thermoelectric properties by crystallizing the chalcogenide glass compositions by excess doping did not yield power factors comparable with the state of the art thermoelectric materials, as these highly electrically conductive crystallized materials could not retain the characteristic high Seebeck coefficient values of semiconducting telluride glasses.

Spatially resolved band alignments at Au-hexadecanethiol monolayer-GaAs(001) interfaces by ballistic electron emission microscopy

We study structural and electronic inhomogeneities in Metal—Organic Molecular monoLayer (OML)—semiconductor interfaces at the sub-nanometer scale by means of in situ Ballistic Electron Emission Microscopy (BEEM). BEEM imaging of Au/1-hexadecanethiols/GaAs(001) heterostructures reveals the evolution of pinholes density as a function of the thickness of the metallic top-contact. Using BEEM in spectroscopic mode in non-short-circuited areas, local electronic fingerprints (barrier height values and corresponding spectral weights) reveal a low-energy tunneling regime through the insulating organic monolayer. At higher energies, BEEM evidences new conduction channels, associated with hot-electron injection in the empty molecular orbitals of the OML. Corresponding band diagrams at buried interfaces can be thus locally described. The energy position of GaAs conduction band minimum in the heterostructure is observed to evolve as a function of the thickness of the deposited metal, and coherently with size-dependent electrostatic effects under the molecular patches. Such BEEM analysis provides a quantitative diagnosis on metallic top-contact formation on organic molecular monolayer and appears as a relevant characterization for its optimization.

Quantitative magnetic imaging at the nanometer scale by ballistic electron magnetic microscopy

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.

k-space spin filtering effect in the epitaxial Fe/Au/Fe/GaAs(001) spin-valve

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.

Band alignments in Fe/graphene/Si(001) junctions studied by x-ray photoemission spectroscopy

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 Gra...

Theoretical and experimental studies of (In,Ga)As/GaP quantum dots.

Abstract(In,Ga)As/GaP(001) quantum dots (QDs) are grown by molecular beam epitaxy and studied both theoretically and experimentally. The electronic band structure is simulated using a combination of k·p and tight-binding models. These calculations predict an indirect to direct crossover with the In content and the size of the QDs. The optical properties are then studied in a low-In-content range through photoluminescence and time-resolved photoluminescence experiments. It suggests the proximity of two optical transitions of indirect and direct types.

Composition dependent nature of the fundamental optical transition in (In, Ga)As/GaP quantum dots

The nature of the ground optical transition in (In, Ga)As/GaP quantum dots is thoroughly investigated through k·p calculations and a supercell tight-binding simulation. Quantum dot morphology is deduced from scanning-tunneling-microscopy images. The strain field has a strong influence on the conduction band states. Indeed, for a pure GaAs QD, the wavefunction of the ground electron state is spatially confined in the GaP matrix, close to the dot apex, in a tensile strain region, having mainly Xz character. Time resolved and hydrostatic pressure photoluminescence experiments strongly support the theoretical conclusions. Promising results from the literature on (In, Ga)As/GaP quantum dot will be reviewed.

Structural and optical properties of (In,Ga)As/GaP quantum dots and (GaAsPN/GaPN) diluted-nitride nanolayers coherently grown onto GaP and Si substrates for photonics and photovoltaics applications

Lattice-matched GaP-based nanostructures grown on silicon substrates is a highly rewarded route for coherent integration of photonics and high-efficiency photovoltaic devices onto silicon substrates. We report on the structural and optical properties of selected MBE-grown nanostructures on both GaP substrates and GaP/Si pseudo-substrates. As a first stumbling block, the GaP/Si interface growth has been optimised thanks to a complementary set of thorough structural analyses. Photoluminescence and time-resolved photoluminescence studies of self-assembled (In,Ga)As quantum dots grown on GaP substrate demonstrate a proximity of two different types of optical transitions interpreted as a competition between conduction band states in X and Γ valleys. Structural properties and optical studies of GaAsP(N)/GaP(N) quantum wells coherently grown on GaP substrates and GaP/Si pseudo substrates are reported. Our results are found to be suitable for light emission applications in the datacom segment. Then, possible routes are drawn for larger wavelengths applications, in order to address the chip-to-chip and within-a-chip optical interconnects and the optical telecom segments. Finally, results on GaAsPN/GaP heterostructures and diodes, suitable for PV applications are reported.