Yann Claveau

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.

Quantum electronic transport in polarization-engineered GaN/InGaN/GaN tunnel junctions

We theoretically investigate GaN/InGaN/GaN tunnel junctions grown along the wurtzite c-axis. We developed a dedicated quantum electronic transport model based on an 8-band k.p Hamiltonian coupled to the non-equilibrium Greens function formalism. We first show that the transmission is dominated by quantum states localized at the heterojunction. We also confirm that, for a thin InGaN layer, current strongly increases with doping. On the other hand, for thick InGaN layers (>8 nm), our results show an unexpected low impact of doping on current. In this latter case, the spontaneous and the piezoelectric polarizations reduce the tunnel-barrier width to the InGaN layer thickness. We conclude that quantum electronic transport in such tunnel junctions is mainly controlled by interfaces with both polarizations and localized states.

Electron transport in ultra-thin films and ballistic electron emission microscopy

We have developed a calculation scheme for the elastic electron current in ultra-thin epitaxial heterostructures. Our model uses a Keldyshs non-equilibrium Greens function formalism and a layer-by-layer construction of the epitaxial film. Such an approach is appropriate to describe the current in a ballistic electron emission microscope (BEEM) where the metal base layer is ultra-thin and generalizes a previous one based on a decimation technique appropriated for thick slabs. This formalism allows a full quantum mechanical description of the transmission across the epitaxial heterostructure interface, including multiple scattering via the Dyson equation, which is deemed a crucial ingredient to describe interfaces of ultra-thin layers properly in the future. We introduce a theoretical formulation needed for ultra-thin layers and we compare with results obtained for thick Au(1 1 1) metal layers. An interesting effect takes place for a width of about ten layers: a BEEM current can propagate via the center of the reciprocal space ([Formula: see text]) along the Au(1 1 1) direction. We associate this current to a coherent interference finite-width effect that cannot be found using a decimation technique. Finally, we have tested the validity of the handy semiclassical formalism to describe the BEEM current.

Type II heterojunction tunnel diodes based on GaAs for multi-junction solar cells: Fabrication, characterization and simulation

In this work, Molecular Beam Epitaxy (MBE) grown tunnel junctions (TJs) based on GaAs(Sb)(In) materials are experimentally and numerically studied. From simple GaAs TJs grown with various n-doping levels, we develop a semi-classical interband tunneling model able to quantify the magnitude of the tunneling current density, which shows that direct interband tunneling is the predominant tunneling mechanism in GaAs tunnel junctions instead of trap-assisted-tunneling mechanisms. Numerical simulations based on non equilibrium perturbation theory through Non Equilibrium Greens Functions (NEGF) and a multi-band kp hamiltonian that includes both gamma and L valleys were performed by the IM2NP (Marseille) and confirmed this result. In order to further improve the performance of the TJs, we are fabricating a type II tunnel heterojunction based on GaAsSb and InGaAs materials.