Electrical spin injection in p-type Si using Fe/MgO contacts

We report the successful electrical creation of spin polarization in p-type Si at room temperature by using an epitaxial MgO(001) tunnel barrier and Fe(001) electrode. Reflection high-energy electron diffraction observations revealed that epitaxial Fe/MgO(001) tunnel contacts can be grown on a (2 x 1) reconstructed Si surface whereas tunnel contacts grown on the (1 x 1) Si surface were polycrystalline. Transmission electron microscopy images showed a more flat interface for the epitaxial Fe/MgO/Si compared to that of the polycrystalline structure. For the Fe/MgO/p-Si devices, the Hanle and inverted Hanle effects were clearly observed at 300 K by using a three-terminal configuration, proving that spin polarization can be induced in the Si at room temperature. Effective spin lifetimes deduced from the width of the Hanle curve were 95 +/- 6 ps and 143 +/- 10 ps for the samples with polycrystalline and epitaxial MgO tunnel contacts, respectively. The observed difference can be qualitatively explained by the local magnetic field induced by the larger roughness of the interface of the polycrystalline sample. The sample with epitaxial Fe/MgO tunnel contact showed higher magnitude of the spin accumulation with a nearly symmetric behavior with respect to the bias polarity whereas that of the polycrystalline MgO sample exhibited a quite asymmetric evolution. This might be attributed to the higher degree of spin polarization of the epitaxial Fe/MgO(001) tunnel contact, which acts as a spin filter. Our experimental results suggest that an epitaxial MgO barrier is beneficial for creating spins in Si.Comment: Paper presented at SPIE Nanoscience + Engineering, Spintronics V session in San Diego, US on August 13th, 201

Spin Accumulation in Nondegenerate and Heavily Doped p-Type Germanium

Spin accumulation induced in p-type germanium from Fe/MgO tunnel contacts is studied as a function of hole concentration p (10^16 - 10^19 cm-3). For all p, the contacts are free of rectification and Schottky barrier, guaranteeing spin injection into the Ge and preventing spin accumulation enhancement by two-step tunneling via interface states. The observed spin accumulation is smallest for nondegenerate doping (p ~ 10^16 cm-3) and increases for heavily doped Ge. This trend is opposite to what is expected from spin injection and diffusion theory. For heavily doped Ge, the observed spin accumulation is orders of magnitude larger than predicted.Comment: To appear in Appl. Phys. Expres

Croissance épitaxiale et propriétés magnétiques d'hétérostructures de Mn5Ge3 sur Ge pour des applications en électronique de spin.

Spin-electronics based on ferromagnetic metal/semiconductor systems offer a pathway toward integration of information storage and processing in a single material. This emerging field aims to create a new generation of electronic devices where two degrees of freedom will be associated: spin and charge of carriers. In this context, the outcome of this thesis is to elaborate a novel ferromagnetic compound, namely Mn5Ge3, on Ge using molecular beam epitaxy method. The interests in this compound are manyfold: it can be stabilized as a unique phase on Ge(111) in the form of epitaxial thin films, it is ferromagnetic until room temperature and it is compatible with Si-based conventional microelectronics. In this work, one major effort was devoted to the epitaxial growth of Mn5Ge3 on Ge using Solid Phase Epitaxy method. By combining structural and magnetic characterizations, we demonstrated high quality epitaxial thin Mn5Ge3 films with good magnetic properties. We also studied the effect carbon incorporation on the structural and magnetic properties of epitaxial Mn5Ge3 films. The carbon-doped films exhibit a high Curie temperature with an atomically smooth interface and a high thermal stability. All these results show that Mn5Ge3 is a promising candidate opening up the ways for spin injection via tunnel effect through the Schottky barrier into Ge.L'intégration de matériaux ferromagnétiques dans des hétérostructures semi-conductrices offre aujourd'hui de nouvelles perspectives dans le domaine de l'électronique de spin. Dans ce manuscrit sont présentés les résultats de la croissance par épitaxie par jets moléculaires d' hétérostructures de Mn5Ge3 sur Ge(111). Le Mn5Ge3 est un composé ferromagnétique jusqu'à température ambiante qui a l'avantage de pouvoir s'intégrer directement au Ge, semi-conducteur du groupe IV. S'agissant d'un matériau relativement nouveau, un des efforts majeurs a porté sur la maîtrise de la croissance des couches minces de Mn5Ge3 par la technique d'épitaxie en phase solide (SPE). Un fort accent a été mis sur les caractérisations structurales, la détermination des relations d'épitaxie avec le Ge(111), afin de les relier aux propriétés magnétiques des films. La seconde partie de ce travail a été consacrée à l'étude des processus cinétiques d'incorporation de carbone dans les couches minces de Mn5Ge3. La combinaison des différents moyens de caractérisations structurales et magnétiques a permis d'aboutir à une augmentation notable de la température de Curie tout en conservant une excellente qualité structurale de la couche et de l'interface avec le Ge et une stabilité thermique jusqu'à 850°C. Tous ces résultats indiquent que les couches minces de Mn5Ge3 épitaxiées sur Ge(111) apparaissent comme des candidats à fort potentiel pour l'injection de spin dans les semi-conducteurs du groupe IV

Voltage tuning of thermal spin current in ferromagnetic tunnel contacts to semiconductors

Spin currents are paramount to manipulate the magnetization of ferromagnetic elements in spin-based memory, logic and microwave devices, and to induce spin polarization in non-magnetic materials. A unique approach to create spin currents employs thermal gradients and heat flow. Here we demonstrate that a thermal spin current can be tuned conveniently by a voltage. In magnetic tunnel contacts to semiconductors (silicon and germanium), it is shown that a modest voltage (∼200 mV) changes the thermal spin current induced by Seebeck spin tunnelling by a factor of five, because it modifies the relevant tunnelling states and thereby the spin-dependent thermoelectric parameters. The magnitude and direction of the spin current is also modulated by combining electrical and thermal spin currents with equal or opposite sign. The results demonstrate that spin-dependent thermoelectric properties away from the Fermi energy are accessible, and open the way towards tailoring thermal spin currents and torques by voltage, rather than material design. © 2014 Macmillan Publishers Limited. All rights reserved.