Etudes par microscopie en champ proche des phénomènes de ...

2ing spin-polarized electrons decreases with injection energyinversely to the increase of the total transmittedcurrent. 9,10,12,18 This result is not contradictory with thespin-valve experiment. Indeed, the magnetic layer is stillspin selective but the incident electron polarization is “diluted”by the secondary electrons before the spin filteroperates. In other words, when spin-polarized incidentelectrons are injected into a metallic film containing asingle magnetic layer, the secondary electrons do not contributeto the spin-dependent transmitted current whichall originates from the spin filtering of the only primaryelectrons. This is an important result as it means thatthe polarization of secondary electrons does not dependon the one of the injected electrons and that the effects ofexchange integral asymmetry on the secondary electronpolarization are negligible.For injection energies higher than 100eV, the extensionof the transport scheme mentioned above does notpredict any particular behavior. However, the situationis again very different, in particular because of theincrease in the electron mean-free-path. Indeed electrontransmission experiments at injection energies ofseveral hundreds of eV exhibits strong deviations fromthe simple transport model which fits at moderate injectionenergy. 22 In the present paper, we report on astudy of spin-polarized electron transport through a ferromagneticmetal/oxide/semiconductor junction, wherethe electron injection energy is varied from a few eVup to 1keV. The experimental configuration can be comparedto a three-terminal device geometry, 15–21 the emitterbeing here the GaAs spin-polarized electron sourceseparated from the metallic base by vacuum. This allowsan easy control of the electron injection energy andof the polarization. At moderate injection energy (from8eV to 100eV above the metal Fermi level), results aresimilar to the one obtained in previous studies. 9,12,14,18But, above 100eV injection energy, the increase in thetransmitted current becomes super-linear and the spindependenttransmitted current rises by several orders ofmagnitude, increasing even faster than the transmittedcurrent. This feature clearly differs from what couldbe predicted from any previous studies performed atmoderate injection energy. We have developed a modelto describe the transport of spin-polarized hot electronthrough the metal/oxide/semiconductor structure. Qualitativeand quantitative agreement with the experimentaldata is obtained over the whole probed injection energyrange. This model is based on the calculation of the electronenergy distribution that results from the secondaryelectron cascade in the metallic layer and of the electrontransfer into the semiconductor collector throughthe junction barrier. Both the energy and velocity relaxationof the incident electrons, by excitation of secondaryelectrons from the metal Fermi sea, are takeninto account. The calculation shows that the increasein the electron transmission and in the spin-dependenttransmission, observed when the injection energy exceedsseveral hundreds of eV, is a combined effect of the broadeningof the electron energy distribution and of the variationwith energy of the electron transfer efficiency at thebase-collector junction. It turns out that a hot-electronspin-filtering device that has a controlled barrier shape atthe base-collector interface and that can be operated atinjection energy of several hundreds of eV exhibits strikingtransport regimes. In particular, a structure havinga thin oxide interfacial layer between the magnetic metalbase and the semiconductor collector combines high spinselectivity (close to unity) and high electron transmission(larger than unity), opening up the possibility to achievelarge magneto-current asymmetry together with currentgain. For other specific base-collector barrier shape, asign reversal of the transmission spin-asymmetry couldeven be obtained at high injection energy due to thespin-dependence of the secondary electron multiplicationefficiency in the magnetic layer.II.EXPERIMENTThe spin-polarized electron transmission experiment isperformed in a UHV chamber with a base pressure ofafew10 −11 Torr. The principle of this experiment isschematized in Fig.1. The sample is a Pd/Fe/oxide/n-FIG. 1: : Schematics of the experimental set-up and principle.GaAs junction. The semiconductor collector is a 1 nmthickn-doped(10 16 cm −3 ) GaAs layer grown on an n + -doped(001) GaAs substrate with an ohmic back contact.A 2nm-thick oxide layer is formed on the surface of theGaAs top layer by exposure to UV-light and ozone. Thisthin oxide layer avoids interdiffusion between the GaAsand the subsequently grown metallic film and contributesto the junction potential barrier. 23 The metallic film ismade of a Fe layer, of thickness d Fe ≃ 3.5nm, coveredby a Pd cap layer, of thickness d Pd ≃ 5nm, which preventsthe iron from oxidation. The Fe layer exhibitsan in-plane magnetization square hysteresis loop, witha coercive field of about 500Oe and a remanent magnetizationm R close to the saturation magnetization m S ,