The photocurrent obtained under polarized optical excitation and the polarized electroluminescence recorded under forward electric bias have been measured in the same hybrid Semiconductor/Ferromagnetic metal structures (Spin-Light Emitting Diode). Systematic investigations have been performed on devices with different ferromagnetic spin injectors, consisting e.g. of MgO tunnel barriers with a CoFeB ferromagnetic layer. Though a very efficient electrical spin injection is demonstrated, very weak polarization of the photocurrent is evidenced: the photocurrent polarization measured under continuous resonant circularly polarized excitation of the quantum well excitons is below 3%. This demonstrates that the investigated devices do not act as efficient spin filters for the electrons flowing from the semiconductor part to the ferromagnetic part of these structures though these systems are very efficient spin aligners for electrical spin injection. We interpret the weak measured photocurrent polarization of the as a consequence of the Zeeman splitting of the quantum well excitons which yields different absorption coefficients for the polarized excitation laser with different helicities. This leads to different intensities of photocurrent collected for the two different circularly polarized excitations. This interpretation is confirmed by an experiment exhibiting the same results for photocurrent measured on a device with a non ferromagnetic electrical contact.
I . INTRODUCTIONBinary information encoded within the spin of carriers can be transferred into corresponding right or left-handed circularly polarized photons emitted from an active semiconductor medium via carrier-photon angular momentum conversion . In order to attain maximized spin-injection at out-ofplane magnetic remanence, a number of material systems have been explored as possible solid-state spin injectors . However the electroluminescence circular polarization (P C ) of emitted light was still limited at 3-4% at remanence . II . SAMPLE STRUCTURE The whole structure of our spin light emitting diode (spin LED) is schematically shown in Fig . 1(a) . The p-i-n multi InGaAs quantum well LED structure was grown by a III-V molecular beam epitaxy (MBE), while the tunnel barrier/ferromagnet contacts were deposited by an inter-connected MBE-sputtering system . In Fig . 1(b), we show the HR-TEM images for a CoFeB/MgO spin injector with perpendicular magnetization anisotropy (PMA) . The spin injector contains 2 .5nm MgO/ 1 .2nm CoFeB/ 5nm Ta, and the inset of Fig . 1(b) shows a good homogeneity and a very low roughness of MgO on GaAs . Moreover, the good continuity of the 1 .2nm CoFeB layer can also be validated in this picture . From the high magnification TEM image, a MgO (001) texture is clearly revealed with an abrupt interface to both the CoFeB and GaAs layers, whereas the interface of Ta/ CoFeB whose quality is less important for PMA appears rather diffusive . II . RESULTS In this talk, I would like to report our recent systematic develop of CoFeB/MgO in-plane and outof-plane spin injector on InGaAs quantum well based spin-LED . For the in-plane injector, we have used different growth techniques (sputtering and MBE) to prepare the MgO tunnel barrier [1] . It is interesting to found that the maximal spin injection efficiency is comparable for both methods . However the bias dependence of P C is quite different . In addition, the effect of annealing is also investigated . Both types of samples show the same trend: an increase of P C with the increase of annealing temperature, followed by a saturation of P C beyond 350°C annealing . Our study reveals that the control of CoFeB/MgO interface is essential important for an optimal spin injection into semiconductor . For out-of-plane injector, we have reduced the thickness of CoFeB down to 1 .2nm on MgO tunnel barrier (2 .5nm) to obtain a strong PMA . The maximum value of Pc measured at zero field is as large as 20% at 25K and still 8% at 300K, which is almost five times higher than any other types of PMA injectors [2] . Our results show that this type of ultrathin perpendicular spin-injectors are of great interest i) to realize the electrical switching of the magnetization of the injector layer owing to the advanced spin-transfer torque properties of CoFeB layer and ii) to be directly embedded in optical cavities for spin lasers due to their very low optical absorption loss . 1) P . Barate, et al ., "Electrical spin injection into InGaAs/GaAs quantum wells: A compari...
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