Optical orientation experiments have been performed in GaAs epilayers with photoexcitation energies in the 3 eV region yielding the photogeneration of spin-polarized electrons in the satellite L valley. We demonstrate that a significant fraction of the electron spin memory can be conserved when the electron is scattered from the L to the Γ valley following an energy relaxation of several hundreds of meV. Combining these high energy photo-excitation experiments with time-resolved photoluminescence spectroscopy of Γ valley spin-polarized photogenerated electrons allows us to deduce a typical L valley electron spin relaxation time of 200 fs, in agreement with theoretical calculations.PACS numbers: 72.25. Rb, 72.25.Fe, 78.55.Cr The electron spin dynamics have been studied in great detail for about 50 years in semiconductors thanks to the optical orientation technique [1, 2]. However all these experiments were performed with optical excitation energies close to the band gap (typically 1.5 -2 eV in GaAs), yielding the photogeneration of spin-polarized electrons in the Γ valley. In addition to its fundamental aspect, the knowledge of the electron spin dynamics of electrons in the upper valleys is of great interest for spintronic devices such as Spin-LEDs or Spin-Lasers, where the electrical spin injection can lead to electrons populating not only the Γ valley but also the satellite L and X valleys whose spin dynamics is almost unknown. In Spin-LEDs based on a Ferromagnetic (FM) layer and Schottky barrier, it was predicted with Monte Carlo simulations that strong electric fields at the interfaces between the FM and the semiconductor layer lead to the redistribution of electrons among several valleys (L and X), where the spin relaxation times have been predicted to be much shorter than the one in the Γ valley [3][4][5][6]. These upper valley electrons thus play a crucial role in the operation of Spin-LED and Spin laser devices [6]. For the demonstration of the Spin Gunn effect predicted a few years ago it is also essential to get information about the spin relaxation times of high energy electrons in the L valley [7]. Very little is known about the spin polarization and the spin dynamics of the high energy electrons in these L valleys in GaAs, though intervalley scattering in zincblende semiconductors has for a long time been a subject of theoretical and experimental interest [8][9][10]. The Dresselhaus intrinsic spin splitting, which is a key parameter for the spin polarization properties has been mainly studied in the close vicinity near the Brillouin zone center k 0 = Γ [11]. The spin-orbit coupling parameters in the upper valleys, for k 0 =L or k 0 =X have been calculated recently by different groups [11][12][13]. Compared to the Γ valley of III-V semiconductors, larger k-dependent spin splittings in the surrounding of the L point were predicted [13].As a consequence the D'Yakonov-Perel spin relaxation mechanism in the L valleys is expected to be very efficient. Multivalley spin relaxation in the presence of high ...
We demonstrate a large electrical spin injection into GaAs at zero magnetic field thanks to an ultrathin perpendicularly magnetized CoFeB contact of a few atomic planes (1.2 nm). The spin-polarization of electrons injected into GaAs was examined by the circular polarization of electroluminescence from a Spin Light Emitting Diode with embedded InGaAs/GaAs quantum wells. The electroluminescence polarization as a function of the magnetic field closely traces the out-of-plane magnetization of the CoFeB/MgO injector. A circular polarization degree of the emitted light as large as 20% at 25 K is achieved at zero magnetic field.Moreover the electroluminescence circular polarization is still about 8% at room temperature.
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