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.

Time-resolved Kerr rotation experiments show that two kinds of spin modes exist in diluted magnetic semiconductor quantum wells: ͑i͒ strongly coupled electron-magnetic ion spin excitations and ͑ii͒ excitations of magnetic ion spin subsystem decoupled from electron spins. The coexistence of these two kinds of spin precession modes cannot be understood in terms of average spins but requires a description, which goes beyond the mean-field approximation.

International audienceWe compare the potentiality of bulk InGaPN and GaAsPN materials quasi-lattice-matched to silicon (Si), for multi-junction solar cells application. Bandgaps of both bulk alloys are first studied by a tight-binding model modified for nitrogen incorporation in diluted regimes. The critical thicknesses of those alloys are then calculated for various compositions. For the same lattice-mismatch and nitrogen amount, the bandgap of bulk GaAsPN is found to be closer to the targeted gap value of 1.7 eV for high efficiency tandem solar cell. GaPN and GaAsPN epilayers are then grown by molecular beam epitaxy on GaP substrate and studied by photoluminescence and X-ray diffraction. A GaAsPN bulk alloy emitting light at 1.77 eV at room temperature is obtained, demonstrating promising properties for further use in III-V/Si photovoltaic multijunction solar cells

Équipe 101 : Nanomagnétisme et électronique de spinInternational audienceAn efficient electrical spin injection into an InGaAs/GaAs quantum well light emitting diode is demonstrated thanks to a CoFeB/MgO spin injector. The textured MgO tunnel barrier is fabricated by two different techniques: sputtering and molecular beam epitaxy. The maximal spin injection efficiency is comparable for both methods. Additionally, the effect of annealing is also investigated for the two types of samples. Both samples show the same trend: an increase of the electroluminescence circular polarization (P-c) with the increase of annealing temperature, followed by a saturation of P-c beyond 350 degrees C annealing. Since the increase of P-c starts well below the crystallization temperature of the full CoFeB bulk layer, this trend could be mainly due to an improvement of chemical structure at the top CoFeB/MgO interface. This study reveals that the control of CoFeB/MgO interface is essential for an optimal spin injection into semiconductor

Population properties and carrier dynamics in a GaAs/(Al,Ga)As double-quantum-well superlattice investigated by time-resolved photoluminescence spectroscopy A new approach is demonstrated for investigating charge and spin diffusion as well as surface and bulk recombination in unpassivated doped semiconductors. This approach consists in using two complementary, conceptually related, techniques, which are time-resolved photoluminescence (TRPL) and spatially resolved microluminescence (lPL) and is applied here to p þ GaAs. Analysis of the sole TRPL signal is limited by the finite risetime. On the other hand, it is shown that joint TRPL and lPL can be used to determine the diffusion constant, the bulk recombination time, and the spin relaxation time. As an illustration, the temperature variation of these quantities is investigated for p þ GaAs. V C 2014 AIP Publishing LLC. [http://dx.

Remanent electrical spin injection into an InGaAs/GaAs based quantum well light emitting diode is realized by using a perpendicularly magnetized MgO/CoFeB/Ta/CoFeB/MgO spin injector. We demonstrate that the Ta interlayer plays an important role to establish the perpendicular magnetic anisotropy and the thickness of Ta interlayer determines the type of exchange coupling between the two adjacent CoFeB layers. They are ferromagnetically or antiferromagnetically coupled for a Ta thickness of 0.5 nm or 0.75 nm, respectively. A circular polarized electroluminescence (Pc) of about 10% is obtained at low temperature and at zero magnetic field. The direction of the electrically injected spins is determined only by the orientation of the magnetization of the bottom CoFeB layer which is adjacent to the MgO/GaAs interface. This work proves the critical role of the bottom CoFeB/MgO interface on the spin-injection and paves the way for the electrical control of spin injection via magnetic tunnel junction-type spin injector.

Remanent spin injection into a spin light emitting diode (spin-LED) at zero magnetic field is a prerequisite for future application of spin optoelectronics. Here, we demonstrate the remanent spin injection into GaAs based LEDs with a thermally stable Mo/CoFeB/MgO spin injector. A systematic study of magnetic properties, polarization-resolved electroluminescence (EL) and atomic-scale interfacial structures has been performed in comparison with the Ta/CoFeB/MgO spin injector. The perpendicular magnetic anisotropy (PMA) of the Mo/CoFeB/MgO injector shows more advanced thermal stability than that of the Ta/CoFeB/MgO injector and robust PMA can be maintained up to 400 °C annealing. The remanent circular polarization (PC) of EL from the Mo capped spin-LED reaches a maximum value of 10% after 300 °C annealing, and even remains at 4% after 400 °C annealing. In contrast, the Ta capped spin-LED almost completely loses the remanent PC under 400 °C annealing. Combined advanced electron microscopy and spectroscopy studies reveal that a large amount of Ta diffuses into the MgO tunneling barrier through the CoFeB layer after 400 °C annealing. However, the diffusion of Mo into CoFeB is limited and never reaches the MgO barrier. These findings afford a comprehensive perspective to use the highly thermally stable Mo/CoFeB/MgO spin injector for efficient electrical spin injection in remanence.