No abstract

International audienceWe have studied the electron spin injection efficiency from a CoFeB/MgO spin injector into AlGaAs/GaAs semiconductor light emitting diodes. The circular polarization of the electroluminescence signal reaches a value as large as 32% at 100 K under a 0.8 T magnetic field. We show that the spin injection efficiency increases with the increase in the MgO barrier thickness from 1.4 to 4.3 nm. Moreover, a higher spin injection efficiency is obtained for MgO barriers grown at 300 °C compared to the ones grown at room temperature. This effect is attributed to the MgO texturing occurring at high temperatures

No abstract

We have demonstrated by electroluminescence the injection of spin polarized electrons through Co/Al 2 O 3 /GaAs tunnel barrier into p-doped InAs/GaAs quantum dots embedded in a PIN GaAs light emitting diode. The spin relaxation processes in the p-doped quantum dots are characterized independently by optical measurements (time and polarization resolved photoluminescence). The measured electroluminescence circular polarization is about 15 % at low temperature in a 2T magnetic field, proving an efficient electrical spin injection yield in the quantum dots. Moreover, this electroluminescence circular polarization is stable up to 70 K.The achievement of spintronic devices necessitates on the one hand the ability to inject spin polarized currents into semiconductors 1 , and on the other hand sufficiently long spin relaxation times to manipulate or store the spin orientation of the injected carriers. The fundamental problem of conductivity mismatch between a ferromagnetic metal and a semiconductor pointed by Schmidt et al. 2 can be solved by the insertion of a tunnel barrier 3,4 which may be achieved either by a thin Schottky barrier 5,6 , or by an insulating layer as aluminium oxide [6][7][8] or Manganese Oxide 9 . In addition to efficient injection, long electron spin relaxation time is a requirement for efficient storage and manipulation. This is indeed the case for semiconductor quantum dots 10 . Electrical spin injection into InAs/GaAs intrinsic QDs has been reported with spin aligner systems based on magnetic semiconductor 11-13 or ferromagnetic metal/semiconductor Schottky barrier. [14][15] In this letter, we demonstrate the efficient electrical spin injection of polarized electrons into p-doped InAs/GaAs quantum dots through an insulating tunnel barrier of Al 2 0 3 . These QDs are very promising for several reasons: (i) the polarization of p-doped QD luminescence is directly related to electron spin polarization even for zero applied magnetic field, because of the cancellation of the anisotropic exchange interaction in the positively charged exciton 16 ; ii) very long electron spin relaxation times were reported recently 16 in these QDs under weak external magnetic field (about 100mT), which screens the effects of hyperfine interaction between electron and nuclear spins; (iii) long polarization decays (under excitation in the GaAs barrier) were measured even at high temperature. 17,18 The Spin-LED (SLED) structure (Fig. 1) was grown by molecular beam epitaxy for the semiconductor part and by sputtering for the tunnel barrier/ferromagnetic spin injector part. It consists in a P-I-N GaAs structure grown on a p-doped GaAs:Zn substrate (p = 2. 10 18 cm -3 ). The p-doped GaAs:Be layer (p = 1. 10 18 cm -3 ) is 2 µm thick. The doping of the n-GaAs layer (thickness: 50 nm) is 10 16 cm -3 to minimize the spin relaxation

No abstract