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Detection of the degree of circular polarization of the electroluminescence of a light-emitting diode fitted with a spin injecting contact (a spin-LED) allows for a direct determination of the spin polarization of the injected carriers. Here, we compare the detection efficiency of (Al,Ga)As spin-LEDs fitted with a (Zn,Be,Mn)Se spin injector in top-and side-emission configuration. In contrast with top emission, we cannot detect the electrical spin injection in side emission from analysing the degree of circular polarization of the electroluminescence. To reduce resonant optical pumping of quantum-well excitons in the side emission, we have analysed structures with mesa sizes as small as 1 µm.-2 -The efficient electrical injection and detection of a spin polarized current into semiconductor heterostructures is a key element for the development of semiconductor spintronics [1,2]. Originally, much effort was devoted to detecting magnetoresistance changes in ferromagnet / semiconductor / ferromagnet interfaces [3,4], where, because of the very small effects observed, it turned out to be difficult to find evidence for spin injection [5]. An important step forward could only be made when zincblende-based light-emitting diodes (LED) (e.g. in the (Al,Ga)As system) were used as detectors for electrical spin injection. In this scheme, already proposed in the 70's by Aranov and Pikus [6], the spin polarized current is converted into circularly polarized electroluminescence; the degree of circular polarization of the electroluminescence of the spin-LED is directly proportional to the spin polarization of the carriers in the detection quantum well (QW). This approach has yielded evidence for spin injection using semi- [7] or ferro-magnetic [8] semiconductors, as well as ferromagnetic metal [9,10,11] contacts on GaAs based LEDs.Spin-LEDs are typically used in a top-emission measurement-geometry, where the external magnetic field is parallel to the current path, and parallel to the wave vector of the emitted light [7,9,10,12,13 ], as depicted in Fig. 1(a).However, for ferromagnetic materials like (Ga,Mn)As where the magnetic moment of the manganese system is typically perpendicular to the growth axis, side-emitted electroluminescence has been used to infer the spin polarization [8]. In this geometry the magnetic field is also parallel to the wave vector of the emitted light, -3 -but at the same time perpendicular to the growth axis of the structure and the confinement axis of the QW (see Fig. 1(a)). This quasi-Voigt geometry gives rise to different selection rules for optical transitions in GaAs based QWs [14] compared with those applicable for top emission [15]. If the selection rules are strictly valid, they may prohibit the determination of electron spin polarization by circular polarized light. Recent work on (Ga,Mn)As based spin-LEDs reveals a remarkable influence of the measurement geometry on the observed spacer layer thickness dependence of the circular polarization of the electroluminescence [16].In view of this, it se...

One major challenge for the development of spintronic devices is the control of the spin polarization of an electron current. We propose and demonstrate the use of a BeTe/Zn1−xSe/BeTe double barrier resonant tunneling diode for the injection of a spin-polarized electron current into GaAs and the manipulation of the spin orientation of the injected carriers via an external voltage. A spin polarization of up to 80% can be observed with a semimagnetic layer of only 3.5 nm thickness. By changing the resonance condition via the external voltage, the degree of spin polarization can be varied, though a complete spin switching has not yet been accomplished.

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We report on optical studies of exciton magnetic polarons in artificial crystals of digital alloys of diluted magnetic ͑semimagnetic͒ semiconductors built of alternating CdTe and Cd 1Ϫx Mn x Te layers, each only a few monolayers thick. Specially designed quantum well structures with the digital magnetic alloy in the quantum well region were grown and studied experimentally by magneto-optical methods. Precise comparative studies of the structures grown simultaneously either on ͑120͒or ͑100͒-oriented substrates were performed by photoluminescence excitation spectroscopy and photoluminescence under selective excitation. While no difference in magnetic properties, as measured by the giant Zeeman splitting of excitonic states, was observed between ͑120͒and ͑100͒-oriented diluted magnetic structures, the exciton magnetic polaron energy was found to be by 10-40 % larger in ͑120͒-oriented structures than in ͑100͒-oriented counterparts. This increase is explained by the effect of the heavy-hole effective mass anisotropy: greater hole mass in ͓120͔ direction causes an enhancement of an initial exciton localization which, in turn, gives rise to an increase of the magnetic polaron energy. Results of model calculations are in good agreement with experimental data. ͓S0163-1829͑98͒00432-9͔ PHYSICAL REVIEW B 15 AUGUST 1998-II VOLUME 58, NUMBER 8 PRB 58 0163-1829/98/58͑8͒/4785͑8͒/$15.00 4785