W. Ossau

Injection and detection of a spin-polarized current in a light-emitting diode

The field of magnetoelectronics has been growing in practical importance in recent years. For example, devices that harness electronic spin—such as giant-magnetoresistive sensors and magnetoresistive memory cells—are now appearing on the market. In contrast, magnetoelectronic devices based on spin-polarized transport in semiconductors are at a much earlier stage of development, largely because of the lack of an efficient means of injecting spin-polarized charge. Much work has focused on the use of ferromagnetic metallic contacts, but it has proved exceedingly difficult to demonstrate polarized spin injection. More recently, two groups have reported successful spin injection from an NiFe contact, but the observed effects of the spin-polarized transport were quite small (resistance changes of less than 1%). Here we describe a different approach, in which the magnetic semiconductor BexMnyZn1-x-ySe is used as a spin aligner. We achieve injection efficiencies of 90% spin-polarized current into a non-magnetic semiconductor device. The device used in this case is a GaAs/AlGaAs light-emitting diode, and spin polarization is confirmed by the circular polarization state of the emitted light.

MOLECULAR-BEAM EPITAXY OF BERYLLIUM-CHALCOGENIDE-BASED THIN FILMS AND QUANTUM-WELL STRUCTURES

A variety of BeMgZnSe–ZnSe‐ as well as BeTe‐based quantum‐well structures has been fabri‐ cated and investigated. BeTe buffer layers improve the growth start on GaAs substrates drasti‐ cally compared to ZnSe/GaAs. The valence‐band offset between BeTe and ZnSe has been determined to be 0.9 eV (type II). Due to the high‐lying valence band of BeTe, a BeTe–ZnSe pseudograding can be used for an efficient electrical contact between p‐ZnSe and p‐GaAs. BeMgZnSe quaternary thin‐film structures have reproducibly been grown with high struc‐ tural quality, and rocking curve widths below 20 arcsec could be reached. Quantum‐well structures show a high photoluminescence intensity even at room temperature.

Laser diodes based on beryllium-chalcogenides

Beryllium chalcogenides have a much higher degree of covalency than other II–VI compounds. Be containing ZnSe based mixed crystals show a significant lattice hardening effect. In addition, they introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures due to their band gaps, lattice constants, and doping behavior. Therefore, these compounds seem to be very interesting materials for short wavelength laser diodes. Here, we report on the first fabrication of laser diodes based on the wide band gap II–VI semiconductor compound BeMgZnSe. The laser diodes emit at a wavelength of 507 nm under pulsed current injection at 77 K, with a threshold current of 80 mA, corresponding to 240 A/cm2.

Electron spin manipulation using semimagnetic resonant tunneling diodes

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.

Detection of electrical spin injection by light-emitting diodes in top- and side-emission configurations

Detection of the degree of circular polarization of the electroluminescence of a light-emitting diode (LED) fitted with a spin injecting contact (a spin-LED) allows the 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 configurations. In contrast to top emission, we cannot detect the electrical spin injection in side emission by analyzing the degree of circular polarization of the electroluminescence. To reduce resonant optical pumping of quantum-well excitons in side emission, we have analyzed structures with mesa sizes as small as 1 μm.

Epitaxy of Zn1 − xMgxSeyTe1 − y on (100)InAs

Abstract ZnTe and CdSe with lattice constants of 6.10 and 6.09 A are nearly lattice matched to InAs with a lattice parameter of 6.06 A. InAs is available as a high quality substrate material for molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED) and X-ray diffraction studies have been performed to investigate the nature of the MBE growth on InAs substrates with and without the epitaxial growth of an InAs buffer layer. For the quaternary Zn 1 − x Mg x Se y Te 1 − y (ZnMgSeTe), we could tune the band gap through the whole visible range. Lattice matched to the InAs substrate, rocking curve widths as low as 38 arcsec for ZnMgSeTe could be obtained. We will present results on structural and optical investigations of these layers and related quantum well structures. A pronounced curvature in the dependence of the band gap on composition could be found not only for ZnSeTe and MgSeTe, but also for ZnMgSe and ZnMgTe. A type-II band alignment between ZnTe and ZnMgSeTe allows us to measure band offsets directly via photoluminescence measurements in particular single quantum well structures.

Magneto-optical determination of the exciton binding energy in GaAs quantum wells

Abstract We measured the photoluminescence of GaAs-(GaAl)As multiquantum well structures in high magnetic fields. The diamagnetic shift of the ground state of the heavy-hole exciton is explained for the whole range of potential well widths studied, with a single reduced mass m = 0.038 m 0 , taking into account the quasi-two-dimensional character of the exciton. The deduced binding energy of the heavy-hole exciton is consistent with the reduced mass obtained from the diamagnetic shift. For well widths less than 80 A we have experimental evidence for the penetration of the exciton wavefunction into the adjacent barrier.

Binding energy of charged excitons in ZnSe-based quantum wells

Excitons and charged excitons (trions) are investigated in ZnSe-based quantum well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of magneto-optical spectroscopy. Binding energies of negatively

Determination of the band offset in semimagnetic CdTe/Cd1−xMnxTe quantum wells: A comparison of two methods

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Two dimensional exciton magnetic polaron in CdTe/Cd1-xMnxTe quantum well structures

and positively