We report transport of electron-hole complexes in semiconductor quantum wells under applied electric fields. Negatively charged excitons (X-), created by laser excitation of a high electron mobility transistor, are observed to drift upon applying a voltage between the source and drain. In contrast, neutral excitons do not drift under similar conditions. The X- mobility is found to be as high as 6.5 x 10(4) cm2 V-1 s-1. The results demonstrate that X- exists as a free particle in the best-quality samples and suggest that light emission from opto-electronic devices can be manipulated through exciton drift under applied electric fields.

We report the structural, electronic, and magnetic study of Cr-doped Sb2Te3 thin films grown by a two-step deposition process using molecular-beam epitaxy (MBE). The samples were investigated using a variety of complementary techniques, namely, x-ray diffraction (XRD), atomic force microscopy, SQUID magnetometry, magneto-transport, and polarized neutron reflectometry (PNR). It is found that the samples retain good crystalline order up to a doping level of x = 0.42 (in CrxSb2−xTe3), above which degradation of the crystal structure is observed by XRD. Fits to the recorded XRD spectra indicate a general reduction in the c-axis lattice parameter as a function of doping, consistent with substitutional doping with an ion of smaller ionic radius. The samples show soft ferromagnetic behavior with the easy axis of magnetization being out-of-plane. The saturation magnetization is dependent on the doping level, and reaches from ∼2 μB to almost 3 μB per Cr ion. The transition temperature (Tc) depends strongly on the Cr concentration and is found to increase with doping concentration. For the highest achievable doping level for phasepure films of x = 0.42, a Tc of 125 K was determined. Electric transport measurements find surface-dominated transport below ∼10 K. The magnetic properties extracted from anomalous Hall effect data are in excellent agreement with the magnetometry data. PNR studies indicate a uniform magnetization profile throughout the film, with no indication of enhanced magnetic order towards the sample surface.

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Spin-dependent transport is investigated in a Ni/Ge/AlGaAs junction with an electrodeposited Ni contact. Spin-polarized electrons are excited by optical spin orientation and are subsequently used to measure the spin dependent conductance at the Ni/Ge Schottky interface. We demonstrate electron spin transport and electrical extraction from the Ge layer at room temperature.

Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane in 3 nm Au/5 nm Ni 80 Fe 20 /GaAs(100) structures with doping density in the range 10 23 to 10 25 m Ϫ3 . At negative bias a helicity-dependent photocurrent dependent upon the magnetization configuration of the film and the Schottky barrier height was detected. The helicity-dependent photocurrent polarization decreases with increasing doping density and has the same variation with photon energy as found for the polarization of photoexcited electrons in GaAs. The results provide unambiguous evidence of spin-dependent electron transport through the NiFe/GaAs interface and show that the Schottky barrier height controls the spin-dependent electron current passing from the semiconductor to the ferromagnet.Since the possibilities for developing a spin-dependent field effect transistor ͑spin FET͒ were first raised by Datta and Das, 1 a great number of studies 2-5 on device structures based on ferromagnet ͑FM͒/semiconductor ͑SC͒ hybrid systems have been carried out. A spin FET for example offers the prospect of fast operation and miniaturization. 2 As the device operation would depend on the injection of a spindependent current into the SC, it is important to clarify the FM/SC interface transport process.The possibility of passing a spin-dependent current through thin film tunnel junctions of both Co/Al 2 O 3 /GaAs and Co/-MnAl/AlAs/GaAs has been discussed by Prins et al. 6 For the former structure, a spin-dependent tunneling current was reported. In the latter structure, however, only magneto-optical effects were seen. A great many studies of spin-dependent tunneling through metal/oxide insulator/ semiconductor ͑MOS͒ junctions have been reported. 7 Some recent experiments suggest 8 that such systems may provide possibilities for spin-polarized scanning tunneling microscopy ͑SP-STM͒. 9,10 However, due to the presence of the oxide layer, the mechanism of the spin-dependent tunneling through the MOS junction is extremely complicated. For the direct FM/SC interface, a Schottky barrier arises which also gives rise to tunneling under appropriate bias conditions. However, it is still not clear whether spin-dependent tunneling at the direct FM/SC interface through the Schottky barrier occurs and is detectable. Evidence of spin-dependent current effects associated with photoexcited electron transport at Schottky diode interfaces based on NiFe/GaAs has been demonstrated recently in forward bias. 11 However, only a small effect, comparable with the estimated magnitude of magneto-optical effects, was observed in reverse bias and therefore no firm conclusions concerning the possibility of spin tunneling could be drawn. In such experiments, the advantage of photoexcitation is that the electron polarization in the SC can be controlled via the light polarization.In this study, in order to clarify the Schottky barrier height dependence of possible spin-polarized transport effects, we fabricated 5 nm thick ferromagnetic layers ...