Spin photocurrents generated by homogeneous optical excitation with circularly polarized radiation in quantum wells (QWs) are reviewed. The absorption of circularly polarized light results in optical spin orientation due to the transfer of the angular momentum of photons to electrons of a two-dimensional electron gas. It is shown that in QWs belonging to one of the gyrotropic crystal classes a non-equilibrium spin polarization of uniformly distributed electrons causes a directed motion of electrons in the plane of the QW. A characteristic feature of this electric current, which occurs in unbiased samples, is that it reverses its direction upon changing the radiation helicity from left-handed to right-handed and vice versa. Two microscopic mechanisms are responsible for the occurrence of an electric current linked to a uniform spin polarization in a QW: the spin polarization-induced circular photogalvanic effect and the spin-galvanic effect. In both effects the current flow is driven by an asymmetric distribution of spin-polarized carriers in k-space of systems with lifted spin degeneracy due to k-linear terms in the Hamiltonian. Spin photocurrents provide methods to investigate spin relaxation and to reach a conclusion as regards the inplane symmetry of QWs. The effect can also be utilized to develop fast detectors for determining the degree of circular polarization of a radiation beam. Furthermore, spin photocurrents under infrared excitation were used to demonstrate and investigate monopolar spin orientation of free carriers. Contents 1. Introduction 936 2. Homogeneous spin orientation-induced photocurrents 938 2.1. Removal of spin degeneracy 938 2.2. The circular photogalvanic effect 942 2.3. The spin-galvanic effect 948 2.4. The spin orientation-induced circular photogalvanic effect versus the spingalvanic effect 952

There is much recent interest in exploiting the spin of conduction electrons in semiconductor heterostructures together with their charge to realize new device concepts. Electrical currents are usually generated by electric or magnetic fields, or by gradients of, for example, carrier concentration or temperature. The electron spin in a spin-polarized electron gas can, in principle, also drive an electrical current, even at room temperature, if some general symmetry requirements are met. Here we demonstrate such a 'spin-galvanic' effect in semiconductor heterostructures, induced by a non-equilibrium, but uniform population of electron spins. The microscopic origin for this effect is that the two electronic sub-bands for spin-up and spin-down electrons are shifted in momentum space and, although the electron distribution in each sub-band is symmetric, there is an inherent asymmetry in the spin-flip scattering events between the two sub-bands. The resulting current flow has been detected by applying a magnetic field to rotate an optically oriented non-equilibrium spin polarization in the direction of the sample plane. In contrast to previous experiments, where spin-polarized currents were driven by electric fields in semiconductor, we have here the complementary situation where electron spins drive a current without the need of an external electric field.

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A nonequilibrium population of spin-up and spin-down states in quantum well structures has been achieved applying circularly polarized radiation. The spin polarization results in a directed motion of free carriers in the plane of a quantum well perpendicular to the direction of light propagation. Because of the spin selection rules the direction of the current is determined by the helicity of the light and can be reversed by switching the helicity from right to left handed. A microscopic model is presented which describes the origin of the photon helicity driven current. The model suggests that the system behaves as a battery which generates a spin polarized current. DOI: 10.1103/PhysRevLett.86.4358 PACS numbers: 73.50.Mx, 68.65.-k, 73.50.Pz, 78.30.Fs The spin of electrons and holes in solid state systems is an intensively studied quantum mechanical property as it is the crucial ingredient for spintronics [1,2] and several schemes of quantum computation [3][4][5]. Among others, current investigations involve the spin lifetime in semiconductor devices [6][7][8] as well as the injection of spin polarized electrons (or holes) from semimagnetic semiconductor materials into semiconductors [9][10][11] or from ferromagnetic into nonmagnetic metals [12,13].It is well known that spin polarized electrons can be generated by circularly polarized light [14,15] and, vice versa, that the recombination of spin polarized charged carriers results in the emission of circularly polarized light [10,11,14]. However, little is known about spin dependent photocurrents when a semiconductor is irradiated by circularly polarized light [15,16]. Helicity dependent photocurrents in semiconductors have been observed in bulk Te utilizing the peculiarities of the valence band structure ("camel back") at the first Brillouin zone boundary and in bulk GaAs subjected to an external magnetic field [15]. A first indication of such a photon helicity dependent photocurrent in semiconductor heterojunctions was found in recent far infrared experiments on p-type GaAs͞AlGaAs heterojunctions containing a two-dimensional hole gas [17]. This preliminary experiment was discussed in phenomenological terms and lacked the microscopic connection to the carriers' spin.The experiments on quantum wells (QWs) described below uncover a novel property of an unbalanced spin polarization: its ability to generate a directed current where the current's direction depends solely on the predominant spin orientation. This effect may be illustrated as an electron analog of mechanical systems where a rotational motion ("spin") is transmitted into a linear one ("current") like a rotating wheel on a hard surface. Below we point out that spin injection into quantum wells of zinc-blende-type material leads always to an electric current in the plane of the quantum well. The reduced dimensionality of quantum wells lowers the crystallographic symmetry and introduces k-linear terms in the Hamiltonian. These k-linear terms lift the spin degenerate of energy bands in k-space which, in...

The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well (QW) structures are extracted from photocurrent measurements on n-type InAs QWs containing a two-dimensional electron gas (2DEG). This novel technique makes use of the angular distribution of the spin-galvanic effect at certain directions of spin orientation in the plane of a QW. The ratio of the relevant Rashba and Dresselhaus coefficients can be deduced directly from experiment and does not relay on theoretically obtained quantities. Thus our experiments open a new way to determine the different contributions to spin-orbit coupling.

The spin-galvanic effect and the circular photogalvanic effect induced by terahertz radiation are applied to determine the relative strengths of Rashba and Dresselhaus band spin splitting in ͑001͒-grown GaAs and InAs based two dimensional electron systems. We observed that shifting the ␦-doping plane from one side of the quantum well to the other results in a change of sign of the photocurrent caused by Rashba spin splitting while the sign of the Dresselhaus term induced photocurrent remains. The measurements give the necessary feedback for technologists looking for structures with equal Rashba and Dresselhaus spin splittings or perfectly symmetric structures with zero Rashba constant.

The problem of electron tunnelling through a symmetric semiconductor barrier based on zincblende-structure material is studied. The k 3 Dresselhaus terms in the effective Hamiltonian of bulk semiconductor of the barrier are shown to result in a dependence of the tunnelling transmission on the spin orientation. The difference of the transmission probabilities for opposite spin orientations can achieve several percents for the reasonable width of the barriers. Lately spin polarized electron transport in semiconductors attracts a great attention.1 One of the major problems of general interest is a possibility and methods of spin injection into semiconductors. A natural way to achieve spin orientation in experiment is the injection of spin polarized carriers from magnetic materials. Although significant progress has been made recently, 2,3,4,5 reliable spin-injection into low-dimensional electrons systems is still a challenge. Schmidt et al. pointed out that a fundamental obstacle for electrical injection from ferromagnetic into semiconductor was the conductivity mismatch of the metal and the semiconductor structure.6 However, Rashba showed that this problem could be resolved by using tunnelling contact at the metal-semiconductor interface.7 On the other hand Voskoboynikov et al.8 proposed that asymmetric nonmagnetic semiconductor barrier itself could serve as a spin filter. It was demonstrated that spin-dependent electron reflection by inequivalent interfaces resulted in the dependence of the tunnelling transmission probability on the orientation of electron spin. This effect is caused by interface-induced Rashba spin-orbit coupling 9 and can be substantial for resonant tunnelling through asymmetric double-barrier 10,11 or triple-barrier 12 heterostructures. However, in the case of symmetric potential barriers, the interface spin-orbit coupling does not lead to a dependence of tunnelling on the spin orientation.In this communication we will show that the process of tunnelling is spin dependent itself. We demonstrate that a considerable spin polarization can be expected at tunnelling of electrons even through a single symmetric barrier if only the barrier material lacks a center of inversion like zinc-blende structure semiconductors. The microscopic origin of the effect is the Dresselhaus k 3 terms 13 in the effective Hamiltonian of the bulk semiconductor of the barrier.We consider the transmission of electrons with the initial wave vector k = (k , k z ) through a flat potential barrier of height V grown along z [001] direction (see

The enhancement of the emission rate of charge carriers from deep-level defects in electric field is routinely used to determine the charge state of the defects. However, only a limited number of defects can be satisfactorily described by the Poole-Frenkel theory. An electric field dependence different from that expected from the Poole-Frenkel theory has been repeatedly reported in the literature, and no unambiguous identification of the charge state of the defect could be made. In this article, the electric field dependencies of emission of carriers from DX centers in Al x Ga 1Ϫx As:Te, Cu pairs in silicon, and Ge:Hg have been studied applying static and terahertz electric fields, and analyzed by using the models of Poole-Frenkel and phonon assisted tunneling. It is shown that phonon assisted tunneling and Poole-Frenkel emission are two competitive mechanisms of enhancement of emission of carriers, and their relative contribution is determined by the charge state of the defect and by the electric-field strength. At high-electric field strengths carrier emission is dominated by tunneling independently of the charge state of the impurity. For neutral impurities, where Poole-Frenkel lowering of the emission barrier does not occur, the phonon assisted tunneling model describes well the experimental data also in the low-field region. For charged impurities the transition from phonon assisted tunneling at high fields to Poole-Frenkel effect at low fields can be traced back. It is suggested that the Poole-Frenkel and tunneling models can be distinguished by plotting logarithm of the emission rate against the square root or against the square of the electric field, respectively. This analysis enables one to unambiguously determine the charge state of a deep-level defect.