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Semiconductor Spintronics
Spintronic > State of Research > Generation of spin-polarized carriers

Generation of spin-polarized carriers

The generation of spin polarized carriers is generally synonymous with the creation of a non-equilibrium spin population, bearing in mind that the spin is not conserved in contrast to the charge, due to the spin-orbit and hyperfine interaction. Spin-polarized carriers can be generated in various ways. The simplest method uses circular polarized light. The optical selection rules in semiconductors for circular polarized light are spin selective due to the spin-orbit interaction [2-8] and allow the generation of spin polarized carriers by excitation of electrons from the valence band into the conduction band. Using the same selection rules it is possible to create entangled electron-hole pairs by excitation with, for example, linear polarized light [9]. Both methods are well established in basic research for some time. For future spintronic devices the optical spin injection is not suitable and electrical spin injection is required.

Electrical spin injection can be achieved in several ways. Very successful is the use of ferromagnetic metal contacts where spin injection has recently been demonstrated with a degree of polarization of 30% at room temperature [10]. These spin injectors are very promising for a large number of experiments and devices, but have two disadvantages. First, due to the large conductivity differences between metal and semiconductor, a good spin injection into the semiconductor can only be achieved when semiconductor and metal are separated by a high-resistance tunnel contact or if the degree of spin polarization in the spin injector is almost 100% [11-15]. The latter is difficult to achieve because magnetic bulk properties differ significantly from the magnetic interface properties. The second disadvantage of metallic spin injectors is that the distance from the spin injector to the area which is responsible for the functionality may be so large that the spin orientation is lost due to spin relaxation during transport. In principle, semiconducting spin injectors are superior to metal spin injectors because the conductivity differences and long transport routes do not exist in semiconducting spin injectors.

Semiconducting spin injectors have the additional advantage that the ferromagnetism in semiconductors is adjustable through the carrier density [16, 17]. Early studies on ferromagnetic semiconductors for spin injection focused on Cr- and Eu-based chalcogenide semiconductors [18]. Current studies can be divided into three different approaches. The first approach successfully uses paramagnetic (II, Mn) VI semiconductors and external magnetic fields for spin injection [19-21]. With this approach the experimentally achieved polarization degrees are nearly 100% and the used magnetic semiconductors can be lattice-matched grown on a non-magnetic semiconductor lattice. However, the used magnetic effects (giant Zeeman splitting) is only sufficiently strong only at low temperatures and the required external magnetic fields pose an additional limitation for the use in devices.

Important Dates:

15. Sept. 2013:
Deadline for the special volume semiconductor spintronics (DFG final report) in physica status solidi b
(further information is sent via email)

30. Sept. - 2. Oct. 2013:
final meeting of the priority program "International workshop on semiconductor spintronics" located in the Residenz Würzburg
(further information)

Recent publication(s):

C. Drexler, S.A. Tarasenko, P. Olbrich, J. Karch, M. Hirmer, F. Müller, M. Gmitra, J. Fabian, R. Yakimova, S. Lara-Avila, S. Kubatkin, M. Wang, R. Vajtai, P. M. Ajayan, J. Kono, and S.D. Ganichev :  "Magnetic quantum ratchet effect in graphene" Nature Nanotechnology 8, 104 (2013)

J.H. Buß, J. Rudolph, S. Shvarkov, H. Hardtdegen, A.D. Wieck, and D. Hägele:  "Long electron spin coherence in ion‐implanted GaN: The role of localization" Appl. Phys. Lett. 102, 192102 (2013)

D.J. English, J. Hübner, P.S. Eldridge, D. Taylor, M. Henini, R.T. Harley, and M. Oestreich:  "Effect of symmetry reduction on the spin dynamics of (001)-oriented GaAs quantum wells" Phys. Rev. B 87, 075304 (2013)

V.L. Korenev, I.A. Akimov, S.V. Zaitsev, V.F. Sapega, L. Langer, D.R. Yakovlev, Yu. A. Danilov, and M. Bayer:  "Dynamic spin polarization by orientation-dependent separation in a ferromagnet–semiconductor hybrid" Nature Communications 3, 959 (2012)

M. Althammer, E.-M. Karrer-Müller, S.T.B. Goennenwein, M. Opel, R. Gross:  "Spin transport and spin dephasing in zinc oxide" Appl. Phys. Lett. 101, 082404 (2012)