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Spin Photocurrents in Semiconductor Quantum Well Structures
for recent review see: (PDF)

Lately, there is much interest to use the spin of conduction electrons in semiconductor heterostructures together with their charge to realise novel device concepts [1]. It is well known that spin polarized electrons can be generated by circularly polarized light  and, vice versa, that the recombination of spin polarized charged carriers results in the emission of circularly polarized light [2]. A new property of the electron spin in a spin-polarized electron gas, is that it can drive an electrical current if some general symmetry requirements are met, i.e the media belongs to the class of gyrotropic materials.

In low dimensional systems optical excitation by circularly polarized light not only leads to a spin polarized ensemble of electrons but also leads to a current whose direction depends on the helicity of the incident light [3]. This effect belongs to the class of photogalvanic effects [4] and represents the circular photogalvanic effect [5]. The coupling of the helicity of the incoming photons to spin polarized final states with a net momentum is caused by angular momentum selection rules together with band splitting in k-space due to k-linear terms in the Hamiltonian [6]. Due to 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.

Moreover achived by any means a net in-plane spin polarization of electrons in a quantum well (QW) based on zinc-blende structure material is inevitebly linked with an electric current in the well. We show that a uniform non-equilibrium spin polarization, generated by electrical or optical spin injection, results in a current flow [7]. This we denote as 'spin-galvanic effect' because of its relationship to other well established transport phenomena like magneto-galvanic or thermo-galvanic effects. This current is caused by spin relaxation due to asymmetric spin flip scattering in QWs with spin-split subbands in k-space due to k-linear terms in the Hamiltonian. The direction of current flow depends on in-plane spin orientation.

Spin photocurrents have been observed in GaAs [3-7], InAs [3], and SiGe [8] QW structures at temperatures varying from 4.2 K to 300 K in a wide spectral range from visible to far-infrared by using a pulsed far-infrared molecular laser delivering 100 ns pulses at wavelengths l from 35 mm to 280 mm [9], TEA CO2 laser at l= 9¸11 mm and Ti:Saphire laser at l = 0.777 mm.

The conversion of spin orientation of carriers into current provides two efficient methods to determine spin relaxation times, ts, of electrons in n-type QWs and of holes in p-type QWs. One of the methods is based on a spin-galvanic effect observed in n-GaAs QWs. A pronounced peak of the current as a function of the magnetic field is observed which is caused by the Hanle effect and allows to determine ts [7]. The second method is based on spin sensitive bleaching of the absorption of far-infrared radiation observed in p-GaAs QW structures [10].

  1. S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S.von Molnar, M.L. Roukes, A.Y. Chtchelkanova, and D.M. Treger, Science 294, 1488 (2001).
  2. Optical orientation, F. Meier, B.P. Zakharchenya, Eds. (Elsevier Science Publ., Amsterdam, 1984).
  3. S.D. Ganichev, S.N. Danilov, J. Eroms, W. Wegscheider, D. Weiss, W. Prettl, and E.L. Ivchenko, Phys. Rev. Lett. 86, 4358 (2001). (PDF)
  4. E.L. Ivchenko and G.E. Pikus, Superlattices and Other Heterostructures. Symmetry and Optical Phenomena, (Springer, Berlin 1997).
  5. S.D. Ganichev, E.L. Ivchenko, H. Ketterl, W. Prettl, and L.E. Vorobjev, Appl. Phys. Lett. 77, 3146 (2000).(PDF)
  6. S.D. Ganichev, E.L. Ivchenko, and W. Prettl, Physica E 14, 166 (2002) (PDF)
  7. S.D. Ganichev, E.L. Ivchenko, V.V. Bel'kov, S.A. Tarasenko, M. Sollinger, D. Weiss, W. Wegscheider and W. Prettl, Nature, 417, 153 (2002). (PDF)
  8. S.D. Ganichev, U. Rößler, F.-P. Kalz, W. Prettl, R. Neumann, K. Brunner, G. Abstreiter, and E.L. Ivchenko, Circular photogalvanic effect in Si/Ge semiconductor quantum wells, MRS Symp. Proc. 690 eds. T.J. Klemmer, J.S. Sun, A. Fert, and J. Bass, F3.11.1 (2001).
  9. S.D. Ganichev, Physica B 273-274, 737 (1999). (PDF)
  10. S.D. Ganichev, S.N. Danilov, V.V. Bel'kov, E.L. Ivchenko, M. Bichler, W. Wegscheider, D. Weiss, and W. Prettl, Phys. Rev. Lett. 88, 057401-1 (2002).(PDF)